A Colorado low you say – only in Manitoba

Understanding heat stress in beef cattle and the 'thermal neutral zone'

There is a great deal of newsprint these days about the relative merits of a “Colorado High,” a subject I will leave readers to explore on their own. In May, however, Manitoba residents were introduced to a “Colorado Low,” an intense weather system that moved up from North Dakota and Montana over the Victoria Day long weekend. This system turned a beautiful spring Saturday into a raging wind- and rainstorm that overnight turned to ice pellets and snow, bringing much of southern Manitoba to a standstill. I write on this topic, as I was over visiting that particular weekend and as I watched this storm unfold it got me thinking about how the environment affects cattle particularly those extensively managed and what adaptations they make to survive and be productive in this crazy country of ours!

In Canada, when we think of environmental stress, we generally think of those mid-winter days where below-zero temperatures and/or wind combine to increase the maintenance energy requirements of cattle. In other parts of North America, particularly the southern U.S. and Mexico, heat stress is an equally serious threat to productivity. To understand how extremes in environmental conditions impact the animal and how it adapts to such stress, it is necessary to explore how an animal regulates its body temperature. Normal body temperature in cattle is 38.5 C and must be maintained within relatively tight limits for normal physiological function. One of the challenges to maintaining a constant body temperature is the need to dissipate heat generated from normal metabolism and digestion, particularly rumen fermentation. The ability of cattle to dissipate such heat is influenced by a number of internal and external factors. Heat is lost from the body via radiation (loss to atmosphere), evaporation (respiration, sweating) or by convection and conduction (air/water movement across or in contact with skin).

Under “normal” environmental conditions, dissipating this body heat is not a large issue and the energy required to do so is considered a part of normal maintenance requirements. However, when environmental temperatures rise to a point where the animal has difficulty dissipating body heat, steps must be taken to actively lose that heat (i.e. seek shade, increase water intake, decrease feed intake, increase respiration rate). Conversely, when the environmental temperature falls, at some point the body heat produced by the animal is no longer sufficient to maintain its core body temperature and steps are taken to generate heat (i.e. shivering, increase feed intake, seek shelter). The range of environmental temperatures where cattle do not have to expend energy to maintain normal body temperature is known as the thermal neutral zone. The points where cattle must actively lose or generate heat are referred to as the upper and lower critical temperatures, respectively.

It is not possible to precisely define the temperatures that define the thermal neutral zone for any group of cattle, as both the upper and lower critical temperatures are a result of complex interactions between environmental (i.e. ambient temperature, wind speed, humidity), animal (i.e. hide thickness, summer versus winter hair coat) and management (i.e. windbreaks; muddy pens, bedding) factors. To understand how these factors interact, let’s look at heat stress. If we use 25 C as a starting point, as the ambient temperature rises and approaches/exceeds 30 C, the animal will experience greater difficulty in dissipating heat. When these high temperatures are combined with humid conditions, the animal’s ability to shed body heat is greatly reduced and the result is heat stress. The higher the humidity and/or ambient temperature, the greater the impact! Heat-stressed cattle show a range of symptoms. In mild cases, cattle will seek shade, or water sources to cool themselves, change or reduce eating/grazing patterns and alter their herd behaviour. Physiologically they will increase their respiration rate. In more severe cases cattle will exhibit very rapid rates of respiration to the point where they are actively panting in order to increase heat loss.

For many Canadian producers, heat stress is not high on their radar, however, it has obvious implications on production, animal welfare and ultimately on economic return. In the feedlot, it will lead to reduced dry matter intakes, lower gains and extended days on feed. In breeding cattle, milk production drops off with implications on weaning weights and breeding programs can be extended, particularly if your bulls are overconditioned. In severe cases, death can result. Alberta Agriculture has an excellent fact sheet that provides practical advice on minimizing heat stress in cattle. As we move into the dog days of summer, it is worth reading.

Cold stress is a much more familiar concept to Canadian producers. Like heat stress, the point where cattle actively take steps to increase heat production to maintain core body temperature is influenced by a number of interacting environmental, animal and management factors. However, since it is only June, it does not seem appropriate to be writing on winter feeding and management strategies. As such, I will save this discussion for a future article. In the meantime, keep your eye on the horizon (or the weather channel) as you never know what system is moving in next.

About the author

Contributor

John McKinnon is a Professor Emeritus at the University of Saskatchewan and a consulting nutritionist who can be reached at [email protected].

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