The South African Weather Service’s website states that if the maximum temperature in a particular area is expected to meet or exceed 5°C above the average maximum temperature of “the hottest month” for that area, and these conditions persist for three days or more, a heatwave may be declared.
This summer, South Africa has already had heatwave conditions declared in various parts.
Prof Michiel Scholtz, a specialist researcher in applied animal breeding at the Agricultural Research Council’s Animal Production division and associate professor in animal breeding at the University of the Free State, says ambient temperature has a particularly significant impact on the productivity of ruminants such as cattle, sheep and goats.
Evolved to stay cool
Dr Jan du Preez, a veterinary specialist for public health, explains that ruminants are homeothermic, which means that their bodies work constantly to maintain the stable core body temperature required for optimal health and productivity.
When the ambient temperature exceeds the maximum upper limit of the animal’s thermoneutral zone, its body undergoes various adjustments to try to compensate for the effects of the higher temperature and cool the animal down.
“For example, blood vessels in the skin will dilate to dissipate heat from the animal into the air; the animal’s rate of respiration will increase to exhale warm air and to inhale cooler air; and sweat glands in the skin will start secreting moisture to help the animal benefit from evaporative cooling,” says Du Preez.
Feeding and drinking
According to Heinz Meissner, professor extraordinaire at the University of Pretoria’s Faculty of Veterinary Science, cattle, sheep and goats reduce their feed intake during a heatwave, but may compensate partially for this by feeding more during the cooler hours of the night.
Reduced feed intake means that the ruminant’s digestive system metabolises less feed. In turn, the animal’s body generates less of the internal heat resulting from the process of metabolism.
“Reduction in feed intake results in less productivity, which will be cumulative depending on the duration and intensity of heatwave conditions, and even for a period thereafter,” he explains.
“Heat-stressed ruminants also drink more water.”
Du Preez concurs with this, pointing out that an extremely heat-stressed high-performance dairy cow can drink up to 150ℓ of water in a single day.
Meissner says that due to its excessive panting and sweating, a ruminant loses electrolytes and ions that are essential for optimal cell and organ functioning.
“Through moisture loss, the animal also loses the bicarbonate that regulates the body’s pH level. This can lead to it experiencing metabolic acidosis that, if left untreated, can cause the animal to go into shock and die.”
KwaZulu-Natal specialist livestock veterinarian Dr Ariena Shepherd explains that if cattle, sheep and goats experience three or more consecutive exceedingly hot days with the intervening nights also remaining hot and humid, the diversion of blood to the skin and other extremities is likely to result in damage to internal organs and processes.
As an example, reduced blood supply to the intestines causes damage to the cells of
the mucosa that, in turn, increases the permeability of intestinal cells, compromising the normal barrier.
Endotoxins (toxins from bacteria inside the body) and harmful organisms that occur in the intestinal lumen (the space within the intestine) can now gain entry into other parts of the body, causing inflammatory reactions that can lead to heat shock and death.
Shepherd points out that cattle struggle more with heat stress than sheep and goats.
“This is because cattle are significantly larger, and have a thicker skin and higher metabolic rate. Dairy cattle and beef cattle in feedlots are particularly susceptible to heat stress. An added advantage held by sheep and goats over cattle is that they store most of their fat in their tail or abdomen, rather than in their muscles, as cattle do, and this helps them cope better with excessive heat.”
Lower reproductivity
According to Du Preez, other important negative effects of physical stress induced by heatwaves include reproductive inefficiencies such as a decline in male animals’ sperm quality and quantity; a decrease in breeding females’ conception rate; an increase in embryo mortalities and resorptions; a slowdown in foetal growth; lower progeny birthweights; and reduced progeny survivability.
Shepherd adds that carcasses of heat-stressed animals “are often prone to dark cutting meat, which greatly reduces its value and shelf life”.
“In dairy animals, heat stress can lower milk production, as well as cause undesirable changes in the milk’s composition.”
Heat stress can decrease milk’s butterfat content by 20% to 40%, explains Du Preez.
Moreover, its fat-free solids can decrease by 10% to 20%, its total milk protein can decrease 10% to 20%, and somatic cell counts can increase 5% to 15%.
Despite these and other problems that can be caused to cattle, sheep and goats by heatwave conditions, there is today considerable information available that can help a farmer mitigate these problems.
Scholtz explains: “During a heatwave, livestock movement needs to be restricted, especially during the hottest hours of each day. In addition, the movement of livestock, especially when they are being provided with emergency drought feed, should be restricted as much as possible to minimise unnecessary energy expenditure that could exacerbate or lead to poor body condition.”
