Heat stress increases enteric methane emissions yield and intensity while impairing rumen function and productivity in lactating dairy cattle

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.