Keep Your Dry Cows Cool for Optimum Milk Production

With the onset of warmer weather just a few months away, keep your dry cows in mind. They will transition into much more productive lactations if you take steps to help them ward off heat stress.

During the dry period, a dairy cow's mammary system undergoes many changes. Tissue grows and extensive cell turnover takes place. This process compensates for cell loss that took place during the previous lactation.

The extent of tissue regeneration 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 lactations since it alters mammary gland regeneration.

Also shown to affect subsequent lactations are environmental factors, such as daily exposure to light, known as the photoperiod, and temperature.

Dry cows exposed to a short-day photoperiod, for example, produce more milk and have better immune functions than similar animals dried-off under a long-day photoperiod.

When it comes to temperature, dairy cows prefer cool weather. As days grow warmer, especially when relative humidity rises, you may notice signs of heat stress among lactating animals: lower dry matter intake and reduced milk production.

Heat and humidity affects dry cows, too. Not only will they exhibit signs of heat stress during the warm spell, but the effects can extend well into their next lactation.

From May to November 2009, University of Florida researchers in Gainesville studied dry cows to evaluate the effect of heat stress on their mammary gland development. Late-lactation cows were dried off 46 days before their expected calving date, separated into two statistically similar groups, 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 two dry cow groups was heat stress exposure. Researchers did not provide the heat-stressed group with means to reduce the effect of warm weather. The other group occupied an area of the barn equipped with fans and sprinklers that would automatically turn on if the ambient temperature exceeded 21.1 degrees Celsius. When activated, the sprinklers were on for 1.5 minutes every six minutes.

Researchers collected various data during the dry period and the first 40 weeks of following lactation.

During the dry period, cows exposed to cooling had a lower body temperature and a lower respiration rate compared with heat-stressed cows. Heat-stressed dry cows had shorter gestations and consequently shorter dry periods. Calves from cooled dry cows were heavier than those from heat-stressed cows.

During the first 40 weeks of lactation after the experimental treatment, cows exposed to cooling during the dry period produced an average five kilograms of milk per day more than heat-stressed cows. In terms of milk yield during the 40 weeks, this represents 1,400 litres and more than $1,000 at current Canadian milk prices.

The analysis of udder tissues sampled during the study provides evidence heat stress during the dry period reduces udder cell regeneration. This in turn reduces milk yield capacity.

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

Mario S. Mongeon is OMAFRA's livestock specialist, based at the Alfred Resource Centre, Alfred, Ont.

Reference: S. Tao , J. W. Bubolz, B. C. do Amaral, I. M. Thompson, M. J. Hayen, S. E. Johnson and G. E. Dahl. Effect of heat stress during the dry period on mammary gland development. J. Dairy Sci. 94 :5976-5986.


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

You have several means available to reduce the effects of warmer temperatures during summer. Providing plenty of drinking water and making it readily available to the dry cows is an excellent first step.

Access to shade, good ventilation and air movement are other simple ways to reduce heat's impact on the herd.

You can also use water to cool your animals. Sprinkler or misting systems are fairly common in large U.S. and New Zealand herds. Most systems are hooked up with fans.

Evaporation induced by the fans enhances the water's cooling effect. These systems are usually installed where cows congregate or above the animals at the feed bunk.

graph showing that during the first 40 weeks of lactation after the experimental treatment, cows exposed to cooling during the dry period produced an average five kilograms of milk per day more than heat-stressed cows.

(Adapted from Tao et AL,2011.)

This article first appeared in the March 2012 Milk producer magazine.

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