Heat stress increases enteric methane emissions yield and intensity while impairing rumen function and productivity...

Emissions by livestock contribute to global warming through greenhouse gases including enteric methane (CH₄). Heat stress (HS) reduces dairy cattle productivity, but its impact on CH₄ emissions is unclear. We hypothesize that HS increases CH₄ intensity in lactating cows. Forty lactating multiparous Holstein cows (mean ± SD; ~173 ± 23 DIM, 703 ± 66 kg of BW, and 46.2 ± 7.6 kg of milk yield) were stratified by lactation number, DIM, and PTA for milk production and then randomly assigned to 2 treatments in a 4-wk crossover design. Twenty cows received treatment sequence of HS abatement via evaporative cooling (shade, fans, and soakers: CL) for 4 wk followed by no cooling (shade only) for another 4 wk, whereas 20 were assigned to a treatment sequence that included 4 wk of no cooling followed by 4 wk of evaporative cooling, all cows had a 2-wk washout period before switching to the opposite treatment. All cows were housed in a freestall barn with Calan gates for daily DMI monitoring and were milked 3 times daily. Enteric gas emissions were measured using a GreenFeed system (GF), and rumination, rumen pH, rumen temperature, and motility were monitored via SmaXtec boluses. The average barn temperature-humidity index during the study was 73.2 (SD = 3.4). Heat stress decreased DMI (19.0 ± 0.3 vs. 24.3 ± 0.3 kg/d). Heat stress decreased rumination time, rumen pH, and rumen motility, whereas ruminal temperature was increased. Consequently, HS reduced milk yield and ECM. Visits of 23/80 replicates were missed due to cows not visiting the GF. Heat stress decreased the daily average number of visits per cow relative to CL cows (2.0 ± 0.1 vs. 1.7 ± 0.1 visits/cow/d). Heat stress reduced daily CH₄ emissions (326.4 ± 8.3 vs. 351.8 ± 7.5 g/d), whereas it increased CH₄ yield (17.0 ± 0.5 vs. 14.5 ± 0.4, g/kg of DMI) and intensity (11.3 ± 0.5 vs. 9.6 ± 0.4 g/kg of ECM). Present results indicate that HS reduces DMI, which compromises ECM yield, but it also affects rumen function and increases CH₄ yield and intensity, reinforcing its negative effect on the environmental footprint and production efficiency of dairy cattle.