Combinations of ruminally degradable and escape protein for implanted finishing steers

One hundred forty-four crossbred medium framed steers (738 lb) were used to compare urea and soybean meal as basal supplemental nitrogen sources and sources of high (blood meal:corn gluten meal; BMCG) or low (soybean meal; SBM) ruminal escape proteins as additional protein supplementation. Basal diets were formulated to contain 1 0.8% crude protein (CP) and were supplemented with either urea or SBM (.91% and 5.55% of DM, respectively). An additional 2 percentage units of CP were either provided or not provided as SBM or as a 50:50 combination (protein basis) of BMCG. Steers were implanted with Revalor-S® and fed experimental diets for 113 days. Steers fed urea diets consumed 3.9% more feed than those fed SBM as the basal N source. Average daily gain was unaffected by treatment. Soybean meal improved feed efficiency 3.8% compared to urea as a basal nitrogen source. Supplying additional CP from SBM increased feed efficiency 4.4% compared to BMCG. Hot carcass weight and dressing percentage were not affected by treatment. Supplementing basal diets with 2 percentage units of CP increased percentage of carcasses grading choice, KPH fat, and yield grade. High dry matter intakes resulted in metabolizable protein intakes above the predicted requirements (760 g/d) for steers in this experiment, which may have precluded a response to supplemental protein.


Summary
One hundred forty-four crossbred medium framed steers (738 lb) were used to compare urea and soybean meal as basal supplemental nitroge n sources and sources of high (blood meal:corn gluten meal; BMCG) or l ow (soybean meal; SBM) ruminal escape proteins as additional pr otein supplementation. Basal diets were formulated to contain 1 0 .8% crude protein (CP) and were supplemente d with either urea or SBM (.91% and 5.55% of DM, respectively). An additional 2 percentage units of CP were either pr ovided or not provided as SBM or as a 50:50 combination (protein basis) of BMCG. Steers were implanted w i th Revalor-S® and fed experimental diets for 113 da y s. Steers fed urea diets consumed 3.9% more feed than those fed SBM as the basal N source. Average daily gain was unaffected by treatment. Soybean meal improved feed efficiency 3.8% compared to urea as a basal nitrogen source. Supplying additiona l CP from SBM increased feed efficienc y 4.4% compared to BMCG. Hot carcas s weight and dressing percentage were not affected by tr eatment. Supplementing basal diets with 2 percentage units of CP increased percentag e of carcasses grading choice, KPH fat, and yield grade. High dry matter intakes resulted in metabolizable protein intakes above the predicted requirements (760 g/d) for steers in this experimen t , which may have precluded a response to supplemental protein.

Introduction
Finishing steers receiving growth promotants , especially the combination of estradiol and trenb olone acetate, have enhanced growth rates, which may increase their requirements for metaboli zable protein. Protein presented to the small i ntestine can be increased by using protein sources suc h as blood meal and corn gluten meal, which escape rumen degradation , whereas the rumen nitrogen requiremen t can be met with sources susceptible to degradation.
Previous studies (Cattlemen's Day, 1994) demonstrate d that supplementing high grain diets with urea enhances rumen organic matter digestio n with little or no improvement in microbia l protein production. A source of natural, ruminally degradable protein may be required to increase m i crobial protein presented to the small intestine and subsequently increase animal performance. Those reports suggested that, in corn-based finishing diets, the level of urea for optimal rat e and efficiency of growth is .91% of dietary dry matter. In this study, our objectives were to compare urea (non-protein nitrogen ) versus soybean meal (natural, degradabl e protein source) as basal, supplemental nitrogen (N) sources and evaluate additional N supplem entation from high-versus low-escap e proteins on performance and carcass characteristics of implanted, finishing steers.

Experimental Procedures
One hundred forty-four cro s sbred, mediumframed steers (738 lb) were allotted to one of four weight replicates and stratified into one of six pens within each replicate in a 2 × 3 factoriall y arranged experiment. Basal diets (90% concentrate, 10.8% CP; Table 1) contained either .91% urea or an equivalent amount of N as SBM (5.55% of diet DM). An additiona l two percentage units of CP were either provided or not provided by a 50:50 combinatio n (protein basis) of BMCG (high escap e) or SBM (low escape). All diets were formulated to con t ain .7% Ca, .35% P, and .7% K. Steers wer e fed 275 mg Rumensin® and 90 mg Tylosin® per head daily. Steers were steppe d up to a n 80% concentrate diet prior to the start of the e xperiment. Initial weights were the average of two consecutive early morning weights taken before feeding. Steers were implanted with Revalor-S® and fed experimenta l diets for 113 days. Final weights were computed from hot carcass weights, assuming a dressing percentage of 62 for calculatio n of daily gain and feed efficiency. Steers were slaughtered at a commercial plant, and carcass data were obtained following a 24hour chill.
Statistical analysis allowed comparison s of: 1) basal supplemental N source, 2 ) basal versus additional supplemental protein (i.e., 10.8 vs 12.8% CP diets), 3) additional N supplementation in the form of high (BMGG) or low (SBM) escape protein, and 4) interaction between basal and additional N sources.

Results and Discussion
Steers fed urea diets consumed 3.9% more feed (P<.07) than those fed SBM as the basal N source (Table 2). Average daily gain was not affected by treatment. Steers supplemente d with SBM as the basal N source were 3.8% more efficient (P<.05) than those supplemente d with urea; additional N supplementatio n from SBM improved feed efficienc y 4.4% (P<.09) versus BMCG. The improvemen t in feed efficiency from SBM suggest s that provision of natural, degradable protein to rumen microorganisms improved fermentation. Hot carcass weight and dressing percentag e (62.6% ) were unaffected (P>.2) by treatment . A basal × additional N source interaction (P<.02) was observed for 12th rib fat thickness. When steers were fed the urea basal diet, fat thickness was increased by additional BMCG , whereas fat thickness o f steers fed the SBM basal diet was decreased by additional BMCG. Loin eye area (LEA) was decreased (P<.05) by the addition of supplemental N to the basal diets, and this depression in LEA was more severe when steers received additional N from BMCG than from SBM. Observed difference s in LEA may be somewhat trivial because of the small differences noted across treatments . Percentage of carcasses grading choice, KPH fat, and calculated yield grade were increased (P<.09) when additional N was provide d to the basal diets. Metabolizable protein requirements for steers in this experiment were calculate d to be approximately 760 g/d. High dry matter intakes resulted in metabolizabl e protein intakes ranging from 949 to 1112 g/d, which may have precluded responses to basal and supplemental N sources in this study.