The Pennsylvania State University. The Graduate School. Department of Animal Science FACTORS THAT AFFECT RUMEN FERMENTATION AND TOTAL

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1 The Pennsylvania State University The Graduate School Department of Animal Science FACTORS THAT AFFECT RUMEN FERMENTATION AND TOTAL TRACT DIGESTION IN PRECISION FED DAIRY HEIFERS A Dissertation in Animal Science by Felipe Pino San Martin 2016 Felipe Pino San Martin Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2016

2 The dissertation of Felipe Pino San Martin was reviewed and approved* by the following: Arlyn J. Heinrichs Professor of Dairy Science Dissertation Advisor Chair of Committee Kevin D. Harvatine Associate Professor of Nutritional Physiology Chad Dechow Associate Professor of Dairy Cattle Genetics Gregory W. Roth Professor of Agronomy Terry D Etherton Distinguished Professor of Animal Nutrition Head of the Department of Animal Science *Signatures are on file in the Graduate School

3 iii ABSTRACT In the last decade farmers and researchers have focused on nutritional methods and management to improve feed efficiency in rearing dairy heifers. Precision feeding has been an interesting alternative to traditional ad-libitum, high forage diets fed to heifers. Precision feeding is an economical way to raise heifers that modifies physiological and nutritional responses, making heifers more efficient without affecting growth, first lactation milk production, or the animal in any way that we are aware. The literature available has provided information about dietary crude protein, optimal N intake, and diet forage-to-concentrate ratio (F:C) in precision feeding, but still more information is necessary. In the present dissertation, three experiments were conducted to evaluate the effect of starch, neutral detergent fiber (NDF) source, fiber digestibility, and rate of passage in precision feeding dairy heifers. The first experiment had two objectives: evaluate effects of starch concentration on digestibility and rumen fermentation and compare two sources of trace minerals (TM; inorganic, ITM, and organic, OTM, form) on digestibility and rumen fermentation. Eight rumen cannulated dairy heifers (15.4 ± 0.8 mo of age and ± kg of body weight) were subject to a split-plot, 4 4 Latin Square design with 19-d periods; 15 d adaptation and 4 d sampling. The whole-plot factor was type of TM; organic as proteinates (OTM) or inorganic sulfates (ITM), and the subplot was starch level (3.5, 12.9, 22.3, and 31.7%). Results of this experiment supported the hypothesis that the type of TM affects rumen bacteria populations and produces responses in ruminal

4 iv fermentation. Digestibility of dry matter (DM), NDF, acid detergent fiber (ADF), hemicellulose, and starch was not affected by treatments. The OTM decreased rumen ph and increased total volatile fatty acid (VFA) production and butyrate concentration. This can be explained by the lower time consuming the ration with OTM, which led to a faster fermentation. Also we hypothesize as the higher bioavailability of OTM suggests a faster utilization of the TM and accelerated replication of ruminal micro-organisms, stimulating ruminal fermentation and VFA production. Butyrate was also linearly increased as starch level increased. In general, TM excretion was not affected by type of TM. Plasma TM concentration was not different by treatment except for Mn, which was higher for OTM. However, mineral intake was reduced in OTM, but blood concentration was not different between TM types. These results suggest that OTM have higher TM absorption compared with the ITM. On the other hand, urine and total manure excretion were higher for ITM, suggesting that ITM stimulated water intake and produce more manure. In summary, the type of TM affected rumen fermentation such that OTM was absorbed to a greater extent than ITM, suggesting higher bioavailability for this form of TM. The objective of the second experiment was to evaluate sorghum silage (SS), including digestibility and fermentation parameters, in precision-fed dairy heifers. Eight Holstein heifers (13.7 ± 0.6 mo of age and ± kg of body weight) fitted with rumen cannulas were used in a replicated 4 4 Latin Square design; treatments were 4 levels of F:C (85:15, 75:25, 65:35, 55:45). When the concentrate proportion of the diet increased, heifers tended to improve feed efficiency, primarily due to lower DM intake (DMI) with the same average daily gain (ADG) over diets with a high proportion of

5 v forage. Rumen ph was affected by F:C, decreasing as the proportion of concentrate increased in the diet since heifers spent less time consuming feed. However, ph was never lower than 5.7 in diets with F:C 55:45, and fiber digestibility was not affected. Volatile fatty acid proportion was slightly influenced by treatment, where butyrate increased as concentrate increased in the diet. Dry matter and starch digestibility were affected by F:C and were improved in diets with more concentrate. Neutral detergent fiber, ADF, and hemicellulose digestibility were not affected by F:C. Wet and dry feces were reduced linearly as F:C decreased, but total manure was not affected by treatment due to increased urine production on high concentrate diets. In the in situ analysis, corn silage had a faster rate of digestion for DM and NDF than SS. This result suggests that the overall digestion of SS was diminished, probably because of the high NDF. Brown mid-rib SS can effectively be fed in precision diets for dairy heifers. Specifically in this study, the 65:35 F:C presented better performance based on rumen fermentation, VFA, rumen ph, digestibility, and feed efficiency. The third experiment was conducted with the objective to compare ad-libitum vs. precision feeding diets with two forages and different levels of NDF to evaluate rumen fermentation, diet digestibility, feed efficiency, and digesta passage rate. Eight Holstein heifers (18.4 ± 0.6 mo and ± kg BW) fitted with rumen cannulas were used in a two-factor, split-plot, Latin Square design with 19-d periods, 14 d of adaptation and 5 d of sampling. The whole-plot factor was feeding system with ad-libitum or precision feeding and 4 heifers in each plot. The subplot included 2 factors: forage quality (low quality: grass hay, LFQ; high quality: corn silage, HFQ) and NDF content (high NDF,

6 vi 48 % HNDF; low NDF, 39.8 %, LNDF). In this study we showed that the reduction in DMI for precision feeding diets improved feed efficiency in comparison with ad-libitum diets for dairy heifers. We observed that HFQ diets increased DMI, resulting in altered feed efficiency due to changes in intake based on fiber intake. Precision-fed diets resulted in a lower minimum rumen ph than ad-libitum diets, but the amount of time spent at the minimum ph was not great enough to reduce fiber digestion or rumen fermentation. Adlibitum diets resulted in lower mean ph than precision-fed diets, but the rumen ph was more consistent throughout the day than in precision feeding, where rapid fermentation resulted when heifers ate much of their daily diet within a small amount of time. This effect was stronger when corn silage was the forage component of the diet. Also, we observed that HNDF diets presented higher minimum ph, suggesting that the presence of additional fiber stimulates rumination and buffers the rumen. Overall, VFA proportions were not affected by the type of diet but were clearly modified by forage quality, where grass hay diets had higher proportions of acetate and corn silage diets higher proportions of propionate. Overall, apparent total tract digestibility was not affected by the type of diet; however, DM digestibility increased with HFQ and decreased with HNDF level. In situ digestibility was affected by forage quality and NDF level, where grass hay diets resulted in a greater 48 h rumen degradation than corn silage. Rate of passage was not affected by type of diet 22 h after feeding, but it was highly affected with the rumen at maximum capacity, 3 to 4 h after feeding. In this study, ad-libitum diets had higher passage rate than precision diets for the nutrients analyzed. Rate of digestion was affected by forage quality in the post-feeding evaluation, indicating that corn silage diets had

7 vii higher digestion rates than grass hay diets. This suggests that higher amounts of indigestible NDF reduced the digestion capacity of the rumen. With the results obtained in this study, we can state that the retention time for precision-fed diets was higher than ad-libitum diets and could lead an increased rumen digestion of nutrients. Also, grass hay diets had a higher retention time compared to corn silage diets. This effect was more significant in the precision-fed heifers. In addition, fluid dilution rate was higher for the ad-libitum diets. Grass hay diets presented a higher fluid dilution rate than corn silagebased diets. In summary, the three factors analyzed in this study affect ruminal fermentation, rumen ph, nutrient digestion, and rate of passage, but the most important result was the difference in feed efficiency presented in the precision feeding diets that could lead to a reduction in the cost of raising dairy heifers. Keywords: Heifers, precision feeding, starch, passage rate, digestibility, rumen fermentation.

