The effect of rumen degradable and rumen undegradable intake protein on feedlot performance and carcass merit in heavy yearling steers 1

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1 Published December 4, 2014 The effect of rumen degradable and rumen undegradable intake protein on feedlot performance and carcass merit in heavy yearling steers 1 J. J. Wagner,* 2 T. E. Engle, and T. C. Bryant *Southeast Colorado Research Center, Colorado State University, Lamar 81052; Department of Animal Sciences, Colorado State University, Fort Collins 80523; JBS Five Rivers Cattle Feeding, Greeley, CO ABSTRACT: A total of 432 crossbred yearling steers (395 kg ± 6.35) were used in a randomized block experiment to study the effects of rumen degradable intake protein (DIP) and rumen undegradable intake protein (UIP) concentration on feedlot performance and carcass merit. The 6 dietary treatments used for this study included 1) a 10.5% CP diet with 5.1% UIP and 5.4% DIP (DIP5); 2) an 11.5% CP diet with 5.1% UIP and 6.4% DIP (DIP6); 3) a 12.5% CP diet with 5.1% UIP and 7.4% DIP (DIP7); 4) a 13.5% CP diet with 5.1% UIP and 8.4% DIP (DIP8); 5) a 14.5% CP diet with 5.1% UIP and 9.4% DIP (DIP9); and 6) a 14.5% CP diet with 6.1% UIP and 8.4% DIP with the additional UIP provided by corn gluten meal. There was a linear increase in final BW and ADG and a trend for a linear increase in DMI associated with increasing DIP concentration within the 5.1% UIP treatments. Feed efficiency and NE recovered from the diet were not influenced by dietary DIP concentration. As dietary DIP concentration increased, carcass fat depth and average yield grade increased linearly and the percentage of yield grade 1 and 2 carcasses decreased linearly. Dietary UIP treatment had no effect on final BW, ADG, DMI, G:F, and calculated NE recovery. For the 14.5% CP diets, marbling score tended to be reduced for steers fed 6.1% UIP as compared with 5.1% UIP. The remaining carcass traits were not affected by dietary UIP. The results of this study show that the DIP requirement in the finishing diet for heavy yearling steers fed steam-flaked corn is greater than 7.4% of dietary DM but likely is not more than 8.4% of dietary DM when dietary UIP is approximately 5.1% of dietary DM. Increasing UIP above 5.1% of dietary DM did not improve feedlot performance or HCW. Expressed on a CP basis, it appears as though the requirement for CP for heavy yearling steers fed steam-flaked corn-based finishing diets is 13.5% of DM, with approximately 62% of CP from DIP. Key words: crude protein, degradable intake protein, feedlot performance, undegradable intake protein, yearling steer 2010 American Society of Animal Science. All rights reserved. J. Anim. Sci : doi: /jas INTRODUCTION Historically, there has been significant variation in BW, frame, flesh condition, and genetic makeup among individuals in each pen of feedlot cattle. Feedlot nutritionists have managed this situation by formulating diets with nutrient concentrations typically above those recommended by the NRC (1996) for the average individual animal within the pen (Vasconcelos and Galyean, 2007). This strategy provides most cattle with ample nutrients to meet their requirements for maintenance and BW gain. Consequently, some cattle may 1 This research was supported by JBS Five Rivers Cattle Feeding (Greeley, CO). 2 Corresponding author: john.wagner@colostate.edu Received May 6, Accepted October 31, not receive adequate nutrients; however, many cattle in the pen receive more nutrients than required. Excess nutrient intake reduces production efficiency and likely results in excess nutrient excretion into the environment. Providing the proper concentration and ratio of rumen degradable intake protein (DIP) to rumen undegradable intake protein (UIP) may improve growth performance and may facilitate a reduction in nitrogen excretion into the environment. The optimal daily MP intake for steers implanted with medium- to high-potency implants was listed at 8.04 g/kg of BW 0.75 (Di- Costanzo and Zehnder, 1999). Results from Klemesrud et al. (2000), Ludden et al. (1995), and Sindt et al. (1993) illustrate that for steers consuming corn-based diets, MP intake generally exceeds requirements, especially for heavy-bw yearling cattle. Experiments conducted to investigate the effect of DIP concentration or 1073

