Breeding for heat tolerance

Heat stress occurs when animals are unable to effectively dissipate heat from metabolism associated with digestion, maintenance and production, plus any heat gained from the environment, the latter being more prominent if the temperature-humidity index (THI) exceeds the comfort zone. In high producing (Holstein) dairy cows, the condition is exaggerated because of the high metabolic load associated with the elevated metabolic demands of lactation. With prolonged exposure to high heat loads, the outcomes are negative to reproduction, production, other physiological functions of dairy cattle, and the economics of the dairy enterprise.

Heat tolerance is the ability of an animal to maintain production and reproduction levels under hot and humid conditions. Because of a global warming escalating phenomenon, heat tolerance need to become a priority trait in genetic and management discussion and implementation strategies. Whereas providing shade, other structures and soaking of cows can be effective, it can be costly or not always practical, and therefore long term solutions with more permanent outcomes such as in the breeding domain should be sought. A summary of results and status of knowledge to that effect, as obtained from the papers cited, provide some pointers:

• Heat tolerance and productivity on average is negatively correlated, which implies that selection for heat tolerance in a herd may reduce milk yields. However, analyses of large data sets in Canada and Australia showed that this is not necessarily true with particular individuals, showing enough variation to work with. Heritabilities for milk yield, fat and protein yield at 15 units above THI threshold though, are relatively low, ranging from 0.15 to 0.27 for milk yield and 0.11 to 0.15 for fat and protein yield respectively.
• The ranking of bulls between the breeding values with no heat stress effect and the breeding values accounting for heat stress, indicated interaction between milk production traits and THI, resulting in substantial re-ranking of the top-ranked sires in Canada for milk production.
• From bullets one and two it follows that adaptation to warmer climates can be and should be addressed by selecting robust or heat-resilient animals and by including resilience indicators in breeding goals. Candidate causal variants related to metabolic functions are relevant for heat tolerance and, therefore, key for improving this trait. This could include adding these variants in the custom SNPs for the normal genomic evaluations or as the basis to design specific agonist or antagonist compounds for lowering core body temperature under heat stress conditions. In fact, it was shown that adding prioritized functionally relevant variants into the 50k SNP panel used in Australia, increased the prediction accuracy of heat tolerance by up to 10% units.
• Crossbreeding with heat tolerant breeds will accelerate heat tolerance in a herd, but it is not really a viable option as milk production will decline. A more appropriate option is introduction of the so-called SLICK gene into high producing dairy cattle. This gene was originally identified in Senepol cattle. Today the SLICK haplotype (http://omia.angis.org.au/OMIA001372/9913/) in cattle confers animals with a short and sleek hair coat. In experiments to determine whether (a) lactating Holsteins with the slick hair phenotype have superior ability for thermoregulation compared to Control, and (b) whether seasonal (summer) depression in milk yield would be reduced in SLICK cows as usually experienced, the results showed in (a): vaginal temperatures and the increases in rectal temperature and respiration rate in slick-haired cows were lower than in Control and they sweated more; in (b): compared with milk yield (average ca 33L/day) of cows calving in winter, milk yield during the first 90 days in milk was lower for cows calving in summer, but the reduction was less pronounced for slick cows than for Control.
• From the above, improved thermoregulatory ability, and thus heat tolerance, should be improved with selection of cows with shorter and sleeker hair coats and higher sweating abilities.