8 viii TABLE OF CONTENTS LIST OF FIGURES... xi LIST OF TABLES... xii LIST OF ABBREVIATIONS... xiii ACKNOWLEDGEMENTS... xiv Chapter Introduction...1 Chapter Literature Review:Implications of Precision Feeding on Nutrient Digestion in Dairy Heifers...5 Traditional heifer feeding management and the impact on dairy farms... 5 Principles of precision feeding in dairy heifers... 8 Improvement of feed efficiency... 8 Physiological changes and metabolic adaptations to precision feeding Reduction in metabolic nutrient cost Passage rate of nutrients and digestibility Factors affecting nutrient digestibility in precision feeding Starch Intake Impact of F:C Effect of NDF on digestion Effect of DMI and passage rate on nutrient digestibility Conclusions... 26

9 ix Chapter Effect of trace minerals and starch on digestibility and rumen fermentation in diets for dairy heifers Abstract Introduction Materials and Methods Results and Discussion Conclusion Chapter Sorghum forage in precision-fed dairy heifer diets Abstract Introduction Materials and Methods Results and Discussion Conclusion Chapter Comparison of diet digestibility, rumen fermentation, rumen rate of passage, and feed efficiency in dairy heifers fed ad-libitum versus precision rations with low and high quality forages and 2 levels of neutral detergent fiber Abstract Introduction Materials and Methods... 71

10 x Animals, Treatments, and Experimental Design Diets Sample Collection and Analysis Statistical Analysis Results and Discussion Conclusions References Chapter Summary and conclusions

11 xi LIST OF FIGURES Figure 5-1. Rumen ph and total VFA production over 24 h in ad-libitum (left column) vs. precision-fed (right column) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF) Figure 5-2. Fermentation end products over 24 h in ad-libitum (left column) vs. precision-fed (right column) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF)

12 xii LIST OF TABLES Table 5-1. Ingredients and chemical composition of diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF) Table 5-2. Body weight, intakes, and feed efficiency in ad-libitum (A-L) vs. precision-fed (P-F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF) Table 5-3. Rumen ph, eating time, rate of eating, and VFA, in ad-libitum (A-L) vs. precision-fed (P-F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF) Table 5-4. Excretion parameters in ad-libitum (A-L) vs. precision-fed (P-F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF) Table 5-5. Apparent total tract nutrient digestibility and in situ digestibility in ad-libitum (A-L) vs. precision-fed (P-F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF) Table 5-6. Pre-feeding rumen digestion kinetics in ad-libitum (A-L) vs. precision-fed (P- F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF) Table 5-7. Post-feeding rumen digestion kinetics in ad-libitum (A-F) vs. precision-fed (P- F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF) Table 5-8. Fluid passage rate in ad-libitum (A-F) vs. precision-fed (P-F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF)

13 xiii LIST OF ABBREVIATIONS ADF Acid detergent fiber ADG Average daily gain BW CP DM DMI DIM HC HFQ ITM LFQ NDF OM OTM RDP RUP SD TM TMR VFA Body weight Crude protein Dry matter Dry matter intake Days in milk High concentrate High forage quality Inorganic trace mineral Low forage quality Neutral detergent fiber Organic matter Organic trace mineral Rumen degradable protein Rumen undegradable protein Standard deviation Trace Mineral Total mixed ration Volatile fatty acid

14 xiv ACKNOWLEDGEMENTS I would like to thank all my family, for supporting me throughout all these years of studies away from home. They are my motivation to continue every day and to take new professional challenges in the future. I would like to thank Natalie, my wife who has been a fundamental part of my life at Penn State. Without her it wouldn t have been possible to complete this stage of my education. I would like to thank my friends from Chile and from all around the world for their help, support and good moments during these years. I am really thankful to my advisor Dr. Heinrichs who was more than an advisor, he was part of my family here in the US. He gave me all the support to develop my projects during these years. Also I want to thank my committee members, Dr. Harvatine, Dr. Roth and Dr. Dechow for the help and guidance during my PhD program and beyond. Thanks to all the undergraduates and collaborators in my research projects, without them it wouldn t be possible to finish all the studies that we did in this period. Special thanks to Susan Strauch for her help and support taking care of Santiago during my last year at Penn State, without her it wouldn t have been possible to finish my PhD.

15 1 Chapter 1 Introduction Finding strategies to raise dairy heifers economically and efficiently is one of the most important topics for dairy farms. The profitability and performance of a dairy farm will depend on the efficiency of managements that maximize milk production while using resources wisely. Currently, a lot of effort is focused in lactating dairy cattle and most of the time, raising dairy heifers is not a priority for dairy farmers. However this situation is contradictory, because heifers are the 2 nd largest contributor to whole farm expenses (Tozer and Heinrichs, 2001). This situation deserves more concern and dedication by dairy farms and researchers, as opportunities to reduce whole farm expenses by reducing the cost of rearing dairy heifers exist. The management cost of raising heifers until initiation of lactation can be reduced by reducing expenses and improving growth rates, minimizing the time that heifers are unproductive (Hoffman et al., 1996). Feed cost represents 60-65% of the total expenses associated with dairy heifer growth until lactation (Gabler et al., 2000). Therefore, it is important to reduce feed cost, improve nutritional management and increment profitability in rearing heifers. Feeding practices that enhance profitability, reduce nutrient losses and produce physiological changes that improve efficiency are required for dairy heifers. Traditionally dairy heifers nutrition is based on low quality forages from weaning until parturition (Heinrichs, 1996). However, based on heifer requirements this traditional

16 2 feed is inefficient if we consider energy and protein nutrition, (Moody et al., 2007; Zanton and Heinrichs, 2009). In the last decade, research has focused on nutritional strategies that increase efficiency of growing dairy heifers. Thus, heifer diets that contain higher nutrient density and highly digestible feeds have been used to improve feed efficiency (Hoffman et al., 1996; Loerch, 1990). Improvements in feed efficiency involve less use of feeds, greater ADG and less waste of nutrients that will be lead in a reduction of expenses and greater profitability. Also, when feeding nutrient dense diets, heifers reduce DMI, and decrease the amount of manure output (Moody et al., 2007). In addition, heifers require less energy for digestion and hence, energy used for growth is enhanced when feeding highly digestible diets (Zanton and Heinrichs, 2007). Limiting the amount of DMI without affecting energy and protein supply is considered limit feeding (Hoffman et al., 1996; Loerch, 1990; Zanton and Heinrichs, 2008). When heifers are limit fed with isonitrogenous and isocaloric diets similar growth and lactation performance are observed compared to ad-libitum fed heifers (Lascano et al., 2009; Zanton and Heinrichs, 2007). Thus, this feeding system allows normal heifer growth, without affecting the mature body size or further milk production (Zanton and Heinrichs, 2007). Also, studies showed an increase in feed efficiency. Lately some studies have evaluated N efficiency and the effect of F:C ratio in limit feeding diets, but digestibility, rumen fermentation, and fiber degradability data are needed to understand the whole scenario in precision feeding dairy heifers. Precision feeding systems involve decreased DMI using highly digestible nutrients and feeding high energy dense diets, according to the requirements. Although precision feeding diets has been commonly used lately in research and farms, it still generates some concern, principally because of

17 3 the high proportion of concentrates that could lead to low rumen ph due to rapid fermentation. There is limited information in the scientific literature that approach precision feeding system to dairy heifers. For that reason, the purpose of this research was to evaluate nutrient utilization in precision fed dairy heifer, and nutritional implications, including fiber digestibility, rumen fermentation, and rate of passage of nutrients. Thus, this research expands our understanding of precision feeding diets and provides changes in the requirements for dairy heifers precision-fed during the growing period.

