Source and level of crude protein for implanted finishing steers Source and level of crude protein for implanted finishing steers

One hundred medium-framed, crossbred steers (738 lb) were used to compare nonprotein nitrogen to natural protein supplementation of finishing diets for implanted steers. Diets were formulate d to contain 11.5 or 13.5% crude protein and were supplemented with either urea or soybean meal. A fifth treatment of cottonseed meal supplementation (13.5% dietary crude protein) was added to evaluate differences between natural sources of rumen degradable protein. Steers were implanted with Revalor® and fed for 132 days. During the first 70 days, daily gain and feed efficiency were improved 8.8 and 6.1%, respectively, for steers supplemented with soybean meal vs urea. No difference was observed with protein level. For the entire feeding period, soybean meal increased dry matter intake 3.8% compared to urea. Protein source and level interacted on daily gain. Increasing dietary protein from 11.5 to 13.5% decreased gain by urea-fed steers 8%, whereas increasing dietary protein fr om 11.5 to 13.5% increased gain 6.1% for steers supplemented with soybean meal. Soybean meal improved feed efficiency 7.6% compared to urea. Protein level had no effect on feed efficiency. Steers supplemented with soybean meal had larger loineye areas than those supplemented with urea. Carcass finish, percentage of carcasses grading Choice, and yield grade were not affected by treatment . Performance and carcass traits of steers fed cottonseed meal were similar to those of steers fed soybean meal. We conclude that urea cannot meet the metabolizable protein needs of implanted finishing steers. Cottonseed meal did not differ from soybean meal as a protein source in this study.


Summary
One hundred medium-framed, crossbred steers (738 lb) were used to compare nonprotein nitrogen to natural protein supplementation of finishing diets for implanted steers.Diets were formulate d to contain 11.5 or 13.5% crude protein and were supplemented with either urea or soybean meal.A fifth treatment of cottonseed meal supplementation (13.5% dietary crude protein) was added to evaluate differences between natural sour ces of rumen degradable protein.
Steers were implanted with Revalor® and fed for 132 days.During the first 70 days, daily gain and feed efficiency were improved 8.8 and 6.1%, respectively, for steers supplemented with soybean meal vs urea.No difference was observed with protein level.For the entire feeding period, soybean meal increased dry matter intake 3.8% compared to urea.Protein source and level interacted on daily gain.Increasing dietary protein from 11.5 to 13.5% decreased gain by urea-fed steers 8%, whereas increasing dietary protein fr o m 11.5 to 13.5% increased gain 6.1% for steers supplemented with soybean meal.Soybean meal improved feed efficiency 7.6% compared to urea.Protein level had no effect on feed efficiency.Steers supplemented with soybean meal had larger loineye areas than those supplemented with urea.Carcass finish, percentage of carcasses grading Choice, and yield grade were not affected by treatment .Performance and carcass traits of steers fed cottonseed meal were similar to those of steers fed soybean meal.We conclude that urea c annot meet the metabolizable protein needs of implanted finishing steers.Cottonseed meal did not differ from soybean meal as a protein source in this study.

Introduction
Growth promotants, especially the combination of estradiol and trenbolone acetate, have the potential to alter nutrient requirements in feedlot steers.Current information concerning requirements of rapidly growing feedlot steers for rumen degradable and metabolizable protein is limited.Soybean meal and urea are two commonly used sources of protei n in finishing diets.The usefulness of urea is limited to the amount that provides sufficient rumen ammonia to maximize microbial protein production and(or) rumen organic matter digestion.Other research presented in this publication suggests that, although urea supplementation increases rumen organic matter digestion, it does not enhance protein flow to the small intestine.Conversely, soybean meal contains a degradable protein fraction to supply ammonia, amino acids, peptides, or other growth factors to rumen microbes, as well as an escape fraction to increase true protein reaching the small intestine.Therefore, soybean meal should be better able to increase the supply of metabolizable protein to rapidly growing steers.Our objective was to evaluate two levels of soybean meal and urea on performance and carcass traits of implanted finishing steers.

