Although summer is winding down and the hot temperatures are subsiding, dairy farmers are encouraged to continue monitoring for signs of heat stress within their valued herd.
The effects of heat stress on dairy farm operations can be costly, with production losses estimated at more than $900 million annually to U.S. dairy herds.
This cost — accompanied by heightened public perception of animal welfare on dairies — increases the importance of developing ways to accurately assess the effects of heat stress on individual dairy operations.
Lactating cattle produce significant amounts of heat during fermentative digestion, and they are fairly efficient at dissipating this heat load under normal environmental conditions, says Todd Bilby, dairy technical services manager for Merck Animal Health. But heat stress develops at temperatures as low as 68 Fahrenheit, which contributes to negative outcomes during much of the year in the United States, even in northern states.
Recent research suggests that the mammary gland is programmed to produce more milk for the entire lactation when heat stress is avoided during late gestation. In addition, when cows are heat stressed in the dry period, there is a concomitant reduction in bodyweight gain and body condition score.
Heat stress also affects calf development. Calves born to heat-stressed dams are smaller at birth, according to research studies, and fail to recover this lost weight even to 1 year old. In addition to weight differences, calves that experience heat stress in utero have altered energy metabolism versus cooled calves. Using insulin and glucose challenges, researchers observed that heat-stressed calves shunt energy toward peripheral tissues in a manner consistent with greater fat accumulation. Thus it appears that the dam is affected acutely by late gestation heat stress, whereas the calf developing in utero becomes programmed to be less productive for life.
Substantial evidence is now available, Bilby says, to support actively cooling dry cows for the entire dry period. Relative to heat stress, cooling during late gestation increases mammary growth, maintains dry matter intake, and improves immune status duringthe transition into lactation. When compared with herd mates that calved in months of significant heat stress, dry cows calving in cooler seasons have greater milk yield, improved reproductive performance and fewer disease incidents.
Further, significant benefits accrue to the calves born to cooled dams relative to those that experience heat stress in late gestation. Calves from cooled dams are larger, have a more robust immune system and ultimately produce more milk in their first lactation compared with herd mates born to heat-stressed dams. Cooling dry cows is an easily implemented management intervention that should lead to improved animal well-being, production and health, and in turn higher financial returns to the dairy, Bilby says.