Dairy animals are considered as biological machines as they consume feed and use as a fuel to produce milk. There are certain changes in environment as well as animal physiology which create stress condition in animals. Stress may be considered as any thing that is applied to an animal from an outside source that has an effect on that animal's normal physiological activity. During stress, the performance of dairy animals is reduced. Yourself (1985) defined stress as magnitude of forces external to the body which tend to displace its system from resting or ground state.
Type of Stress
Stress can be categorized in four major categories as follows:
Toxic metal pollutants
Handling & Transportation
Among these stressors, heat stress causes more losses to dairy animals. In present publication, the effects of heat stress on animal production and reproduction has been discussed.
High environmental temperature and high relative humidity are the primary factors that cause heat stress in dairy animals. Moreover, radiant energy from the sun contributes to stress if animals are not properly shaded. As the environmental temperature increases, the reliance on evaporative cooling (sweating and panting) to dissipate body heat increases. However, high relative humidity reduces the effectiveness of evaporative cooling and during hot, humid summer weather dairy animals can't eliminate sufficient body heat and body temperatures rise. Lactating cows and buffaloes create a large quantity of metabolic heat and accumulate additional heat from radiant energy. Buffaloes tend to be more uncomfortable because they have fewer sweat glands under the skin than cows. Thermal environment is a major factor that can negatively affect production and reproduction in dairy animals, especially in animals of high genetic merit.
There is a particular temperature zone in which lactating dairy cows feel comfort and produce at an optimal level, Lactating dairy cows feel heat stress when the rectal temperature is higher than 39.4 oC.
Temperature Humidity Index (THI)
One way to measure the combined effect of temperature and humidity is the use of a temperature humidity index. The THI is calculated from the wet and dry bulb air temperature (WMO, 1989) for a particular day according to the following:
THI = 0.72 (W+D) +40.6,
Where W = Wet bulb temperature oC and D= Dry bulb temperature oC
The THI values of 70 or less are considered comfortable, 75-78 stressful and a value greater than 78 causes extreme distress with lactating cows being unable to maintain thermo regulatory mechanisms or normal body temperatures.
Effects of Heat Stress on Physiology
Heat stressed cows generally exhibit altered blood acid-base chemistry as a result of the shift in cooling from conductive, convective, and radiation to evaporative cooling (Kibler and Brody, 1950). Panting and sweating increase as the reliance on evaporative cooling increases. Panting sharply increases the loss of CO2 via pulmonary ventilation, reducing the blood concentration of carbonic acid and upsetting the critical balance of carbonic acid to bicarbonate necessary to maintain blood pH, resulting in a respiratory alkalosis (Benjamin, 1981). Compensation for the respiratory alkalosis involves increased urinary bicarbonate excretion (Benjamin, 1981), leading to a decline in blood bicarbonate concentration.
Effect of Heat Stress on Feed Intake, Digestion and its Utilization
Increased environmental temperature causes reduced feed intake. High environmental temperature may affect the rumen micro organisms that synthesize Vitamin B complex, amino acids and fatty acids on which the nutrition of ruminants largely depends. In addition, reduction of blood flow to rumen epithelium (Hales et al., 1984) and reduction of rumination is noticed during dehydration and heat stress (Aganga et al., 1990). The fractional rate of digesta passage in the GI Tract of heat stressed animal is slower than that of animals in thermo neutral zone.
Effect of Heat Stress on Milk Production
The reduction in milk production during heat stress may be due to decreased nutrient intake and decreased nutrient uptake by the portal drained viscera of the cow. Blood flow shifted to peripheral tissues for cooling purposes may alter nutrient metabolism and contribute to lower milk yield during hot weather.
For lactating dairy cows the ambient temperatures above 25oC are associated with lower feed intake, drops in milk production and reduced metabolic rate (Berman, 1968). Heat stressed dairy cows in one chamber experiments consumed less feed (13.6 vs 19.4 kg/d), more water (86.0 vs 81.9 l/d) and produced less milk (16.5 vs 20.0 kg/d) than cows in a thermal neutral environment (Schneider et al., 1988). McDowell et al., (1976) suggested that milk production is reduced by 15 % accompanied by a 35% decrease in the efficiency of energy utilization for productive purposes, when a lactating Holstein cow is transferred from an air temperature of 18 to 30oC. Milk fat, SNF and milk protein percentage also decreases due to heat stress. Sharma et al., (1983) concluded that during 60 days of parturition (early lactation), climatic conditions appear to have maximum influence and cows are less able to cope with heat stress.
Effect of heat stress
Effect of Heat Stress on Reproduction
The effects of heat stress can be directly related to the increase in body temperature in heat-stressed cows. Ulberg and Burfening (1967) showed that small increases in maternal body temperature would cause decreased pregnancy rates in cattle (Figure 1). The increase in body temperature affects the reproductive tract and the early embryo. These changes in the reproductive tract influence the ability of a cow to become pregnant during heat stress.
Figure 1. Pregnancy rates for cattle with different rectal temperatures at the time of breeding (Ulberg and Burfening, J. Ani. Sci. 1967).
The somatic cells within the follicles (theca and granulosa cells) can also be damaged by heat stress. Heat stress damages ovarian follicles and causes a decrease in estradiol synthesis Wilson et al., (1998).
This decrease in estradiol synthesis could influence expression of estrus, ovulation, and the corpus luteum.
Heat Stress Management
There are many tools available to help the dairy farmer combat heat stress. During heat stress, environmental cooling can maintain feed intake and nutrient density of feed can optimize nutrient intake to maintain milk production. There are certain strategies to minimize the effects of heat stress.