2 1074 intake on feedlot performance generally demonstrated that results depended on the dietary grain source (Milton et al., 1997; Shain et al., 1998; Cooper et al., 2002; Gleghorn et al., 2004; Block et al., 2005). DiCostanzo (2007) reported that for cattle consuming steam-flaked corn-based diets, the amount of DIP required for maximum ruminal OM digestion and microbial nitrogen flow (MNF) was 640 g/animal daily, or 7.29% of DMI. The objectives of this research were to investigate the effect of dietary DIP and UIP concentrations on feedlot performance and carcass merit in heavy yearling steers consuming steam-flaked corn-based finishing diets. MATERIALS AND METHODS This study was conducted at Continental Beef Research (CBR) located in Lamar, CO, before the facility was gifted to Colorado State University. Feeding and care of experimental cattle during the study described in this manuscript were in accordance with guidelines published by FASS (1999). Cattle Source, Processing, and Allotment A total of 748 crossbred yearling steers with individual feedlot processing BW greater than or equal to 390 kg were selected for the study from various sources of new cattle that arrived at 2 commercial feedlots from mid-july through early August. Steers were processed, individually weighed, and tagged with consecutively numbered ear tags as they were sorted at their feedlot of origin. Processing procedures consisted of the typical programs used at each facility and included treating for parasites with Dectomax (Doramectin, Pfizer Animal Health, Exton, PA), vaccinating with Bovi-Shield 4 respiratory vaccine (bovine rhinotracheitis, virus diarrhea, parainfluenza 3, respiratory syncytial virus vaccine, Pfizer Animal Health), and implanting with Revalor-S (24 mg of estradiol benzoate and 120 mg of trenbolone acetate, Schering-Plough/Intervet Animal Health, DeSoto, KS). Because steers were fed fewer than 140 d, they were not reimplanted. Immediately postprocessing, steers were transported approximately 200 km to CBR. Upon arrival, steers had overnight access to longstemmed grass hay and water. Data collected during processing and sorting at the feedlot of origin were stratified within feedyard origin by ascending individual processing BW into 4 BW block replicates, for a total of 8 BW block replicates (BW block replicates 1 to 4 for feedyard 1 and 5 to 8 for feedyard 2). Within each feedlot origin group BW block replicate, individual steers within each set of 10 steers, in ascending BW order, were randomly assigned to 1 of 10 treatments. By following this procedure, four 9-animal pens per treatment were available for the trial for each of the 2 feedlot origins. Steers were fed ad libitum amounts of a starting diet [47.5% steam-flaked corn, 30% alfalfa hay, 17.4% corn silage, 3% condensed corn distillers solubles (CCDS), 1.2% soybean meal, Wagner et al. and 0.9% supplement; DM basis] for either 5 or 9 d, depending on origin, from arrival at CBR until the study beginning dates. On the trial beginning dates, steers were weighed individually and were tagged with visual identification tags. Steers were then sorted into their respective treatment pens and the study was begun. Dietary Treatments Four of the 10 treatments set up during randomization were used for another study not reported in this manuscript. Six dietary treatments were used for the present study: 1) a 10.5% CP diet with 5.1% UIP and 5.4% DIP (DIP5); 2) an 11.5% CP diet with 5.1% UIP and 6.4% DIP (DIP6); 3) a 12.5% CP diet with 5.1% UIP and 7.4% DIP (DIP7); 4) a 13.5% CP diet with 5.1% UIP and 8.4% DIP (DIP8); 5) a 14.5% CP diet with 5.1% UIP and 9.4% DIP (DIP9); and 6) a 14.5% CP diet with 6.1% UIP and 8.4% DIP, with the additional UIP provided by corn gluten meal (CGM). Experimental Diets