18 4 References Gabler, M. T., P. R. Tozer, and A. J. Heinrichs Development of a Cost Analysis Spreadsheet for Calculating the Costs to Raise a Replacement Dairy Heifer1. J Dairy Sci 83: Heinrichs, A. J Nutrition and management of replacement cattle. Animal Feed Science and Technology 59: Hoffman, P. C., N. M. Brehm, S. G. Price, and A. Prill-Adams Effect of Accelerated Postpubertal Growth and Early Calving on Lactation Performance of Primiparous Holstein Heifers. J Dairy Sci 79: Lascano, G. J., G. I. Zanton, F. X. Suarez-Mena, and A. J. Heinrichs Effect of limit feeding high- and low-concentrate diets with Saccharomyces cerevisiae on digestibility and on dairy heifer growth and first-lactation performance1. J Dairy Sci 92: Loerch, S. C Effects of feeding growing cattle high-concentrate diets at a restricted intake on feedlot performance. 68: Moody, M. L., G. I. Zanton, J. M. Daubert, and A. J. Heinrichs Nutrient utilization of differing forage-to-concentrate ratios by growing Holstein heifers. J Dairy Sci 90: Tozer, P. R., and A. J. Heinrichs What Affects the Costs of Raising Replacement Dairy Heifers: A Multiple-Component Analysis1. J Dairy Sci 84: Zanton, G. I., and A. J. Heinrichs The effects of controlled feeding of a highforage or high-concentrate ration on Heifer growth and first-lactation milk production. J Dairy Sci 90: Zanton, G. I., and A. J. Heinrichs Digestion and nitrogen utilization in dairy heifers limit-fed a low or high forage ration at four levels of nitrogen intake. J Dairy Sci 92: Zanton, G., and J. Heinrichs Precision feeding dairy heifers: strategies and recommendations. College of Agricultural Sciences, DAS:

19 5 Chapter 2 Literature Review: Implications of Precision Feeding on Nutrient Digestion in Dairy Heifers Traditional heifer feeding management and the impact on dairy farms It is known that dairy heifers are the future of herd milk production, but not enough investigation has been done in this area. Adequate heifer nutrition is key for optimization of body weight gain before calving, for proper development of the mammary gland, and for future milk production. Because heifers are in a unproductive period, many farmers do not take time to focus the appropriate management of these animals. In addition, many farmers are not aware of the impact of that heifer nutrition can have on future production of dairy cows. Compared to dairy cow nutrition research, very little work has been done in dairy heifers over the past 50 years, and the majority of dairy replacement research is focused on colostrum and calf nutrition (Eastridge, 2006). Even though dairy cows provide the major farm income, heifers represent the second or third largest cost towards the production of milk (Harsh et al., 2001; Tozer and Heinrichs, 2001) and also comprise a large proportion of animals in the farm inventory. Changes in heifer management can impact farm profitability and productivity (Hutjens, 2004; Zanton and Heinrichs, 2005). These features make heifer growth, nutrition, and reproduction interesting areas for research; where outcomes could reduce expenses in raising dairy heifers. The cost of raising heifers often represents 15 to 20% of the total annual expenses in a dairy farm, and nutrition is 60 to 70% of this cost (Gabler et al., 2000; Harsh et al.,

20 6 2001). Traditionally, dairy heifers are fed ad-libitum with high-forage, low-energy diets to meet their requirements; however, the amount of fiber consumed limits DMI. A tremendous disadvantage of this ad-libitum system is that heifers select feedstuffs in the ration, providing a non-homogeneous intake by a group of heifers, which could potentially affect rumen health. Also, heifers are physiologically inefficient in relation to the digestion and utilization of forages to meet their requirements, therefore feeding adlibitum diets increases waste of nutrients, contributing negatively to the environmental efficiencies in a dairy farm (Zanton and Heinrichs, 2009b). One of the main reproductive objectives of dairy farms to reduce expenses is to decrease the age at first calving to 22 to 23 mo (Heinrichs, 1993); however, to achieve this goal it has been shown to be necessary to improve growth performance (Hoffman et al., 2007). Tozer and Heinrichs (2001) estimated that when the age at first calving is reduced from 25 to 21 mo (by increasing diet energy density), the cost of raising dairy heifers is reduced by 18%. However, increased energy in the ration is not the only change that it is necessary to achieve this goal. Several studies show that when higher energy diets are offered, heifers increase prepuberty growth rate, but when animals are over conditioned has negative effects on time to conception, age at first calving, and difficulties at first calving, while first lactation milk yield is reduced (Little and Kay, 1979; Foldager and Sejrsen, 1991). Little and Kay (1979) observed milk yield decreased between 15 to 48% in first lactation heifers when high energy diets were used to increase ADG without controlling DMI, and Foldager and Sejrsen (1991) reported a 10 to 25% reduction in milk yield when pre-pubertal growth rate increased 0.6 kg/d over the control ADG (0.8 kg/d). When switching heifer diets from high forage to high concentrate (HC) by using a readily

21 7 digestible carbohydrate (ground wheat), Tremere et al. (1968) observed accumulation of rumen lactic acid, a decline in rumen ph under 5.0, and a shift in rumen fermentation that reduced fiber digestion and VFA concentration. Tajima et al. (2001) agreed with this information, while observing a depression in abundance of cellulolytic bacteria when feeding HC diets. Calsamiglia et al. (2008) also stated rumen ph affects the pattern of VFA production and true digestion, where HC diets that reduce rumen ph, reduce the concentration of acetate and butyrate produced and also reduce OM and NDF digestibility, reducing efficiency of nutrient utilization. However, as a reduction in the age of calving is desired, researchers have been recently investigating how energy and DMI affect heifer growth without affecting production potential, health, or welfare (Hoffman et al., 2007; Moody et al., 2007; Lascano and Heinrichs, 2009; Zanton and Heinrichs, 2009b; Pino and Heinrichs, 2016). Recent investigations have focused on nutritional changes that modify and increase feed efficiency of dairy heifers using energy dense diets, increasing energy density while reducing DMI (limit-feeding), without affecting rumen health and further milk production (Zanton and Heinrichs, 2005; Hoffman et al., 2007; Hall, 2008; Zanton and Heinrichs, 2009b). A limit-fed, energy dense diet that provides nutrients required for optimal growth in dairy heifers is a feeding system called precision feeding. Precision diets provide energy and protein to meet the requirements, reduce growth energy expenses, and improve feed efficiency in dairy heifers (Zanton and Heinrichs, 2009b). It has been demonstrated that this feeding system improves feed efficiency, reduces nutrient losses, and decreases manure production (Hoffman et al., 2007, Moody et al., 2007; Lascano et al., 2009; Zanton and Heinrichs, 2009b; Pino and Heinrichs, 2016). Zanton

22 8 and Heinrichs (2008a) observed that in precision feeding systems each kg of reduction in DMI, manure output decreased 2.6 kg. The reduction in manure reduces labor and expenses related to the management of manure and its disposal, and what is most important is that nutrient losses are also reduced. Principles of precision feeding in dairy heifers As mentioned, feed cost is the principal expense associated with rearing dairy heifers (Gabler et al., 2000). To substantially reduce this cost, a reduction in the age at first calving through increased ADG is necessary. However, ration costs need to be reduced also. The best method to reduce feeding costs is optimizing nutrient intake, by feeding animals to meet their requirements (precision feeding). This way, nutritional requirements are covered while nutrient losses are minimized (Hoffman et al., 2007; Zanton and Heinrichs, 2008a). Thus, precision feeding improves feed efficiency through a reduction in DMI, while keeping a constant ADG (Loerch, 1990; Galyean et al., 1999; Hoffman et al., 2007; Zanton and Heinrichs, 2008a). Precision feeding has also been reported in beef cattle as the most traditional way to reduce expenses (Koch et al., 1963; Loerch, 1990; Galyean et al., 1999). Improvement of feed efficiency Feed efficiency can be affected by several factors such as genetics, nutrient digestibility, forage quality, growth rate, age, body condition, gestational stage, temperature, and level of exercise, among others (Zanton and Heinrichs, 2008b). Genetic

23 9 selection of cows towards greater milk production has also increased average body size, simultaneously increasing DMI, energy, protein, and other nutrient requirements (Gabler et al., 2000). However, in recent years feed efficiency has been included in the parameters of bull selection, with a heritability of 0.37 (Van Arendonk et al., 1991). In beef cattle, feed efficiency has been selected for longer time and has a higher heritability than in dairy cattle (Arthur et al., 2001). The effect of DMI on feed efficiency has been extensively studied in dairy heifers. Traditional low-energy, high-forage diets limit energy intake because of the high fiber content (NRC, 2001) and prevent fat deposition in the pre-calving heifers. However, feeding dairy heifers with NRC (2001) recommendations greatly exceeds the optimum ADG, generating over-conditioned dairy heifers (Hoffman et al., 2007; Anderson et al., 2015; Akins, 2016). Limiting feed intake and providing nutrient dense diets that cover requirements is another way to improve feed efficiency (Loerch, 1990; Hoffman et al., 2007). In dry cows, limiting feed intake improves digestibility of DM and reduces feed cost (Driedger and Loerch, 1999). Similar observations were reported in dairy heifers, where reducing feed intake controlled growth rates without affecting first lactation milk yield (Lammers et al., 1999). This feeding system has been successfully used in beef cows (Loerch, 1996), ewes (Susin et al., 1995), and beef heifers (Wertz et al., 2001) without affecting production or animal performance. The relationship between energy intake and energy retention is not linear. Thus, maximum feed efficiency does not occur at maximum energy intake (Ferrell and Jenkins, 1998). This is the main justification of limit feeding, where feed efficiency is improved by managing nutrient utilization (Loerch, 1990; Galyean et al., 1999).

