Interactions of Dietary Magnesium, Monensin and Potassium in Dairy Cattle THESIS. the Graduate School of The Ohio State University

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1 Interactions of Dietary Magnesium, Monensin and Potassium in Dairy Cattle THESIS Presented in Partial Fulfillment of the Requirements for the Degree Masters of Science in the Graduate School of The Ohio State University By Alexander W. Tebbe, B.S. Graduate Program in Animal Sciences The Ohio State University 2017 Master's Examination Committee: Dr. William P. Weiss, Advisor Dr. Jeffrey L. Firkins Dr. Chanhee Lee

2 Copyrighted by Alexander W. Tebbe 2017

3 ABSTRACT Two studies were conducted to evaluate the dietary interactions of magnesium, monensin and potassium on the milk fatty acid profile, nutrient digestibility and mineral balance in lactating dairy cattle. In the first study, a series of two experiments investigated the time course of urinary Mg excretion after abrupt diet changes. Total collection of urine was also used to validate using creatinine to estimate urine production and calculate Mg output (g/d). The first experiment in this study evaluated the interaction of Mg (basal vs. basal + supplemental MgO; total diet Mg: 0.20 vs 0.42% of DM) and K (basal vs. basal + supplemental K 2 CO 3 ; total diet K: 1.60 vs 2.57% of DM). The second experiment evaluated the interaction of monensin (0 vs 14 mg/kg monensin) and Mg source (MgO vs MgSO 4 ; total diet Mg: 0.36% of DM) under antagonism from increased dietary K levels (2.11% of DM). Based on spot sampling, urinary Mg reached steady state after 2 d when dietary K increased regardless of Mg level whereas increases in only supplemental Mg did not appear to stabilize Mg excretion by 7 d. Using a 4-d composite sample from total collection of urine in experiment 2 (n = 34 cow-periods), the calculated excretion of creatinine per kg of BW was similar to previous estimates [29.0 (± 1.16) mg creatinine/kg of BW] but was variable cow-to-cow (RMSE 4.9; 13% of mean). However, using creatinine also reduced the true variation in urine production and could result in false ii

4 treatment differences when not accounting for the prediction error. The Mg excretion measured with creatinine and a Mg kit underestimated excretion 23% or about 1.8 g/d. The underestimation was partially caused by method of measuring Mg (Mg kit vs. ICP). Increasing supply of Mg increased branched chain FA percentage in milk fat but also altered rumen biohydrogenation in diets not predisposed to milk fat depression. The second study was a continuation of experiment 2 evaluating total-tract digestion and mineral balance. Eighteen multiparous cows were used in a split-plot replicated Latin square with two 21-d periods and total collection of urine and feces on d of each period. During the entire experiment (42 d), cows remained on the same monensin treatment (0 or 14 mg/kg) but received a different Mg source for each of the two 21-d periods. Diets were designed to differ only by source, contain about 40% of the total Mg (0.36% of DM) as supplemental Mg, and have similar concentrations of all other major nutrients (NDF 29.7%; CP 16.8%). Feeding supplemental MgSO 4 decreased DMI (26.9 vs 25.7 kg/d; P < 0.1) and tended to decrease milk yield (40.2 vs 39.3 kg/d; P < 0.09) compared to MgO. Feeding MgSO 4 with monensin decreased NDF digestibility compared to diets without monensin or with MgO (46.7 vs. 50.2%). The MgSO 4 with monensin diet also decreased apparent absorption of Mg compared to diets without monensin (12.5 vs. 16.1%) whereas MgO with monensin increased apparent absorption (19.8%). Cows consuming MgSO 4 increased apparent Ca absorption (28.4 vs 23.8 %) without concomitant increases in urine excretion. The diet with MgSO 4 without monensin increased long-chain FA percent in milk suggesting increased lipolysis and BW loss or decreased de novo FA synthesis in the mammary gland. Overall, feeding MgSO 4 at a iii

5 similar dietary Mg level as MgO was not effective for production and did not improve digestibility or absorption of most nutrients in lactating dairy cattle. iv

6 ACKNOWLEDGEMENTS I would first like to give my sincere appreciation to the many people of my pregraduate career who have given me their time, support or knowledge leading up to my current academic success. Looking back, I wish I could have appreciated it more at that time, but in some way, shape or form you have all been influential to where I am today. Secondly, I would like to thank current and past members of the OARDC Dairy Nutrition Lab. Dr. Mat Faulkner, thank you for being such a great friend and mentor. I have always valued your advice and look forward to our continued friendship. Donna Wyatt, your expertise in the lab and conducting experiments has been essential to my current and future work. Also, thank you for the stories, laughs, memories and how you have never yelled at me no matter what the mistake; all have made working here really enjoyable. Dr. Bill Weiss, I cannot thank you enough for seeing my potential, pushing me the extra mile, and always making me a top priority regardless of how busy you were. Your work ethic, dedication and intelligence are truly an inspiration, and I will always be indebted to you for the opportunities you have given me. I look forward to continuing my PhD research within this lab. To my committee members, Drs. Jeff Firkins and Chanhee Lee, I truly value your expertise in the planning and completion of this thesis; thank you for all your time. To the v

7 current and past OARDC farm crew, I thank you for all of your hard work and help during these research trials. To my roommates and colleagues, Danielle, Logan and Audrey, thanks for all your help inside and out of research and the good times we ve shared. Without you, this endeavor would not have been possible nor would I have been able to keep my sanity. Also, thank you to the rest of my fellow graduate students and other OSU faculty and staff. Lastly to my friends and family, thank you for your continued support, encouragement and endless love through it all. Especially to my mom and dad, there is no way I can describe how thankful I am for the great life you have given me, and for the inspiration to work hard and be the best I can be every single day. vi

8 VITA May Mater Dei Catholic High School May B.S. Animal Sciences, University of Illinois Champaign-Urbana 2015 to Present...Graduate Research Associate, Department of Animal Sciences, The Ohio State University PUBLICATIONS Tebbe, A. W., M. F. Faulkner, and W. P. Weiss Effect of partitioning the nonfiber carbohydrate fraction and neutral detergent fiber method on digestibility of carbohydrates by dairy cows. J. Dairy Sci. 100: Lee, C., A. W. Tebbe, J. M. Campbell, and W. P. Weiss Effects of spray-dried plasma product on transition and early lactation dairy cows American Dairy Science Association Annual Meeting. Morris, D. L., A. W. Tebbe, W. P. Weiss, and C. Lee Effects of drying procedures of milk, urine, and fecal samples on nitrogen losses and its effects on nitrogen secretion and excretion in dairy cows American Dairy Science Association Annual Meeting. Tebbe, A. W., and W. P. Weiss Transient effects of supplemental potassium and magnesium in dairy cattle American Dairy Science Association Annual Meeting. FIELD OF STUDY Major Field: Animal Sciences vii

