August 1, 2014 11:36
By Sean Thompson, BSc, AAg, Regional Livestock Specialist
While we have the option to go indoors or dress appropriately during the hot summer months, our livestock can only cope with the heat by changing their behaviour or through physiological responses. It is important that livestock have access to adequate water and shade in order to prevent heat stress throughout the summer.
What is heat stress?
Heat stress occurs when animals experience warm environmental conditions which prevent them from adequately dispersing body heat. The ideal ambient temperature for cattle is between 5 and 25°C, with variation among animals depending on certain factors (e.g. stage of growth, production, age). As the outside temperature increases beyond this range, heat loss becomes much more difficult. Cattle maintain a constant body temperature by regulating heat loss. They primarily dissipate heat from the surface of their skin, either directly to the air (radiation and convection), to a cool surface (conduction), or through evaporation of sweat. However, when conditions become extreme a cow's ability to lose heat is limited. Significantly warm temperatures will decrease the amount of heat that an animal can lose to its surroundings and high humidity reduces the evaporative cooling potential.
Negative effects on livestock
Cattle will generally not graze under extreme heat since 1) eating increases heat production and 2) grazing usually takes place in non-shaded pasture. If warm temperatures extend into the evening or last for several days at a time, this can result in significant reductions in animal performance, including lower calf gains, less milk production, and poor feed conversion. As well, prolonged exposure to heat can affect reproductive efficiency. If livestock are overheated and spend the day trying to cool off, normal breeding behaviour may be negatively affected. The heat can also cause physiological effects, such as reduced embryo survival in cows and decreased sperm production and quality in bulls. The consequences for herd bulls are particularly important to consider as it may take several weeks for them to fully recover, which would result in cows not being bred during that time.
Signs of heat stress
There are several symptoms that you should watch for to determine if your cattle are experiencing heat stress. Open mouth panting with the head lowered and extended is an obvious sign of overheating. This will be characterized by heavy "puffing" and noticeable sweat on the neck and body. In severe cases animals will also begin to slobber. When grazing cattle become heat stressed they seek relief by finding shade or water to wade in. One or any combination of these signs in your herd could indicate that animals are struggling to maintain a constant body temperature. Special attention should be taken when sudden or abrupt hot conditions occur because cattle are more susceptible to overheating in these situations compared to gradual increases in ambient temperature.
Two necessities for livestock on pasture are adequate shade and water. Shade can be provided by either trees or man-made structures, which is important because direct sunlight can increase the temperature an animal experiences by 3-4 degrees above the air temperature. A clean supply of water is also essential for grazing cattle. Beef cows will drink approximately 40-60 liters of water per day, but that can nearly double when extreme heat conditions exist. Not only quality but quantity and access to water is important; a rule of thumb is that cattle should not have to travel more than a ½ mile in hilly terrain or 1 mile on flat ground to access water. Additionally, taking measures to ensure fly control will reduce overcrowding and thus the risk of overheating. Lastly, avoid working or transporting cattle during the day as the increased heat production from handling will only add to their heat stress. If cattle must be worked or moved it should be done in the early morning.
For more information on livestock management, contact your Regional Livestock Specialist, or call the Agriculture Knowledge Centre at 1-866-457-2377.