Although dairy producers are paying more attention to their milking cows during heat stress, many are not aware of the negative impact of heat stress during the dry cow period.
Research in the United States (Ferreira et al., 2016) revealed could cost as much as A$130 per cow per year.
This amount only accounts for the loss of milk production and not the impact on the offspring.
Dairy cows adapt to heat stress by modifying their 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 kilograms 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 also played an important role (Tao et al. 2018).
Preparing the mammary gland for the next lactation period required the mammary gland involution for the destruction of the inactive mammary cells and the proliferation of active mammary cells.
Studies have shown that those process are impaired when dry cows are under heat stress due to some change at the cell and hormonal level.
Heat stress in the dry cow period also reduces the immune response pre and post calving.
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 that 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 5kg 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 were 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.
Figure 1. Effect of heat stress during the dry period on next milk production (Macko et al., 2017)