Midwinter doesn’t seem like a very good time to be writing about the impact of summer heat on our dairy cows but a couple of subjects have come up in conversations with dairy farmers in a few provinces that have made me think of it. One situation is that a lot of dairy farms seem to be at the peak stocking level. This is happening because dairy farmers are trying to increase their farm’s production to meet incentives and increases in quota. If cows, especially fresh cows, are crowded, they can be more susceptible to the impact of hot, humid summer weather.
Dairy cows seem to be especially vulnerable to heat stress. They produce large amounts of heat energy as they digest their feed and their metabolic rate is quite high in order to produce activity to produce milk. Because they do not eat as much and are not producing milk, it stands to reason that dry cows may be less susceptible to heat stress than lactating cows.
Like other animals, cows react to heat by trying to decrease the amount of heat produced and by trying to increase heat loss. They do this through changes in their metabolism and by changes in their behaviour. Some of the most obvious changes are an increase in breathing, maybe even panting and sweating, increased drinking, a decrease in the amount they eat and a change to eating more when the barn is cooler. Heat stress causes cows to stand for longer than normal. This is thought to be because standing leaves a larger surface area where heat can be lost. The impact of heat stress is lower milk production, reduced fertility and increased culling.
Changes in feeding behaviour and in dry matter intake seem to be big contributors to the decrease in milk production. It is also likely that the fall in milk also occurs because cows need to use more energy trying to keep cool, energy that could have gone to milk production. If farmers reformulate the ration to offset the fall in dry matter intake by increasing its protein and energy density of the ration, it may actually increase the impact of heat stress because of the impact on the cow’s metabolism.
Some of the common ways that farmers try to deal with the impact of heat stress on cows is to increase cooling by using water spray or misting and by increasing air movement in the barn, especially in critical areas like feed bunks and holding areas. Both strategies increase costs and deliver diminishing benefits if the relative humidity is already high. Sprayers and misters also require a reliable source of water and will likely significantly increase the amount of water in the barn.
It does look as if the climate will reverse the trend to higher average temperatures even though we will likely still experience some cooler-than-average weather patterns. Maybe part of the long-term fix is to select for sires and dams that are better adapted to a warmer climate. As recently pointed out by researchers at the University of British Columbia, this may not be an option for Canadian dairy farmers because their cows need to be productive during our winters too. If genetic selection for better heat tolerance leads to reduced cold tolerance, not much may be gained.