and Feeding Protocol Table 1 shows the assumed values used for CP, DIP, UIP, and NPN for each feed commodity used in the formulation of the diets. Degradable intake protein and UIP concentrations for most of the feed commodities were based on values presented in NRC (1996). Because CCDS is not listed in NRC (1996), the DIP and UIP values assigned for CCDS were based on estimates provided by 3 feedlot nutritionists (M. L. Galyean, Texas Tech University, Lubbock, TX; S. A. Bachman, Steve Bachman Consulting, Amarillo, TX; M. D. Miller, Mark Miller Consulting, Elk City, OK, personal communication). The concentration of CCDS was the same for all diets, constituting only 3% of dietary DM (Table 1). Crude protein and NPN values were historical values at CBR obtained from laboratory analyses conducted at a commercial feed testing laboratory (SDK Laboratories, Hutchinson, KS), using standard AOAC procedures (method for CP and method for NPN; AOAC, 2005) for each feed commodity except CGM. Corn gluten meal was assumed to contain 66.3% CP on a DM basis (NRC, 1996). A beginning diet, step-1 diet, and step-2 diet, each containing increasing concentrations of steam-flaked corn (0.36 kg/l) and decreasing concentrations of roughage and 16.5 mg of monensin (Rumensin, Elanco Animal Health, Greenfield, IN) per kilogram of DM were used to acclimate the steers to grain. The finishing diets (Table 2) contained monensin and tylosin (Tylan, Elanco Animal Health) at 33 and 11 mg/kg of DM, respectively. Corn silage was used as the roughage source in the finishing diets from the beginning of the trial through d 59. Sorghum silage was used as the roughage source in the finishing diets from d 60 through slaughter. As urea replaced steam-flaked corn in the diet, the theoretical NE m and NE g concentrations for the DIP5, DIP6, DIP7, DIP8, and DIP9 diets were 2.25 and 1.59,

3 Dietary degradable and undegradable intake protein and 1.55, 2.24 and 1.54, 2.23 and 1.54, and 2.22 and 1.53 Mcal/kg of DM, respectively. The theoretical NE m and NE g concentrations for the CGM diet were 2.23 and 1.54 Mcal/kg of DM, respectively. All diets were formulated to contain a minimum of 0.70% calcium, 0.26% phosphorus, 0.25% magnesium, 0.70% potassium, 3,300 IU of vitamin A/kg of DM, and 33 IU of vitamin E/kg of DM. All diets were fed twice daily at 0700 and 1230 h. Diets were manufactured immediately before feeding, using a stationary 4-auger mixer (Model 203, Harsh Manufacturing, Dodge City, KS) located in the feed mill at the research facility. Finishing diets and feed commodities were sampled (approximately 1 kg) every week during the experiment. Duplicate subsamples (approximately 100 g each) were dried at 60 C for 48 h in a forced-air drying oven at CBR. The remaining portion of each sample was composited by month and split into 2 portions of similar size. One portion was used for routine CP (method ; AOAC, 2005); NPN (method ; AOAC, 2005); NDF (method 6, NDF in Feeds: Filter Bag Technique, Ankom Technology, Fairport, NY); calcium, magnesium, and potassium (method ; AOAC, 2005); and phosphorus (method ; AOAC, 2005) analyses at a commercial laboratory (SDK Laboratories). The second portion was stored frozen at CBR as a backup. Loose supplements were manufactured at CBR at the beginning and throughout the duration of the trial as needed, according to standard operating procedures. Supplements contained macrominerals, trace mineral premix, vitamin premixes, urea, mineral oil, monensin premix (176 g/kg), and tylosin premix (220 g/kg). Feed bunks were examined at 0600 h and the target was to have only a few crumbles of feed left in each bunk at this time. If bunks were slick (devoid of feed) for 2 consecutive mornings, the feed offered was increased by 0.23 kg of DM per animal. Feed refusals were weighed and sampled for DM determination whenever feed became spoiled because of adverse weather conditions and at the conclusion of the experiment. Feed refusal samples were evaluated for DM content at CBR by drying the sample for 48 h in a 60 C forced-air oven. Dry