Protein requirements of growing steers limit-fed corn-based diets Protein requirements of growing steers limit-fed corn-based diets

Seven steers (513 lb) were used in an exper-iment to investigate optimal levels and sources of protein in diets limit-fed to allow gain of 2.2 lb/day. Treatments were: a negative-control diet (urea; su pplemented, 11.7% crude protein) and six diets containing either 13.5, 15.4, or 17.2% crude protein with either solvent-ex-tracted (SSBM) or expeller-processed (ESBM) soybean meal, in which the soybean meal replaced corn in the control diet. Diets provided 75, 87.5, 100, or 112.5% of estimated crude protein requirement for a gain of 2.2 lb/day. The basal diet contained 83% rolled corn, 15% alfalfa, and .2% urea. Nitrogen (N) retention was increased linearly (P<.01) by SBM addition with no differences between sources. Because N retention increased to the highest level offered, the steers apparently required more protein than estimated by the 1984 National Research Council' s Nutrient Requirements of Beef Cattle .


Summary
Seven steers (513 lb) were used i n an experiment to investigate optimal levels and sources of protein in diets limit-fed to allow gain of 2.2 lb/day.Treatments were: a negative-control diet (urea; su pplemented, 11.7% crude protein) and six diets containing either 13.5, 15.4, or 17.2% crude protein with either solvent-extracted (SSBM) or expeller-processed (ESBM) soybean meal, in which the soybean meal replaced corn in the control diet.Diets provided 75, 87.5, 100, or 112.5% of estimated crude protein requirement for a gain of 2.2 lb/day.The basal diet contained 83% rolled corn, 15% alfalfa, and .2%urea.Nitrogen (N) retention was increased linearly ( P<.01) by SBM addition with no differences between sources.Because N retention increased to the highest level offered, the steers apparently required more

Introduction
Calves can be grown efficiently on highconcentrat e diets fed at restricted intakes.Benefits of restrictedly feeding high-grain diets relative to ad libitum feeding of high-roughage diets include more stable intake patterns, more predictable perf ormance, and reductions in cost of gain.Growing programs aim to restrict energy-allowabl e gain without restricting other nutrients required to support that gain.Thus, compared to a low -energy diet fed at ad libitum intake, a limit-fed high-energy diet requires higher concentrations of dietary protein.However, restricted feeding alters ruminal function, which may influen ce optimal levels and sources of protein.
Our objectives were to 1) investigate optimum levels of dietar y protein, and 2) compare solvent-extracte d soybean meal (SSBM) and expeller-processe d soybean meal (ESBM) as source s of supplemental protein for limit-fed steers.

Experimental Procedures
Seven Angus-cross steers (513 lb) were used in a nitrogen balance experiment to determine optimal levels of dietary protein and source of supplemental protein in a high-concentrate diet fed at restri cted intakes designed to allow gain of 2 . 2 lb/day.The experiment was a 7 × 4 incomplete Latin square.Treatments consisted of six diets in a 3 × 2 factorial arrangemen t plus a negative-control diet containing only urea as s upplemental crude protein.
The two main factors were source of supplemental protein and level of protein in the diet.Protein so urces were SSBM and ESBM (Super Soy®, Delavan Processing).These protein sources replaced corn i n the basal diet (negative control) at levels of approximately 5, 10, and 15% (as-fed basis).Diets (Table 1) were formulate d to provide 75, 87.5 , 100, and 112.5% of the recommendation for protein required by a 513 lb steer gaining 2.2 lb/day, according to the 1984 National Research Council' s Nutrient Requirements of Beef Cattle .
Steers were implanted with Compudose® 200 and housed in metabolism crates.Each period contained a 9-day a daptation and a 5-day collection phase.Feed allocated to each steer was fed in equal portions twice daily (7 a.m. and 7 p.m.).Feed allocations were adjusted at the start of each new period for projected changes in body weight.
Feed samples, urine, and feces were collected and analyzed for N in order to calculate the amount of N retained by the steers.Nitrogen retention was used as an in d icator of protein accretio n (lean growth) in the steers.A jugular blood sample was taken 5 hiyrs after the morning feeding on the last day of each period to measure plasma urea and amin o acid concentrations.
Estimates of the undegraded intake protein (escap e protein) in SSBM and ESBM were obtained by incubating samples with protease enzymes and measuring the protein resistant to degradation.

Results and Discussion
On average, steers consumed 9.9 lb of dietary dry matter per day.Nitrogen balance data are presented in Table 2.No significant interactions occurred between level and source of protein, and effects on N balance were similar for SSBM and ESBM.Urinary N excretion increase d linearly (P<.01), whereas fecal N excretio n remained constant with increasing levels of protein.Nitrogen retention was increased linearly (P<.01) by increasing levels of protein up to the highest level of supplementation (112.5% of the 1984 NRC recommendation).This suggests that, u n der the conditions of our study, the protein requirement of steers was underestimated by the 1984 NRC.Concentration s of total amino acids ("amino nitrogen) and urea in plasma in-creased as protein intake increased (Table 2).The inc reases in urea concentrations are reflective of the higher dietary protein concentration, whereas the increases in amino acid concentrations are reflective of increased supply of absorbable amino acids to the small intestine.
Enzymati c digestion of SBM samples in vitro esti mated undegraded intake protein to be 32% of SSBM-protein and 48% of ESBMprotein .The relatively small difference in ruminal escape between t he two protein sources resulted in relatively small differences between sources i n the supply of protein to the intestine; this could explain the similarity in N balance response s for SSBM and ESBM.Also, when ESBM with its higher escape value was used instead of SSBM, degradable protein supply may have become marginal for ruminal microbes; this could decrease microbial protein synthesis, thereby eliminating any advantage gained by using a protein source with a higher escape value.The conclusion that total supply of protein to the intestine was not affected markedly by so urce of protein also is supported by the similar increases in plasm a "-amino N (amino acid) concentrations betwee n the SBM sources.
Previous studies that investigated supplemental protei n for limit-fed growing cattle have yielded equivocal results.In our study, N retention of limit-fed growing steers was improved when corn-urea diets were supplemented with SBM, and N retenti o n increased up to the highest level of protein supplementation (112.5 % of estimated crude protein requirement).This suggests th at requirements of limitfed steers for dietary protein are higher than predicted by the 1984 NRC.

Table 2 . Nitrogen Balance and Plasma Metabolites of Steers Fed Different Levels of Solvent-Extracted (SSBM) or Expeller-Processed Soybean Meal (ESBM)
a Linear effect of level of expeller soybean meal (P<.05).bA measure of total amino acids in plasma.c