9 TABLE OF CONTENTS ABSTRACT... ii ACKNOWLEDGEMENTS... v VITA... vii PUBLICATIONS... vii FIELD OF STUDY... vii TABLE OF CONTENTS... viii LIST OF TABLES... xi LIST OF FIGURES... xiii CHAPTER THESIS INTRODUCTION... 1 CHAPTER REVIEW OF LITERATURE... 5 OPTIMIZING MINERAL NUTRITION IN DAIRY CATTLE... 5 INTRODUCTION... 5 MINERAL EXCRETION OF DAIRY CATTLE... 6 Environmental Concerns... 6 Assessing Excretion of Minerals... 8 Phosphorus... 9 Potassium Sulfur Divalent Cations: Cu, Mg, and Zn INCREASING THE EFFICIENCY OF MINERAL ABSORBTION Phosphorus Intake Potassium Intake Sulfur Intake Magnesium Intake viii

10 Copper and Zinc Intake Ionophores Controlling Variation of Feeds Organic and Specialty Minerals CONCLUSIONS CHAPTER TRANSIENT EFFECTS OF SUPPLEMENTING MAGNESIUM, MONENSIN AND POTASSIUM ABSTRACT INTRODUCTION MATERIALS AND METHODS Cows and Treatments Samples and Analyses Calculations and Statistical Analysis RESULTS Experiment Experiment Total Collection of Urine DISCUSSION Evaluation of Spot Sampling Milk Fatty Acid Profile CONCLUSIONS TABLES FIGURES CHAPTER EFFECTS OF MAGNESIUM SOURCE AND MONENSIN ON NUTRIENT DIGESTIBILITY AND MINERAL BALANCE IN LACTATING DAIRY COWS ABSTRACT INTRODUCTION MATERIALS AND METHODS Cows and Treatments Digestion Collection Samples ix

11 Milk and Blood Samples Calculations and Statistical Analyses RESULTS AND DISCUSSION Production Nutrient Intake and Apparent Digestibility Macro-mineral Metabolism FA Metabolism and Milk FA Profile CONCLUSIONS TABLES CHAPTER THESIS CONCLUSIONS COMPLETE LIST OF REFERENCES APPENDIX SUPPLEMENTARY TABLES SUPPLEMENTARY FIGURES x

12 LIST OF TABLES Table 1. The total number and partitioning of milking cows for two different herd sizes in the United States since Table 2. Ingredient composition of diets fed during experiment Table 3. Nutrient analysis of diets fed during experiment Table 4. Ingredient composition of diets fed during the first period of experiment Table 5. Nutrient analysis of diets fed during the first period of experiment Table 6. Effect of supplemental Mg and K on production during Experiment Table 7. Effect of supplemental Mg and K on intake, blood, and urine excretion during Experiment Table 8. Effect of supplemental Mg and K on milk fatty acids during Experiment Table 9. Effect of monensin and Mg source on intake, blood, and urine excretion during period 1 of Experiment Table 10. Effect of monensin and Mg source on milk fatty acid profile during period 1 of Experiment Table 11. Descriptive statistics during total collection of urine and feces of experiment Table 12. Effect of monensin and Mg source on urine excretion during the total collection of urine and feces of Experiment Table 13. Ingredient composition of the diets Table 14. Nutrient composition of diets and forages Table 15. Mineral analysis of water during digestion trials Table 16. Effects of supplemental monensin and Mg source on production during the entire period xi

13 Table 17. Effects of supplemental monensin and Mg source on nutrient intake and digestibility and water intake Table 18. Effects of supplemental monensin and Mg source on nitrogen metabolism Table 19. Effect of supplemental monensin and Mg source on macro-mineral metabolism Table 20. Effect of supplemental monensin and Mg source on macro-mineral excretion Table 21. Fatty acid composition of diets and forages Table 22. Effects of monensin and Mg source on milk fatty acids Table 23. Concentrations of dietary NDF fractions using 4 different NDF methods Table 24. Effect of monensin and Mg source on NDF digestibility Table 25. Effect of monensin and Mg source on FA intake and digestion Table 26. Effect of monensin and Mg source on micro-mineral balance Table 27. Effect of monensin and Mg source on micro-mineral excretion xii

14 LIST OF FIGURES Figure 1. Total acreage in the United State receiving animal manure by species as a proportion of all acres receiving manure Figure 2. Relationship between body weight and creatinine excretion per day during total collection of Experiment Figure 3. Analyzed urinary excretion of Mg from spot samples on d 0, 2, 4 and 7 of experiment Figure 4. Estimated urinary excretion of Mg in diets with varying supplemental levels of Mg and K measured on d 0, 2, 4 and 7 of experiment Figure 5. Urine measures using composite samples collected during the total collection of urine and feces for 4 d in experiment Figure 6. Actual urine production and predicted urine production using creatinine as a marker Figure 7. Calculated Mg excretion from actual urine production and ICP versus predicted Mg excretion from creatinine, BW and Mg kits Figure 8. Urinary creatinine from cows fed diets varying in supplemental levels of Mg and K measured on day 0, 2, 4 and 7 of experiment xiii

15 CHAPTER 1 THESIS INTRODUCTION Magnesium is a macro-mineral essential for all live forms. In ruminants, Mg is unique in that it is primarily absorbed in the rumen and not the small intestine like other macro-minerals (Tomas and Potter, 1976). Available Mg depends on intake from feedstuffs or supplements, quantity absorbed, and the amount endogenously lost in feces, urine, or milk. However, absorption of Mg varies immensely because of the rumen s complex environment and the dissimilar availabilities of sources. Consistent supply of absorbable Mg is essential to prevent deficiencies and maintain productivity. While moderate overfeeding Mg does not pose significant risks to the environment or the cow, Mg supplementation in excess inflates feed costs, decreases intake of other nutrients, and ultimately lowers producer profits. The Nutrient Requirements of Dairy Cattle (NRC, 2001) established Mg requirements using a factorial approach by estimating the total amount of Mg needed for maintenance, growth, lactation, and gestation. Thus, to predict the cow s Mg demand, body weight, growth rate, milk production, and stage of pregnancy of the animal must be known. The model s predicted Mg demand is then compared to the total absorbable 1

