Impact of Extreme Climate Variability on Dairy Cattle vis-a-vis Mitigation Strategies
Authors: Chandan Kumar Rai¹, Arti2, Manish Kushwaha² and Abul K. Azad^1
1Ph.D. Scholar, Dairy Extension Division, NDRI, Karnal-132001, Haryana, India
²Ph.D. Scholar, DES&M, NDRI, Karnal-132001, Haryana, India
³Ph.D. Scholar, Forage Research & Management Centre, NDRI, Karnal-132001, Haryana, India

Climate variability and climate change is very complex, uncertain and a dynamic process. It affects all aspects of climate e.g. making rainfall less predictable, changing the seasons, and increasing the occurrence of extreme events such as heat waves, cold waves, drought, flood etc. In 2007, the Intergovernmental Panel on Climate Change (IPCC) reported that the decadal earth’s temperature increases by 0.2°C per decade and also predicted that the global average surface temperature would be increased to 1.8-4.0°C by 2100. The livestock in the subtropical, tropical and semi-arid regions are exposed to extreme climatic factors viz., high air temperature, relative humidity, solar radiation, rain fall and wind speed for extended periods in a year. All these external climatic factors have a direct impact on the performances (milk and wool production, reproduction and growth) of animals through producing heat stress. Heat stress is defined as the sum of forces external to the animal that act to displace body temperature from a set point.

IMPACT OF HEAT STRESS ON CATTLE

Livestock production system is sensitive to climate change and at the same time itself a contributor to the phenomenon, climate change has the potential to be an increasingly formidable challenge to the development of the livestock sector in India. This sector has a very important role to play in the economic progress of the country as it contributes over one-fourth (26.00%) to the agricultural GDP and provides employment to 18 million people in principal or subsidiary status. Responding to the challenge of climate change requires formulation of appropriate adaptation and mitigation options for the sector.

Impacts on production traits : The intensive selection for high milk production makes the animals more susceptible to heat stress because of both higher metabolic heat production and high environmental temperature. All these factors limit the ability of the lactating cows to dissipate heat and as a result the cows come under heat stress. The temperature humidity index (THI) was commonly applied to know the degree of heat stress in animal, which uses the combined effect of air temperature and relative humidity. There is antagonistic relationship between THI and milk production as the THI and milk production in dairy cattle are negatively correlated. The decline in milk production was estimated as 21% with increase in THI from 68 to 78. The milk yield was found decreased by 0.41 kg per cow per day with per unit increase in THI above 69 (Bouraoui et al., 2002). The milk production in dairy cows is not affected by heat stress when mean THI values are between 35 and 72 (Du Preez et al., 1990). However there is sharp decrease in milk production when THI values are above 76.

Impacts on Reproduction traits: High temperature in summer months combined with a high level of humidity has negative influence on reproductive performance in cattle. Heat stress was also found to be associated with reduced fertility of dairy cows in summer through poor expression of oestrus. The negative effects of heat stress on conception rate of Holstein cows seem to appear when THI ≥75 on 3 days prior to artificial insemination and the effects of heat stress are more evident in the form of declining conception rate from 30.6% to 23% when THI was above 80 (Garcia Ispierto et al., 2007). The low reproductive activities were also observed in animals during summer.

Impacts on nutrition: Physiological parameters like rectal temperature (RT) and respiration rate (RR) are the most sensitive indices of heat tolerance among the physiological reactions studied (Verma et al., 2000) which usually increase at high ambient temperature and humidity and reflect the degree of stress imposed on animals by climatic parameters. High relative humidity reduces the effectiveness of the evaporative cooling in animals. Determining rectal temperature on group of cows in the afternoon can be a quick way to get a precise judgment of the degree of heat stress and the efficiency of any cooling system integrated into cow housing (West, 2003). Respiration rate is also a sensitive indicator of heat stress in animals. There is increase in respiration rate of buffaloes and crossbreds cattle during summer compared to other seasons.

