Combinations of nonprotein nitrogen and natural protein affect Combinations of nonprotein nitrogen and natural protein affect performance of finishing steers fed flaked corn diets performance of finishing steers fed flaked corn diets

Two hundred crossbred steers (7 85 lb) were used to evaluate the effects of the relative proportion of supplemental nitrogen derived from soybean meal (SBM) and urea. Treatments included an unsupplemented negative control and four 13% CP diets containing SBM:urea proportions (nitrogen basis) of 100:0, 67:33, 33: 67, and 0:100. Steers fed the control diets gained 38% slower (P<.01); ate 4% less feed (P<.10); were 33% less efficient (P<.01); and had lighter carcasses (P<.01) with less backfat (P<.01), less kidney, pelvic, and heart fat (KPH; P<.12), less ribeye area (REA; P<.11) , and less marbling (P<.01) than nitrogensupplemented steers. Among steers fed supplemented diets, feed intake increased linearly as proportion of SBM increased (P<.01) . Daily gain (P<.05) and feed:gain (P<.05) responded quadratically and was best for steers fed combinations of the nitrogen sources. Similarly, hot carcass weights and backfat thickness were greater (P<.06) for steers fed the mixed supplements. There was a tendency for a linear increase in KPH as proportion of urea in the diet was increased (P<.14). Yield grade, ribeye area, and marbling were unaffected by SBM:urea proportions. In high-concentrate finishing diets, at least some of the supplemental nitrogen should be derived from a natural, degradable-protein source.


Introduction
Nitrogen (protein) supplementation of feedlot diets can be viewed as meeting three "requirements" : 1) ammonia for ruminal microbes , 2) peptide/amino acid for ruminal microbes , and 3 ) postruminal digestible amino acids (metabolizable protein) for the animal.Urea can supply ruminal ammonia and, indirectly, digestible amino acids as microbial protein.Because a portion of SBM protein is degraded in the rumen, it can contribute to all three requirements.Som e research has shown that starch-digesting bacteri a derive two-thirds of their nitrogen from peptides and(or) amino acids.Because corn protein is largely resistant to ruminal degradation, supplying a natural, degradabl e protein in high-corn finishing diets may be beneficial.
Although nitrogen supplementatio n of feedlot diets has both qualitativ e and quantitative dimensions, practica l formulation still centers on a total nitroge n (crude protein) system with limited consensu s on ruminal degradability and on the usefulnes s of supplying natural, degradable protein to the ruminal ecosystem.Previous researc h (1994 KSU Cattlemen's Day) showed that urea can meet only part of the total protein needs of feedlot steers.Our objective was to better establish the relative contributions of nonprotei n nitrogen and natural protein to performance of stee rs fed diets based on steamflaked corn.

Experimental Procedures
Two hundred Continental × British steers (785 lb) were allocated to one of four weight blocks.Within each block, steers were allotted to one of five pens.Treatments included an unsupplemente d negative control and four 13% CP diets containing SBM:urea proportions (nitrogen basis) of 100:0, 67:33, 33:67, and 0:100 (Table 1).Steers were processed using standard procedures, implanted with Revalor, and stepped up to the final rations in 21 days.Initial weights were the averages of two consecutive , early morning weights.Because treatment affected dressing percentage (control vs. others; P < .01),final weights were calculate d from hot carcass weights using the average dressing percent (61.54%).These adjusted final weights were used to calculate overall gain and feed efficiency.Hot carcass weights were taken immediately after slaughter and carcass data obtained following a 48-hour chill.The heaviest block was slaughtered after 126 days on feed, and the remaining three blocks after 150 days.The experiment was conducted from July 1 to Novemb e r 28, 1994 at the Southwest Resear ch-Extension Center, Garden City.Preplanned orthogo n al contrasts compared control vs. supplemented diets and the linear, quadratic, and cubic effects of altering SBM:urea nitrogen.
Within the supplemented diets, daily gain responded quadratically (P<.05), with great-er gains for diets containing both protein sources.As the proportion of SBM nitrogen increased , feed intake increased linearly (P<.02).
Consequently, feed efficiency improve d curvilinearly (P<.05) as the proportion of urea in creased.The lowest feed:gain ratio was at 33% SBM:67% urea.The increase d gain on the mixed-nitrogen supplement s translated into 17 lb additional carcas s weight or 28 lb additional final live weight (P<.06) compared to those supplemented with only SBM or urea.Backfat also responde d quadratically (P<.05) to the increasing propo rtion of urea, with 20% greater fat depth for stee rs fed SBM-urea combinations vs. SBM or urea alone.In contrast, KPH tended to increase linearly (P<.14) as urea increased.Although small incr e ases occurred in ribeye area for steers fed the mixed-nitrogen sources , they were not statistically significant.Yield grades, marbling, and percent grading choice were similar among steers fed the supplemented diets.
Steers fed combinations of SB M and urea in 13% CP flaked corn-based diets performed better than those supplemented with SBM or urea alone.Although fat thickness also increase d for these intermediate levels, yield grade was unaffected.Because carcass weight also increased, these data suggest that the compositio n of the extra gain is similar to gains of steers fed single-source supplements.The greater feed intake observed with SBM agrees with previous research (1994 KSU Cattlemen's Day).Soybean meal has less utilizable energy than steam-flaked corn and this likely caused poorer efficiency a s the proportion of SBM was increased .The diet supplemented with only urea proba bly had greater utilizable energy, but performanc e was improved by small additions of SBM, which may have contributed to the supply of protein postruminally or alternatively may have provided needed peptides and(or) amino acids to the ru men microbes.These data sugges t that, in high-concentrate finishing rations, at least some of the supplemental nitroge n should be derived from a natural, degradable-protein source such as SBM.

Table 1 . Composition of Experimental Diets a
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