ANIMAL MANAGMENT DURING SUMMER STRESS

Introduction

Dairy animal produce milk most efficiently in environments where they can maintain their body temperature at around 38oC. Tissue and cellular metabolism and the underlying biochemical reactions that sustain life and productive functions need body temperature to be maintained within very narrow limits. Relatively small increases in body temperature of at least 1oC result in detectable, deleterious effects on metabolism and tissue integrity, in particular, the breakdown of body protein and a significant depression in production. The condition resulting in this, where heat load on the body of the animal exceeds heat dissipation is called heat stress. The dairy animal, dissipates heat by two distinct methods; evaporative and non-evaporative pathways.

Evaporative cooling is mainly effected through sweating and respiration. Heat stress itself is a function of time, temperature and humidity, because animal rely on water evaporation via sweating and panting to dissipate an excess of heat they have generated metabolically or absorbed from the environment. The primary factors that cause heat stress in dairy animal are high environmental temperatures and high relative humidity. In addition, radiant energy from the sun contributes to stress if animals are not properly shaded. As the environmental temperature increases, the difference between the temperature of the animal surroundings and their body decreases and her 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, the animal cannot eliminate sufficient body heat and her body temperature rises. The tremendous amount of body heat that the high yielding dairy cow produces is helpful in cold climates but is a severe liability during hot weather and in hot climates. The physical heat production of an animal is also

controlled by both internal factors and external factors of an animal. Internal heat load comes from basic functions such as respiration, digestion, as well as other daily maintenance requirements. These factors will be influenced by stage of lactation, production levels, as well as quantity, quality and type of feed consumed. External physical heat loads are management factors that affect physical activity and performance. Animal comfort, layout of facilities, stocking densities and fly control can all impact on the animal external physical heat load. Buffaloes are more prone to heat stress than cattle due to scarcely located sweat glands, black color and sparse hair on body surface. The sweat glands of buffalo skin have a low blood supply, number of sweat glands per unit area of skin is about one third of that of cattle and the thickness of corneum layer and epidermis is about double that of cattle.

The thickness and the black pigment of the buffalo skin help in absorption of more heat and leads to disproportionate convective and radiative heat losses from the extremities during exposure to solar radiation.As mentioned earlier, heat load will increase as temperature, humidity and solar radiation increase and air movement decreases.

Effect of heat stress on animals

Under heat stress, a number of physiological and behavioral responses vary in intensity and duration in relation to the animal genetic makeup and environmental factors. The degree to which an animal resists rise in temperature varies with species because of difference in their heat regulation mechanism. When animals are exposed to rising air temperature, the first response observed was increase in respiration rate. Air temperature up to around 300C has little effect on respiration rate and rectal temperature. At higher temperature above 410C, rectal temperature of animal will increase rapidly, while the respiration rate rises rapidly to about 3-4 times the normal values. Under thermal stress, animal employs moderate level of sweating and resort to open mouth panting. Thermal stress lowers feed intake of animal which in turn reduces their productivity in terms of their milk yield, body weight and reproductive performance. Under thermoneutral environmental conditions, most of the large domestic animals are able to maintain equilibrium between heat production and heat loss. High heat loads may lead to energy deficit, even when they do not induce a marked reduction of feed intake in animals.

Strategies for ameliorating heat stress

The effects of heat stress are costly to dairy farmers, but there are opportunities to recover some of the losses to hot weather. Physical modifications of environment, genetic development of breeds that are less sensitive to heat and nutritional management are the three major key components to sustain production in hot environment.

1. Shelter management

With the help of managemental tools, it is possible to modify the microenvironment to enhance heat dissipation mechanism to relieve heat stress.

(a)Scientific construction of animal shed so the air movement will not be hampered

(b) Provide comfortable micro-environment to animals.

(c) Use of cooling ponds, water sprinkler system and whole body bathing.

(d) Use of cooler for cooling of animal sheds.

There is no doubt that shading is one of the cheapest ways to modify an animal's environment during hot weather. Although shade reduces heat accumulation, there is no effect on air temperature or relative humidity and additional cooling is necessary for farm animals in a hot humid climate.

2. Nutritional management

It has been documented that both low and high ambient temperature cause oxidative stress. Oxidative damage, as a result of heat stress may be minimized by antioxidant defense mechanisms that protect the cells against cellular oxidants and repair system that prevent the accumulation of oxidatively damaged molecules. Antioxidants, both enzymatic and non-enzymatic, provide necessary defense against oxidative stress as a result of thermal stress.

I.Non enzymatic antioxidants in reducing oxidative stress

a)Vitamins

Both vitamin C and vitamin E have antioxidant properties. Antioxidant vitamins have proved to protect the biological membranes against the damage of ROS and the role of vitamin E as an inhibitor –“chain blocker”- of lipid peroxidation has been well established. Like vitamin E,ascorbate is also a chain breaking antioxidant. It prevents lipid peroxidation due to peroxyl radicals. It also recycles vitamin E. It protects against DNA damage induced by H O radical. Vitamin C has a paradoxical effect as it can also produce ROS by its action on transition metal ions. Both ascorbate and zinc are known to scavenge reactive oxygen species (ROS) during oxidative stress. Vitamin C was found to assist in absorption of folic acid by reducing it to tetrahydrofolate, the latter again acts as an antioxidant. Use of folic acid is impaired when vitamin C is deficient.

b)Minerals and trace elements

Zinc and other trace elements like copper and chromium act as typical antioxidants as they work indirectly. Zinc is a catalytic cofactor for Cu/Zn SOD and catalyzes dismutation of superoxide anion, producing molecular oxygen and H2O2, the latter product is usually metabolized by GPx and CAT. The activity of Cu/Zn SOD, CAT and GPx is decreased in copper deficient animals. It is also reported that normal copper levels are necessary to maintain the structural integrity of DNA during oxidative stress. Supplementation of electrolytes is one among the nutritional strategies to combat heat stress in animals. Addition of Na+, K+ and Cl- is benefited in heat stressed dairy cows in terms of milk yield, acid base balance and altered temperature. Supplementation of sodium and potassium in the form of bicarbonate/carbonate also help in better regulation of acid-base balance in the blood.

3.Amelioration through immunomodulation by dietary supplement

The immunostimulant effect of antioxidant depends on age and immune state of organisms.The effect of heat stress can be neutralized by complex antioxidant system that can organism develops. The antioxidant system can be booked by supplementing antioxidants in diet. Vitamin C and trace minerals like zinc have proved to play a vital role as modulators of antibody response and enhances of wound healing in domestic animals.

Ajeet Kumar & Anil Gattani, Assistant Professor,

Department of Veterinary Biochemistry, Bihar Veterinary College, Patna