STRATEGIES TO MINIMIZE EFFECTS OF HEAT STRESS

In order to overcome the effect of heat stress, the following strategies are useful for livestock to sustain the interest of dairy farmers in the context of extreme climate situations as well as to maintain sound production and reproduction status of their livestock:

Nutritional and feeding strategies : When animals are exposed to heat stress, the biological functions are affected which include depression in feed intake and utilization, disturbances in the metabolism of protein, energy and mineral balances, enzymatic reactions, hormonal secretions and blood metabolites (Marai et al., 2009), resulting in the impairment of production and reproduction performances. The dry matter intake is (DMI) reduced under heat stress conditions, as a result the negative nitrogen balance may occur (West, 1999). The most limiting nutrient for animals during summer is usually energy intake and a common approach to increase energy density of the diet and to reduce forage proportion in the diet. Diets high in grain and low in fiber cause less heat stress in animals. During hot weather, declining DMI and high lactation demand requires increased dietary mineral concentration. Increasing potassium to 1.3 to 1.5%, sodium to 0.5 to 0.6% and magnesium to 0.3 to 0.4% may result in less heat stress by allowing the animals to dissipate heat.

Modifications of micro-environment : One of the first steps that should be taken to moderate the stressful effects of a hot climate is to protect the animals from direct and indirect solar radiation through provision of shade. Shades are of numerous types i.e. trees and artificial shades made up of metal and synthetic materials. In the humid climate, the orientation of the shade should be north-south direction. Though the evaporative cooling strategies are costly, but they are more useful to alleviate the heat stress in animals. Evaporative cooling systems use the energy from the air to evaporate water and evaporation of water into warm air reduces the air temperature. Evaporative cooling systems use high pressure, fine mist and large volumes of air to evaporate moisture and cool the air surrounding the animals. The milk production and reproductive performances of dairy cattle were improved using an evaporative cooling system. Fogging systems use very fine droplets of water and these water droplets are immediately dispersed into the air stream and quickly evaporate, thus cooling the surrounding air. Misting systems generate larger droplets than fogging systems, but cool the air by the same principle. The sprinklers are different from foggers and misters. The sprinklers do not cool the air rather than the large droplet arising from them wet the hair coat and skin of the cows and buffaloes and then water evaporates to cool the hair and skin. This system is very effective in combination with air movement. The mechanical air cooling is possible by using the evaporative cooling pad and fan system which are very useful in reducing the rectal temperature and respiratory rate in cows and buffaloes.

Breeding strategies: The National Genetic Evaluation of milk yield for heat tolerance was conducted and the results indicated that the most thermo-tolerant sires transmit lower milk yields with higher fat and protein contents in milk than the least thermo-tolerant sires. Daughters of the most heat-tolerant sires had higher total performance index, longer productive life and higher daughter pregnancy rate (Bohmanova et al., 2005). The inclusion of THI as a selection criterion in a selection index is recommended, especially for dairy cattle raised in hot environments. Heat tolerance is considered one of the most important adaptive aspects in cattle. Considerable variation exists for heat tolerance between individual species/breeds and even between individuals within a species/breed. The identification of heat-tolerant animals within high-producing breeds may be useful because these animals are able to maintain both high productivity and survivability when exposed to heat stress conditions (Gaughan et al., 2009). The bos indicus cattle are more heat-tolerant than bos taurus cattle. Improving animal adaptation to climatic stress can be achieved either by selection in stressed conditions or by introgressing ‘heat adaptation’ genes from a local breed into a commercial breed. Some easily measurable morphological/anatomical traits could be used to select heat-tolerant animals. These traits are generally related to change in the animal’s ability for heat dissipation. In cattle, slick hair coat plays an important role in heat tolerance. Selection for quantitative traits related to thermal adaptation, such as lower metabolic heat production, could also increase animal heat tolerance.

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About Author / Additional Info:
I am currently pursuing PhD in Dairy Extension from NATIONAL DAIRY RESEARCH INSTITUTE, Karnal, Haryana (INDIA).