289 Efficacy of increasing levels of a urea-biuret mixture on beef cattle performance and enteric methane emissions.

Abstract Supplementation with non-protein nitrogen (NPN) has been used as an alternative to supply nitrogen in low-protein diets for ruminants. Urea is the primary source of NPN supplemented to cattle, but its rapid degradation rate in the rumen limits the inclusion level due to risks of ammonia toxicity. However, novel NPN formulations that include a mixture of urea and biuret (UB) result in lower degradation rates, which may allow for higher inclusion levels than urea alone. Thus, it was hypothesized that by using UB, the inclusion of NPN can be increased in beef cattle diets without impairing production efficiency or safety. The objective of this study was to evaluate urea supplementation at 1% and increasing inclusions of UB on beef steers performance and enteric methane (CH4) emissions. To this end, 128 Angus-crossbred steers were used in a generalized randomized block design. Treatments were composed of a corn-silage basal diet supplemented with urea at 1% of the diet dry matter (DM) (U1), or supplemented with UB at 1.12% (UB1), 2.24% (UB2), or 4.48% (UB4) of the diet DM. The experimental period consisted of 24 d of adaptation, followed by 86 d of performance data collection, and 5 d of enteric CH4 emissions measurements using the sulfur hexafluoride tracer technique. No differences in dry matter intake were observed among treatments (P = 0.41). The UB4 treatment decrease average daily gain (ADG) when compared with U1 (P = 0.05), while no differences were observed among U1, UB1 and UB2. Similarly, gain to feed ratio decreased for the UB4 treatment when compared with U (P = 0.005), though no differences were observed among U1, UB1 and UB2. Total CH4 emission rate (g/d) and CH4 intensity (g/kg ADG), were not affected by treatments (P > 0.05). In conclusion, UB supported equivalent performance to U, when included at 1.12% or 2.24% of the diet DM without compromising efficiency or increasing CH4 emissions. However, the highest inclusion level (4.48%) impaired feed efficiency. These results demonstrate that UB formulation allows NPN supplementation at higher dietary inclusion levels than conventional urea, thereby expanding the safety margin of NPN utilization in beef cattle diets.