16 supply to determine adequacy, which is determined by summing the product of each source of Mg in the diet times a specific absorption coefficient (AC). At that time, limited published data existed on Mg digestion in cattle. In effect, the NRC (2001) subcommittee did not use mean apparent digestibility to estimate the AC; rather, they reduced the mean by one standard deviation and gave all feedstuffs an AC = Because of the low feedstuff AC, inorganic sources are often supplemented to increase absorbable supply that can have AC ranging from depending on the source s solubility. However, actual availability of Mg from a common source such as MgO (AC = 0.70) can range from (Jesse et al., 1981; Van Ravenswaay et al., 1989; Xin et al., 1989) and should not be considered constant. Most of Mg AC were also derived from sheep data and may underestimate Mg absorption in lactating cattle. This is because sheep and cattle are similar anatomically, but their digestive physiologies differ. Nutrient metabolism should then be carefully evaluated before extrapolating between cattle and sheep. For example, sheep are expected to have 1.75 times greater Mg apparent absorption compared to cattle, probably because of the sheep s greater ruminal surface area in comparison to total ruminal contents (Shockey et al., 1984). All NRC (2001) AC for Mg are constants and currently do not account for dietary interactions known to alter absorption. The most common dietary interaction is the antagonistic effect of K. Increasing dietary K above requirements increases milk fat production and feed efficiency (Erdman et al., 2011; Iwaniuk et al., 2015) and can alleviate intake depressions caused by heat stress (West et al., 1987). Common diets also range in K levels inherently because of the variable K content in forages. Equations have 2

17 been derived to quantify K antagonism but are primarily from studies feeding MgO. In addition, these equations neglect other nutrient interactions on Mg absorption including sodium deficiency (Kemp et al., 1961), high calcium (>1.3% of DM; Roche et al., 2002), high phosphorus (>0.6% of DM; Schonewille et al., 1994), and diets high in added fat (NRC, 2001) or rumen degradable protein (Head and Rook, 1955; Martens and Schweigel, 2000). Since the FDA s approval of Rumensin (monensin) in 2004 for dairy cattle, many US rations contain monensin to improve feed efficiency and decrease methane emissions (Odongo et al., 2007). Monensin may improve digestion and retention of nutrients in cattle (Spears, 1990; Duffield and Bagg, 2000; Ipharraguerre and Clark, 2003). More specifically, monensin may improve Mg absorption subject to K antagonism (Greene et al., 1986), but few if any data exist in lactating dairy cattle. In summary, the problems about the current estimation of Mg balance and supply (or model) include: 1. The current NRC system does not quantitatively address interactions of nutrients, mineral sources or common feed additives in the estimation of AC (e.g., the level of K, monensin). 2. The NRC (2001) AC for Mg sources are primarily derived from studies involving sheep and probably should not be extrapolated to high producing dairy cattle. 3. Monensin and different Mg sources may benefit nutrient digestion and absorption, but the interaction in lactating cattle has not been evaluated. 3

18 4. Mg absorption is highly variable between sources and within sources. Currently, urine Mg concentration is the most practical method for determining Mg absorption but the time-course of Mg absorption and subsequent excretion after a diet change is limited. Establishing the timecourse and finding a more robust system to estimate absorption would be beneficial in determining adequate Mg intake. The first objective of this research was to develop a system to evaluate Mg absorption by estimating urinary excretion of Mg over time. The experiments fed different concentrations of K and Mg or fed different Mg sources and monensin in lactating dairy cattle. The second objective was to investigate ways to improve Mg absorption and the digestion of nutrients for a diet subject to antagonism from an elevated dietary K concentrations. The experiment was an extension of the trial evaluating the interaction of monensin and Mg source and was conducted via total collection of urine and feces. The last objective was to provide insight on ruminal changes or shifts in microbial populations by investigating the milk fatty acid profile in both studies. 4

19 CHAPTER 2 REVIEW OF LITERATURE OPTIMIZING MINERAL NUTRITION IN DAIRY CATTLE INTRODUCTION Public pressure, government regulations and the motivation to co-exist with other live forms has caused agricultural systems such as the dairy industry to reconsider the impact animal operations may have on the environment (Hristov et al., 2006). In the same context, animal agriculture is essential to the conversion of wastes generated from the energy, manufacturing, and other agricultural industries into edible products for the global food supply. Currently, the dairy industry provides an estimated million metric tons of milk products and 67.1 million metric tons of meat each year (FAO et al., 2011). By the year 2050, the demand for these products is expected to rise approximately 158 and 173%, respectively (FAO, 2011). Increasing productivity and simultaneously reducing the environmental impact of the dairy industry through aspects such as nutrition will be vital to global sustainability. Appropriate mineral supplementation for dairy cattle is essential for health, production, and animal welfare (NRC, 2001). As production intensifies, balancing minerals, confounded by different chemical and physical forms, and interactions with 5

20 other nutrients, becomes as difficult as providing adequate protein and energy (Strusinska et al., 2003). Currently, most dairy and beef cattle nutritional models used in the US rely on the NRC (2001) system to balance minerals in the diet (Weiss, personal communication). The NRC (2001) system is adequate to prevent deficiency of most minerals; however, diets are often formulated to contain excess minerals (i.e., safety factors) to overcome absorptive antagonism and reduce the risk of deficiencies. Feeding most minerals in marginal excess of requirements can be done with minimal risk of toxicity to the animal, but overfeeding increases the amount of minerals excreted in manure and can be an environmental hazard. Therefore, to better understand why and how to optimize the supplementation of minerals to dairy cattle, this literature review will first identify current environmental concerns associated with minerals and assess mineral excretion from dairy cattle. This review will then discuss ways to reduce overfeeding minerals and by identifying factors affecting absorptive efficiency of minerals commonly supplemented. MINERAL EXCRETION OF DAIRY CATTLE Environmental Concerns As the scale of dairy production has intensified, the large amounts of minerals excreted in animal manure and its potential to endanger the environment has gained awareness by consumers and producers. Historically, dairy cattle were grown and maintained solely on farm-produced feeds or pasture. The manure generated was used to maintain the fertility of the same crop land and was considered minimal to the acreage available (Jongbloed and Lenis, 1998). In today s intensified operations, importing large amounts of nutrients from off the farm (primarily as feed) is required; however, the land 6

