Dry-period heat stress increases methane emission intensity by reducing milk yield in the subsequent lactation

Multiple studies have documented the negative effects of heat stress (HS) on milk production and intake in dairy cattle, including during the dry period. However, its impact on enteric methane (CH₄) production has only been evaluated in lactating cows, whereas carryover effects from the dry period on the subsequent lactation remain unexplored. We hypothesized that HS in the dry period would decrease milk yield and increase CH₄ intensity in the subsequent lactation. Forty dry cows were enrolled and housed in a freestall barn equipped with individual feed bunks. Cooled cows (DCL, n = 19) had access to shade, soakers, and fans, whereas heat-stressed cows (DHT, n = 21) had shade only. After calving, all cows were moved to a neighboring pen with individual feed bunks and provided with shade, soakers, and fans. At ~80 d in milk, a GreenFeed unit was placed in the barn to measure enteric gas output over 4 wk in a randomized complete block design, conducted in 3 cohorts of 4-wk periods. Individual DMI, milk yield, daily CH₄, CH₄ yield, and CH₄ intensity were recorded. Dry matter intake was similar between groups (DCL = 27.4 ± 0.7, DHT = 26.8 ± 0.6 kg/d ± SEM), whereas DHT decreased milk yield (DCL = 53.5 ± 2.1, DHT = 45.8 ± 1.9 kg/d ± SEM). However, energy-corrected milk was similar between groups. Daily CH₄ did not differ between treatments (DCL = 318.7 ± 10.6, DHT = 325.1 ± 9.8 g/d ± SEM), whereas DHT increased CH₄ intensity (DCL = 6.0 ± 0.3, DHT = 7.2 ± 0.3 g/kg of milk yield ± SEM). The results of our study confirm the negative effects of heat stress during the dry period on the next lactation milk yield and CH₄ intensity.