PSI-2 Investigating dietary protein’s role in mitigating enteric emissions from finishing beef cattle.

Abstract Methane (CH4) emissions in ruminant systems are a loss of energy intake and a major contributor to climate change. This study evaluated the effects of increasing levels of non-protein nitrogen (NPN; 0.75, 1.5, and 3.0% dry matter basis) on enteric methane emissions and growth performance in finishing diets fed to beef steers. One hundred and eighty steers (IBW= 455 ± 41.5 kg) from 4 different sources were stratified by body weight (BW) and source and then randomly assigned to 1 of 4 NPN treatments. Steers were housed at Colorado State University’s Climate-Smart Research Facility. Daily gas flux and dry matter intake (DMI) were individually determined using SmartFeed and GreenFeed technologies (Rapid City, SD). Steers were acclimated for 35 days before a 13-day covariate period, where baseline CH4 emissions and feed intake data were collected. During the covariate period, a common finishing ration was fed (1.5% NPN treatment). Then, steers were fed their respective treatment for 53 days, and intake and methane emissions were measured continuously. Animal body weights were collected two consecutive days prior to beginning the NPN treatment and at the end of the experiment. Data were analyzed using R statistical software (4.4.3) and JMP (18.1.2), where NPN level, source, the interaction of source and NPN level, and covariate emissions and DMI were included in the model. Significance was declared at P < 0.05. Methane emissions were not different (P = 0.26) among treatments. However, CH4 yield (g/kg DMI) was greater (P < 0.05) for steers supplemented with 1.5% of NPN. Methane intensity (g/kg ADG) was greater (P< 0.05) for steers supplemented with 1.5% NPN treatment compared to those supplemented with 3.0% NPN. Steers supplemented with 0 and 0.75% of NPN had greater (P < 0.05) DMI than those supplemented with 1.5% NPN. Steers supplemented with 3.0% of NPN (P < 0.05) had intermediate DMI. Average daily gain (kg/d) was not different (P = 0.40) among treatments. Steers supplemented with 1.5% of NPN had a greater (P < 0.05) gain-to-feed ratio (G:F). In conclusion, increasing NPN levels in finishing diets can affect DMI, G:F, methane yield, and methane intensity without affecting ADG or enteric methane production.