21 base to subsequently apply the manure produced is often too small causing nutrient accumulation. This is problematic largely because farm size has rapidly increased and is continuing to increase even though the total number of cows in the US has slightly decreased (Table 1). For example, in 1992, there were about 560 concentrated dairy operations (>1000 lactating cows) accounting for 10% of the 9.5 million total milking cows in 1992 (USDA-NASS, 1992). In 2012, the number tripled to over 1800 concentrated dairy operations accounting for nearly 50% of the 9.25 million dairy cows (USDA-NASS, 2012). Although the total number of cows has decreased and perhaps output of manure, there is now a higher risk of nutrient (mineral) accumulation, which can endanger the surrounding environment (Hristov et al., 2007). The dairy industry, in particular, receives attention because of the predicted 300 million metric tons of animal manure produced annually (EPA-OAP, 2005) and spread on crop land, 56% of the total amount of manure applied is from dairy cattle (Figure 1; USDA-ERS, 2009; Carter and Kim, 2013). Excretion of minerals in manure and the subsequent application onto croplands is not harmful per se. Rather, environmental hazards evolve when the flux of animal derived inputs exceed a soil-plant systems capacity to utilize the nutrients (i.e., an imbalance; Tamminga, 1996). The environmental concerns associated with an imbalance of nutrients can be divided into three main categories: soil accumulation, water eutrophication, and air quality (e.g. global warming, odors; Jongbloed and Lenis, 1998). Soil accumulation of minerals is caused by the excessive application of highly concentrated manure. A high concentration of minerals in the soil can potentially have negative health indices on all consumers in the subsequent food chain: organisms in the 7

22 soil (e.g., microbes, earthworms; Van Rhee, 1974), vegetation grown in the soil, herbivores that eat those plants (Henkens, 1975), and potentially prey animals that consume those plant-eaters. Soil accumulation of minerals can also cause water eutrophication and ground water contamination via leaching and run-off, which can occur for decades after external mineral additions (Carpenter, 2005). The development of water eutrophication (i.e., increased nutrient supply to aquatic vegetation) is detrimental because it can form toxic algal blooms. Toxic algal blooms are of major concern because they cause abrupt reductions in ecosystem biodiversity and can have residual effects on both aquatic and human health (Smith et al., 1999). However, the distance from the source at which water eutrophication can occur is much greater compared to soil accumulation. The production of gaseous compounds from minerals (e.g., H 2 S) can also cause harm at long distance from the source, but in most scenarios, gas production is minimal compared to gas produced from organic compounds (e.g., CO 2, CH 4, NH 3, and NO 2 ) in cattle husbandry. Assessing Excretion of Minerals Because of the large amount of environment concerns thought to be associated with minerals and animal agriculture, the US Environmental Protection Agency (EPA) has developed programs to manage wastes. For example, dairy farms with >700 lactating cows must have a National Pollutant Discharge Elimination System permit and Nutrient Management Plans to be in operation (Pfeffer and Hristov, 2005). The regulations have led to the idea of a mineral footprint, where farm imports (e.g., feed, bedding, fertilizer) and exports (e.g., animal products, manure, forages) are quantified to identify imbalances 8

23 (Suttle, 2010). The extent of accumulation occurring on farm is then used as a means to hold operations accountable or to identify farming operations requiring intervention. Minerals that have been evaluated and considered to leave behind a significant mineral footprint on dairy farms include P, K, S, Cu, Mg and Zn. These minerals are considered to have significant footprints because of their mass of imports onto a farm and exports off farms. Moreover, it is not uncommon for farms to overfeed or over supplement these minerals because of 1) fear of deficiencies and 2) their low cost in rations compared to macro-nutrients such as protein (Sink et al., 2000). Greater inclusion is problematic because of the low absorption of these minerals ultimately increasing their excretion in manure. The extent each of these minerals contributes to mineral footprints varies amongst farms, which makes quantifying the footprint a useful, indirect measure of a farm s environmental impact (Hristov et al., 2006, 2007). Phosphorus Among minerals, P has long been recognized to be overfed and have the largest impact environmentally (Ondersteijn et al., 2002; Dou et al., 2003). Concentrated animal agriculture is thought to contribute up to 40 to 70% of total P loads in fresh water (Smith and Alexander, 2000; Longabucco, 2001) and be the primary source causing water eutrophication and growth of toxic algal blooms (Sharpley, 1995). The extent of P pollution is also a profound problem in both Ohio (e.g. Lake Erie) and across the US; however, P pollution is not solely from animal agriculture. Purchased feed is the single largest source of imported P on dairy farms and accounts for 65 to 85% of the total of P imported annually per animal (28 to 51 kg/yr per cow; Cerosaletti et al., 2004). The exportation of P is fairly constant, with the largest 9

24 portion exported in the form of animal products (~50%; meat and milk) and manure (40-45%), and the remainder partitioned to cash crops (~5%). The actual efficiency of P is highly variable and often times results in a lower efficiency because of the low P retention in the cow and the increased proportion of P excreted in feces as inclusion increases (Morse et al., 1992). In early research using mathematical models, P was used with 44 to 66% efficiency (total farm exports/ total farm imports 100; Van Horn et al., 1996; Kuipers et al., 1999). When using actual farms to measure P footprint, P surplus (surplus = imports exports) per cow ranged from 4 up to 41 kg/year, with whole farm efficiencies ranging from 42 to 90% (Klausner et al., 1998; Spears et al., 2003; Hristov et al., 2006). Because of the extremely large variation of P efficiency and potentially large surplus, the dairy industry is then considered a major contributor of P pollution. To optimally use P, management on all levels of production must be considered. Contrary to common thought, growing crops can actually increase the P footprint and decrease efficiency. Higher P footprints with growing crops is because of additional imports as fertilizer and lower emphasis of efficiently utilizing P inputs from the herd s manure (Spears et al., 2003). Properly crediting manure nutrients is also important to control P balance (VanDyke et al., 1999). For example, the N to P ratio of manure spread on fields (3:1) is typically half of what is recommended for growing corn in the Midwest (6:1). Subsequently, it is not uncommon for farmers to over-apply P in the soil as a consequence of trying to meet N requirements rather than P (Knowlton et al., 2004). Regulating the density of P in diets based on production level is the most crucial aspect of increasing whole farm efficiency. Regulating diet P is important because nearly 60 to 70% of P intake is excreted in feces and will increase as dietary inclusion rises 10