24 10 The metabolic nutrient cost of digestion in animals is higher as DMI increases. Nutrient digestion and absorption carried out by the gastrointestinal (GI) tract requires intense oxidative metabolism and uses a great portion of dietary energy. Remaining energy will be used for maintenance, growth, and productivity. Importantly, the GI tract, liver, spleen, and pancreas together use around 40 to 50% of body oxygen consumption. As the amount of nutrients to digest is larger, metabolic activity and oxygen consumption will increase, increasing nutrient utilization (Huntington and Reynolds, 1983; Reynolds et al., 1991b). In growing steers feed efficiency improved by 30% when intake was reduced 20 or 30% from ad libitum diets (Loerch, 1990). Importantly all diets kept the same net energy for maintenance and growth, and the animals maintained the same ADG. The improvement in feed efficiency observed when reducing DMI is explained by a reduction in rumen passage rate (Tamminga et al., 1979), allowing increased digestibility (Loerch, 1990). Increased digestibility is accompanied by reduction in feed waste (Hoffman et al., 2007); and reduced DMI is accompanied by a reduction in gut and liver size (Reynolds et al., 1991b) that reduces energy requirements for maintenance and increases energy available for growth (Loerch, 1990; Hoffman et al., 2007). These observations have been found in heifers fed energy dense diets; as DMI decreased, feed efficiency was improved (Wertz et al., 2001). A 10 or 20% reduction in intake allowance reduced manure output by 12.9 and 34.6%, respectively, while feed efficiency improved 23.7 and 28.9% compared to ad-libitum diets (Hoffman et al., 2007). Thus, by restricting DMI passage rate is reduced, while nutrient digestion and absorption are increased, and nutrient waste and manure are reduced. Overall, dairy

25 11 heifer feed efficiency is improved without negative effects on growth, health, or future milk production (Zanton and Heinrichs, 2009b). Physiological changes and metabolic adaptations to precision feeding Precision feeding involves the correct use of the nutrients, principally highly digestible nutrients, to provide a controlled and optimum ADG (Zanton and Heinrichs, 2005) and supply energy above maintenance to stimulate growth. To achieve this target, precision feeding systems reduce feed intake but use nutrient dense diets reduce metabolic nutrient costs (Reynolds et al., 1991a,b). Reduction in metabolic nutrient cost Limit feeding of dietary DM reduces the cost of energy used for nutrient digestion. Because energy consumption is based on requirements, fat deposition is limited by not overfeeding, and energy is partitioned to maintenance and growth, thereby increasing overall metabolic efficiency (Jarrett et al., 1976; Owens et al., 1995; Harsh et al., 2001). Energy dense diets (those with a higher proportion of concentrates), result in higher retention of energy in the tissues and reduced heat energy production. This was observed by Reynolds et al., (1991a,b) when comparing beef heifers fed a constant level of ME using 2 diets; 75% concentrate or 25 % concentrate. They also observed that heifers that received high concentrate diets had increased DM digestibility and feed efficiency. With a reduction in feed intake, nutrients are partitioned to metabolic and biological processes required for growth and maintenance (McLeod et al., 2007). Also,

26 12 fat storage is limited, which is beneficial as it is known that increased fat and growth decrease first lactation milk production (Zanton and Heinrichs, 2005). Improvement in feed efficiency is also achieved because as DMI decreases, visceral size and weight is reduced, and therefore energy used by the portal drained viscera for digestion processes is also reduced (Ferrell et al., 1986; Ferrell and Jenkins, 1998; McLeod and Baldwin, 2000; McLeod et al., 2007). The metabolic energy demand of the GI tract for digestion and absorption is very high; therefore, if GI tract size is reduced, oxygen and energy consumption by the portal drained viscera will be lower and more dietary energy will be available for other tissues (Reynolds et al., 1991b). Also Reynolds (2002) found that splanchnic tissues are responsible for 40 to 50% of total body oxygen consumption; therefore, heat increment and energy utilization will increase as splanchnic tissue mass is greater. In sheep, higher DMI was associated with greater weight and size of internal organs, where internal organs weighed up to 34% of BW (Colucci et al., 1989; Burrin et al., 1990). When feeding isoenergetic diets but with different F:C, higher intake in the high forage diets affected internal organ mass, increasing the energy used by the GI tract (McLeod and Baldwin, 2000). Also, in restricted fed lambs, organ mass was reduced as compared to ad-libitum, increasing feed efficiency (Fluharty and McClure, 1997). In beef heifers, as energy density increases (higher proportion of concentrates), tissue energy retention is higher and heat increment due to digestion is reduced (Reynolds et al., 1991a,b).

27 13 Passage rate of nutrients and digestibility As is described by Tamminga et al. (1979) and Wertz et al. (2001), limit feeding diets decrease rumen passage rate and increase diet retention time in the rumen (Lascano and Heinrichs, 2009). This allows increased exposure of diet components to rumen microorganisms, and therefore, increased rumen digestion and fermentation (Tamminga et al., 1979; Firkins et al., 1986; Merchen et al., 1986; Dijkstra, 1992). Bell (1971) stated that gut capacity remains a constant fraction of BW, but as BW increases some metabolic activities decrease, affecting efficiency. Clauss and Hummel (2005) stated that at high intakes the ratio of organ and gut surface to gut volume remains constant, but when intake decreases volume will decrease and the ratio will be greater. In this scenario, nutrients are retained for a longer time in the rumen, allowing more extensive degradation and utilization by rumen microbes. In addition, the surface area for digestion and absorption in the gut becomes proportionally higher, improving the interaction between nutrients and enzymes. This situation makes digestion and absorption more efficient. Firkins et al. (1986) and Merchen et al. (1986) observed that as DMI increases, rumen digestibility decreases and the pattern of ruminal fermentation changes. In diets where intake was reduced by changes in energy content (energy dense diets), propionate increased at the expense of acetate. Also, with HC diets microbial N and the efficiency of microbial protein synthesis increases (Merchen et al., 1986; Colucci et al., 1990; Zanton and Heinrichs, 2008a). In dairy heifers, the reduction in passage rate will reduce microbial protein flow to the small intestine, which is compensated by higher protein digestion and N retention (Zanton and Heinrichs, 2008a).

28 14 Changes in DM digestibility are variable and depend on diet F:C (Tyrrell and Moe, 1975). Experiments in sheep and beef cattle, Colucci et al., (1990) compared different F:C and observed that nutrient digestibility increases as concentrates increase in the diet. Pino and Heinrichs (2016b) and Lascano and Heinrichs (2011) observed the same effect in dairy heifers. Importantly, increased nutrient digestibility is observed with a reduction in total intake. In the last decade many studies have proven that HC diets that reduce DMI do not affect rumen ph, rumen health, and fiber digestion in precision-fed heifers (Moody et al., 2007; Lascano and Heinrichs, 2009; Lascano et al., 2014; Ding et al., 2015; Pino and Heinrichs, 2016). In addition, maximum rumen ph was higher for precision-fed dairy heifers than ad-libitum systems (Chapter 5). Increased digestibility is accompanied by a reduction in methane emissions and output of feces and urine, and therefore reduced nutrient loss (Reynolds et al., 1991b). Reduced manure production decreases farm expenses associated with manure management. In beef and dairy heifers, N intake is higher when feeding low concentrate diets; however, N efficiency and retention is greater with HC diets (Reynolds et al., 1991a; Zanton and Heinrichs, 2008a; Lascano and Heinrichs, 2011). Also, fecal DM, urine, and urinary N excretion were mostly lower when feeding HC diets. Amino acids, urea N, and glucose released by splanchnic tissues to peripheral organs was greater when animals received HC diets, making more nutrients available to other tissues and for growth in the case of heifers (Huntington et al., 1996; Zanton and Heinrichs, 2007). Also, Zanton and Heinrichs (2007) found that maximum N efficiency was at 1.8 g N intake/kg BW 0.75 in dairy heifers and concluded that N use is more efficient when feeding HC diets.

