Heat stressed dry cows have long-lasting impacts

Occasionally, I find myself driving around at 2 a.m. on call and reaching for a podcast to stay awake. My recent pick, the Real Science Exchange by Balchem Animal Health and Nutrition, featured Dr. Jimena Laporta with a very timely discussion on heat abatement in dry dairy cows.

For years, the industry has discussed the effects of heat stress on lactating dairy cows and proposed strategies to mitigate it. More recently, the heat abatement conversation has begun to include dry dairy cows as well. Dairy cows in nearly every state of the U.S. will experience heat stress, though some regions will have shorter seasons. In fact, one review found that cooling dry cows was profitable across all states except Alaska, assuming new facility construction was not needed. Data comparing heat-stressed versus cooled dry cows has been shown to benefit the milk production of the cooled dry cow in her next lactation as well as benefit her daughters, granddaughters and possibly great-granddaughters.

Multiple researchers have demonstrated that the offspring of cows who experience “late-gestation hyperthermia” or heat stress in the dry period experience permanent effects. These permanent effects are often referred to as prenatal or fetal programming. There is a great deal of cell development and programming for the calf that happens in the last trimester of pregnancy, especially the immune and mammary systems.

In Dr. Laporta’s work, they found that calves born to heat-stressed dry cows exhibit the fol-lowing:

— Reduced birth weights.

— Reduced weaning weights (incidentally, some calf raisers our clinic works with have complained about a drop in average daily gain in calves born July through October).

— Reduced absorption of colostral immunoglobulin G after birth (possible accelerated gut closure is suspected).

— Early calving dates (up to four days early).

— Alterations in the mammary gland of the developing fetus, which result in reduced milk production in the first lactation and subsequent lactations (up to 120 kilograms or 264 pounds per year) for three consecutive lactations.

Additionally, Dr. Laporta’s group found in their 10-year data set that the average dairy cow born to a heat-stressed dam in the U.S. would have a 5-month shorter productive life compared to a cow born to a cooled dam. Other findings include reduced skin thickness, more sebaceous glands and more distant sweat glands from the skin surface.

Some basic facts to keep in mind: one, heat stress includes both temperature and humidity, often referred to as THI (temperature humidity index); two, the average milk yield loss in the dam’s next lactation if heat-stress abatement is not provided during the dry period is approximately 5 kilograms per day (about 10 pounds). Why this loss? The dry period is responsible for the regeneration and proliferation of mammary tissue for the next lactation, and heat stress interferes with this. Three, the definition of cooled cows in this article is based on Dr. Geoffrey Dahl’s data with cooled dry cows — soakers on one minute every five minutes when the temperature is greater than 72 degrees, and fans on when 70 degrees. Four, cooling pre-fresh cows only did not reduce the negative impacts of heat stress on the fetus.

Just a side note. I would not routinely use 70 degrees as my base point for when fans turn on for lactating cows, at least in the summer months in the Midwest. With humidity included, that set point tends to be too high, and lactating cows will experience heat stress before fans kick on. I would trend toward 65 degrees or lower for fans to kick on in lactating groups.

If you have a cow monitoring system such as SCR or CowManager, there are modules to help you monitor heat stress through different behaviors. You could use these modules to help guide you to the right fan and soaker set points, especially if your barns have high stocking densities. Another reference for cooling specifics is the Elanco heat abatement guide that can be found online.

Can heat stress additives help enough to skip using fans and/or soakers in dry cows? I doubt the return on investment is as good as soakers and/or fans. In speaking with a dairy nutritionist, these feed additives range from vasodilators to cellular hydration compounds. He explained that evaporative cooling is an excellent heat stress management tool and would have a better return on investment than the cost of nutritional aids every year.

Megan Weisenbeck is one of six veterinarians at Northern Valley Livestock Services in Plainview, MN. She practices primarily dairy production medicine in Minnesota and Wisconsin. Megan can be reached at [email protected].