Impact of Genetics on Heat Tolerance
Gary W. Rogers
University of Tennessee

Heat stress in many parts of the world has a significant impact on dairy production. In areas within 40 degrees latitude north and 40 degrees latitude south of the equator, heat stress is especially significant. High humidity levels in many areas along with high temperatures exacerbate the problem. South Africa and the southern part of the US are alike in many respects when it comes to heat stress. The hot summer months create a situation where cow comfort, production, dry matter intake, reproduction and health suffer, even when the best methods of heat abatement are used.

In some areas of the world, some breeds of dairy cattle or crossbred dairy cattle involving specific breeds predominate partly because of their perceived ability to cope with heat stress. Producers perceive that some breeds are more tolerant to heat stress than other breeds. If this is true, then can we take this one step further and ask the question, “Are some families within a breed more tolerant to heat stress than other families?”

Many studies have looked at how various breeds respond to heat stress. Milk production, feed intake, body temperature, respiration rates, reproduction and health have all been addressed. Although no single study has definitively characterized how modern dairy breeds respond to heat stress, we can draw some conclusions when we consider all the relevant studies. The first conclusion that can be made is that Bos indicus or Zebu type cattle generally handle heat stress better than most Bos taurus type cattle (includes the European breeds). Zebu type cattle apparently have more and larger sweat glands that are closer to the skin. However, no strain of Zebu type cattle is available that is similar in dairy output to our prominent highly selected European dairy breeds. Sahiwal cattle, a Zebu type, have been used extensively in India and Southeast Asia and to some extent in Australia. Sahiwal are well adapted to hot climates and produce moderate amounts of milk. Red Sindhi is another Zebu type that has had some selection for milk production.

Differences in heat tolerance among our highly selected European breeds of dairy cattle also exist. Jersey and Brown Swiss cattle appear to cope with heat stress somewhat better than Holsteins. Milk production, reproduction and feed intake are less affected by heat stress in Jerseys and Brown Swiss when compared to Holsteins. Holsteins may benefit from heat abatement more than Jerseys or Brown Swiss. Jerseys are popular in some locations in the southern US due to their ability to handle the heat stress. This situation exists even though the milk market in the southern US is relatively less favorable to Jerseys where most of the milk goes for fluid consumption and producers are not rewarded for the high protein content in Jersey milk. Heat tolerance in Ayrshires, European Red and White breeds, Simmental type cattle and Guernseys is probably less well documented than in Holsteins or Jerseys but some of these breeds may have good heat tolerance. Crosses of the heavily selected European breeds or crosses of the heavily selected European breeds with Sahiwal may also be more heat tolerant than many of the pure breeds. The genotype (specific cross or purebred strain) that will work best in a high heat stress environment will depend on many factors including the nature of the production system and the availability of inputs. Purebreds from the heavily selected European breeds or crosses of these breeds will likely be the best choices in hot climates for production situations where inputs are not severely limiting and heat abatement is practiced. Such is the situation in the US, Europe and most of South Africa and as a consequence Holsteins and Jerseys with some crossbreds dominate our herds. Dairy production systems with very limited inputs and limited heat abatement will require something different from heavily selected purebred European breeds. Crosses with Sahiwal might work well in some of these low input circumstances.

What about differences in heat tolerance within our heavily selected European breeds? Do some Holstein families handle heat stress better than others? Do all Jerseys handle heat stress better than all Holsteins? At this point, we really do not have definitive answers to these questions and we certainly do not have efficient ways to discriminate between families that are more or less heat tolerant. Some recent work from University of Georgia here in the US using data primarily from Georgia and Florida indicates that families do respond differently to heat stress. Families rank one way in cool and moderately warm environments but rank differently in hot (high temperature and high humidity) environments. When temperatures are consistently near or higher than 30 degrees C and humidity is 50% or more some families produce and reproduce relatively much better than other families. In theory this means that eventually we may be able to select sires whose daughters produce and reproduce well in all climates but especially produce and reproduce better than daughters of other sires under heat stress conditions. Some other work from Florida indicates that a gene for short or slick hair may be useful for enhancing heat tolerance in our dairy cattle, although we are a ways from getting this gene incorporated into our modern dairy cattle families. Some other studies demonstrate that Holsteins with similar genetic levels but with more white on them handle heat stress better than those that have more black. This makes sense because the white reflects heat better. However, white animals are also more prone to skin related problems when shade is not available all the time. We know that some families are better able to handle heat stress and over time we may be able to incorporate this information into our breeding programs. At this time, however, it is very difficult to make breeding decisions specifically designed for hot environments. Currently, breed selection or crossing with a productive and heat tolerant breed may be the best way to genetically alter heat tolerance in very hot climates but be sure to consider all the other attributes of a new breed or breed cross.