29 15 Factors affecting nutrient digestibility in precision feeding An important feature of nutrient digestion in ruminants is that enzymatic hydrolysis is the principal mechanism of digestion (Van Soest, 1994). Ruminants and rumen microbes coexist through a symbiotic relationship, where dietary components are utilized and fermented by rumen microorganisms, and their end products are nutrients used by the ruminant (Van Soest, 1994). Even though a high proportion of concentrates are used in the dairy industry, forages still are the principle source of nutrients for ruminant production systems. However, low DM digestibility due to plant cell walls is associated with limited energy in forage-based diets. This low digestion of nutrients leads to a less efficient animal in the dairy industry (Jung, 1989; Galyean and Goetsch, 1993). There are many factors that affect nutrient digestibility in ruminants. Some of the most important are: chemical composition of feedstuffs, vegetative stage of the plants or grains at harvest, type of grain processing, dietary load, rate of digestion, passage rate, nutrient interactions, rate of fermentation, particle size, and F:C among others (Jung, 1989; Van Soest, 1994). The interaction of these factors can modify digestion of nutrients; even when comparing diets with equal nutritional value, digestibility of feedstuffs could be very different. This review will focus on discussing some important factors that are not well studied in precision feeding diets for dairy heifers and are in close relation with the studies done recently. The factors analyzed below are starch intake, F:C, NDF, DMI, and passage rate.

30 16 Starch Intake Carbohydrates are the principal source of energy for maintenance, growth, and productivity in animals (Armstrong, 1965). Houseknecht et al. (1988) observed that beef heifers fed low energy-low starch diets presented lower plasma insulin-like growth factor and reduced growth, ADG, and backfat and ribeye area. However, feeding highly digestible nutrients, specifically rapidly fermentable non-structural carbohydrates, to dairy heifers is still a concern for dairy farmers and researchers (Nocek, 1997; Kmicikewycz and Heinrichs, 2014). High starch diets are often associated with changes in the rumen bacteria that shift toward amylolytic microbes, thus decreasing rumen fiber digestion (Tremere et al., 1968; Hoover, 1986; Nocek, 1997). Precision feeding diets in dairy heifers contain highly digestible nutrients to reduce DMI, increase digestibility, and improve feed efficiency (Lascano and Heinrichs, 2009; Zanton and Heinrichs, 2009b). However, precision-fed dairy heifers do not present sub-acute ruminal acidosis (SARA) or depression of DM digestibility. High intake cows are exposed to large amounts of highly fermentable carbohydrates, often in rations that are fiber deficient. Fiber deficient diets reduce rumination and salivation; therefore, flow of salivary buffer to the rumen is decreased and susceptibility for SARA increases (Kmicikewycz and Heinrichs, 2014). In cows, increments of readily fermentable carbohydrates depress fiber digestion due to a marked preference of microbes for carbohydrates instead of fiber; this leads to a decrease in rumen ph and a reduction of cellulolytic microorganisms, and therefore of fiber digestion (Hoover, 1986). However, DMI and the total amount of highly digestible carbohydrates (principally starch that is more associated with SARA) in precision-fed dairy heifers is reduced to cover requirements; hence rumen ph does not decrease to

31 17 levels that affect rumen digestion (Moody et al., 2007; Lascano and Heinrichs, 2009; Zanton and Heinrichs, 2009b; Pino and Heinrichs, 2016). It is important to state that dietary starch concentration can affect rumen ph and fiber digestibility if the inclusion of the starch is abrupt and without an adaptation period (Tremere et al., 1968; Hoover, 1986). This occurs due to the rapid increment of Streptococcus sp. and the large proportion of lactic acid released in the rumen within a couple of h after feeding (Cerrilla and Martinez, 2003). However, if the starch increment is gradual over time and the microbial population shifts to more amylolytic bacteria, it is possible to expect no changes in fiber nor DM digestion and, importantly, no ruminal acidosis (Tremere et al., 1968). Starch digestion per se depends on different factors, for example: starch source and type, DMI, dietary composition, processing, and adaptation of microbes (Huntington, 1997). Different grains have a different digestion capacity and time of rumen fermentation (Church, 1988; Huntington, 1997). In general, ground grains have higher digestibilities than whole (Moe and Tyrrell, 1977) and flaked corn is more rapidly fermentable in the rumen than rolled corn (Schuh et al., 1970). Effective starch degradability is also different between grains. Steam-flaked corn and sorghum have the highest starch degradability, while wheat or barley have the highest degradability when the grain is ground (Offner et al., 2003). Barley, wheat, and oats have a high fraction of immediately soluble starch in comparison to corn. These characteristics affect rumen fermentation and rumen ph and modify nutrient digestion (Offner et al., 2003). Pino and Heinrichs (2016) evaluated the effect of 4 different starch concentrations (3.5, 13, 22.3, and 32%) in precision-fed dairy heifers. Mean ph was higher for the 2

32 18 highest starch concentrations, and ph never decreased under 5.7. Dry matter digestibility was not depressed by any treatment, and DM and starch digestibility were higher at higher dietary starch concentrations. Also, hemicellulose digestion decreased as starch intake was higher; however, Lascano et al. (2012) observed a tendency for hemicellulose digestion to increase with higher starch in the diet. In addition, Lascano and Heinrichs (2009) and Moody et al. (2007) reported a lower minimum rumen ph in precision-fed dairy heifers fed different F:C, but in none of them was DM or fiber digestibility decreased when compared to traditional ad-libitum diets. Also, Lascano and Heinrichs (2011) observed higher DM, NDF, and starch digestibility in HC-high starch diets. Addition of feed additives can modify nutrient digestion. Lascano et al. (2012) reported that regardless of starch concentration (16.7 vs. 28%), DM, OM, NDF, and starch digestibility were not different as the dose of dietary yeast (Saccharomyces cerevisiae) increased; however, DM, OM, hemicellulose, NDF, and ADF digestibility increased quadratically with increasing dose of yeast. Colucci et al. (1989) evaluated the effect of intake and dietary concentrate on digestibility in cows and sheep. Here, DM, starch, and hemicellulose digestibility increased with higher dietary starch in sheep, and DM digestibility increased in cows regardless of DMI (10, 22, and 32.4% starch). When comparing low and high DMI, DM, CP, NDF, and hemicellulose digestibility increased in sheep and cows with low intake and high starch diets. Similar are the results of Pino and Heinrichs (2016b), where although NDF, ADF, and hemicellulose digestibility were not affected, DM and starch digestibility increased linearly as starch replaced dietary forage.