25 (NRC, 2001). Increasing dietary P also increases land needed to recycle P and overall farm balance. For example, increasing dietary P by 26% (0.38 to 0.48% of DM) has been estimated to increase the cropland needed to recycle P by up to 39% (Powell et al., 2001) and increase whole farm balance by up to 9% (Wang et al., 1999) or about 11.8 kg/yr per cow (Cerosaletti et al., 2004). Feeding diets with higher P or with supplemental P also increases the amount of water soluble P in feces, which is more prone to run off (Dou et al., 2002). Potassium The K footprint has primarily been quantified because of its large content in cattle diets. The actual environmental concerns of K, however, are unclear. Similarly to P, K can buildup in soil and easily leach into water because of K s high solubility in manure (Soberon et al., 2015). High K leaching can then accumulate in groundwater but rarely reaches levels that may pose a risk to consumers (WHO, 2009). Rather, K is commonly added to treat potable water, and high intakes of K have even been found to reduce the prevalence of hypertension and heart disease in humans (UKEVM, 2003). Significant K footprints could, however, affect the dairy cow. For example, a build-up of K in soils will elevate the K content of forages and can negatively affect herd nutrition programs or cow health (e.g., dry cow diets, hypomagnesia; Fisher et al., 1994). However, high K content in lactating diets in the US is uncommon because typical diets contain large amounts of corn silage rather than legumes and grasses, which are forages more prone to K accumulation. Excess K is also often beneficial to both the lactating herd (Erdman et al., 2011; Iwaniuk et al., 2015) and crop yields (Zörb et al., 2014). Therefore, it seems the only reason to quantify the K footprint of US farms would be to improve profitability. 11

26 On average, the mass of K imports (140 kg/yr per cow) and exports (55 kg/yr per cow) are substantially greater than those of P (Hristov et al., 2006). The larger import and exports of K are because of the higher K content in both animal (i.e., milk) and plant products. However, the imports, exports and subsequently efficiency of K can vary drastically because of varying forage production and can even lead to K efficiencies over 100% for certain farms (i.e., exports exceed imports; Hristov et al., 2006). The largest K import is in the form of purchased feed (50 to 95% of total K imports) followed by bedding (5 to 11% of total K imported; Cerosaletti et al., 2004; Hristov et al., 2006). Manure and milk (55 and 25% of total K exports, respectively) make up the majority of exports. The efficiency of the cows will obviously not surpass 100% but will decrease considerably as dietary K increases. Although K has a high absorption (~90 to 100%; NRC, 2001), the K content of meat and milk is essentially constant regardless of intake (Sasser et al., 1966). However, K also increases inputs (i.e., DM intake and water) and outputs (i.e., manure). For every 1% of DM increase in dietary K, water intake and manure can increase up to 17.5 L/d (Fraley et al., 2015) and 14 kg/d, respectively (Weiss et al., 2009b). In effect, if a true environment issue can be attributed to K, the primary emphasis should be put on effective dietary K levels to optimize water intake and manure excretion for a given production level. Sulfur Sulfur is the only mineral in animal nutrition known to transform and produce significant quantities of gas that could pose an environmental threat. The gas, H 2 S, is toxic to vertebrates and is produced by sulfate-reducing bacteria either in the rumen or in manure. Microbes can also produce other S-containing compounds that contribute to odor 12

27 (O'neill and Phillips, 1992). The S unaffected by microbes can also indirectly cause the accumulation of toxic compounds such as methylmercury in fish, wildlife and humans (Bates et al., 2002). According to Hristov et al. (2007), the majority of imported S is from feed (>90%), with minor contributions from fertilizer, bedding and imported animals. Sulfur has several major exports including manure (60%) and milk (25%) with minor contributions from animals or home-grown feeds. The resulting efficiency of farms ranges from 26 to 57%; however, S exported and efficiency per se may actually be much higher. This is because S is inevitably lost as H 2 S, even though it is an unusable export. The underestimation of S exports would likely be greater for farms that import large amounts of distillers grains because of its high inorganic S content (Liu and Han, 2011) and potential to be reduced to H 2 S as inclusion increases (Neville et al., 2011). Drinking water also contains on average about 36 mg/l sulfate but can be over 1000 mg/l in some areas (Beede, 2006). In effect, the actual S footprint on farms is unknown and has more facets than originally considered. Future footprint estimates should account for the volatization of S and the contribution of sulfate from water for accurate assessments. Divalent Cations: Cu, Mg, and Zn Although many divalent cations are recognized in ruminant mineral nutrition, the elements Cu, Zn, and potentially Mg have been recognized as potential threats to the environment from dairy farms. These divalent cations have lower absorption rates compared to the elements previously mentioned (20 to 30%, Mg; 1 to 5%, Cu; 15%, Zn; NRC, 2001) and can cause manure to have higher concentrations than some synthetic fertilizers (McBride and Spiers, 2001). The accumulation of Zn and Cu in soil can be 13

28 toxic to both plants and foraging animals (Ferket et al., 2002), whereas high Mg in manure can reduce Ca-P bonds and increase P solubility and leaching (Josan et al., 2005). Similar to K, the threat of these minerals to humans or environmental pollution is not large compared to P. In fact, Mg, Zn and Cu are often deficient, and dietary supplementation has been shown to improve immune function (Zn and Cu; Mirastschijski et al., 2013) or reduce insulin resistance in humans (Mg; Guerrero Romero and Rodríguez Morán, 2011). Assessing the footprint of these minerals, however, may still be valuable because these minerals are routinely overfed and could accumulate to hazardous levels overtime. Hristov et al. (2007) reported on-farm efficiency of Mg, Cu, and Zn averaging a modest 67, 62 and 56%, respectively. The average import of these minerals was greatest in feed at 25.9 kg/yr per cow for Mg and was much lower for Cu and Zn at 99 and 744 g/yr per cow. However, much like S, Mg can be increased in water (averages 14 mg/l; Beede, 2006) and may be a considerable input when assessing the Mg footprint. The largest export of these elements was manure at 13.4 kg/yr per cow for Mg and 57.2 and 295 g/yr per cow for Cu and Zn, respectively. The mass of exports as milk were 1.45 kg/yr for Mg, and 1.4 and 39 g/yr for Cu and Zn. The low exports can be expected because of the small concentrations of these minerals in meat and milk (milk: g/kg, Mg; 0.15 mg/kg, Cu; 4 mg/kg, Zn; NRC, 2001). INCREASING THE EFFICIENCY OF MINERAL ABSORBTION All life forms require minerals in the proper quantity and proportion to other nutrients to maximize productivity and maintain vigor. Minerals consumed above or below these optimal amounts are considered losses or inefficiencies if not secreted into 14

