Too Hot to Handle - The Detrimental Effects of Heat Stress

We are all aware of the detrimental effects that heat stress can have on our cows, and are often quick to run to the cow sheds and collecting yards to put measures in place to mitigate this.

Whilst extensive literature has highlighted the negative consequences of heat stress on lactating and dry dairy cow productivity, health, and reproduction (Bernabucci et al., 2010; Tao and Dahl, 2013), less information is available characterising its effect on the calf.

The dry period coincides with late gestation when the foetus grows at the fastest rate and accumulates around 60% of its birth weight (Bauman and Currie, 1980).

Consequences of calves exposed to prenatal heat stress:

• Weigh less at birth, weaning, and throughout first year of life (in part to a shorter gestation length and retarded foetal growth (Tao et al., 2012; Monteiro et al., 2014, 2016a)
• Calves have compromised passive immune transfer, decreased total plasma protein, hematocrit, and impaired cellular immune functions (Tao et al., 2012)
• Calves are less likely to survive to their first lactation compared with prenatal cooled calves
• If prenatal heat-stressed heifers do reach lactation, they produce approximately 2 to 5 kg/d less milk across the entire first lactation compared with heifers exposed to prenatal cooling, partially attributed to perturbations in mammary gland microstructure and cellular turnover
• Heifers exposed to prenatal heat stress had; reduced rectal temperatures, sweating rates and increased skin temperatures when exposed to heat stress as mature lactating cows, compared with prenatally cooled counterparts (Ahmed et al., 2017)

Pre-weaned calves are often not considered for heat stress abatement due to their larger surface/mass ratio and smaller heat loads as a functionally non-ruminant and non-lactating animal (This is in part due to Collier et al., 1982; Broucek et al., 2009; Dado-Senn et al., 2019). However, as we have discussed before, early life experiences can have long-term consequences on future productivity of an animal.

The upper critical temperature of the calf thermoneutral zone is anything over 24°C, and the physiological range of calf body temperatures (BT) lies between 38.1 to 39.2°C.

When considering the thermoneutral zone, we must be measuring ‘effective ambient temperature’, which is the actual temperature felt by the calf – this can vary greatly from air temperature. Measure the microclimate immediately surrounding the calf. A calf in a clean, dry hutch can have an effective ambient temperature 8-10°C warmer than the air temperature on the day.

Calves exposed to high ambient temperature and humidity can lead to:

• Elevated calf rectal temperatures, skin temperatures, and respiration rates
• Reduced ADG and weaning weight compared with calves provided active heat stress abatement or born in winter months (Hill et al., 2011; López et al., 2018). This
  is in part due to a decline in feed and nutrient intake and an increase in water intake

Heat stress in calves will lead to dehydration, reduce feed intake and in turn, can lower the immune system. During periods of high heat stress, nutrients consumed will go towards trying to drive off heat from the calf’s body rather than using the nutrients to grow.

As a result, it would not be advisable to reduce milk feed rates during the summer months.