376 Awardee Talk: Consequences of Heat Stress During Late Gestation for the Dairy Cow and Her Calf

Abstract

As global temperatures continue to rise, heat stress emerges as a growing threat to animal productivity and well-being. Because of their high feed intake and metabolic heat production, dairy cows are particularly vulnerable to heat stress and the complications of that environmental insult. A significant knowledge base exists around the impact of heat stress during lactation, but more recent emphasis about impacts on the dry cow, and by extension on the developing fetus, have yielded new insights to inform management of heat stress throughout the production cycle. Heat stress at dry-off slows involution and reduces mammary cell proliferation as parturition approaches. Immune responses, both innate and acquired, are reduced with heat stress and can persist into lactation. Calves whose dams experience heat stress late in gestation have lower birthweights, poorer passive transfer and altered carbohydrate metabolism relative to calves from cooled dams. Calves from heat-stressed dams leave the herd at a higher rate before puberty compared with those from cooled dams, and thus fewer complete the first lactation. More importantly, in utero heat stress causes epigenetic changes in methylation patterns of liver and mammary tissue, and those differences are associated with yield reductions that are transferred through at least two generations, producing a drag on animal performance long after the initial in utero insult. Therefore, appropriate management of heat stress in the dry cow is critical to optimize performance of the cow and calf.