29 the desired product or if production decreases. Unfortunately, mineral utilization in ruminant production is inherently an inefficient process compared to other forms of agriculture (e.g., crops) because of the large amount imported, required daily by the cow, and excreted in manure instead of in meat or milk (Ondersteijn et al., 2002). From the above discussion, increasing mineral efficiency is important for reducing the environmental hazards attributed to intensified dairy production. Because the diet is the ultimate source, optimizing the delivered minerals is crucial because not all dietary minerals are considered available for absorption (NRC, 2001). Understanding what affects mineral availability and accurately estimating intake of absorbable mineral is needed to meet and not drastically exceed requirements. Phosphorus Intake Measuring the absorption of P in ruminants is complicated because of the significant amounts of P incorporated into microbial mass (e.g., phospholipids and nucleic acids; Durand and Kawashima, 1980) and the variable amounts of P recycled in saliva (Breves and Schröder, 1991). However, the absorption of P in ruminants compared to non-ruminants is higher because rumen microbes have the capacity to break down phytate P (Clark et al., 1986; Morse et al., 1992), which can make up to 70% of the total P in grains (Nelson et al., 1968). Inorganic P is also a large source of P fed to dairy cattle and typically has a high, relatively constant absorption (80 to 85%; NRC, 2001; Suttle, 2010). Inorganic supplements with high absorption rates (80 to 90%, NRC, 2001) such as di-calcium phosphate, bone meal, and monosodium phosphate are commonly used to meet requirements. The absorbed P requirement for non-pregnant, lactating cattle set by NRC (2001) is 1.0 g/kg of DM intake plus an additional 2 mg/kg of BW for maintenance 15

30 and 0.9 g/kg of milk produced. Dietary P around 0.34 to 0.36% of the DM should be adequate for average Holsteins (600 to 700 kg of BW) to maintain milk production and reproductive efficiency (Satter and Wu, 1999; Knowlton et al., 2004). Phosphorus is a highly reactive atom and can form inorganic complexes with cations and have reduced solubility (Silveira et al., 2006). When feeding Ca and Mg 50% above their requirements, P absorption decreased 18% in sheep (Field et al., 1983). However, some claim Ca and Mg antagonism on P absorption is not true for cattle, and high Ca and Mg are actually beneficial for reducing P run-off. For example, the apparent absorption of P in cattle was unaffected by Ca and Mg when fed to exceed NRC requirements (0.80 to 1.0 and 0.4 to 0.43% of DM, respectively), but the excess Ca and Mg reduced water soluble P by up to 5 g/d (Herrera et al., 2010). Besides being uneconomical, increasing dietary Mg and Ca would also increase their excretion. In addition, soluble P does not make up a large portion of the P excreted. However, it may be beneficial in areas under strict government regulations or with high soil P content, but other alternatives should be considered. Feeding phytase to increase phytate P availability increased P apparent absorption in dairy cattle (Kincaid et al., 2005; Knowlton et al., 2005) and was beneficial to diets already low in P (Jarrett et al., 2014). However, the general increase in P absorption was small (1 to 2 percentage unit increase in apparent absorption). Exogenous enzymes are also expensive, and justifying their cost in typical rations with moderate concentrate levels may be difficult. The greatest loss of P efficiency is from feeding P above requirements (Morse et al., 1992; Wu et al., 2001; Dou et al., 2002). Precision feeding P by feeding closer to 16

31 requirement; obtaining frequent, accurate feed analysis; grouping based on DM intake and milk yields; and practicing good management strategies is the most effective way to increase P efficiency to dairy cattle (Cerosaletti et al., 2004). Assessing P adequacy of a diets can be done by measuring the readily soluble inorganic P content in feces (Dou et al., 2002). Soluble inorganic P in feces makes up to 95 98% of total P excretion in dairy cattle (Morse et al., 1992). A readily soluble inorganic P benchmark of % of fecal DM suggests adequate dietary P. If above the range, dietary P should be reduced by 0.05% of DM for every 0.07% of fecal DM (Dou et al., 2002). Potassium Intake Potassium is the most variable mineral in feedstuffs (Eriksson and Rustas, 2014). The variation occurs simply because all crops have unique growth rates confounded by inconsistent fertilization practices, soil types, and portions of the plant harvested (e.g., corn silage vs. corn grain). The most common example found in feedstuffs is the low K content of corn silage or grains and the high K content of grass-legume forages. In the ruminant, K is highly available from all feedstuffs and supplements (>90%; NRC, 2001). The absorbed K requirement for non-pregnant, lactating cattle set by NRC (2001) is 2.6 g/kg of DM intake plus an additional 38 mg/kg of BW for maintenance and 1.5 g/kg of milk produced. Rations with K content above 1% of DM are adequate for the average Holstein (600 to 700 kg of BW) to maintain milk production, but dietary K below 1.5% of the DM is rare (NRC, 2001). The absorption of K is relatively unaffected by other minerals and nutrients. Rather, high K is more known as an antagonist to other minerals such as Mg (later discussion). A high dietary K level can also be unfavorable because of its role in the 17

32 dietary cation and anion difference (DCAD, meq/kg = Na + + K + Cl - S 2- ) and Ca metabolism. In the late prepartum cow, high K (and DCAD) can increase the incidence of hypocalcemia by increasing blood ph and reducing both Ca absorption and bone mobilization at the onset of lactation (Goff and Horst, 2003). Conversely, high DCAD approaches are often used in lactating cow diets to enable greater acid excretion when highly fermentable diets are fed (Hu et al., 2007). A DCAD of 426 meq/kg of DM is suggested to maximize feed efficiency, but Na may actually be more effective than K (Iwaniuk et al., 2015). Increased K content may also be beneficial to cows during heat stress to maintain DM intake and milk yields (Beede and Collier, 1986; Sanchez et al., 1994). However, as previously mentioned, K increases water intake and manure output and should be considered when evaluating production efficiency. No known methods to access the adequacy of K in the diet are currently available. However, for diets altering DCAD to control metabolic acid-base balance, urine ph maybe an effective tool to monitor adequate dietary K. The K content of urine or feces may also be effective. Sulfur Intake The absorption of S primarily takes place in the small intestine as S incorporated into amino acids, B-vitamins and sulfate (NRC, 2001) or in the rumen as a sulfide anion (Bray and Hemsley, 1969). In typical lactating diets, inorganic S sources are typically not supplemented because basal S meets the requirement. However, inorganic S can be included when meeting cationic requirements (e.g., sulfate minerals) or to maintain adequate microbial protein synthesis when non-protein N is high (Bouchard and Conrad, 1973). Water can also be a significant source of S and needs to be included when 18

