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

Rumen function in the dairy cow is negatively affected by heat stress: Changes such as decreased rumination time and lower pH have been reported, as well as lower rumen motility. Altogether, these induce changes in rumen fermentation and gas outcomes, including VFA profiles, and consequently in enteric methane (CH₄). However, the effects of heat stress on CH₄ emissions have only been evaluated in lactating dairy cows, and the question is what happens in the dry period, since research has shown that heat stress during the dry period can have lasting impacts, for e.g. by reducing milk yield in the following lactation. If cows have been exposed to heat stress in the dry period, to what extent does it carry over to CH₄ emissions and milk yield during the subsequent lactation? The authors cited hypothesized that heat stress during the dry period would decrease milk yield and increase CH₄ intensity, and therefore their objective in this study was to evaluate the carryover effects of dry-period heat stress exposure on CH₄ emissions across the subsequent lactation.

Forty dry cows were enrolled and housed in a barn equipped with individual feed bunks. Nineteen cooled cows (DCL) had access to shade, soakers, and fans, whereas twenty-one heat-stressed cows (DHT) had shade only. After calving, all cows were moved to a neighbouring pen with individual feed bunks and provided with shade, soakers, and fans. At about 80 days in milk, a GreenFeed unit was placed in the barn to measure enteric gas output over four weeks in a randomized complete block design, conducted in three cohorts of four-week periods. Individual dry matter intake (DMI), milk yield, daily CH4, CH4 yield, and CH4 intensity were recorded.

Dry matter intake was similar between groups (DCL = 27.4 ± 0.7, versus DHT = 26.8 ± 0.6 kg/day), whereas DHT decreased milk yield (DCL = 53.5 ± 2.1, versus DHT = 45.8 ± 1.9 kg/day). However, energy-corrected milk was similar between the treatment groups. The daily CH4 did not differ between treatments (DCL = 318.7 ± 10.6, versus DHT = 325.1 ± 9.8 g/day), but in the DHT treatment CH4 intensity was increased (DCL = 6.0 ± 0.3, versus DHT = 7.2 ± 0.3 g/kg of milk yield). The results of the study confirmed the negative effects of heat stress during the dry period on the next lactation milk yield and CH4 intensity.

Concluding remarks: The effects of heat stress in dairy cattle are clearly not limited to the period during which the stress occurs. Indeed, heat stress has prolonged negative consequences, particularly when cows are exposed during the dry period. These findings, along with other research, provide evidence of the carryover effects of dry-period heat stress on productivity during the subsequent lactation, as cows exposed to heat stress produce less milk and exhibit lower feed efficiency compared with those maintained under cooling conditions during the dry period. Moreover, these results suggest that cows exposed to heat stress during the dry period increase methane intensity due to decreased milk yield. Thus, these results underscore the importance of implementing cooling measures during the dry period to prevent production losses and mitigate increases in methane emissions intensity.