Heat stress in gestating cows weakens future generations

Heat stress has a significant effect on dairy cows and negatively impacts milk and reproductive performance and can lead to illness and shortened lifespans. In the US, this equates to losses of about US$1.5 billion annually. However, according to study* conducted in the US, the impact of heat stress is lasting and continues to affect the performance of the offspring of a heat-stressed cow for 2 generations.

The impact of heat stress is lasting and continues to affect the performance of the offspring of a heat-stressed cow for 2 generations. Photo: Dreamstime.

Previous research has found that heifers born to cows that are heat-stressed during late gestation grow to be smaller and produce on average 5kg per day less milk in their first lactation, compared with heifers born to dams that were cooled during the hottest days of the year.

According to Jimena Laporta, senior author from the Department of Animal Sciences at the University of Florida in Gainesville, this suggests a permanent effect of foetal environment on genetic expression in adulthood.

Productivity and reproductive performance of 2 generations studied

The authors had 2 objectives during the study, namely:

1. To measure the carryover effects of maternal exposure to heat stress during late gestation on milk yield, reproductive performance, and survival rates of daughters and granddaughters.
2. To estimate the economic losses related to those outcomes across the US.

Over a 10-year period, data was collected on the performance of Holstein cows in Florida, the US state with the greatest number of heat stress days per year. Information was collected on the lifespans, productivity, and reproductive performance of 2 successive generations of cows born to dams exposed to heat stress during pregnancy and those born to dams that were provided active cooling during heat stress periods.

Figure 1 – Milk yield in the first (a; n = 108), second (b; n = 54), and third (c; n = 19) lactation of daughters (F1) born to dams under cooling (CL; access to fans, shade, and water soakers) or heat stress (HT; only access to shade) during late gestation (~46 d). All daughters had access to active cooling during their first, second, and third lactations.

Negative performance of daughters and granddaughters of heat-stressed cows

Daughters and granddaughters of heat-stressed cows showed negative effects in rates of survival from birth to first calving, length of productive lifespan, as well as milk performance, including milk yields and nutrient contents through the first 3 lactations. These effects translated into considerable annual costs to dairy producers in the country, with nationwide losses, based on the US average milk price from 2010 to 2015, of up to US$595 million annually.

Number of dairy cows and heat stress days The number of dairy cows (dry and milking) per state (USDA-ERS, 2019) and the number of heat stress days per state (NOAA, 2019). Taller bars represent more cows within each cow number range.

Interestingly, Laporta notes that lactating cows tend to be the focus of heat reduction strategies, possibly because the effects of overheating are more immediately evident among them than among non-lactating pregnant cows, for which the damage may become apparent only later when they resume milking. However, considering the hidden costs that carry over to future generations of cows and the likelihood of increased heat stress due to ongoing climate change, Laporta and colleagues consider cooling techniques for dry cows – such as the use of fans and sprinkler systems – crucial.

*This study, ‘Late-gestation heat stress impairs daughter and granddaughter lifetime performance’, was published in the Journal of Dairy Science and was conducted by scientists J. Laporta, F.C. Ferreira, V. Ouellet, B. Dado-Senn, A.K. Almeida, A. De Vries, and G.E. Dahl from the University of Florida and the University of California, Davis, in the US.