ALTHOUGH DAIRY producers are paying more attention to their milking cows during heat stress, many times they will not be aware of how heat stress during the dry cow period has a huge negative impact on the cow and her offspring.
This implies economic losses for the dairy producer; as an example, it represents an average loss of A$130 per cow per year in United States dairy herds (Ferreira et al., 2016) when dry cows are under heat stress.
This amount is only considering the loss of the subsequent milk production and not the impact on the offspring.
Dairy cows will adapt to heat stress by modifying her metabolism to increase body heat loss and reduce heat production.
Part of the adaptive mechanism also involves a shift in glucose use to prioritise the immune system at the expense of other uses, such as milk production and reproduction.
A summary of various studies has demonstrated that when dry cows are under heat stress during either the close up or the entire dry cow period, their subsequent milk production dropped by 2.2 or 4 kg per day respectively during their next entire lactation period (Tao et al. 2016; Macko et al., 2017).
Although the reduction in pre-calving dry matter intake (1 to 1.5 kg) explains part of the decrease, the compromised mammary development is also playing an important role (Tao et al. 2018).
Preparing the mammary gland for the next lactation period require the mammary gland involution for the destruction of the inactive mammary cells and the proliferation of active mammary cells.
Studies have shown that those processes are impaired when dry cows are under heat stress because of some change at the cell and hormonal level.
The immune response pre and post calving will be reduced when dairy cows are under heat stress during the dry cow period.
Studies have shown a lower immune cells (neutrophils) function after calving and a lower plasma immunoglobulin (IGg) level during the dry cow period (do Amaral et al., 2011).
This will create more health problems after calving.
To add to the negative impact of heat stress during the dry cow period, the calves born from those cows will have lower immunity and reduced milk and reproductive performance.
Heat stress during late gestation will trigger hormonal changes at the placenta and fetal level which will result in lower birth weight (Macko et al., 2017) and lower weight and height at 12 months of age.
The mammary gland development in the fetus will be impaired and the future milk production of those first calf heifers was shown to be reduced by 5 kg per day for at least the first 245 days in milk (Dahl et al., 2016).
These heifers will become pregnant later and will require more services per conception than heifers from cows that were not under heat stress conditions during the dry period.
What can be done to reduce the impact of heat stress during the dry cow period?
The use of cooling systems and a reduction of stocking density are sound management tools to reduce the extent of the heat stress.
Providing clean water along with some nutritional adjustment to the ration will alleviate the risk of reduced performance.
The supplementation of specific protected B vitamins in the diet before and after calving is also beneficial to reduce the negative impact of heat stress.
B vitamins are essential nutrients, being enzyme cofactors with specific functions in the energy and protein metabolism and in the immune response of the dairy cow.
Riboflavin (B2) play specific role in immune cells (neutrophils) production and function, even in reducing oxidative stress along with folic acid (B9).
Liver function is crucial for glucose synthesis, immune response and some specific hormones needed for reproduction.
B vitamins, such as folic acid (B9), riboflavin (B2), B12 and choline, are nutrients involved in the mechanism to reduce liver fat infiltration.
Those same B vitamins when fed as a protected blend during the transition period was shown to increase dry matter intake before calving and up to four weeks after calving (Morrison et al., 2018).
An increase in dry matter intake during this crucial period would be beneficial to reduce the impact of heat stress.
Take home message
- Heat stress during the dry cow period will impact negatively the cow health status and next milk production and reproduction.
- Dairy calves born from cow suffering from heat stress in late pregnancy will have lower birth weight, reduced immunity and will produce less milk in their first lactation. Their reproductive performance will also be reduced.
- The reduced performance of the cow and her offspring will result in a loss of profit for the dairy producer.
- Supplementing a blend of protected B vitamins (Choline, Folic acid, B12 and Riboflavin) before and after calving is an innovative tool to provide ammunitions to dairy cows to counteract the heat stress impact on health, reproduction and milk production.
- The benefit of supplementing protected B vitamins during heat stress is even higher than under non-stressful condition.