The summer season is here and the knowledge and understanding of the effects of heat stress on cow production and how to mitigate these effects are important for dairy operations. The combined effects of high heat and high humidity make a very uncomfortable environment for both farmers and lactating dairy cows. The decrease in milk production as a result of heat stress is readily seen, but there are less immediate negative effects of heat stress such as reduced fertility. Previous iGrow articles have addressed heat stress, but it is about time to refresh the basic concepts concerning heat stress, its implications, and provide an update on new genetic research which may lead to a more permanent solution to the problem.

Heat Stress: Symptoms & effects

Individual cows that are heat stressed are easy to identify by the symptoms of panting and drooling, which are the mechanisms they use to rid themselves of excess heat in an attempt to maintain their normal body temperature. In addition, heat-stressed dairy cows reduce feed intake and are unable to mobilize body fat reserves to compensate for their lower energy intake. At the same time, there is a 25 to 30% increase in maintenance requirements for the extra activities associated with combating heat stress such as muscle movements for panting, greater sweating, and production of heat shock proteins to maintain the cell functions. The combination of these factors puts tremendous stress on the high-producing dairy cow.

Cows under heat stress may experience:

  • Drop in milk yield and fat and protein content.
  • Metabolic alkalosis and ruminal acidosis.
  • Reduced fertility.
  • Impaired milk coagulation properties, reduced cheese yield, and alteration of organoleptic characteristics of milk products.

Managing Heat Stress

Fresh quality water availability is required at all times but is especially important during the summer to aid the cow in dissipation of body heat by sweating and breathing. Water intake increases in parallel to the ambient temperature. Sprinklers over cows in loafing and feeding areas as well as voluntary “cow showers” are additional methods of reducing heat stress. However, large dairy operations need to be able to ask for a prompt decision at a population level. It is imperative to recognize the ability of animals to adapt during long-term exposure to high temperature environments.

Temperature-Humidity Index (THI)

Heat load exposure of cows, estimated by the Temperature-Humidity Index (THI) that combines temperature and relative humidity, is widely used as an indicator of the animal-perceived outdoor conditions and as guide for taking appropriate measures. In a practical context, values of THI:

  • 74° F is the threshold where heat stress may be present among cows,
  • Greater than 74° F would be uncomfortable for 50% of cows,
  • At approximately 80° F most cows would be severely affected by heat stress.

However, the THI has its limitations since it does not take into account individual animal characteristics (e.g. low vs. high producing cows, metabolic heat production, skin water loss), as well as ambient airflow, housing type, heat duration and degree of night cooling.

Increases in dairy cow productivity by genetic selection will make them even more susceptible to heat stress in the future. Modern high yielding dairy cows lose the ability to regulate their body temperature at an ambient temperature of 77 to 84° F.

Genetic Selection: A future heat stress solution?

Besides the known heat stress alleviating strategies such as watering (more available drinking water), feeding (buffer and probiotic additives, fat supplementation) and cooling (shades, fans, sprinklers, etc.), the selection of animals more resistant to heat stress may be an additional tool in the dairy producer’s toolbox Even with environmental modification to relieve the negative effects of heat stress, it is clear that some cows are better at regulating their body temperature than others, which suggests a genetic component to heat tolerance.

In 2013, researchers in Florida estimated that 13 to 17% of the variation in rectal temperature in cows during heat stress is due to genetic differences. This heritability (h2 = 0.13 to 0.17) is lower than the heritability of milk yield (h2~0.30), but it is high enough to allow selection for lower rectal temperature under heat stress.

Methods of estimating genetic merit for heat-tolerance have been developed in Australia with clear results in sire selection. For example in the most extreme sires, when the THI went from 60 to 90° F, daughters of the least heat-tolerant sires had a decrease in milk yield from about 40 to 28 lb/day (0.4 lb/THI unit). In contrast, daughters from the most heat-tolerant sire did not decrease milk production at all as the THI increased.

Another interesting approach would be the introduction of genes from other breeds into our productive dairy herds that may increase their heat-tolerance. A dominant “slick” gene first described in Senepol breed of beef cattle in the Virgin Islands, that causes very short hair growth was introduced into Holsteins cows in Florida, and the resulting offspring were better able to regulate their body temperature during heat stress than cows with normal hair.

In this same manner, studies on the immunological responses of a heat-tolerant (Romosinuano breed) and a heat sensitive-breed (Angus) demonstrated differences in metabolic response (i.e., immuno-resistance, hormone levels, etc.) between breeds under changes in ambient temperature, which may help understand differences in productivity among cattle breeds in response to heat stress. A less dense hair coat in the Romosinuano breed compared to the Angus, may play an important role in its ability to tolerate higher temperatures. The genetic variation for rectal temperature and hair coat density observed in these studies suggests the future possibility of producing cows with greater heat tolerance from sires selected on genetic merit for this trait.

Advantages & Disadvantages

The advantage of selection for heat-tolerance is that the reduction in milk yield and fertility during the summer could be minimized. However, the possible disadvantages of this selection, such as producing cows less resistant to cold stress or negative relationships between heat tolerance and other economically important traits, must be investigated before heat tolerance is incorporated into selection criteria of dairy cattle.

Monitoring Heat Stress

It is really important to monitor weather changes and the animal’s body language (e.g. increased respiration rate, sweating, panting and reduction in eating) as show in Figure 1. When extended high temperatures are expected during the summer time, take proper actions to mitigate heat stress. Tracking temperature and humidity records and calculating the temperature-humidity index are part of proper cattle care management.


Figure 1. Detection of thermal stress and actions that should be taken under extensive production systems. Both climate and animal data should be monitored for the detection of thermal stress situations. Once thermal stress situation is detected, mangers should take measures to alleviate the impact of thermal stress on animals. Salama et al., 2016. Chapter 2: Thermal stress in ruminants: Responses and strategies for alleviation. Pages 11-36 In Animal Welfare in Extensive Production Systems. J. J. Villalba, ed. 5M publishing, Sheffield, United Kingdom. (ISBN: 9781910455548, In Press).

Take-Home Message

  • Controlling heat stress is a challenge for a dairy farm. It is important to recognize symptoms of heat stress so that environmental interventions can be put in place to reduce the negative consequences of heat stress.
  • Individual inherent variation between animals in their response to heat stress opens the window for selection of thermo-tolerant animals.
  • The genetic tolerance to heat stress in Holsteins, through identifying specific genes or gene markers that are related to thermo-tolerance, could be the key to overcoming these real economic losses related to heat stress.