Experimental Procedures
One hundred medium-framed crossbred steers (738 lb) were stratified by weight into one of four blocks.Within each block, steers were allocated to o ne of five pens in a 2×2+1 factorially arranged ex periment.Diets (Table 1) contained supplemental protein from urea or soybean meal and were formulated to provide 11.5 or 13.5% (dry matter basis) crude protein.A cottonseed meal diet, formulated to provide 13.5% dietary crude protein, was used as an additional treatment to determine differences between the two natural sources of rumen degradable protein.
All diets were formulated (dry matter basis) to contain .7%Ca, .35%P, .7%K, 25 g/ton Rumensin®, and 10 g/ton Tylosin®.Initial weights were the averages of two consecutive early morning weights.Steers were implanted with Revalor and stepped up to final rations in 14 days.Steers were fed the experimental diets for 132 days.Hot carcass weights adjusted by a 62% dressing percent were used as final weights fo r calculation of gain and feed efficiency.Steers were slaughtered at a commercial plant with carcass data being obtained following a 24hour chill.The statistical analysis allowed comparisons of: 1) level of crude protein, 2) source of crude protein, 3) interaction between level and source of crude protein, and 4) soybean meal vs cottonseed meal at 13.5% dietary protein.

Results and Discussion
During the first 70 days, daily gain (P<.05) and feed efficiency (P<.10) were improved 8.8 and 6.1%, respectively, for steers supplemented with soybean meal vs urea (Table 2).Despite rapid gains (3.6 to 4.0 lb/day), increasing dietary protein above 11.5% did not improve daily gain or feed efficiency.
Steers supplemented with soybean meal consumed 3.8% more feed (P=.23 ) over the entire feeding period than those supplemented with urea.A level by source interaction (P<.05) was observed for daily gain.Daily gain by steers fed urea was decreased 8.0% by increasing supplementation to 13.5% dietary crude protein, whereas gain by steers fed soybean meal increased 6.1% when crude protein levels were increased from 11.5 to 13.5%.Steers supplemented with soybean meal were 7.6% more efficient (P<.03) than those supplemented with urea; feed efficiency was not affected by dietary level of crude protein.
The improvement in performance from soybean meal probably was due to improved feed intake, increased metabolizable protein supply, and(or) improvements in fermentation from the provision of rumen degradable amino acids and peptides.Dietary crude protein level and sourc e interacted (P<.05) to affect hot carcass weight.As dietary crude protein level from urea increased from 11.5 to 13.5%, hot carcass weight decreased 2.5%, whereas increasing dietary crude protein level from 11.5 to 13.5% with soybean meal increased hot carcass weight 2.4%.Compared to urea, soybean meal supplementation increased (P<.10) loineye area, but no effect of dietary protein level was observed.These data suggest that ureafed steers were deficient in metabolizable protein and that this was at least partially corrected by supplementing with soybean meal.Dressing percentage, fat thickness (12th rib), marbling score, yield grade, and percentage of carcasses grading choice were not affected by treatment.
Results from previous research suggest that urea supplementation serves to enhance organic matter fermentation in the rumen, with little or no increase in metabolizable protein supply to the animal.Improvements in performance and incr eased carcass weights and loineye areas, with little increase in carcass finish, suggest that supplementing high grain diets with soybean meal increases the total supply of metabolizable protein to the animal.This may be mediated via increased feed intake, increased microbial protein production, or escape of soybean meal protein to the small intestine.Steers fed cottonseed meal had performan c e and carcass traits similar to those of steers fed soybean meal.Whether benefits from soybean and cottonseed meals result from the degradable or escape fractions, or a combination of the two, is unclear.However, nonprotein nitrogen evidently cannot meet the metabolizable protein requirement of rapidly growing steers.
Feed cost of gain and economic return to level and source of crude protein are presented in Table 3. Relative to urea, ration costs were increased when soybean meal or cottonseed meal was fed.However, because of improved daily gain and feed efficiency, cost of gain was similar and economic returns were increased when soybean meal or cottonseed meal was fed.To provide dietary levels of 1500 I U /lb Vitamin A, 20 IU/lb Vitamin E, .7%Ca, .35%P, .7%c K, 25 g/ton Rumensin®, and 10 g/ton Tylosin®.

Table 2 . Effect of Level and Source of Crude Protein on Performance and Carcass Traits of Implanted Finishing Steers
a Final wt = Hot carcass wt÷.62.b Urea vs soybean meal (P<.05).cUrea vs soybean meal (P<.10).dFeed/gain was analyzed as gain/feed and reported as the reciprocal.eCrude protein level by protein source interaction (P<.05).f 4 = slight, 5 = small, 6 = modest.gMeans in a row lacking a common superscript differ (P<.10).hij

Table 3 . Effect of Level and Source of Crude P r otein on Economic Return in Implanted Finishing
a Calculated $.35 /head/day.bCash price $73.00 per cwt.cInitial cost $85.00 per cwt.d