matter consumption for each pen was calculated by subtracting the amount of DM weighed back from the amount of DM delivered and dividing the result by animal days for the pen. Weighing Conditions The initial BW used in the analyses was a single full BW obtained at CBR on the trial beginning date. Final BW was an average of 2 individual full BW obtained on 2 consecutive days before slaughter. Interim pen BW were obtained on d 28, 56 (replicates 1 to 4), 58 (replicates 5 to 8), 84 (replicates 5 to 8), 85 (replicates 1 to 4), and 112 (replicates 1 to 4 only). A 4% pencil shrink was applied to all BW before data analysis. Table 1. Crude protein, NPN, and assumed rumen degradable intake protein (DIP) and rumen undegradable intake protein (UIP) concentrations for each feed commodity used to formulate the experimental diets Ingredient CP 1 NPN 1,2 DIP 3 UIP 4 Alfalfa hay Corn silage Sorghum silage Flaked corn CCDS Corn gluten meal Soybean meal Urea All values are from laboratory analyses except values for corn gluten meal. 2 NPN, % of CP equivalent. 3 DIP, % of CP; from NRC (1996). 4 UIP, % of CP; from NRC (1996). 5 Condensed corn distillers solubles. The DIP and UIP reflect an average of estimates obtained by personal communication from S. A. Bachman (Steve Bachman Consulting, Amarillo, TX), M. D. Miller (Mark Miller Consulting, Elk City, OK), and M. L. Galyean (Tech University, Lubbock). 6 NRC (1996). Energy Recovery Net energy for maintenance and NE g requirements were calculated using equations published by NRC (1996). Final shrunk BW (FSBW) was assumed to equal the actual FSBW for each pen. Shrunk BW (SBW) was calculated as the average of the initial and FSBW for each pen. Shrunk relative BW of 478 kg, SBW, and FSBW were used to calculate equivalent SBW (NRC, 1996). The NE m and NE g content derived from the diet for each pen was calculated from pen performance and requirements for NE m and NE g using the quadratic formula described by Zinn et al. (2003); however, the shrunk relative BW approach was used instead of the medium-framed steer equations shown in NRC (1984). Slaughter Procedures and Carcass Data Steers were shipped to a commercial slaughter plant (JBS Swift, Cactus, TX) after 127 d on feed for the BW block replicate 1 to 4 steers and after 120 d on feed for the BW block replicate 5 to 8 steers. On the day of slaughter, steer slaughter order was recorded and carcass identification numbers were matched with ear tag number. After a 36-h chill, routine carcass measurements were obtained by the Cattlemen s Carcass Data Service (West Texas A & M University, Canyon). Preliminary yield grade, ribeye area, KPH fat percentage, and marbling score were recorded. Hot carcass weight, the incidence of dark cutters, and USDA quality and yield grades were obtained from the carcass kill sheets supplied by the packing plant.

4 1076 Wagner et al. Table 2. Dry matter composition of the finishing diets used for the rumen degradable intake protein (DIP) and rumen undegradable intake protein (UIP) treatments Treatment 2 DIP5 3 DIP6 3 DIP7 3 DIP8 3 DIP9 3 CGM 4 Item 1 Phase 1 Phase 2 Phase 1 Phase 2 Phase 1 Phase 2 Phase 1 Phase 2 Phase 1 Phase 2 Phase 1 Phase 2 Steam-flaked corn Corn silage Sorghum silage CCDS Yellow grease Soybean meal 50% Corn gluten meal 60% Urea Limestone Magnesium limestone Potassium chloride Salt Mineral oil Trace mineral premix Vitamin A premix Vitamin E premix Monensin premix Tylosin premix Percentage of DM. 2 DIP5 = 5.4% DIP; DIP6 = 6.4% DIP; DIP7 = 7.4% DIP; DIP8 = 8.4% DIP; DIP9 = 9.4% DIP % UIP. 4 Corn gluten meal, 8.4% DIP and 6.1% UIP. 5 Corn silage diets were fed through d 59 (phase 1). Sorghum silage diets were fed from d 60 through slaughter (phase 2). 6 Condensed corn distillers solubles. 7 Min Ad Inc. (Amarillo, TX; 21.45% calcium and 11.68% magnesium, DM basis). 8 Trace mineral premix: cobalt, 340 mg/kg; copper, 7.7%; manganese, 6%; zinc, 22.4%; selenium, 300 mg/kg million IU/kg ,238 IU/kg. 11 Rumensin (Elanco Animal Health, Greenfield, IN; g/kg). 12 Tylan (Elanco Animal Health; g/kg).

