Heat and Cold Stress of Farm Animals

Farm animals, like humans perform best when the temperature is neither too hot nor too cold. Research shows that within a preferred temperature range, animals will not need to expend any energy keeping themselves warm or cool, are under no temperature induced stress, and therefore are able to maximise their productivity. Ideally, farmers should aim to keep all their animals within this ‘comfort zone’ all the time. In cases where animals are housed in sheds this is possible.

The depiction of the typical comfort zones for common farm animals suggests that animals grazing in an open paddock may find themselves in less than ideal conditions during much of their life. This can reduce weight gain, milk production, birth weights and survival while making stock management more difficult.

Comfort Zones For Adult And Newborn Animals

Reference: Reid, R. and Bird, P.B. (1990),'Shelter' in Trees for Rural Australia, ed. K.W. Cremer, Inkata Press Melbourne, pp 319-335.

The range of the comfort zone depends upon the species of animal, its size, genetic condition, health, energy reserves, age, condition of its coat and any additional stressful conditions such as pregnancy or lactation. In the graph, the lower limit (to the left of the grey bar) represents the critical temperature below which cold stress begins to affect behaviour and productivity. This causes animals to shiver, consume more feed and seek shelter. At effective temperatures above the upper limit (to the right of the grey bar) animals may sweat, pant, drink more water, reduce their consumption or seek shade.

Under prolonged or extreme conditions, the effect on animal health can be very significant resulting in irreversible losses in productivity or even death. Trees have a role to play in reducing the wind chill factor that dramatically reduces the effective temperature and by reducing exposure to direct sunlight.

Heat Stress And Farm Animals
Exposure to direct radiation can dramatically increase heat stress in stock and most will actively seek shade on hot days. Much of the research into heat stress has occurred within the dairy industry. High heat loads in cattle lead to depressed feed intake, decreased milk yield, milk fats and protein %, elevated somatic cell counts as well as increased risk of mastitis, weight loss and reduced reproduction. Once the heat load on a dairy cow becomes severe the animal may show signs of distress. This can include desperately seeking shade, refusing to lie down, reducing feed intake, crowding water holes, splashing water from toughs, laboured breathing, excessive salivation, convulsions and even death.

Although wool provides good insulation, sheep behaviour and productivity may also be affected by heat stress. Research has shown that ram fertility can be affected for many weeks as a result of a period of intense heat stress, while ewes are less likely to become pregnant or may be more likely to abort if stressed. The weight of lambs born to heat-stressed ewes has been shown to be lower, making them more susceptible to death from stress, mismothering or predators.

Trees for Stock Shade

Wind Speed and the Windchill Factor
Wind chill is the degree to which wind speeds reduce the effective temperature. For example, if the temperature is 4°C with still air, the effective temperature is lowered to -4°C at a modest wind speed of 20 km per hour.

Reference: Reid, R. and Bird, P.B. (1990), 'Shelter' in Trees for Rural Australia, ed. K.W. Cremer, Inkata Press Melbourne, pp 319-335.

A shelterbelt that reduces wind speeds by more than 50% may be capable of dramatically increasing the effective temperature that an animal experiences. In the above example, if the wind speed in the open was 20km/hr and the ambient temperature was 2oC then the animals in an open paddock would be experiencing an effective temperature of around -6oC. A shelterbelt that reduced the wind speed by 50% might increase this effective temperature by 4oC.

Another critical factor influencing cold stress of stock is rainfall. A small amount of rain can reduce the insulation value of an animal’s coat. Goats, for example, don’t have the waterproofing lanolin of sheep wool so when their hair gets wet it loses much of its insulation value. The same is true for cattle and freshly shorn sheep. A wet coat also induces evaporative cooling which further chills animals. Newborn stock that are unable to dry off are particularly susceptible to cold stress in windy conditions. A chill index model developed by Donnelly (1984) relates temperature, rainfall and wind speed to lamb mortality. Donnelly calculated that if the chill index exceeds 1100, newborn lambs are more likely to die.

Disregarding the effect of rainfall, the effect of wind speed on the chill index is shown in the graph below. A chill-index of more than 1100 is considered a real risk to lamb survival. If the air temperature is a relatively mild 9oC, the chill index can reach a critical level if the wind speed is more than about 25km/hr.

The Effect of Wind Speed on Chill Index

Reference: Cleugh, H. (1997), 'Trees for Shade and Shelter' in Design Principles for Farm Forestry: A guide to assist farmers to decide where to plant trees and farm plantations on farms eds. Abel, N. et al, RIRDC, Canberra, pp.39-52

Trees for Wind Shelter

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