Forage intake and enteric methane emissions in N-fertilized or grass-legume pastures during cool and warm seasons

• Enteric CH 4 emissions and intensities were not reduced by legumes in grazing systems during cool or warm seasons. • Methane intensity varied by season; legume-rich forage likely reduced CH 4 emissions during the cool season. • The forage quality in the cool season likely drove the reduction in CH 4 intensity relative to the warm season. Ruminant livestock are a major source of enteric methane ( CH 4 ) emissions, which impact the environment by increasing greenhouse gas levels. In addition to its negative impact on the environment, CH 4 production also signifies a considerable loss of dietary energy in ruminants. The primary drivers of CH 4 output in ruminants are the composition of their diet and the amount of feed consumed, with a positive relationship observed between feed intake and CH 4 . Our objectives were to assess the effect of different pasture systems on cattle performance and feed intake, to measure cattle enteric CH 4 emissions using the Sulfur hexafluoride ( SF 6 ) tracer technique, and to evaluate the relationship between forage intake and enteric CH 4 emissions. Responses were measured using yearling steers grazing three different pasture systems (treatments) from 2016 to 2018, across both cool and warm seasons. Treatments were grass monocultures or a grass-legume mixture (during the warm season) overseeded during the cool season, and included (1) N-fertilized bahiagrass (Paspalum notatum Flügge) during summer overseeded with a mixture of cereal rye ( Secale cereale L.) and oat ( Avena sativa L.) during the cool season ( Grass-N ); (2) unfertilized bahiagrass during summer overseeded with the rye-oat mixture plus a mixture of clovers during winter ( Trifolium sp.) ( Grass+CL ); and, (3) rhizoma peanut ( Arachis glabrata B.) and bahiagrass mixture during summer overseeded with the same rye-oat-clover mixture described in 2 above ( Grass+CL+RP ). Treatments were replicated three times in a randomized complete block design. Treatments had no effect on DM intake (7.8 and 6.9 kg/d for cool and warm seasons, respectively; P ≥ 0.18). Enteric CH 4 emissions did not differ among treatments in either season (354 and 451 g/ha per day for cool and warm seasons, respectively). There was a season × treatment interaction for emission intensity, and emissions during the cool season were 58% lower than in the warm season across treatments. This suggests improved production efficiency and a reduced environmental impact per unit of output during the cool season. The increased forage quality in the cool season likely contributed to the decreased CH 4 emission intensity observed in grazing systems relative to the warm season.