HEAT STRESS RELATED FACTORS AFFECTING DAIRY COWS

Heat stress has been identified as one of the major challenges for livestock production. Global temperatures are steadily increasing, with South African temperatures increasing at nearly twice the global rate. Of the livestock used for food production, dairy cows are the most sensitive to thermal changes, which have detrimental effects on their health, welfare, and overall productivity. Several abiotic factors that influence the heat load experienced by the cow are not commonly included in thermal indices used to measure heat stress; these include solar radiation, wind speed, and soil quality. These need to be addressed in future research. Furthermore, the thermal comfort zone of cows has been altered by years of intense selection for increased milk yield, causing cows to become heat stressed at lower temperatures. Considering the abiotic and biotic factors affecting the cow’s heat load, it can be argued that dairy cows in tropical and subtropical climates are experiencing heat stress most of the time. In the review cited, the abiotic and biotic factors influencing the heat load are discussed.

Abiotic factors:

Apart from ambient temperature and humidity which are considered in a THI, several other abiotic factors affect the heat dissipation of dairy cattle: rainfall (precipitation), feed (quality and composition), relative humidity, cloud cover, shade or shelter, wind speed, geographical location, soil quality, atmospheric pressure, photoperiod and solar radiation. The main factors are discussed briefly:

Solar radiation: The consequences of increased solar radiation include increase in respiration rate, increased panting score, increased occurrence of DNA damage due to chromosome dissociation and fragmentation and altered DNA-repair signalling, increased metabolic rate, decreased feeding, increased shade-seeking behaviour, increased body and rectal temperature, and in the longer term, decreased fertility and follicular activity.

Shade, shelter and wind speed: Shade and shelter can alleviate some of the heat stress by decreasing the heat load and improving heat dissipation, whereas wind speed may have a positive effect by reducing the ambient temperature experienced by the animal, and may thus provide some short-term relief. Wind improves heat loss through convection by replacing hot air near the surface of the cow’s skin with cooler air, whether the cow is standing or lying down.

Geographical location: The geographical location of cows (e.g. altitude) determines several environmental factors that can influence heat load, including photoperiod, atmospheric pressure, and rainfall patterns all of which have influences either alone or in combination.

Soil and feed: Soils with P deficiency lead to impaired energy metabolism, further increasing their energy requirements and making it even harder for them to cope under heat stress conditions, whereas poorer quality fodder increases heat production during rumen fermentation, making it difficult to dissipate heat.  

Biotic factors with a genetic base:

The thermo-neutral zone (TNZ) of a lactating dairy cow is reported as ranging from −5 to 22 °C. Each individual cow has a heat stress threshold beyond which a decline in performance will be observed. Biotic factors of significance are:

Production potential: A cow’s genetic production potential has a direct impact on her ability to tolerate hot climates. The TNZ shifts to lower temperatures for higher-producing cows, because of the inverse relationship that exists between the metabolic production of internal heat and heat tolerance. Once the upper critical limit has been exceeded, the cow has to use more energy to cool down, thereby depleting the energy reserves needed for milk production.

Breed: Breed plays a significant role in determining the heat dissipation capacity of cows. Holstein cows are particularly sensitive to heat stress and exhibit a significant decrease in production performance under conditions of moderate (Temperature-Humidity Index [THI] = 72–75) heat stress. In contrast, Jersey cows only experience a decrease in production performance during severe (THI = 75–80) heat stress, and remain unaffected during moderate heat stress.

Skin factors: Several breed-specific factors affect the rate of energy exchange, including the type of hair coat (length and thickness), hair colour, and skin pigmentation. Cattle breeds with short hair have a higher tolerance to heat stress than those with long hair, mainly due to their increased evaporative ability. The thickness of a cow’s hair is positively correlated with rectal temperature, with thick hair acting as a layer of insulation that reduces the ability of the cow to dissipate heat. Cows with dark-coloured coats experience higher rates of solar absorption, which compromise their ability to lose heat through convection or evaporation. Consequently, the dark-coloured breeds often exhibit higher respiratory rates, panting scores, and skin surface temperatures. Holsteins typically have thicker and darker coats, and, as a result, they tend to experience a greater degree of thermal discomfort.

Cow size: Cow frame size is positively correlated with intake capacity and, as a result, larger cows have higher feed intakes and higher maintenance energy requirements, which impacts their ability to regulate body temperature. Larger cows also have a higher metabolic rate and produce a larger amount of internal heat. The heat load of large cows is exacerbated further when they need to walk long distances while grazing to fulfil their maintenance requirements. Smaller cows also have greater sweating rates and a higher surface area-to-volume ratio, making them lose heat more efficiently through evaporation. Moreover, smaller cows can increase their respiratory rate more efficiently than larger cows can.

Biotic factors with a physiological base:                                                                                                                                                                                                                                                                                  Several factors of with physiological consequences influence the thermoregulatory abilities of cattle:

Previous exposure to heat stress: This will affect the cow’s susceptibility to subsequent heat stress exposure, with the extent of this effect depending on the intensity and duration of the exposure, as well as the acclimatisation ability of the cow. The circadian and seasonal rhythms of cows become altered during episodes of heat stress, causing variation in the production and metabolism of glucose, non-esterified fatty acids, urea, and cholesterol, thereby decreasing the energy available to dissipate heat. Seasonal fluctuations can also disrupt the autonomic nervous systems of cows, resulting in altered internal signalling and cardiac fluctuations.

Body condition score: A higher body condition score is indicative of a higher amount of body fat. Cows with high body condition scores have an increased rate of metabolic activity, which generates a larger amount of internal heat and places them at risk of experiencing heat stress. Not only do these cows produce more internal heat, but they also have a reduced ability to dissipate heat.

Cow age in terms of number of lactations: Cow age plays a significant role in determining susceptibility to heat stress – determining not only the upper critical limit, but also the thermo-neutral range. Multi-lactation cows are more susceptible to heat stress than their first-lactation counterparts. This susceptibility is evident through a larger decrease in milk production, lower quality colostrum due to decreased concentrations of immunoglobulin G, solid non-fat, protein and fat, and a higher increase in somatic cell count. Heat-stressed multi-lactation cows also tend to decrease the time spent ruminating more, shows a lower threshold for respiration rate and rectal temperatures, and they have a higher proportion of cows that have to be re-bred because of failed insemination.

Physiological state of the cow: This will determine the severity of heat stress that she experiences. Pregnant cows have higher energy demands than dry cows and, as a result, they have higher metabolic heat production. The stage of lactation will also affect the cow’s heat dissipation rate, as lactating cows have more heat to dissipate than non-lactating cows. Also, mid-lactation cows experience heat stress more severely than cows in the early- or late-lactation stages, and early-lactation cows produce less metabolic heat per kilogram of milk yield because of the mobilisation of stored tissue. Even though dry cows are more tolerant of heat, exposure to heat stress will affect their subsequent lactation period, primarily by decreasing milk production. 

Concluding comments: The health, welfare, and overall productivity of dairy cows are severely affected by heat stress and, as a result, short-term amelioration strategies are employed to modify the cow’s microclimate through environmental management. However, many dairy farms worldwide employ pasture-based systems, and many of the suggested mitigation strategies (both physical and nutritional) are regarded as impractical for these systems. Long-term, holistic strategies should thus be considered, bearing in mind that increased production remain paramount, despite the negative association between the level of production and heat tolerance. As a result, researchers are striving to develop breeding values for heat tolerance that can be incorporated into selection indices. Also, non-breeding methods should be further explored to improve heat tolerance, as dairy cows will have to face increasing harsh conditions.