Suppress the effects of heat stress with forward thinking

by Kristen Davis, GippsDairy

Changes in the Australian climate, with trends towards more adverse weather events, prompts forward thinking to mitigate the impacts on dairy cattle.

Increases in heat and humidity challenge cows to balance their metabolic and environmental heat gain, which can pose adverse effects on productivity when not achieved.

Continual generation of heat occurs internally in cattle due to metabolisation. Offloading of this heat is require to allow for the maintenance of a core body temperature between 38.6°C and 39.3°C. A perpetual cycle occurs through the gain and loss of heat from the environment. If the cow exceeds the threshold of what can be expelled through thermo-regulation, the heat load of the cow builds initiating innate responses such as increased breathing rate and sweating to restore equilibrium. As temperatures rise above 25°C, cows are exposed to an environment which is outside their thermoneutral zone, therefore active regulation of body temperature must occur to maintain the optimal range.

The exchange of heat between the cow and the environment occurs through radiation, conduction, convection and evaporation. The greater the temperature difference, the faster the flow of heat, therefore, increases in both air temperature and humidity reduce the cow’s ability
to self-cool.

When cows are under heat stress and reach a critical point of heat load, changes will begin to occur in metabolism, hormonal regulation and
feed intake. This consequently has adverse effects on milk production, milk quality, fertility and health. These productivity losses can be impacted both during lactation and the dry period.

Interestingly, researchers have found that calves whose dams experienced heat stress during pregnancy go on to become maiden heifers that are less fertile. This means a heifer that is produced has both lower production and more difficulty getting in calf.

Now through breeding and genetics we have the opportunity to select and breed cows that are more heat tolerant. The Heat Tolerance Australian Breeding value (ABV) allows you to identify animals with greater ability to tolerate hot and humid conditions with less impact on milk production.

To improve heat tolerance in your herd, select animals with a high Balance Performance Index (BPI) and a Heat Tolerance ABV of greater than 100. The Heat Tolerance ABV has a base of 100 and is expressed as a percentage. For example, an animal with a Heat Tolerance ABV of 105, is 5% more tolerant than average to hot and humid conditions. Conversely, an animal with an ABV of 95, is 5% less tolerant.

You may notice an increase in fertility and a decline in production when selecting for heat tolerance, this is due to the trait being favourably linked with fertility and unfavourably with production. Therefore, when increasing selection pressure on heat tolerance within your bull team, compromises may be seen in production, whilst improvements may be seen in fertility.

A moderate heritability of 12% is seen for heat tolerance, which emphasises the importance in incorporating the use of this technology with other management practices due to its environmental influences. The high impact of environmental conditions reminds us of the critical significance of utilising the correct combination of cooling methods based on the farm operation in order to reduce heat stress.

The Heat Tolerance ABV is a useful tool complementary to your management practices to help mitigate the impact heat stress can have on your herd, through the breeding of animals that are more tolerant to hot and humid conditions with subsequently less impact on cow productivity.

For further information on minimising the effects of heat stress within your herd, visit the Cool Cows section of the Dairy Australia website, or DataGene for further information on ABV’s.

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