Select Sires

Physiology of Heat Stress and How it Relates to Reproduction
Transcript of Select Sires' Reproductive Moment Program
on DairyLine Radio Which Aired July 31, 2003
With Jim Spain, Associate Professor of Animal Science,
University of Missouri, Columbia

Jim, each year stress, associated with high temperatures, and humidity wreck havoc on reproductive performance of dairy herds nationwide. Today you are discussing the thermal balance of cows.

When we look at thermal load, or the input from heat sources, we have got the internal heat source we know this cow is going to experience. We need to look at how we are housing that cows, as far as crowding, and then we need to look at what other heat sources the cows are exposed to. The sun and solar radiant heat load are the biggest challenge that many of our operators experience in looking at how they manage facilities and where they house cows. The next thing we need to understand, or consider, when we think about helping this cow manage through periods of hot weather and thermal stress is how does this cow lose her body heat. One of the early signs that we will see is that this cow will begin to pant. She will begin to breathe heavier more shallow breaths, she is trying to loose heat through evaporative cooling by moving more air over the wet passages of the respiratory tract. The other thing she does that we really cannot see, but is really potentially going to have an impact on us; she begins to change where her blood flows. She begins to shift blood flow from her core body to the rumen, stomachs, and reproductive tracts moving it from her core body to the peripheries. What she is trying to do is allow her limbs, ears, and tail to act as a radiator. The other thing she does to facilitate that is to perspire. She is trying to loose heat through evaporative cooling. We know that cows suffer the greatest heat stress when it doesn’t cool off at night, when those cows don’t have the opportunity to lose the heat they have gained during the day to the environment. Heat stress affects the fertility of this cow and her reproductive function, from the standpoint of her ability to have a normal calving, and then her ability to have high fertility both during a period of heat stress as well as for some months after heat stress. So, heat stress affects not only milk production and feed intake in a very visible way that producers see and measure every day on the farm. Heat stress also plays an effect as it relates to animal health and normal immune function, as well as animal reproductive function in her ability to have a normal calving, normal gestation length and more desirable fertility at that breeding, either during or following heat stress. Heat stress is an important issue more and more producers are becoming aware of because of the great advances we have made in the cows genetic ability to produce milk.

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