33 19 Impact of F:C Precision feeding requires high energy, limit-fed diets; however, dairy farmers preferentially feed dairy heifers with ad-libitum, low energy, forage-based diets. Although limit-fed, high energy diets contain more concentrates, they are more cost effective per unit of energy and protein than forages (Zanton and Heinrichs, 2007). High concentrate diets have been shown to have greater efficiency of ME use (Blaxter and Wainman, 1964). Also, high energy diets reduce DMI in dairy heifers to cover requirements and reduce the nutrient loss that occurs in ad-libitum diets. Reduced DMI and reduced loss of nutrients in feces and refusals benefit farm economy as it reduces the cost of rearing. Also, the reduction in DMI reduces visceral organ size that reduces the metabolic cost of digestion (Reynolds et al., 1991b). This allows more energy to be used by the rest of the organs for growth (Huntington et al., 1996), and leads to an increase in feed efficiency and a reduction in dairy heifer rearing cost (Zanton and Heinrichs, 2009b). Additionally, HC diets are more digestible due to the lower DMI and higher rumen retention time, prolonging the interaction with microbes that degrade nutrients to a larger extent (Wertz et al., 2001; Lascano and Heinrichs, 2009). The concept of using higher concentrate proportions in ruminant diets has been widely described before (Colucci et al., 1989; Reynolds et al., 1991b; Huntington et al., 1996; Zanton and Heinrichs, 2008a; Lascano and Heinrichs, 2009; Suarez-Mena et al., 2015; Lascano et al., 2016). Reynolds et al. (1991) and Huntington et al. (1996) observed that at the same level of ME intake, steers that consumed HC diets had reduced heat production and more energy was used for growth. Also, as diet concentrate increased, nutrient digestibility was higher. Reynolds et al. (1991b) observed that with higher

34 20 concentrate in the diet, apparent digestion of DM, energy, CP, ether extract, ash, OM, NDF, ADF, and hemicellulose increased in steers that consumed a low F:C (25:75). Similar results were observed by Murphy et al. (1994) in lambs, when as diet concentrate proportion increased (up to 92% of the ration) apparent digestibility of DM, OM, ADF, NDF, and starch increased linearly. Colucci et al. (1989) found that apparent digestibility of DM and energy increased at low and high DMI as dietary concentrate increased in cow and sheep diets. Also, NDF digestibility increased linearly as dietary concentrate increased, only at low DMI. In cows at low intakes, NDF, ADF, and hemicellulose digestion increased linearly as dietary concentrate increased (Colucci et al., 1989). Thus an interaction between F:C and DMI exists. In dairy heifers apparent digestion for DM, OM, and ash increased with HC diets (Zanton and Heinrichs, 2009a; Lascano and Heinrichs, 2011; Suarez-Mena et al., 2015; Lascano et al., 2016; Zanton and Heinrichs, 2016). Lascano and Heinrichs (2011, 2016) and Pino and Heinrichs (2016b) observed that apparent starch digestibility increased with HC diets, and Lascano and Heinrichs (2011, 2016) observed that NDF and ADF digestibility increased with HC diets. The other authors did not observe increments in NDF, ADF, and hemicellulose digestion or found a reduction in digestion as in Zanton and Heinrichs (2009a). This could be explained by a shift in rumen bacteria towards amylolytic bacteria at expenses of fibrolytic bacteria, limiting fiber digestion (Mertens and Loften, 1980; Calsamiglia et al., 2008). Zanton and Heinrichs (2009a) observed higher CP and N digestibility in diets with a low proportion of forages. Lascano et al. (2016) did not observe differences in N digestion, but N excretion was reduced and N retention was higher in heifers fed HC

35 21 diets. Thus, researchers have observed that the reduction in excretion and the increase in N retention leads to increased N efficiency and use by animals fed with HC diets (Murphy et al., 1994; Moody et al., 2007; Zanton and Heinrichs, 2009b). In general, as the F:C decreases, DM, OM, and starch digestion increases due to more energy available in the rumen and rapid growth of microbes that can degrade nutrients faster than in diets with high proportion of forages (Merchen et al., 1986). Also, in precision-fed dairy heifers HC rations reduce DMI and stimulate rumen retention that will provide a higher digestion response (Zanton and Heinrichs, 2009b). Effect of NDF on digestion Animal performance depends on intake and digestibility of nutrients. But, intake and digestibility of nutrients are closely related to the amount and digestibility of dietary NDF (Van Soest, 1967; Mertens, 2009). The physical properties and source of NDF are important factors associated with ruminal degradation of nutrients. Forages and concentrates have different NDF proportions, but due to physical and chemical characteristics they also present differences in rumen digestion (Sarwar et al., 1991). Diets with the same amount of NDF could have different nutrient digestibility depending on the source of NDF (forages or concentrates; Mertens, 2009) or differences in the rate and extent of NDF digestion (Varga and Hoover, 1983). It has been demonstrated that NDF content and the variation in digestion are some of the most important factors associated with changes in DM digestibility. When low energy, high fiber rations are fed, ruminants regulate intake based on rumen fill; however, when high energy, low fiber diets are fed, ruminants limit their

36 22 intake based on maintenance and production energy requirements (Colucci et al., 1982; Van Soest, 1994; Mertens, 2009). Fiber content and its digestibility have a large impact on nutrient digestion because fiber is the least digestible component of the diet. Intake is regulated by the amount of rumen undigested NDF and also by rapidly digested NDF. Cell walls that degrade rapidly in the rumen promote a greater rate of digestion and nutrient rate of passage, leading to a greater DMI (Mertens and Loften, 1980; Varga and Hoover, 1983; Oba and Allen, 2000; Mertens, 2009). Oba and Allen (2000) observed that at higher DMI, rate of passage was higher and NDF digestibiliy was lower. However, this does not occur in limit-fed heifers. As DMI is limited, nutrients have a longer exposure time to rumen microbes, and degradation and digestion are increased (Colucci et al., 1989; Zanton and Heinrichs, 2009b). Sarwar et al. (1990) studied the replacement of forage NDF with soyhulls or corn gluten in dairy heifer ad-libitum diets and found that the diet with soyhulls or corn gluten decreased rumen ph 3 h after feeding and decreased rumen NDF digestion when compared to control (forage NDF). However, total tract NDF digestibility was higher and OM digestibility was lower when feeding soyhulls or corn gluten compared to control. The drop in OM digestion is because of lower rumen ph and its effect on fibrolytic bacteria. Similar results were reported by Calsamiglia et al. (2008) using in vitro studies. However, in precision-fed dairy heifers, as intake is reduced, ph does not affect digestibility to a large extent. Minimum ph (reported in some precision feeding trials) never fell below 5.5 and did not have effects on NDF digestibility (Lascano et al., 2009; Pino and Heinrichs, 2016; Zanton and Heinrichs, 2016).

37 23 To demonstrate that nutrient digestibility increased with low intakes, Colucci et al. (1989) evaluated low and high intake of HC diets in sheep and cows. For both species, DM, energy, and CP digestibility increased in diets with low and high intake; however, NDF, ADF, and hemicellulose digestibility only increased with low intake. Therefore, low intake and HC diets increased fiber digestion due to higher fiber retention time of the diet in the rumen. Also, NDF digestibility has been shown to increase in low intake situations using low and high concentrete diets (Colucci et al., 1989). Ding et al. (2015) observed that low quality forages (high in NDF) used in precision-fed heifers presented lower DM and OM digestion than high quality forage diets. In precision feeding diets, studies have demostrated that fiber digestibility does not decrease with HC, because there is no drop in rumen ph and hence no reduction in fibrolytic bacteria (Pino and Heinrichs, 2016; Zanton and Heinrichs, 2016). Data available suggests that the amount of dietary NDF does not directly affect nutrient or fiber digestibility. Even though fiber digestibility depends of many factors (Mertens, 2009), more studies are necessary to compare precision feeding diets to traditional ad-libitum diets to evaluate how these factors are affected with low intake. Effect of DMI and passage rate on nutrient digestibility The nutritional value of ruminant diets is affected by the rate of nutrient degradation and evacuation from the rumen. These 2 factors will determine the release of nutrients by microbial fermentation and feed intake. Increasing feed intake usually increases rate of liquid and particle passage through the rumen and the GI tract in ruminants (Balch and Campling, 1965; Colucci et al., 1990). Higher amounts of dietary

38 24 fiber will increase the rate of passage principally by the increase in rumen load and stimulation of evacuation (Clauss and Hummel, 2005). Bell (1971) determined that gut capacity of herbivores remains a constant fraction of BW, and as BW increases there are some specific tissue metabolic rates that decrease. In ad-libitum diets, the ratio between organs and gut surface to digesta volume stays constant, but in precision feeding this ratio increases due to changes in the GI tract volume. This allows a higher nutrient retention time in the rumen and also greater surface of contact with gut enzymes for digestion and absorption, and thus, greater digestibility (Clauss and Hummel, 2005). As stated before, precision-fed heifers consume and digest fewer nutrients than when fed traditional ad-libitum diets; reducing heat production due to digestive metabolism and retaining more energy that can be used by tissues for growth (Reynolds et al., 1991b). In ad-libitum fed heifers, passage rate is greater when high proportions of concentrates are fed in the diet. This is explained by the smaller particle size of these diets in comparison to forage-based diets (Van Soest, 1994).With HC ad-libitum diets, retention time is reduced, together with decreased rumen digestion of nutrients (Colucci et al., 1982, 1990). However, in precision feeding heifers DMI is controlled to cover energy and N requirements with energy dense diets, and then rumen retention time is prolonged (Colucci et al., 1989; Murphy et al., 1994). With the reduction in intake, rate of passage is reduced, increasing exposure time of feedstuffs to microbes, leading to an improvement in nutrient degradation by rumen microbes; overall, nutrient digestibility is increased in precision feeding dairy heifers (Colucci et al., 1990; Zanton and Heinrichs, 2008a; Zanton and Heinrichs, 2009b; Lascano et al., 2016).