33 formulating rations. Milk primarily requires S in the form of amino acids and vitamins but has no inorganic S requirement. The current NRC (2001) requirement is set at 0.2% of DM for all diets. Lactating diets commonly have excessive S, and research regarding factors affecting S absorption are lacking. Similar to K, excessive S consumed can significantly decrease the absorption of other elements (e.g., Se, Mn, Zn and Cu; later discussion) and will decrease DCAD; all of these imbalances can reduce milk production or cow health. However, using inorganic S sources to decrease DCAD in prepartum diets and reduce the incidence of hypocalcemia is common; approximately 27% of herds feed negative DCAD diets (USDA, 2014). Adequacy of dietary S or DCAD levels in prepartum diets can be evaluated by measuring urine ph (optimal ph 6.3 to 6.5). Magnesium Intake Magnesium is unique among macro-minerals because most of the absorption occurs in the rumen (Tomas and Potter, 1976). Continuous, absorbable Mg supply is important to prevent the rapid onset of hypomagnesia. The Mg absorption of feedstuffs is predicted to be low (16%; NRC, 2001); thus, inorganic sources such as MgO, MgSO 4, MgCl 2 or MgPO 4 are commonly supplemented to maintain adequate absorbable supply. These sources vary in solubility and have predicted absorptions of 70, 90 and 90%, respectively (NRC, 2001), although they are probably much lower. Magnesium oxide is the most common form supplemented because it can buffer the rumen (Erdman et al., 1982) and has a higher concentration of Mg (54-56%) compared to MgSO 4 and MgCl 2 (9% and 18%, respectively; Goff, 2014). Anionic sources can also have lower palatability, decrease intake, and cost more. The availability of MgO is highly variable 19

34 among sources and can differ based on particle size, origin, and degree of Calcination (Jesse et al., 1981; Van Ravenswaay et al., 1989; Xin et al., 1989). Calcination at ~1200 C and fine particle sizes are thought to have the greatest solubility and ruminal absorption (Goff, 2014). The NRC (2001) maintenance requirement for Mg is estimated to be 3 mg/kg of BW for adult cattle and 0.12 to 0.15 g/kg of milk for lactation. However, the actual Mg requirement of milk may be slightly lower than 0.12 g/kg (Suttle, 2010). Rations with dietary Mg from 0.20 to 0.25% will be adequate to sustain milk production in most scenarios. The absorption of Mg is largely dependent on the electrical potential difference of rumen epithelial cells and ruminal solubility of the Mg source. As a result, a number of nutrients are thought to alter the absorption of Mg either directly or through changes of rumen physiology. High K intake is the most well-known and can significantly reduce blood and urine Mg (Kemp et al., 1961). Increasing Mg in the diet to overcome the inhibition and supply enough digestible Mg was described by Weiss (2004): Intake of digestible Mg (g/d) = Mg intake (g/d) 4.4 K content (% of DM) and Schonewille et al. (2008): Intake of digestible Mg (g/d) = Mg intake (g/d) 0.08 K content (% of DM) A deficiency of Na triggers identical inhibition of Mg uptake as high K. Dietary P >0.6% of DM (Schonewille et al., 1994), diets high in added fat (Rahnema et al., 1994; NRC, 2001), or high rumen degradable protein (Head and Rook, 1955; Martens and Schweigel, 2000) also decrease Mg absorption. 20

35 Assessing Mg status of animals can be done by measuring urine excretion of Mg because the kidney is the primary regulation site of blood Mg homeostasis (Kemp, 1983). Urine concentrations >107 mg/l are considered adequate but the range of concentration is large (21 to more than 107 mg/l). Concentration will also be unreliable unless total volume of urine is collected or estimated. The time course of Mg excretion is also not well established and may affect the perception of adequate dietary levels. Copper and Zinc Intake Dietary Cu and Zn absorption occurs in the small intestine (Wang et al., 2011) and can be stored in the liver. Inorganic sources of Cu include CuO, CuCO 3 and CuSO 4, which have low AC of 1, 5, and 5% (NRC, 2001). Inorganic sources of Zn include ZnO, ZnCO 3, ZnSO 4 and ZnCl 2 with higher AC of 12, 10, 20, and 20% (NRC, 2001). Organic complexes or chelates of amino acids as well as hydroxy Cu and Zn complexes are also available for supplementation and are thought to have greater absorption rates compared inorganic sources, but this is controversial (see later discussion). The current NRC (2001) maintenance requirement for absorbed Cu is mg/kg of BW and 0.15 mg/kg of milk for lactating cows. The Zn requirements are higher than Cu at mg/kg of BW for maintenance and 4 mg/kg of milk. Rations with Cu and Zn at and mg/kg of DM are recommended to maintain immune function, but the recommendations are dependent on the amount of antagonists. Dietary Cu has many antagonists, a small range between toxic and clinically deficient levels, and high variability in diets. The combined problems makes utilizing dietary Cu efficiently a challenge. The most common antagonist is the interaction of Cu with molybdenum and S. Basal levels of molybdenum and S in diets are typically 1 21

36 mg/kg and 0.20% of DM, respectively. However, increases of molybdenum and S to 4 mg/kg or 0.30% of DM, respectively, can reduce Cu bioavailability 40 to 70% (Suttle, 1975). High reduced iron content (800 to 1,000 mg/kg) and potentially high calcium (>0.90% of DM; Kirchgessner, 1965) can also decrease copper status (Mullis et al., 2003). Antagonism requires more dietary Cu, but the quantity needed is unknown. Cadmium, Zn, and Mn may also decrease Cu retention or absorption, but even less is known about these interactions (Suttle, 2010). The type of fiber and Cu sources can also affect absorption. According to Faulkner et al. (2017), hydroxy Cu compared to CuSO 4 has a higher absorption in diets high in corn and soybeans feedstuffs, whereas the reverse is true in a diet high in non-forage fiber (by-products). The absorption of Zn is subject to less antagonism than Cu. Similar to P, Zn absorption can be decreased significantly by elevated levels of phytate in non-ruminants (NRC, 2001; Suttle, 2010), but phytate antagonism is not a factor to ruminants because of microbial phytases (Spears, 2003). The efficiency of Zn absorption will also decrease as inclusion of dietary Zn increases. The Cu and potentially Zn concentrations in the liver are good indicators of status. A Cu liver content of 50 to 112 mg/kg on a wet basis are considered adequate (Cunha and McDowell, 2012). This method to assess adequate levels may seem inhumane and expensive but has become relatively simple and less invasive to conduct. Ionophores The use of ionophores such as monensin for dairy cattle production has grown immensely since approval in Monensin is commonly fed because it increases feed efficiency and has the potential to reduce metabolic diseases (Duffield et al., 2003). The 22

