Dry Cows are Susceptible to Heat Stress Too!

Summer is just around the corner and soon, the herd will head back to pasture. The warm summer months can be challenging for the milking herd when temperatures and relative humidity soar. We tend to forget that dry cows can suffer from heat stress too.

It is well known that the dry period in dairy cattle is critical in many ways. Optimum conditions during the dry period will influence positively the outcome of the subsequent lactation. When dry cows are exposed to heat stress, the effects may extend well into the following lactation.

During the dry period, the mammary system undergoes many changes. Tissue grows and extensive cell turnover takes place. This process is required to compensate for the cell loss that took place during the previous lactation. The extent of the regeneration process dictates the number of milk producing cells as well as their production capacity. The absence of a dry period is associated with decreased milk production in the subsequent lactation since the mammary gland regeneration process is altered.

Environmental factors such as photoperiod and temperature have been demonstrated to affect subsequent lactation as well. For example, cows exposed to short-day photoperiod during the dry period produce more milk and have improved immune function than similar animals dried-off under long-day photoperiod.

Temperature is another important environmental factor. Dairy cows prefer cool temperatures. As the temperature gets warmer, especially if the relative humidity is high, signs of heat stress may start to show up: lower dry matter intake and reduced milk production for lactating animals. Dry cows can be negatively affected by warm temperatures as well and this is a less known fact. Furthermore, not only will dry cows exhibit signs of heat stress during the warm spell but the effects may extend well into the following lactation.

From May to November of 2009 a study involving dry cows was conducted at the University of Florida in Gainesville. The objective of the project was to evaluate the effect of heat stress on the development of the mammary gland of dairy cows. Cows in late lactation were dried off about 46 days before the expected calving date, separated in 2 groups statistically similar, exposed to a fixed photoperiod of 10 hours of darkness, fed the same ration and housed in the same barn.

The only difference between the 2 dry cow groups was exposure to heat stress. The heat stressed group was not provided with means to mitigate the effect of heat whereas the area of the barn housing the other group of dry cow was equipped with fans and sprinklers that would automatically turn on if the ambient temperature exceeded 21.1°C. The sprinklers, when activated, were on for 1.5 minutes every 6 minutes.

Various data were collected during the dry period as well as the following lactation for the first 40 weeks.

During the dry period, cows exposed to cooling had a lower body temperature as well as a lower respiration rate compared to cows in heat stress. Furthermore, heat stressed dry cows had a shorter gestation length and consequently shorter dry period. The calves from the cooled dry cows were heavier than those from cows exposed to heat stress.

During the first 40 weeks of lactation following the experimental treatment, cows that were exposed to heat stress during the dry period produced on average 5 kg of milk per day less than the cows exposed to cooling. In term of milk yield during the 40 weeks, this represents 1400 liters of milk and more than a thousand dollars at current milk prices in Ontario.

The analysis of udder tissues sampled during the study provides evidence that heat stress during the dry period alters cell proliferation in the udder. Fewer secretory cells are produced and this in turn reduces milk yield capacity.

This study emphasizes the necessity to take measures to reduce the impact of heat stress on cows. When temperatures rise above the animal's comfort zone during the dry period, it could compromise the development of the mammary system, which decreases milk production in the following lactation.

During the summer months, several means are available to mitigate the effects of warmer temperatures. Providing plenty of drinking water and making it easily available for the dry cows is an excellent first step. Make sure that the dry cows have access to shade and good air movement. These are simple means to reduce the impact of heat on the herd.


Effect of heat stress on dry cows:

  • Decreased milk production in the following lactation
  • Decreased milk solid yields
  • Negatively affect liver functions in early lactation
  • Decreased immune functions during the transition period

Figure 1. Lactation curves for 2 groups of dairy cows following exposition to heat stress or cooling during the dry period

Lactation curves for groups of dairy cows after heat stress and cooling during the day


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