5 Dietary degradable and undegradable intake protein 1077 Pen Observations Pens were checked daily shortly after the morning feeding to monitor cattle for health problems. Cattle exhibiting symptoms of injury or disease were removed from the pen, treated according to the appropriate treatment schedule, and immediately returned to the pen. Steers with persistent health problems were removed from the study and sold for salvage once pharmaceutical withdrawal requirements were met. Experimental Design and Data Analysis Data were evaluated using mixed model procedures appropriate for a randomized block design as outlined by the MIXED procedure (SAS Inst. Inc., Cary, NC.). Pens constituted the experimental units for all the data evaluated. All variables in the models were considered class variables. Models evaluating feedlot performance and continuous carcass data included treatment as a fixed effect and BW block replicate as a random effect. Quality grade, yield grade, liver abscess, HCW category, and carcass maturity distribution data were evaluated as categorical data by using the GLIMMIX procedure of SAS, assuming a binomial distribution. The Link = Logit option of the model statement and the ILINK option of the LSMEANS statement were used to calculate the likelihood ± SEM that an individual within each pen qualified for a specific category. Degradable intake protein treatment means, among only the 5.1% UIP treatments, were compared using linear and quadratic contrasts. Orthogonal contrasts were used to evaluate the effects of increased UIP. Contrasts of interest were DIP8 vs. the means of DIP9 and CGM to evaluate the effects of increasing CP from 13.5 to 14.5% and DIP9 vs. CGM to determine if the effects observed were a function of additional UIP vs. DIP. Statistical significance was declared using an α level of 0.05, with tendencies discussed when P-values were between 0.05 and RESULTS AND DISCUSSION Feed Ingredient and Diet Nutrient Composition Table 3 displays the DM nutrient concentrations, as determined by laboratory analyses, for the feed commodities used during the trial. Nutrient concentrations for most of the commodities used were similar to the assumed values used when formulating the diets. The CP concentration for the CGM (76.87% of DM) appeared greater than the assumed value (66.30). Table 4 shows the DM nutrient composition, as determined by laboratory analyses, for the finishing diets used for the trial. The analyzed CP and NPN concentrations observed for the DIP5 diet were numerically greater than the formulated values of 10.5 and 0.68% of DM, respectively. The analyzed CP and NPN concentrations observed for the DIP6, DIP7, DIP8, and DIP9 finishing diets were numerically less than formulated values of 11.5 and 1.58, 12.5 and 2.55, 13.5 and 3.52, and 14.5 and 4.49% of dietary DM, respectively. The lesser analyzed NPN concentrations, as compared with formulated values, may have been due to volatilization of some ammonia during the sampling and drying process before NPN analysis. Polan et al. (1967), as cited by Huber et al. (1968), reported that recovery of urea nitrogen from urea-treated high-dm corn silage (37 to 44% of DM) averaged only 70% when samples were dried at 85 C for 48 h before Kjeldahl analysis; however, urea nitrogen losses from samples that were subjected to Kjeldahl analysis without prior drying were negligible. Routine NPN analysis conducted by the commercial laboratory included 24-h drying at 50 C. The CP concentration observed for the CGM finishing diet was greater than the formulated value. This was due to the greater than expected CP concentration for CGM. The NPN content of the CGM finishing diet was numerically less than the observed NPN concentration for the treatment DIP9 finishing diet. This suggests that the UIP concentration for the CGM treatment was indeed greater than that observed for DIP9. Steer Health Seven steers from various treatments were treated for toe abscesses early during the trial. Respiratory problems during the trial were minimal. Of 3 steers treated for respiratory symptoms, 2 failed to recover and were sold for salvage. Three additional steers died during the trial because of respiratory disease. One steer was diagnosed with polioencephalomalacia and was removed from the trial. One steer was removed from the trial because of a pen injury. Feedlot Performance Table 5 shows the effect of dietary DIP and UIP concentrations on BW and feedlot performance. Initial BW for the steers averaged 395 kg and did not differ (P > 0.20) among treatments. Differences for DIP8 vs. DIP9 and CGM or for DIP9 vs. CGM were not significant for all the performance variables examined; therefore, increasing the dietary UIP concentration above 5.1% had no effect on growth, feed intake, BW gain efficiency, or NE recovery. Linear effects of increasing DIP on final BW (P = 0.05) and ADG (P = 0.03) were significant. Final BW appeared to reach a maximum within the range of 7.4 to 8.4% DIP. Quadratic effects of increasing DIP on final BW and ADG were not significant (P > 0.10). A linear (P = 0.06) increase in daily DMI was associated with increasing DIP concentration. For each percentage unit increase in DIP, DMI increased by approximately kg/animal. Quadratic effects of DIP on DMI were not observed. Dietary DIP concentration had no significant (P > 0.10) effect on G:F or dietary NE recovery.