37 benefits of monensin may stem further than just energy metabolism and may increase the absorption of certain minerals. The results, however, are not always consistent (Spears, 1990) and have yet to be confirmed or quantified in dairy cattle. Apparent absorption of P in beef cattle increased 12 20% with the addition of monensin (Starnes et al., 1984; Spears et al., 1989). Inclusion of monensin was also shown to increase Mg absorption up to 10% in steers (Greene et al., 1986). Ionophores may also affect trace mineral metabolism. In lambs, the apparent absorption and retention of Zn was increased 50 and 45%, respectively, upon the addition of monensin (Kirk et al., 1985). Similar positive effects were seen in steers from which monensin increased the retention of Zn and Se 43 and 75%, respectively (Costa et al., 1985). In diets with high S, monensin may be beneficial because of its potential role in controlling H 2 S but the actual benefit of monensin is controversial. In steers fed monensin, the production of H 2 S was decreased in diets high in S from distillers grains and no negative production effects were found (Felix et al., 2012). In in vitro studies, H 2 S production increased (Kung et al., 2000) or did not change (Quinn et al., 2009) when monensin was included in a high S treatment. If a decrease of gas production were to actually occur, this would reduce environmental hazards and health risks but also would be beneficial to the animal in the prevention of polioencephalomalacia. More research in dairy cattle needs to be conducted using different sources and levels of S to validate this hypothesis. Controlling Variation of Feeds From field to bunk, many sources of variation can affect the actual nutrients delivered to the cow. There is variation among and within feedstuffs, variation when 23

38 mixing or delivering the feed, and variation of what the cow actual eats (i.e., sorting; Stone, 2003; Rossow and Aly, 2013). Knowing the main sources of added variation and how to limit them is crucial to optimize mineral nutrition. The mineral composition of feeds is variable (St-Pierre and Weiss, 2015) and can negatively affect diet formulation if average mineral concentrations are not known with relative certainty. Less confidence in the composition will also lead to greater safety factors or supplemental levels to reduce the risk of deficiency (Black and Hlubik, 1980). Chemically analyzing feed ingredients in a ration is crucial to accurate quantification of mineral concentrations in feeds. Larger herds (St-Pierre and Cobanov, 2007), forages, and byproducts (St-Pierre and Weiss, 2015) will require more frequent chemical analysis to reduce variation and improve precision of mineral nutrition. However, the frequency needs to be feasible in terms of cost. Controlling the variation between formulated and consumed rations is largely influenced by the person mixing the ration and equipment available (Stone, 2003). Analyzing the TMR delivered is not accurate for evaluating minerals (Weiss et al., 2016). Therefore, the feeder must recognize the importance of accuracy and where variation exists (Stone, 2003). Because of the small inclusion rates, blending minerals into larger concentrate mixes (if the right equipment is available) and properly designed feed facilities can also prevent overfeeding (Brouk and Harner III, 2016). Organic and Specialty Minerals The use of organic trace minerals complexed or chelated to organic ligands (amino acids or polysaccharides) has increased substantially in the past 25 years. Hydroxy trace minerals have also recently entered the market for their ability to remain 24

39 relatively insoluble in the rumen and have a high solubility in acid upon passing through the abomasum to the intestines. The advantages of feeding organic or specialty minerals over inorganic sources as a means to increase bioavailability seems clear in terms of improving productivity and reproductive performance (Rabiee et al., 2010); however, this may only benefit immunologically compromised cows such as cows subject to mastitis, poor hoof health, or transition cows (Andrieu, 2008; Osorio et al., 2016). These problems all are multifaceted and can be largely influenced by management and facilities. Rather, the apparent absorption (Spears, 1996) of organic and inorganic minerals is probably similar. When Mn methionine and MnSO 4 were fed to dry cows, no differences in Mn apparent absorption were observed (Weiss and Socha, 2005). Feeding Cu lysine or Cu proteinate compared to CuSO 4 were equally effective at maintaining liver Cu concentrations in heifers and cows fed increased dietary S and Mo (Rabiansky et al., 1999; Chase et al., 2000; Sinclair et al., 2013). In another study evaluating a cassette of minerals (Cu, Mn, Se and Zn) from inorganic and organic sources to heifers fed different starch levels, there were no differences in macronutrient digestion, and sulfate sources actually had greater net retentions and numerically greater absorption (Pino and Heinrichs, 2016). A recent experiment in our lab also observed no differences in apparent absorption between inorganic and organic sources of Zn and Mn (Faulkner et al., 2017). However, Cu apparent absorption was higher for hydroxy sources when included in a diet high in corn and soybeans ingredients compared to sulfate, whereas the reverse was true for a diet higher in fiber from byproduct ingredients. 25

40 More research using individual elements as well as administering treatments under different circumstances (e.g., differences in carbohydrate content, stage of lactation, antagonist) is needed to fully elucidate the differences in absorption between organic and inorganic sources. At this time, there does not appear to be a clear environmental advantage. Rather, feeding organic sources may only be beneficial to herds with prevalent health disorders. CONCLUSIONS The excessive amounts of minerals excreted in manure and subsequently accumulated in the soil poses a potential environmental risk that has grown public awareness. In effect, the government and researchers both have begun to address the growing concerns by quantifying mineral footprints of dairy farms. The majority of research has focused on P excretion; however, other minerals can also produce a significant mineral footprint, but currently do not appear to pose a large threat. Future estimates of farm mineral footprints should be more conclusive of all true imports and exports of minerals; however, the current approaches are still useful tools to gauge efficiency and finding areas for improvement on individual farms. The diet is the single largest factor determining the extent of mineral excretion. Understanding the interactions and sources of antagonism among elements is crucial to prevent over supplementing minerals yet provide adequate dietary levels. The use of ionophores and monitoring feed variation may be beneficial in improving mineral utilization. The environmental benefits of supplementing diets with organic or specialty minerals, however, are not clear-cut, and more research is needed to quantify their absorption and excretion. 26

41 Table 1. The total number and partitioning of milking cows for two different herd sizes in the United States since 1992 (from USDA-NASS). < 100 cows > 1000 cows Year Farms % of total % of total Total number Farms cows cows of milk cows , ,492, , ,095, , ,104, , ,267, , ,252,000 27

42 Figure 1. Total acreage (in thousand acre units) in the United State receiving animal manure by species as a proportion of all acres receiving manure (adapted from Carter and Kim, 2013). 28