Optimal urea level in corn-based finishing diets containing alfalfa Optimal urea level in corn-based finishing diets containing alfalfa as the roughage source as the roughage source

One hundred medium-growth potential, crossbred yearling steers (766 lb) were used to identify the optimal level of urea in finishing diets, based on growth and carcass traits. The corn-base d diets had no urea o r contained .35, .70, 1.05, or 1.40% urea (dry matter basis) and no other supplemental protein. Alfalfa hay (10% of DM) was the roughage source and contained 16% crude protein. Feed efficiency and gain were improved by .35% urea, with little improvement from higher additions. Regression analysis indicated that the optimal level of urea for gain and feed efficiency was .5% of dietary dry matter. Hot carcass weight and dressing percentage responded quadratically, being highest for steers receiving .7 and 1.05% urea. Fat thickness and yield grade responded cubically to the addition of urea; these trai ts were also highest for steers receiving .7 and 1.05% urea. Loineye area decreased linearly with increased urea in the diet. Marbling scores and KPH fat were unaffected by urea addition. The increased growth, carcass weight, and finish, with no improvements in loine ye area, indicate that urea enhanced diet digestibility but did not increase metabolizabl e protein supply. Optimal urea levels (.5%) were less than those previously indicated for diets containing prairie hay as the roughage source (.9%), suggesting that alfalfa may supply a portion of the rumen degradable nitrogen requirement when utilized as a source of roughage in high grain diets.


Summary
One hundred medium-growth potential, crossbred yearling steers (766 lb) were used to identify the optimal level of urea in finishing diets, based on growth and carcass traits.The corn-base d diets had no urea o r contained .35,.70,1.05, or 1.40% urea (dry matter basis) and no other supplemental protein.Alfalfa hay (10% of DM) was the roughage source and contained 16% crude protein.Feed efficiency and gain were improved by .35%urea, with little improvement from higher additions.Regression analysis indicated that the optimal level of urea for gain and feed efficiency was .5% of dietary dry matter.Hot carcass weight and dressing percentage responded quadratically, being highest for steers receiving .7 and 1.05% urea.Fat thickness and yield grade responded cubically to the addition of urea; these trai ts were also highest for steers receiving .7 and 1.05% urea.Loineye area decreased linearly with increased urea in the diet.Marbling scores and KPH fat were unaffected by urea addition.The increased growth, carcass weight, and finish, with no improvements in loine ye area, indicate that urea enhance d diet digestibility but did not increase metabolizabl e protein supply.Optimal urea levels (.5%) were less than those previously indicate d for diets containing prairie hay as the roughage source (.9%), suggesting that alfalfa may supply a portion of the rumen degradable nitrogen requirement when utilized as a source of roughage in high grain diets.

Introduction
Current information r egarding requirements of finishi ng cattle for rumen degradable protein and metabolizable protein remains limited.Urea is a common source of rumen degradable protein in finishing diets.Previous research (Cattlemen' s Day, 1994) suggested that the optimal level of urea for rate and efficiency of gain in corn-based finishing d iets utilizing native grass hay for roughage (10% of dietary DM) was .91% of dietary dry matter.
Alfalfa is used commonly in finishing diets as a source of dietary roughage.Compared to native grass hay, alfalfa contains more rumen degradable protein.Althou gh typically included as 10% or less of finishing diets, alfalfa may reduce the amount of dietary urea needed to meet microbial demands for ammonia.Our objectiv e was to identify the optimal level of urea for performance and carcass traits of finishing yearling steers fed high grain diets containin g alfalfa hay as the roughage source.

Experimental Procedures
One hundred medium-growth potential, crossbred steers (766 lb) were received from Flint Hills grass in July 1994.Steers were allotted to one of four weight replicates and stratified into one of five pens within each replicate.They were stepped up to an 80% concentrat e diet prior to beginning the experiment.A single initial weight was taken following a 3-day period of equalized intake.Steers were implanted w i th Revalor-S® and fed diets without urea or containing .35,.70,1.05, or 1.40% urea (dry matter basis).Diets contained no other supplemental protein.All diets contained alfalfa (16% crude protein) at 10% of the dietary dry matter and were formulated to conta i n .75%Ca, .35%P, .7%K, and a 10:1 N:S ratio.Steers were fed 275 mg Rumensin® and 90 mg T ylosin® per head daily .Crude protein levels ranged from 9.9 to 13.9%.Experimental diets were fed for an averag e of 144 days.The largest weight block was slaughtered following 109 days on feed, and the three remaining blocks were fed for an additional 44 days.Because the smallest block was not ready for slaughter when the trial was terminated, carca ss data were collected for only three of the four weight blocks.Final weights were pencil shrunk 4% for calculation of daily gain and feed efficiency.Steers were slaughtered at a commercial plant, and carcass data were obtained following a 24-hour chill.

Results and Discussion
Daily gain (P<.05), fee d intake (P=.11), and feed efficienc y (P<.05) responded quadratically to the addition of urea (Table 1).Both daily gain and feed efficiency were improved by the first increment of urea (.35%) with little or no improvemen t from subsequent urea additions.As dietary urea increased, hot carcass weight responded qu adratically (P<.05), being greatest at .7% dietary urea.A quadratic trend (P=.11) also was observed for dressing percentage, which was also greatest at .
7% dietary urea.Fat thickness (12 rib) and calculated yield grade th responded cubically (P<.05) to the addi-tion of urea, a reflection of lower intake and performance for the highe r urea levels.Loineye area decreased linearly (P<.02) with dietary level of urea.Kidney, heart, and pelvic fat and marbling scores were unaffected (P>.18) by the additio n of urea.Predicted crude protein requirements of steers in thi s study (1.87 lb/day) were met by the control diet.Improvements in performanc e and increases in carcass weight and finish, with no improvem e nt in loineye area, sugges t that urea enhanced energy utilization (diet digestibility) but did not increase protein supply to the smal l intestine.These results are consisten t with previous research conducted at Kansas State (Cattlemen's Day, 1994).
The Iowa State metabolizable protein system predicted the urea f ermentation potential for the basal diet in this study to be .7%.Regressio n analysis (model Y=urea + ure a) 2 predicted the optimal level of urea for gain (r =.30) and feed efficiency ( r=.40) to be .5% 2 2 of dietary dry matter.When final weight was determined as carcass weigh t adjusted by a 62% dressin g percentage, the optimal level of urea for gain and feed eff i ciency was .57% of dietary dry matter( r = .27and .35,respectively).
2 Optima l levels of urea (.5%) were less than those previously reported for diets containing prairie hay as the roughage source (.91%), suggestin g that alfalfa, used as source of roughag e in high grain diets, can supply a portion of the degradable protein requirement for finishing steers.