Shade and water
According to Du Preez, livestock drinking troughs should always be placed in the shade of vegetation or man-made structures, and should offer sufficient space and water for as many animals as possible to drink simultaneously.
The shade will help to keep the water temperature below 21°C. The higher the water temperature increases above 21°C, the less likely that cattle, sheep and goats are to drink the water. This could result in the animals dehydrating and dying.
“Where it’s possible to do so, such as with intensive dairy or beef feedlot operations, shower the animals with water at least once a day,” he adds.
“Livestock managers should ensure that the water penetrates the animals’ coats down to the skin. In housing barns, the wet animals can then be further cooled with industrial fans.”
Meissner also stresses the importance of shade, saying that during a heatwave, livestock should be given as much access as possible to any form of shade available.
Where a farm has dams and streams, the farmer should allow animals access to these so that they can stand in the water to cool themselves down.
“Do not shear sheep in summer when temperatures may soar. Wool is insulating, while bare skin is less so,” he adds.
According to Shepherd, sheep should ideally have a wool length of between 40mm and 60mm in summer. If the wool is longer than this during the hottest times of year, it will add to the animals’ heat stress.
Feeding to maintain body condition
Meissner says that under hot conditions, animals will shift to feeding at night, when it is cooler.
“The feed should ideally contain added minerals and vitamins to replace electrolytes lost via panting and sweating. During heatwaves, supplementary feed should also contain more concentrate rations than fibrous feed such as hay and pasture. When an animal ruminates and ferments fibrous feed, more body heat is generated.”
Shepherd points out that even after a heatwave has passed, its impact can continue for some time afterwards, depending on the severity of the temperature and the animals’ tolerance.
To facilitate recovery as much as possible, ad-lib access to water should continue, feed supplements should replace energy and protein lost by the animals, and farmers should
keep a greater lookout for diseases and parasites that may suddenly flourish, given that the animals’ immunity might have been weakened by the excessive heat.
“If possible, wait a week or so after a heatwave before conducting any potentially stressful procedures, such as vaccinations, transport or slaughtering, with the animals.”
Farmers are also advised to utilise the genetics of more heat-tolerant cattle, sheep and goat breeds as one of the best strategies to lessen the effects of heatwaves on livestock production over the long term.
Scholtz cites the case of the 2015/2016 summer, which was the hottest and driest season ever recorded in South Africa.
“In the preceding nine summers there were, on average, 1,9 heatwaves totalling 6,5 heatwave days each. In the 2015/2016 summer, there were 12 heatwaves, totalling 71 heatwave days,” he says.
“A beef cattle crossbreeding project, in which the indigenous and locally developed Afrikaner, Bonsmara and Nguni cows are mated to Afrikaner, Bonsmara, Nguni, Angus and Simmentaler bulls, is being conducted at the Vaalharts Research Station in the hot, semi-arid Northern Cape.”
At this project, according to Scholtz, the pre-weaning growth rates of calves between the cooler and wetter 2014/2015 summer and the extreme 2015/2016 summer were compared, and these comparisons revealed interesting and scientifically significant results in terms of the pre-weaning performances of the different crosses.
The average pre-weaning growth rate of calves sired by Afrikaner and Nguni bulls was 11% lower during the 2015/2016 season, while that of calves sired by Bonsmara, Angus and Simmentaler bulls was 36% lower.
The hot, dry 2015/2016 summer also affected the post-weaning growth of these genotypes. The average daily gain of the Angus-types and Simmentaler-types decreased 17% compared with the 2014/2015 summer, whereas that of the Nguni-types and Afrikaner-types decreased 9%.
Proper planning
“Livestock in Southern African countries has to adapt to higher ambient temperatures, lower nutritional value of the grass in some cases, and an expansion of pests and diseases, especially of ticks and tick-borne diseases, because of global warming,” Scholtz says.
“As a result, matching livestock genotypes and production systems, such as crossbreeding, with production environments will become crucial, requiring the utilisation of diverse genetic resources with the appropriate genetic potential for growth, meat, milk and fibre production, resistance to disease, and fertility.”
Scholtz is of the opinion that effective early-warning systems that can project changes in heat stress due to climate change should be developed. These would help livestock farmers plan ahead more effectively.
He adds that seasonal warning systems of six to 12 months ahead would enable farmers to make proactive decisions on whether or not to decrease herd sizes and manage stocking rates ahead of and during heatwaves and droughts.
“Given that heat stress can compromise the fertility of male ruminants, especially in beef cattle, timely warnings will enable farmers to plan using multi-sire breeding and/or males from tropical-adapted genotypes to mitigate against possible reduced fertility in breeding males,” he says.
（November 7, 2020. From “Farmer’s Weekly”)
Send your message to us:
Post time: Nov-08-2020