270 Seasonal variation in enteric methane emissions in cows grazing native sagebrush steppe.

Abstract Cattle are the largest emitters of greenhouse gases (GHG), specifically methane (CH4), due to their ruminant digestive system. Cows are unable to directly digest the complex carbohydrates in the plant tissues they consume. Instead, they rely on enteric fermentation from microorganisms in the rumen to break down these tissues. During this process, hydrogen and carbon dioxide in the rumen are converted into CH4, which is expelled through eructation. Methane production is energetically inefficient to the animal, with cattle typically losing 6% of their ingested energy as enteric CH4. Research on enteric CH4 emissions of grazing cattle is limited, therefore deserving more exploration. The goal of our study was to characterize the seasonal variation in enteric CH4 emissions of beef cows grazing native sagebrush steppe. The study began in spring of 2024 with twenty-two cows (Angus × Hereford, 545 ± 17 kg) from the Eastern Oregon Agricultural Research Center (EOARC; Burns, Oregon). Methane emissions (g/day) were measured using the Greenfeed Pasture System monitoring units (C-Lock). Prior to the grazing season, all cows were acclimated the monitoring units. During the acclimation period, they were maintained as a group and provided meadow hay ad libitum. Fifteen cows were successfully acclimated. After the acclimation period, cows and monitoring units were relocated to the Northern Great Basin Experimental Range (NGBER; Riley, Oregon) for the measurement of enteric CH4 emissions during the grazing season over summer and fall. The monitoring units were placed near water sources and programmed to record up to 8 visits per cow each day, with alfalfa pellets used as an enticement. Cows were fitted with virtual fence collars (Vence) to assist with grazing management and composite forage samples were collected for forage nutrient analyses. Preliminary data for CH4 emissions during spring, summer and fall were analyzed as repeated measures with the MIXED procedure of SAS (SAS Inst. Inc., Cary, NC, USA). Statistical significance was defined at P ≤ 0.05. Spring, summer, and fall forage CP and TDN were 8.5% and 56%, 3.5% and 57%, and 2.8 % and 55%, respectively. Approximately 9,000 CH4 emission readings were collected from spring through fall. Methane emissions were greatest (P < 0.0001) during summer (213 ± 1.06 g/cow daily), and similar between spring and fall (198 ± 1.40 g/cow daily; P = 0.51), illustrating the seasonal variation in emissions of grazing beef cattle.