Most beef producers know intuitively that when the weather gets colder their cows need more energy to maintain their condition and productivity. The questions are when do cows start experiencing cold stress and then how much more energy do they need?
When we’re considering cold stress, we need to factor in both the actual temperature and the wind speed to determine the effective temperature. In Table 1 you can see wind speed can dramatically lower the effective temperature the cattle experience. Any kind of available protection, whether natural or man-made, can be very valuable in reducing the amount of wind chill.
The second consideration is just exactly when does a cow begin to feel cold stress? The point of cold stress, or lower critical temperature, depends in large part on the amount of insulation provided by the hair coat. As shown in Table 2, that insulation value changes depending on the thickness of the hair coat and whether it is dry or wet.
As a general rule, for every degree that the effective temperature is below the lower critical temperature, the cow’s energy needs increase by 1 percent. For instance if it is 15° F with a 5 mph wind, the energy needs of a cow with a heavy winter coat are about 10% higher than they would be under more moderate conditions. That requirement jumps up to 50% higher if the hair coat is completely wet or matted down with mud so there’s no insulation value.
Ignoring the energy costs of long-term cold stress greatly increases the risk of problems down the road during calving and subsequent re-breeding performance.
Also, any steps that we can take to lower the cold stress the cows have to contend with will lower her maintenance requirements and costs.