Soybean meal replaced by slow release urea in finishing diets for beef cattle

Eight crossbred steers (average body weight of 418 kg) fitted with ruminal and abomasal cannula were used to evaluate the effects of replacing soybean meal (SBM) with slow-release urea (SRU) in beef cattle diets containing two concentrate levels. The experimental design included two 4×4 Latin squares, which were run simultaneously. Each Latin square received one level of concentrate [400 or 800 g/kg on a dry matter (DM) basis]. Within each Latin square, the four replacement levels of soybean meal protein with slow-release urea were applied to the animals (0%, 33%, 66% and 100% of substitution on N basis). The DM intake as well as organic matter (OM) intake and crude protein (CP) intake decreased linearly (P<0.05) as SBM was replaced with SRU. Ruminal digestibility coefficient of OM tended to be greater (P=0.074) for the 40 % concentrate diet. DM and OM passage rate (kp) were greater (P<0.05) on the 80% concentrate diet. A cubic effect (P<0.10) of SBM replacement with SRU on ruminal ammonia (NH3-N) concentration in relation to time was detected. A quadratic effect on pH was observed (P<0.10) when replacing SBM with SRU. Nitrogen intake, nitrogen excreted in the feces, nitrogen balance and efficiency of nitrogen use decreased linearly (P<0.10) as SRU increased in the diet, whereas the total nitrogen excreted in urine increased linearly (P=0.007). The production of microbial nitrogen and microbial efficiency were not affected by the experimental treatments (P>0.10). A lower intake of DM, OM, and CP was observed when cattle were fed SRU compared to SBM. However, the use of SRU did not change the digestibility and digestion rate (kd) and kp of DM, OM, CP and neutral detergent fiber corrected for ash and protein (NDFap). In summary, SRU provides higher concentrations of NH3-N throughout a day than SBM in cattle fed low concentrate diets.