6 1078 Wagner et al. Table 3. Analyzed nutrient composition of feed commodities used in the experimental diets Feed commodity Item 1 Flaked corn Alfalfa hay Corn silage Sorghum silage Soybean meal CGM 2 CCDS 3 DM ± ± ± ± ± ± ± 0.88 CP 9.26 ± ± ± ± ± ± ± 0.56 NPN ± ± ± ± ± ± ± 0.15 NDF 8.78 ± ± ± ± ± ± ± 0.10 Ether extract 3.92 ± ± ± ± ± ± ± 0.30 Calcium 0.05 ± ± ± ± ± ± ± 0.01 Phosphorus 0.26 ± ± ± ± ± ± ± 0.06 Potassium 0.43 ± ± ± ± ± ± ± 0.08 Magnesium 0.13 ± ± ± ± ± ± ± Percentage of DM unless stated otherwise. Mean ± SEM. 2 Corn gluten meal. 3 Condensed corn distillers solubles. 4 Percentage of as-fed. 5 NPN, CP equivalent. Carcass Merit Table 6 shows the effect of DIP and UIP concentrations on carcass merit. Increasing dietary UIP above 5.1% did not affect HCW, dressing percentage, and most other carcass measurements. Despite similar final BW, ADG, and HCW, the mean LM area for the DIP9 and GCM treatments was increased (P = 0.03) compared with the DIP8 treatment. Carcasses from the DIP9 and CGM treatments tended to exhibit a greater (P = 0.10) likelihood of qualifying for quality grades that were USDA Standard or less as compared with DIP8 carcasses. Dressing percentage was not influenced (P > 0.10) by treatment and averaged 63.0%. A trend for a linear increase (P = 0.08) in HCW was associated with increasing DIP concentration. There were no treatment differences for the likelihood that an individual carcass within a pen qualified for the heavy ( 431 kg) HCW category. Carcass fat depth measurement increased (linear, P = 0.04; quadratic, P = 0.10) as DIP concentration increased. Despite differences in final BW, ADG, and HCW, linear and quadratic DIP effects for LM surface area were not significant (P > 0.10). Perkins et al. (1992) suggested that a significant portion of the variation for LM area was explained by BW. Yield grade calculated from carcass measurements increased (linear, P = 0.04; quadratic, P = 0.06) and the likelihood that an individual carcass qualified for the USDA yield grade 1 or 2 categories decreased (linear, P = 0.02; quadratic, P = 0.04) with increasing DIP concentration. A quadratic effect (P = 0.02) of DIP concentration was found for the percentage of USDA yield grade 4 or 5 carcasses. Because final BW, ADG, HCW, and fat depth were greater for the DIP7 and DIP8 treatments, as compared with the DIP5 and DIP6 treat- Table 4. Analyzed DM nutrient composition of the finishing diets 1 Treatment 2 Item DIP5 DIP6 DIP7 DIP8 DIP9 CGM 3 Theoretical CP Theoretical DIP Theoretical UIP DM ± ± ± ± ± ± 0.71 CP ± ± ± ± ± ± 0.17 NPN ± ± ± ± ± ± 0.21 NDF ± ± ± ± ± ± 0.22 Ether extract 6.66 ± ± ± ± ± ± 0.09 Calcium 0.79 ± ± ± ± ± ± 0.03 Phosphorus 0.29 ± ± ± ± ± ± 0.01 Potassium 0.71 ± ± ± ± ± ± 0.01 Magnesium 0.26 ± ± ± ± ± ± Mean percentage of DM ± SEM, unless stated otherwise. 2 DIP5 = 5.4% DIP; DIP6 = 6.4% DIP; DIP7 = 7.4% DIP; DIP8 = 8.4% DIP; DIP9 = 9.4% DIP. 3 Corn gluten meal. 4 Rumen degradable intake protein. 5 Rumen undegradable intake protein. 6 Mean percentage of as-fed ± SEM. 7 NPN, CP equivalent.

