Heat Stress

What is heat stress?

Cattle in modern animal production accomplishes high performance which in turn requires enormous demands on metabolic performance. High metabolic performance is accompanied by high body heat production due to digestive processes and production of milk or meat, respectively. Under moderate climatic conditions, this heat is released to the environment. However, when ambient temperature increases up to a particular level, release of body heat production to the environment does not work adequately. In high yielding animals which are already loaded by high performance, situation is intensified as basal metabolic body heat production rises with increased performance (Brown-Brandle et al., 2004; Spiers et al., 2004). According to Rhoads et al. (2009), Holstein cows are sensitive to heat stress when temperature is above 25 °C and temperature humidity index (calculated from ambient temperature and relative humidity) exceeds 72.

What are indicators for heat stress?

When heat stress in cattle occurs, indicators can be the following: higher breathing frequency, reduced feed intake and body weight and thus, lowered performance.

Consequences of heat stress …

… on health

As feed intake is reduced, health suffers. Particularly dry cows close to calving should be under special observation as they could already be in a negative energy balance and immune response is often depressed (Ingvartsen and Moyes, 2012). Thus, they are already more vulnerable to metabolic disorders and diseases, respectively. Furthermore, fetal growth and milk production can be affected.

… on performance

As a consequence of reduced feed intake, performance (milk yield, growth) decreases which in turn results in economic loss. For example, at 35 °C energy requirement for maintenance increases by 20% over thermo neutral conditions (NRC, 2008) and is often at the expense of yield. Beside effects on milk yield, also milk quality is adversely affected as can be observed in an increase of somatic cell counts within summer months (Igono et al., 1988). Moreover, reproduction can be affected due to delayed ovulation.

How can heat stress be prevented?

Most important measure is to ensure free access to clear and fresh water, avoid direct solar irradiation, especially during midday, as well as insufficient air ventilation. Installation of fans or sprinklers can avoid accumulation of heat during hot phases. Furthermore, stress should be prevented, e.g. by avoiding too high stocking density. To minimize risk of infections, hygiene management in stable and milking parlour should be adequate.

If heat stress is present – which measures help?

Due to reduced feed intake, energy density should be elevated by supplementation of fat. Furthermore, intake of forages, especially low quality forages, should be decreased as their fermentation causes high heat production. As reduced intake of fibremay lead to a decline of pH and thus, acidification of the gastrointestinal tract, buffering substances should be fed. Also protein content should be decreased as degradation of protein and detoxification of Ammonia costs lots of energy. Feeding of minerals balances loss due to transpiration. Submitting the ration several times a day avoids heating of the feed which could result in energy loss and further fermentation.

A trial conducted on an experimental dairy farm in Germany during hot summer showed that neither feed intake nor milk yield (see figure) was affected by high temperature when SANGROVIT® was supplemented. SANGROVIT® has shown to stimulate feed intake due to its bitter substances. These bitter substances might also indirectly act as buffer against decreasing rumen pH as they stimulate saliva production. Furthermore, due to its digestive properties, SANGROVIT® improves nutrient supply and availability of nutrients.

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Heat Stress