One of the first steps that should be taken to moderate the stressful effects of a hot climate is to protect the cow from direct and indirect solar radiation. It was estimated that total heat load could be reduced from 30 to 50% with well-designed shade (Bond and Kelly, 1955), and shading is one of the more easily implemented and economical methods to minimize heat from solar radiation. The design and management of shade for dairy cattle vary in different areas and climates. Cows in a shaded versus a no shade environment had lower rectal temperatures (38.9 and 39.4ºC) and reduced respiratory rate (54 and 82 breaths/min), and yielded 10% more milk when shaded (Roman-Ponce et al., 1977).
Hahn et al., (1981) recommended protection of livestock in hot weather by trees or adequate solid roofed shades (1.8 to 2 m2/cattle) preferably white painted sheet metal for artificial shade. Trees are the most effective shade producers. They combine protection from the sun with the radiation sink effect created by cool leaves evaporating moisture. If artificial shade is used, placing about 2.5 cm of insulation directly beneath the metal roof reduces the radiation heat load on the cow.
Air movement (fans), wetting the cow, evaporation to cool the air, and shade to minimize transfer of solar radiation are used to enhance heat dissipation. In hot and humid environments, shading alone does not provide adequate heat stress relief. Additional cooling in the form of fans and sprinklers is usually beneficial and is easily installed in the barns. Sprinkling (not misting) the cattle with water to fully wet their bodies and using fans to evaporate the water cools the cows and encourages greater feed intake and milk production. Sprinklers and fans are usually placed next to the feed bunk so that the feeding area is the coolest place on the farm, helping to encourage greater feed intake. Water from sprinklers must not be blown into free stalls by fans, as the mist creates a mastitis hazard, or onto the feed, which increases spoilage. Sprinkler water can be confined to a desired area by choosing the right nozzle and placing nozzles correctly in the barn. To avoid water from being blown, fans and sprinklers do not run at the same time. Cows cooled with ducted air and spray for 20 min on, 10 min off, yielded 2 kg/d more milk than shaded controls, maintained rectal temperature near normal (below 39ºC), and maintained higher plasma growth hormone compared with shaded controls (Igono et al., 1987). Large droplets from a low-pressure sprinkler system that completely wets the cow by soaking through the hair coat to the skin were more effective than a misting system (Armstrong, 1994).
Cooled cows had sharply reduced respiratory rate (57 versus 95 breaths/min), and efficiency of production (kg milk per kg DMI) was improved for cooled cows, probably due to lower energy expenditures for body cooling. Moreover, Roussel et al., (1970) studied the effect of zone cooling (head and neck) during the summer months on milk yield. He found 19% more dairy milk production as compared to control.
Clean drinking water is arguably the most important nutrient for the dairy cow. Water intake is closely related to DMI and milk yield. It should be available as fresh and clean. Water tanks/bowls should be cleaned regularly and should be conveniently located to encourage drinking. Texas research work demonstrated that offering chilled drinking water enhanced milk yield for lactating cows (Milam et al., 1986) by reducing body temperature through absorbed heat energy.
Energy is a critical nutrient because of the decline in feed intake which occurs during hot weather. Because energy is usually the nutrient which is most limiting in dairy diets, especially during high production and heat stress, the diet must be made more energy dense to provide sufficient energy to maintain milk yield. Increasing the energy in the diet can be achieved by increasing concentrates (grains) and decreasing forages in the diet. However increasing concentrates to greater than 55 to 60% of the diet dry matter is risky and can result in depressed milk fat content, acidosis, cows going off feed, laminitis, and reduced efficiency of nutrient use.
Added dietary fat is an excellent way to increase energy content of the diet, especially during summer when feed intake is depressed. Fat is high in energy (about 2.25 times as much as carbohydrate), does not add starch to the diet (minimizing rumen acidosis), and may reduce heat load in summer. Added dietary fat often boosts milk fat test a point or two. Addition of rumen protected fat like EnerFATTM is a good option to add more energy to the ration. One rule of thumb when high fat addition is required is that 1/3 comes from natural feed ingredients, 1/3 comes from oilseeds, and 1/3 comes from rumen bypass fats.
Minerals play a very important role in reducing heat stress. Care should be taken to supplement extra minerals as there will be a reduction of minerals available to the animals due to low DMI. Chromium (Cr) has been studied as an essential element in livestock animals. A number of studies confirm the association between Cr and the metabolism during increased physiological, pathological and nutritional stress. Chromium demand in animals increases during periods of higher stress - e.g. fatigue, trauma, gestation and different forms of nutritional (high-carbohydrate diet), metabolic, physical, and emotional stress as well as environmental effects. A number of authors confirm decreased sensitivity to stress in Cr supplemented animals through a reduced concentration of cortisol in blood. Al-Saiady et al. (2004) also found that adding chelated chromium to the diet of dairy cows under heat stress improved milk yield and feed intake without affecting milk components (Sano et al., 1999; 2000a, b). Mertz (1992) reported that under stressor influence, secretion of the cortisol increases, acting as an insulin antagonist through increasing blood glucose concentration and reduction of glucose utilization by peripheral tissues. Increase in blood glucose levels stimulate the mobilization of the Chromium reserve, Cr being then irreversibly excreted in urine. Products like Chromcal TM, an ideal combination of chromium propionate and calcium propionate improved milk production during heat stress by reducing the bad effects of heat stress and by supplementing extra minerals.
Nutrition and management are the two very important tasks by which we can minimize the loss caused by heat stress. Small corrections in nutrition and management during the summer can give comfort for the cows and more profit to the dairy owners.
Authors: Dr. R. Gnanasekar, B.V.Sc&A.H, M.Sc (Animal Nutrition), Product Manager - Ruminant Nutrition; and Dr. Deepak K. Dubey,Ph.D (Animal Nutrition), Regional Technical Manager - Ruminant Nutrition - Kemin Industries South Asia Private Ltd.