39 25 Zanton and Heinrichs (2008a) offered 4 different levels of DMI (high forage diets) and evaluated the rate of passage in dairy heifers. They observed that as DMI increased up to ad-libitum levels, rumen passage rate also increased. Also the researchers observed that DM, OM, and NDF digestibility increased as intake decreased, leading to higher feed efficiency in limit-fed heifers. Lascano et al. (2016 ) showed that low forage diets had lower turnover rate for solid and liquid fractions than higher fiber diets, and also that rate of passage increased linearly as dietary fiber increased. These changes in retention time allowed greater DM, OM, NDF, ADF, cellulose, and starch digestibility in low forage diets, while DM, OM, and cellulose digestion decreased linearly as dietary fiber increased. Retention time of rumen digesta is also affected by F:C. Lascano and Heinrichs (2009) evaluated 3 F:C with 3 different levels of intake. Heifers that consumed the smallest F:C presented a lower DM turnover rate leading to a higher rumen retention time. Pino and Heinrichs (2016a) evaluated diets with 4 different starch concentrations and 4 different DMI dairy heifers. As dietary starch concentration increased, DMI decreased linearly. In this study, DM, hemicellulose, and starch digestibility increased linearly as starch concentration increased; however, treatments did not affect NDF and ADF digestion. Zanton and Heinrichs (2016) observed that heifers fed with low energy diets and high DMI presented a greater ruminal passage rate and lower OM digestibility at 8 and 20 mo of age. Also, they observed that heifers that received high energy diets with lower DMI presented higher N digestion and retention when compared to low energy, high intake diets. Colucci et al. (1989) observed that sheep and cows fed with low intake diets presented greater digestion performance due to longer retention time in the rumen.

40 26 Conclusions New information and more focused research about precision feeding in dairy heifers has been published in the last 10 years. Studies support that this feeding system improves feed efficiency through a reduction in DMI, energy dense diets, and highly digestible feedstuffs, covering the requirements of growing animals. By reducing DMI, the metabolic expenses of the portal-drained viscera, liver, and kidneys was decreased by a dramatic reduction in oxygen and glucose consumption. The reduction in DMI also changes the passage rate of nutrients in the rumen, where rumen turnover is lower as DMI decreases. Thus, feedstuffs stay in the rumen longer, and microbes have more time to degrade nutrients, increasing diet digestibility. Also, precision feeding systems can reduce the cost of rearing heifers. As less feedstuffs are used there are no refusals or nutrient losses and there is a reduction in manure output. The objective of this literature review was to analyze metabolic adaptations of dairy heifers to precision feeding systems and to expose the effect of some nutrients on precision feeding and digestibility. In the last decade seminal studies on precision feeding have been performed; however, more research is needed to evaluate the impact of specific nutrients in this system and compare digestibility and rumen fermentation in precision feeding vs. ad-libitum diets.

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44 30 Gabler, M. T., P. R. Tozer, and A. J. Heinrichs Development of a cost analysis spreadsheet for calculating the costs to raise a replacement dairy heifer. J. Dairy Sci. 83: Galyean, M. L., and A. L. Goetsch Utilization of forage fiber by ruminants. Pages in Forage Cell Wall Structure and Digestibility. H. G. Jung, D. R. Buxton, R. D. Hatfield, and J. Ralph, ed. Am. Soc. Agron., Crop Sci. Soc. Am., Soil Sci. Soc. Am., Madison, WI. Galyean, M. L., E. E. Hatfield, and T. L. Stanton Review: Restricted and programmed feeding of beef cattle definitions, application, and research results. Prof. Anim. Sci. 15:1-6. Hall, M. B Determination of Starch, Including Maltooligosaccharides, in Animal Feeds: Comparison of Methods and a Method Recommended for AOAC Collaborative Study. J. AOAC Int. 92: Harsh, S. B., C. A. Wolf, and E. Wittenberg Profitability and production efficiency of the crop and livestock enterprises of Michigan dairy operations: 1998 summary and analysis. Michigan State University, Department of Agricultural, Food, and Resource Economics. Heinrichs, A. J Raising dairy replacements to meet the needs of the 21st century. J. Dairy Sci. 76: Hoffman, P. C., C. R. Simson, and M. Wattiaux Limit feeding of gravid Holstein heifers: Effect on growth, manure nutrient excretion, and subsequent early lactation performance. J. Dairy Sci. 90:

45 31 Hoover, W. H Chemical factors involved in ruminal fiber digestion. J. Dairy Sci. 69: Houseknecht, K. L., D. L. Boggs, D. R. Campion, J. L. Sartin, T. E. Kiser, G. B. Rampacek, and H. E. Amos Effect of dietary energy source and level on serum growth hormone, insulin-like growth factor 1, growth and body composition in beef heifers. J. Anim. Sci. 66: Huntington, G. B Starch utilization by ruminants: From basics to the bunk. J. Anim. Sci. 75: Huntington, G. B., and P. J. Reynolds Net volatile fatty acid absorption in nonlactating Holstein cows. J. Dairy Sci. 66: Huntington, G. B., E. J. Zetina, J. M. Whitt, and W. Potts Effects of dietary concentrate level on nutrient absorption, liver metabolism, and urea kinetics of beef steers fed isonitrogenous and isoenergetic diets. J. Anim. Sci. 74: Hutjens, M. F Accelerated replacement heifer feeding programs. Adv. Dairy Technol 16: Jarrett, I. G., O. H. Filsell, and F. J. Ballard Utilization of oxidizable substrates by the sheep hind limb: Effects of starvation and exercise. Metabolism. 25: Jung, H. G Forage lignins and their effects on fiber digestibility. Agron. J. 81: Kmicikewycz, A. D Effects of diet particle size and supplemental hay on mitigating subacute ruminal acidosis in high-producing dairy cattle. PhD Dissertation. The Pennsylvania State University, State College.

46 32 Koch, R. M., L. A. Swiger, D. Chambers, and K. E. Gregory Efficiency of feed use in beef cattle. J. Anim. Sci. 22: Lammers, B. P., A. J. Heinrichs, and R. S. Kensinger The effects of accelerated growth rates and estrogen implants in prepubertal Holstein heifers on estimates of mammary development and subsequent reproduction and milk production. J. Dairy Sci. 82: Lascano, G. J., and A. J. Heinrichs Rumen fermentation pattern of dairy heifers fed restricted amounts of low, medium, and high concentrate diets without and with yeast culture. Livest. Sci. 124: Lascano, G. J., and A. J. Heinrichs Effects of feeding different levels of dietary fiber through the addition of corn stover on nutrient utilization of dairy heifers precision-fed high and low concentrate diets. J. Dairy Sci. 94: Lascano, G. J., A. J. Heinrichs, and J. M. Tricarico Substitution of starch by soluble fiber and Saccharomyces cerevisiae dose response on nutrient digestion and blood metabolites for precision-fed dairy heifers. J. Dairy Sci. 95: Lascano, G. J., L. E. Koch, and A. J. Heinrichs Precision-feeding dairy heifers a high rumen-degradable protein diet with different proportions of dietary fiber and forage-to-concentrate ratios. J. Dairy Sci. 99: Lascano, G. J., G. I. Zanton, F. X. Suarez-Mena, and A. J. Heinrichs Effect of limit feeding high- and low-concentrate diets with Saccharomyces cerevisiae on digestibility and on dairy heifer growth and first-lactation performance. J. Dairy Sci. 92:

47 33 Lascano, G., J. Heinrichs, and J. Tricarico Saccharomyces cerevisiae live culture affects rapidly fermentable carbohydrates fermentation profile in precision-fed dairy heifers. Can. J. Anim. Sci. 95: Little, W., and R. M. Kay The effects of rapid rearing and early calving on the subsequent performance of dairy heifers. Anim. Sci. 29: Loerch, S. C Effects of feeding growing cattle high-concentrate diets at a restricted intake on feedlot performance. J. Anim. Sci. 68: Loerch, S. C Limit-feeding corn as an alternative to hay for gestating beef cows. J. Anim. Sci. 74: McLeod, K. R., and R. L. Baldwin Effects of diet forage:concentrate ratio and metabolizable energy intake on visceral organ growth and in vitro oxidative capacity of gut tissues in sheep. J. Anim. Sci. 78: McLeod, K. R., R. L. Baldwin, M. B. Solomon, and R. G. Baumann Influence of ruminal and postruminal carbohydrate infusion on visceral organ mass and adipose tissue accretion in growing beef steers. J. Anim. Sci. 85: Merchen, N. R., J. L. Firkins, and L. L. Berger Effect of intake and forage level on ruminal turnover rates, bacterial protein synthesis and duodenal amino acid flows in sheep. J. Anim. Sci. 62: Mertens, D. R., and J. R. Loften The effect of starch on forage fiber digestion kinetics in vitro. J. Dairy Sci. 63: Mertens, D. R Impact of NDF content and digestibility on dairy cow performance. Pages in Proc. Western Canadian Dairy Sem., Red Deer, AB. University of Alberta, Edmonton.

48 34 Moe, P. W., and H. F. Tyrrell Effects of feed intake and physical form on energy value of corn in timothy hay diets for lactating cows. J. Dairy Sci. 60: Montgomery, S. P., J. S. Drouillard, E. C. Titgemeyer, J. J. Sindt, T. B. Farran, J. N. Pike, C. M. Coetzer, A. M. Trater, and J. J. Higgins Effects of wet corn gluten feed and intake level on diet digestibility and ruminal passage rate in steers. J. Anim. Sci. 82: Moody, M. L., G. I. Zanton, J. M. Daubert, and A. J. Heinrichs Nutrient utilization of differing forage-to-concentrate ratios by growing Holstein heifers. J. Dairy Sci. 90: Murphy, T. A., S. C. Loerch, and F. E. Smith Effects of feeding high-concentrate diets at restricted intakes on digestibility and nitrogen metabolism in growing lambs. J. Anim. Sci. 72: Nocek, J. E Bovine acidosis: Implications on laminitis. J. Dairy Sci. 80: National Research Council Nutrient Requirements of Dairy Cattle. 7th rev. ed. Natl. Acad. Press, Washington, DC. Oba, M., and M. S. Allen Effects of brown midrib 3 mutation in corn silage on productivity of dairy cows fed two concentrations of dietary neutral detergent fiber: 3. Digestibility and microbial efficiency. J. Dairy Sci. 83: Offner, A., A. Bach, and D. Sauvant Quantitative review of in situ starch degradation in the rumen. Anim. Feed Sci. Technol. 106: Owens, F. N., D. R. Gill, D. S. Secrist, and S. W. Coleman Review of some aspects of growth and development of feedlot cattle. J. Anim. Sci. 73:

49 35 Pino, F., and A. J. Heinrichs Effect of trace minerals and starch on digestibility and rumen fermentation in diets for dairy heifers. J. Dairy Sci. 99: Reynolds, C. K Economics of visceral energy metabolism in ruminants: Toll keeping or internal revenue service. J. Anim Sci. 80:E74-E84. Reynolds, C. K., H. F. Tyrrell, and P. J. Reynolds. 1991a. Effects of diet forage-toconcentrate ratio and intake on energy metabolism in growing beef heifers: Net nutrient metabolism by visceral tissues. J. Nutr. 121: Reynolds, C. K., H. F. Tyrrell, and P. J. Reynolds. 1991b. Effects of diet forage-toconcentrate ratio and intake on energy metabolism in growing beef heifers: Whole body energy and nitrogen balance and visceral heat production. J. Nutr. 121: Sarwar, M., J. L. Firkins, and M. L. Eastridge Effect of replacing neutral detergent fiber of forage with soyhulls and corn gluten feed for dairy heifers. J. Dairy Sci. 74: Schuh, J. D., J. O. A. Lima, W. H. Hale, and B. Theurer Steam-processed flaked grains versus steam-rolled grains for dairy calves. J. Dairy Sci. 53: Suarez-Mena, F. X., G. J. Lascano, D. E. Rico, and A. J. Heinrichs Effect of forage level and replacing canola meal with dry distillers grains with solubles in precision-fed heifer diets: Digestibility and rumen fermentation. J. Dairy Sci. 98: Susin, I., S. C. Loerch, and K. E. McClure Effects of feeding a high-grain diet at a restricted intake on lactation performance and rebreeding of ewes. J. Anim. Sci. 73:

50 36 Tajima, K., R. I. Aminov, T. Nagamine, H. Matsui, M. Nakamura, and Y. Benno Diet-dependent shifts in the bacterial population of the rumen revealed with realtime PCR. Appl. Environ. Microbiol. 67: Tamminga, S., C. J. Van Der Koelen, and A. M. Van Vuuren Effect of the level of feed intake on nitrogen entering the small intestine of dairy cows. Livest. Prod. Sci. 6: Tozer, P. R., and A. J. Heinrichs What affects the costs of raising replacement dairy heifers: A multiple-component analysis. J. Dairy Sci. 84: Tremere, A. W., W. G. Merrill, and J. K. Loosli Adaptation to high concentrate feeding as related to acidosis and digestive disturbances in dairy heifers. J. Dairy Sci. 51: Tyrrell, H. F., and P. W. Moe Effect of intake on digestive efficiency. J. Dairy Sci. 58: Van Arendonk, J. A. M., G. J. Nieuwhof, H. Vos, and S. Korver Genetic aspects of feed intake and efficiency in lactating dairy heifers. Livest. Prod. Sci. 29: Van Soest, P. J Development of a comprehensive system of feed analyses and its application to forages. J. Anim. Sci. 26: Van Soest, P. J Nutritional Ecology of the Ruminant. 2nd ed., Cornell Univ. Press, Ithaca, NY. Varga, G. A., and W. H. Hoover Rate and extent of neutral detergent fiber degradation of feedstuffs in situ. J. Dairy Sci. 66:

51 37 Wertz, A. E., L. L. Berger, D. B. Faulkner, and T. G. Nash Intake restriction strategies and sources of energy and protein during the growing period affect nutrient disappearance, feedlot performance, and carcass characteristics of crossbred heifers. J. Anim. Sci. 79: Zanton, G. I., and A. J. Heinrichs Meta-analysis to assess effect of prepubertal average daily gain of Holstein heifers on first-lactation production. J. Dairy Sci. 88: Zanton, G. I., and A. J. Heinrichs The effects of controlled feeding of a highforage or high-concentrate ration on heifer growth and first-lactation milk production. J. Dairy Sci. 90: Zanton, G. I., and A. J. Heinrichs. 2008a. Rumen digestion and nutritional efficiency of dairy heifers limit-fed a high forage ration to four levels of dry matter intake. J. Dairy Sci. 91: Zanton, G., and J. Heinrichs. 2008b. Precision feeding dairy heifers: Strategies and recommendations. DAS:08-130, College of Agricultural Sciences, The Pennsylvania State University, State College. Zanton, G. I., and A. J. Heinrichs. 2009a. Digestion and nitrogen utilization in dairy heifers limit-fed a low or high forage ration at four levels of nitrogen intake. J. Dairy Sci. 92: Zanton, G. I., and A. J. Heinrichs. 2009b. Review: Limit-feeding with altered forage-toconcentrate levels in dairy heifer diets. Prof. Anim. Sci. 25:

52 38 Zanton, G. I., and A. J. Heinrichs Efficiency and rumen responses in younger and older Holstein heifers limit-fed diets of differing energy density. J. Dairy Sci. 99:

53 Chapter 3 Effect of trace minerals and starch on digestibility and rumen fermentation in diets for dairy heifers A paper published in the Journal of Dairy Science 1 F. Pino 2 and A. J. Heinrichs* 3 A reprint is contained in the following pages. 1 Reprinted with permission of J. Dairy Sci., : Primary researcher and author. 3 Author for correspondence. *Department of Animal Science, The Pennsylvania State University, University Park, PA 16802

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