Level of urea in high grain diets: finishing steer performance

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Summary
Eighty-eight medium-framed crossbred steers (731 lb) were used to identify the optimal level of urea in finishing diets for growth and carca ss traits.Diets contained no urea or .5, 1.0, or 1.5% urea (dry matter basis) and no other supplemental protein.Feed efficiency and gain were improved substantially by the first increment of urea (.5%), with little or no improvement from subsequent urea additions.Pooled across level, urea improved feed efficiency by 5.6% and gain by 8.9%, whereas dry matter intake declined 3.3% compared to controls.Regression analysis indicated that the optimal level of urea for gain and feed efficiency was .91% of dietary dry matter.Dressing percentage and hot carcass weight responded quadratically, being higher for steers receiving .5 or 1.0% urea.Fat thickness, yield grade, and KPH fat increased linearly with level of urea.Percentage choice carcasses tended to increase, a lthough no differences in marbling score were observed with increased urea.Because of increased carcass weight and finish, with no increase in loineye area, these data sugge st that adding urea increased energy utilization (diet digestibility) rather than metabolizable protein supp ly to the small intestine.

Introduction
Current information concerning requirements of finishing cattle for ru men degradable protein and metabolizable protein is limited.In order to establish metabolizable or net protein systems, a requirement for ruminal degradable protein needs to be established.Urea is a common source of rumen degradable nitrogen in f inishing diets and, therefore, our objective was to identif y the optimal level of urea for performance and carcass traits of finishing yearling steers.

Experimental Procedures
Eighty-eight medium-framed crossbred steers (731 lb) were received from Flint Hills grass in July 1993.A single initial weight was taken foll owing a 3-day equalized intake period of prair ie hay and protein supplement.Steers were stratified into three weight blocks; implanted with Revalor®; and stepped up to the final ration without urea or containing .5, 1.0, or 1.5% urea (dry matter basis).The step-up period of 14 days began on the day initial weights were taken.Diets (Table 1) contained no supplemental protein other than urea.Al l diets were formulated to contain .7%Ca, .35%P, .7%K, 25 g/ton Rumensin®, and 10 g/ton Tylosin®.Steers were fed experimen tal diets for an average of 131 days.The two largest weight blocks were slaughtered followi ng 119 days on feed, but the smallest weight block required an additional 35 days on feed to reach a desirable finished weight.Hot carcass weights and a 62% dressing percent were used to determine final weight for calculation of gain and feed efficiency.Steers were slaughtered at a commercial plant, and carcass data were obtained following a 24-hour chill.

Results and Discussion
Dry matter intake responded cubically (P<.10) to the addition of urea, being lower for steers supplemented with .5 or 1.5% urea.The reduction in intake was probably associated with an increased starch fermentation rate.Daily gain (P=.10) and feed efficiency (P<.02) responded quadratically to the addition of urea.Both daily gain and feed efficiency were increased substantially by the first increment of ur ea (.5%), with little or no improvement from subsequent u rea additions.Pooled across level, urea supplementation improved daily gain 5.6% and feed efficiency 8.9% and reduced dry matter intake 3.3% compared to the control diet.As dietary urea increased, dressing percentage responded quadratically (P<.01).A quadratic trend (P=.16) also was observed for hot carcass weights.Fat thickness (12th rib) increased (P<.04) and KPH fat tended (P=.14) to increase linearly wit h level of urea.Loin eye area and incidence of liver ab scesses were not affected by dietary level of urea.Calculated yield grade increas ed linearly (P<.10) with level of urea as a result of increased 12th rib fat and KPH fat.Percentage of carcasses grading Choice tended (P=.17) to increase as dietary level of urea increased, although urea level had little effect on marbling sc ores.Predicted crude protein requirements of steers in this study (1.88 lb/day) were met by the control diet.Improvements in performance and increased carcass weight and finish, with no improvement in loineye area, suggest that urea enhanced energy utilization (diet digestibility) by the animal, rather than metabolizable protein supply to the small intestine.The Iowa State metabolizable protein system predicted the urea fermentation potential for the basal diet in this study to be 1.09%.Regression analysis (model Y= urea + urea ) 2 showed the optimal level of urea for gain (r =.31) and feed efficiency (r =.40) to be 2 2 .91% of dietary dry matter .

Table 1 . Diet Composition a
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