May 15 2013 06:37 AM

Heat stress during late gestation impacts not only cow health and production but calf performance and immunity, as well.

As temperatures rise, so does the need for cow cooling. On some farms, milking herd facilities may be equipped with heat abatement systems including fans and sprinklers, but dry cow housing may not be as adequately cooled.

It is well documented that heat stress during the dry period affects a cow's mammary gland development, metabolism and immunity, leading to suppressed immune function during the transition period and less milk production during the subsequent lactation. Research has also shown that heat stress during the dry period leads to lower birth weight of calves.

What we don't know a lot about is how heat stress during late gestation affects calf performance after birth. To take a closer look at this, the University of Florida conducted a study at their Dairy and Calf Unit from July to November 2010. Cows in the study were housed in a freestall barn and exposed to either heat stress (HT) or cooling with sprinklers and fans (CL).

The birth weight of all calves was recorded, but only heifer calves were used in measurements of growth and immune status after birth. There were nine calves born to HT cows and 12 calves born to CL cows. All calves were managed under identical conditions from birth forward, which included a colostrum feeding 4 hours after birth, pasteurized milk after Day 1 and weaning at 2 months of age.

Data collection included gestation length, birth weight, monthly body weight and withers height measurements up to 7 months, hematocrit and plasma total proteins, total serum IgG, and blood cortisol concentrations. Results of the study were published in the December 2012 Journal of Dairy Science.

It was found that cows exposed to heat stress had a gestation period 4 days shorter than cows in the cooling treatment group. Birth weight was higher for CL cows (93 pounds) over HT cows (80 pounds). Body weight at weaning was also greater for CL calves (173 pounds) than for HT calves (145 pounds).

Immune response indicators differed between treatment groups as well. Compared to calves in the CL group, HT calves had less total plasma protein (6.3 versus 5.9 g/L), total serum IgG (1,577.3 versus 1,057.8 mg/dL), and decreased hematocrit (33 versus 30 percent).

In contrast, body weight and withers height were very similar between both treatment groups from months 3 to 7. Treatments also did not affect the blood cortisol concentration levels after weaning.

Based on these results, the researchers concluded that heat stress of the dam during the dry period impacted fetal growth, calf growth and immune function from birth to weaning. Improved calf performance and health is simply another reason why dry cows should not be ignored during hot summer weather.
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The author is an associate editor and covers animal health, dairy housing and equipment, and nutrient management. She grew up on a dairy farm near Plymouth, Wis., and previously served as a University of Wisconsin agricultural extension agent. She received a master's degree from North Carolina State University and a bachelor's from University of Wisconsin-Madison.