Penn State Extension Dairy Team
Cows that experience heat stress during their dry period produce less milk, are more prone to disease, and are more difficult to breed back than cows that do not suffer from heat stress just prior to calving.
These negative impacts of heat stress on cows are well known. But what about calves these cows are carrying?
Previous research has determined that calves born to cows that experience heat stress have lower birth weights, and colostrum IgG content has been shown to be reduced as well.
A recent study conducted at the University of Florida offers some new insight into calves born to cows that experienced heat stress during the dry period. This study was published in the December 2012 issue of the Journal of Dairy Science.
For this experiment, 34 dry cows were housed in a freestall barn for 45 days prior to calving and were exposed to ambient environmental conditions or provided with cooling from fans and sprinklers.
Measurements of rectal temperatures and respiration rates, as well as reduced feed intake, during the dry period and lower milk production during the subsequent lactation confirmed that the cows that were not cooled did indeed experience heat stress.
All calves were weighed at birth, and subsequent measurements were evaluated on heifer calves only.
Calves born to heat-stressed dams weighed 13 pounds less at birth and 28 pounds less at weaning than calves born to dams with access to cooling. Average daily gain from birth to weaning was not different for the two groups of calves. Body weight and withers height from weaning through 7 months of age were not impacted by heat stress.
In this study, colostrum IgG content was not affected by heat stress. However, calves born to heat-stressed dams were less efficient in absorbing IgG from colostrum and had lower serum IgG concentrations for the first 28 days of life than calves born to cooled cows, indicating reduced passive transfer of immunity.
Calves exposed to heat stress before birth also had a compromised T-cell response, as measured by the number of monocytes in circulation at 7, 28, 42, and 56 days of age.
The response of B-cells was evaluated at 4 weeks of age by an ovalbumin challenge and did not differ between calves in the two groups. Effects of heat stress on B-cell function before this time cannot be determined from this study.
These results confirm that calf body weight can be significantly impacted by heat stress during the final weeks of gestation. In addition, both passive transfer and cell-mediated immunity were compromised in calves exposed to heat stress.
With summer just around the corner, now is the time to evaluate cooling strategies for all the animals in your herd. Don’t forget about those dry cows.