Heat Stress - Does Breeding Have an Answer?

About the job

With rising temperatures in the summer months, performance losses become noticeable, fertility problems occur more frequently, and the animals are generally more restless. Dairy farmers are challenged. Management and feeding must be adjusted. But are there also breeding measures?

The summer months are generally already hotter than the rest of the year. With climate change, the problem becomes even more pronounced because average temperatures are rising. Looking back a little more than 15 years and considering, for example, the nearest official meteorological station for Zug in Lucerne, an upward trend is clearly visible for the whole year as well as for the summer (Figure 1). Furthermore, there are increasingly more temperature and weather fluctuations. From these facts, it can be deduced that our dairy cows are increasingly exposed to heat situations. In addition, production performance has increased in recent years. The resulting increased metabolic performance is associated with the production of more body heat. Furthermore, age and, of course, stage of pregnancy also influence metabolic performance.

It gets hot from 16°C

Various studies have shown that in the range of about 4 to 16 °C ambient temperature, a dairy cow on average has to expend the least so-called regulatory measures to maintain body temperature. This is the thermoneutral zone. Below 4 °C, the dairy cow must expend energy to avoid freezing. When air temperatures rise above 16 °C, excess body heat must be released to the environment through complex regulatory mechanisms. This also costs energy. Beyond a certain point, heat dissipation is no longer possible, and the cow automatically switches to internal physiological adjustments: milk production is reduced, fertility suffers, or susceptibility to disease increases; a classic example is the increased susceptibility to mastitis. When these signs appear, it is referred to as heat stress.

Humidity is also important

Figure 2: THI values (Temperature-Humidity Index (NRC, 1971)) depending on temperature and relative humidity

For these stress situations, not only the ambient temperature is responsible; humidity also plays a role. To consider both factors simultaneously, temperature and humidity are converted into an index called THI (Temperature-Humidity Index). Various calculation formulas exist in the literature. In the present calculations, a THI application (NRC, 1971) was chosen, which is often used for genetic evaluations. The interplay of temperature and humidity and the resulting effects on the THI and the cow are shown in Figure 2.

Breeding for heat tolerance

To avoid these heat stress situations as much as possible, dairy farmers must adjust management and feeding. Fans in the barn, sprinkler systems, or switching to night grazing are just a few keywords. In current research worldwide, breeding for heat tolerance is a burning issue. It turns out that breeds react differently to heat stress. Within breeds, individual animal differences can be identified. Australia was the first country to develop a breeding value estimation for heat tolerance and include it in routine evaluations. In Italy, breeding values for heat tolerance have been published for Holstein cattle for almost a year. Intensive research and development work is underway in Spain, the USA, and Canada in this field. Heritabilities range from 0.1 to 0.2, showing that this trait is genetically manageable. It is expected that further routine breeding values will be introduced abroad.

Slick gene against heat stress

In the Holstein population, a gene has been identified that contributes positively to heat tolerance. This gene is dominantly inherited and causes animals to have shorter and smoother coats. Thus, they are more resistant to heat and react less quickly to it. Originally, this gene was found in the Senepol breed. Meanwhile, there are Holstein breeding programmes specialising in this slick gene. This is an answer that can make a positive contribution to breeding for heat tolerance. However, the slick gene is probably only one cause in the whole complex of heat tolerance or stress resistance in general, and other gene loci also play important roles.

Heat stress in Switzerland

As part of an internship at Qualitas AG, it was investigated at which THI value performance losses occur in Swiss dairy cows and whether breed differences exist. For this purpose, all test day results since 2007 were used and linked with the weather data of the respective location. The weather data come from the official stations of MeteoSwiss. Using a statistical method developed at the University of Guelph in Canada, the point on the THI scale was determined at which performance losses become significant. Since cows drink much more water at the beginning of heat stress periods, performance reductions do not immediately show in milk quantity but in the components. This is illustrated by protein in Figure 3. First-lactation Brown Swiss (BS) cows show first significant losses at a THI of 54.4, Original Braunvieh (OB) at 55.8. In mature cows, BS and OB react practically identically. The reason why BS and OB react differently in the first lactation could have various causes. What is more surprising is the fact that performance losses occur relatively early, even if they are initially small.

Figure 3: Thresholds on the THI scale at which daily protein quantity performance losses occur

What next

With the methods described above, it will now be further investigated whether these reaction patterns are similar for other traits. Additionally, the extent of individual animal differences at the genetic level will be evaluated. Whether there will ever be a breeding value "heat tolerance" in Switzerland must be decided by the breeding organisations.

BORIS ZANDONA and BEAT BAPST