7 Dietary degradable and undegradable intake protein 1079 Table 5. Least squares means showing the effect of rumen degradable intake protein (DIP) and rumen undegradable intake protein (UIP) concentration on feedlot performance Treatment 1 DIP contrast 2 CGM 3 contrast Item DIP5 DIP6 DIP7 DIP8 DIP9 CGM SEM Linear Quadratic DIP8 vs. DIP9 CGM DIP9 vs. CGM Theoretical CP Theoretical DIP Theoretical UIP Pen replicates Initial BW, kg Final BW, kg ADG, kg DMI, kg G:F, g/kg NE m, 4 mcal/kg NE g, 4 mcal/kg DIP5 = 5.4% DIP; DIP6 = 6.4% DIP; DIP7 = 7.4% DIP; DIP8 = 8.4% DIP; DIP9 = 9.4% DIP. 2 DIP contrasts were among the DIP5, DIP6, DIP7, DIP8, and DIP9 treatments only. 3 Corn gluten meal. 4 Calculated from feedlot performance using the relationships described in NRC (1996). ments, more yield grade 4 or 5 carcasses for the DIP7 and DIP8 treatments were expected; however, it is not known why fewer carcasses categorized as yield grade 4 or 5 were found for the DIP9 treatment as compared with the DIP7 and DIP8 treatments. Marbling score and the distribution of USDA quality grades were not influenced (P = 0.20) by DIP concentration. Block et al. (2001) demonstrated that greater fat depth was re- Table 6. Least squares means showing the effect of rumen degradable intake protein (DIP) and rumen undegradable intake protein (UIP) concentration on carcass merit Treatment 1 DIP contrast 2 CGM 3 contrast Item DIP5 DIP6 DIP7 DIP8 DIP9 CGM SEM Linear Quadratic DIP8 vs. DIP9 CGM DIP9 vs. CGM Theoretical CP Theoretical DIP Theoretical UIP Pen replicates HCW, kg HCW, kg Dressing percentage Fat depth, cm LM, cm KPH, % Yield grade Yield grades 1 and Yield grade Yield grades 4 and Marbling, 6 units Quality grades Choice and Prime Quality grade Select Quality grade sub-select 4, B and C maturity 4, Liver abscesses DIP5 = 5.4% DIP; DIP6 = 6.4% DIP; DIP7 = 7.4% DIP; DIP8 = 8.4% DIP; DIP9 = 9.4% DIP. 2 DIP contrasts were among DIP5, DIP6, DIP7, DIP8, and DIP9 treatments only. 3 Corn gluten meal. 4 Percentage likelihood of an individual within each pen qualifying for the described category. 5 Calculated from carcass measurements. 6 Marbling score units, 300 = Slight 00, 400 = Small Quality grades Standard, Commercial, and Utility. 8 Maturity B and C carcasses. 9 Percentage likelihood of an individual liver within each pen showing signs of abscesses.

8 1080 Wagner et al. Table 7. Level I model calculations for each treatment (NRC, 1996) Treatment 2 Item 1 DIP5 DIP6 DIP7 DIP8 DIP9 CGM 3 Theoretical CP, % Theoretical DIP, % Theoretical UIP, % MP m required, 4 g MP g required, 5 g MP total required, g MP bacteria, 6 g MP intake, 7 g ,023 MP balance, 8 g DIP required, 9 g DIP intake, 10 g DIP balance, 11 g DIP = rumen degradable intake protein; UIP = rumen undegradable intake protein. 2 DIP5 = 5.4% DIP; DIP6 = 6.4% DIP; DIP7 = 7.4% DIP; DIP8 = 8.4% DIP; DIP9 = 9.4% DIP. 3 Corn gluten meal. 4 MP required for maintenance. 5 MP required for BW gain. 6 MP from microbial protein synthesis. 7 MP intake from the diet. 8 MP balance, MP intake MP required. 9 Rumen DIP required. 10 Rumen DIP from the diet. 11 Rumen DIP balance, DIP intake DIP required. lated to heavier HCW, larger LM area, greater percentage of fat in the commercial rib section (ribs 6 to 12), and greater marbling scores. The liver abscess rate decreased (linear, P < 0.01) with increasing DIP concentration. This result is consistent with the idea discussed by Zinn et al. (2003) that dietary urea can act as a transitory buffer in the rumen. Cattle consuming diets with increased urea may be more likely to cope successfully with increased acid production and decreased rumen ph associated with feeding steam-flaked corn. The effects of UIP on liver abscess rate were not significant (P > 0.10). The minimum DIP concentration required in steamflaked corn-based finishing diets for heavy yearling steers is between 7.4 and 8.4% of dietary DM when considering final BW, ADG, DMI, or HCW, with the 8.4% DIP diet resulting in improved performance over the 7.4% DIP diet. Dry matter intakes of 9.83 and kg/d, DIP concentrations of 7.4 and 8.4% of DM, and a UIP concentration of 5.1% of DM for both diets results in MP intakes of 934 and 953 g/d as calculated by the Level I model of NRC (1996) for the DIP7 and DIP8 treatments, respectively (Table 7). When expressed per unit of metabolic BW, the resulting values of 8.81 and 8.89 g/kg of BW 0.75, respectively, are greater than the 8.04 g/kg of BW 0.75 value suggested by DiCostanzo and Zehnder (1999) for heavy yearling steers that were implanted with moderate- to high-potency implants. Total DIP intake by steers fed the DIP7 and DIP8 diets was 723 and 840 g/d, respectively. The value for the DIP7 treatment is less than the requirement and the value for the DIP8 treatment is greater than the DIP requirement, indicated by Level I of NRC (1996) as 864 and 880 g/d for the DIP7 and DIP8 treatments, respectively. Zinn and Shen (1998) found that a minimum of 100 g of DIP/kg of total digestible OM was required to maximize ruminal OM digestion and MNF for steam-flaked corn-based diets. Using performance data from 29 studies and extrapolating from Zinn and Shen (1998), DiCostanzo (2007) reported that for cattle consuming steam-flaked corn-based diets, the amount of DIP required for maximum ruminal OM digestion and MNF was 640 g/d or 7.29% of DM. Applying the same assumptions (93.4% OM in the diet and 78% total tract OM digestibility) as DiCostanzo (2007) suggests that the DIP required for maximized ruminal OM digestibility and MNF was 716 and 732 g/d for the DIP7 and DIP8 treatments, respectively, corresponding to approximately 7.3% of DM. Shain et al. (1998) and Milton et al. (1997) demonstrated a limited effect of supplemental DIP from urea on feedlot performance when diets contained dry-rolled corn; however, other workers have noted a relationship between performance and supplemental DIP from urea when steam-flaked corn was the primary grain in the diet. Cooper et al. (2002) observed quadratic improvements in DMI, ADG, and NE calculated from feedlot performance associated with increasing DIP concentration and concluded that DIP requirements for steamflaked corn-based diets were between 7.1 and 9.5%, with an average of 8.3% of DM. Gleghorn et al. (2004) examined steam-flaked corn diets ranging from 6.07 to 9.7% DIP and reported no differences in DMI; however, maximum ADG and G:F were found for diets with 8.2% DIP. Block et al. (2005) studied DIP requirements of cattle fed steam-flaked corn diets containing from 0 to 40% wet corn gluten feed and concluded that the DIP requirement was 9.6% for maximum ADG and 9.2% for

9 Dietary degradable and undegradable intake protein 1081 maximum G:F. These authors found no effect of DIP on DMI. Cole et al. (2006) and Vasconcelos et al. (2009) observed increases in DMI associated with increased dietary DIP. Zinn et al. (2003) examined steam-flaked barley-based diets containing approximately 6.29, 7.10, 8.65, and 9.06% DIP and concluded that improvements in ADG associated with increasing DIP were due to increases in DMI and not the result of improved ruminal OM digestion or increased MNF to the small intestine. Gain-to-feed ratio was not improved, but DMI was increased for steers consuming increased DIP in the current study. The increased ADG and HCW observed in the current study may simply be the result of increased DMI and the resulting increase in NE intake; however, Level I calculations of NRC (1996) indicate that the DIP5, DIP6, and DIP7 diets were deficient in DIP, perhaps limiting ruminal OM digestion or MNF to the small intestine. Metabolizable protein balance (MP intake MP required), as determined by Level I of NRC (1996), was 59, 64, 70, 72, and 76 g/d for the DIP5, DIP6, DIP7, DIP8, and DIP9 treatments, respectively. Providing additional UIP above 5.1% when DIP was 8.4% of DM did not improve ADG, DMI, or HCW. These results are consistent with the idea that for diets containing corn or corn-based by-products, the UIP requirement is easily met, especially for heavy-bw yearling steers. Rumen degradability of the protein in steam-flaked corn, CGM, and wet distillers grain is 43.0, 41.0, and 33.4% of CP, respectively (NRC, 1996). Although differences observed in the current study were not statistically significant, providing excess UIP may negatively affect ADG and DMI. Zinn and Owens (1993) expressed concerns with oversupply of UIP, which may lead to increased energy expenditures caused by increased ammonia detoxification and nitrogen recycling. 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