By Prof Michiel Scholtz
According to António Guterres, secretary-general of the UN, the era of global warming is over and we are now in a period where the earth is boiling. He refers to it as “global boiling”.
What does the hot summer mean for cattle farmers?
A hot summer will inevitably cause cattle more heat stress, which can be defined as a physiological condition in which the core body temperature is higher than normal. Core body temperature is the result of the total heat load (internal heat production and environmental heat), minus the animal’s ability to dissipate heat.
If the heat load is higher than the animal can dissipate, it will change its behaviour and physiological changes will occur.
The immediate response to increased heat load includes increased respiration rates, reduced feed intake and increased water intake.
Further changes include increased blood flow to the skin surface, decreased metabolic rate and changes in water metabolism.
Heat stress also reduces reproductive capacity in both sexes. This effect on summer fertility can reduce pregnancy rates by 20% to 27%.
Heat stress is a common cause of reproductive inefficiency. In the case of cattle, the fertility of the bull is affected, as semen quality decreases. Sperm concentration and mobility are reduced, and there is a higher percentage of morphologically abnormal sperm in an ejaculation.
It takes about eight weeks after a period of heat stress for semen quality to return to normal, contributing to the carry-over effect of heat stress on fertility. Just one day of heat stress can reduce semen quality and potential fertility for two months.
If bulls cannot increase the rate of heat loss from the body when exposed to elevated temperatures, semen quality and potential fertility will decrease.
Heat stress also affects the fertility of cows. A high environmental temperature has a significant impact on cow pregnancy, as heat stress affects the development of the oocyte (egg cell).
On the other hand, the critical period for embryo survival is the first seven days after fertilisation.
Variation of up to 42% in weaning weight
Heat stress also affects the growth of beef cattle. Heat stress in cows during late pregnancy can influence the performance of offspring during the suckling period, and heat stress impairs the growth of suckling calves.
It has been found that the maximum daily temperature during calves’ suckling period explains up to 42% of the variation in weaning weight in dry and hot areas of South Africa.
During times of heat stress, the average daily gain of imported beef cattle breeds (British and European breeds) decreased by 17%, while that of indigenous Sanga breeds (Afrikaner and Nguni) decreased by only 9%.
When temperatures are high, animals reduce their feed intake so the heat produced in the digestive system can be minimised.
Grazing time is also reduced, because animals do not graze during the hot hours of the day, while sweating and water intake increase.
Other factors involved in thermal comfort include the animal’s skin and haircoat (thickness, structure, thermal insulation, absorption and reflection), and body conformation (shape, size and surface area).
Nutritional stress has the greatest indirect impact on the grazing animal during heat stress, and the fact that higher temperatures cause the loss of certain mineral metabolites (anions and cations) is often overlooked.
This leads to a loss of appetite, reduced feed intake and lower production (milk, growth, wool, etc.). Adjusting the mineral content in supplementary feeding for grazing animals can reduce the impact of heat and limit production losses.
Solutions for farmers
Reduce livestock numbers. Livestock numbers should be reduced as a preventative measure. This is particularly important because there may be less food available than usual during winter. Farmers should not wait until the livestock are thin before marketing them. A good approach is to start disposing of older and less productive cows now.
Mineral levels in nutrition. It is important to change the anion-cation balance in lick supplements in line with expected temperatures. If the intake of mineral metabolites can be manipulated to minimise the losses caused by heat stress, it can lead to a balanced level of anions and cations. This can help cattle withstand high temperatures. Balancing electrolyte levels (cation and anion) through supplementation to alleviate heat stress in grazing animals can help to maintain production levels. Suppliers of licks should note this and provide farmers with the necessary advice and guidance.
Test semen quality. While it may be tempting to suggest keeping bulls cool during mating seasons in hot weather, it is not practical for extensive cattle farming. A more practical approach would be for farmers to test the semen quality of bulls after heatwaves. Bulls with compromised semen quality can then be replaced during the mating season.
Multiple bull matings. Another strategy is to use multiple bull matings in stud and commercial herds. For stud herds, parentage needs to be confirmed. Another option is to use bulls of heat-tolerant genotypes in breeding seasons when high temperatures are predicted.
Alternative feed. Farmers can consider using alternative feed such as “bush feed”. This involves hammermilling bushes, shrubs and tree branches to use as livestock feed together with rumen supplements. It was done in the 2015-16 season, which was also hot and dry. Livestock often perform quite well on bush feed.
Access to water and shade. During times of drought and extreme heat, livestock’s thermal regulation and their ability to cope with heat stress are impaired. Farmers should ensure their livestock always have access to water and shade where practical.
Limit movement. The movement of livestock should be restricted, especially during the hot hours of the day. Additionally, movement of livestock during the provision of drought feed should be limited as much as possible, as animals allowed to move excessively will use more energy. This is unproductive and places a burden on the maintenance needs of the animal.
Provide night-time feeding. During times of extreme heat, feed should also be provided at night because livestock tend to eat more during the night.
Monitor animal health. Combat parasites and provide adequate salt and mineral mixes during drought, as would be done under normal conditions. Livestock under nutritional and heat stress are less resistant to parasites than under normal conditions.
Prof Michiel Scholtz is a retired specialist researcher in applied animal science at the Agricultural Research Council Animal Production in Irene, where he is contracted. He is also an affiliated professor in animal science at the University of the Free State. Acknowledgement is given to the Department of Agriculture, Land Reform & Rural Development and the National Research Foundation for their financial contribution to research on climate change and animal production.
