Summer Stress Impact and Management in Livestock

Summer Stress Impact and Management in Livestock

Dr. Ajit Kumar

PhD Scholar, Department of Livestock Production Management, WBUAFS, Kolkata

https://www.pashudhanpraharee.com/summer-stress-management-in-livestock-4/

Climate change associated heat stress was established to be the crucial factor, which negatively influences animal production. The animals possess several adaptive mechanisms which are helpful for their survival in harsh environmental conditions, but while doing so their productive performances are compromised. Among the various mechanisms, which help to maintain homeostasis in animals, Physiological adaptability was considered one of the primary response mechanisms by which the heat stressed animals survive the heat stress. Respiration rate (RR), pulse rate (PR), rectal temperature (RT), sweating rate (SR) and skin temperature (ST) are the cardinal physiological variables which helps to maintain the heat balance and homeostasis in the stressed animals.

Rising global earth surface temperature is one of the most intriguing factors emerging from the changing climate. Globally, about 17% of the kilocalorie consumption and 33% of the protein consumption is contributed from the various livestock commodities. According to FAO (2015), the global demand for livestock products are expected to double by 2050 due to the improved standard of living and the rising population.

Climate change emerged as one of the main threats to the livestock sector. Further, the climate change induced heat stress has established as one of the crucial factors affecting livestock production. Heat stress may decrease milk production and quality, meat production, reproductive efficiency and animal health. Dairy and beef industry are especially affected by heat stress in monetary terms. Adaptation to prolonged heat stress may lead to production losses.

Environmental factors, animal factors and thermoregulation play important role in energy exchange between environment and the animal. Environmental factors include ambient temperature, relative humidity, solar radiation and wind speed. Further, animal factors comprise breed, color, stage of lactation and health status. Finally, thermoregulatory mechanisms include circulatory adjustments, sweating and panting. Heat stressed animals, physiological adaptation is composed of two components: 1) heat load due to metabolism, heat exchange, radiation, and convection with the environment; 2) heat dissipation that is loss of heat loaded through sweat evaporation. Some of the physiological determinants of adaptations to heat stress are RT, PR, RR that may result in reduction of productive and reproductive potential in animals.

The cattle and buffaloes are known for their milk production and they contribute approximately 96% to total milk production in India. Though milk production in India has been reached to 187.75 million tonnes in 2019 with a growth rate of 6.5%, but there is high demand of milk and it is projected that by 2030 India will be able to produce >200 million tonnes of milk. This target will be achieved if there is the optimum balance between productivity and fertility. Fertility is a very broad term which is influenced by various factors including genetic, nutritional, hormonal, physiopathology, management and environment or climate.

The main natural physical environmental factors affecting livestock system includes air temperature, relative humidity (RH), solar radiation, atmospheric pressure and wind speed (WS). All these environmental factors are pooled to produce heat stress on animals, which is defined as any combination of environmental variables producing conditions that are higher than the temperature range of the animal’s thermo neutral zone (TNZ). Heat stress has an adverse effect on reproduction traits of dairy cattle and buffaloes. The negative influence of heat stress on reproduction traits of cattle and buffaloes can be quantified through formulating temperature humidity index (THI). The THI is a single value which incorporates the both of the air temperature and RH in the index. Heat load index (HLI) is another index to measure the level of heat stress in feedlot cattle through incorporating the RH, wind speed and black-globe temperature (BGT). A negative correlation exists between reproduction traits of cattle and buffaloes with THI and animals experience the adverse effects of heat stress when the THI crosses a threshold level. The thermo neutral zone (TNZ) of dairy animals ranges from 16°C to 25°C, within which they maintained a physiological body temperature of 38.4-39.1°C.

Table 1: Classification of zones based on THI values in cattle and buffalo

NHSZ=Non-heat stress zone, CHSZ=Critical heat stress zone, HSZ=Heat stress zone, THI=Temperature humidity index

Table 2: Classification of zones based on THI values in cattle with THI model

THI=Temperature humidity index

Effect of Heat Stress (HS) on Production Performance of Dairy Animals

Heat Stress (HS) adversely affects milk production and its composition in dairy animals; especially animals of high genetic merit, heat loads above 35°C activate the stress response systems in lactating dairy cows. In response dairy cows reduce feed intake which is directly associated with negative energy balance, which largely responsible for the decline in milk synthesis. Moreover, maintenance requirements of energy also increased by 30% in HS dairy animal therefore, energy intake would not be enough to cover the daily requirements for milk production. A reduction in DMI by 0.85 kg with every 1°C rise in air temperature above a cow’s TNZ, this decrease in intake accounts approximately 36% of the decrease in milk production. THI negatively correlated to milk yield, as increase of THI value from 68 to 78 decreases DMI by 9.6% and milk production by 21%. HS during the dry period (i.e., last 2 months of gestation) reduced mammary cell proliferation and so, decreases milk yield in the following lactation.

Moreover, HS during the dry period negatively affects the function of the immune cell in dairy cows facing calving and also extended to the following lactation. The stage of lactation is an important factor for severity of imposed HS and animals which were in mid-lactation is mostly heat sensitive compared to early and late lactating counterparts. Hot and humid environment not only affects milk yield but also effects milk quality. When THI value goes beyond 72, milk fat and protein content declines. In addition, the analysis of protein fractions also showed a reduction in percentages of casein, lactalbumin, immunoglobulin G (IgG and IgA). 80% of these were associated with loss of productivity and 20% with health issues which might be due to disruption of internal homeostasis mechanism.

Effect of Heat Stress (HS) on Reproductive Performance of Dairy Animals

          High air temperature and humidity affects cellular functions by direct alteration and impairment of various tissues or organs of the reproductive system in both the sexes of the animal. HS reduces the length and intensity of estrus besides increases incidence of anestrous and silent heat in farm animals. Multifactorial mechanisms involved in reducing fertility of dairy animals depending on the magnitude of HS. HS reduces oocyte development by affecting its growth and maturation. It increases circulating prolactin level in animal’s results to acyclicity and infertility. Moreover, 80% of estrus may be unnoticeable during summer which further reduces fertility. A period of high-temperature results to increase secretion of endometrial PGF-2α, thereby threatening pregnancy maintenance leads to infertility. Plasma follicle-stimulating hormone (FSH) surge increases and inhibin concentrations decrease during HS leading to variation in follicular dynamics and depression of follicular dominance that could be associated with low fertility of cattle during the summer and autumn. However, FSH secretion is elevated under HS condition probably due to reduced inhibition of negative feedback from smaller follicles which ultimately affect the reproductive efficiency of dairy animals. Conception rates were drop from about 40% to 60% in cooler months to 10-20% or lower in summer, depending on the severity of the thermal stress. About 20-27% drop in conception rates or decrease in 90-day non-return rate to the first service in lactating dairy cows were recorded in summer.

Bull is recognizing as more than half of the herd and hence, bull’s fertility is equally or more important for fertilization of oocyte to produce a good, viable and genetically potential conceptus. It is well known that bull testes must be 2-6°C cooler than core body temperature for fertile sperm to be produced. Therefore, increased testicular temperature results from thermal stress could changes in seminal and biochemical parameters leads to infertility problems in bulls. The significant seasonal difference in semen characteristics was reported by several studies.

Strategies for Ameliorating Heat Stress

            Physical modifications of the environment, nutritional management and genetic development of thermo tolerance breeds are key components for sustainable livestock production in tropical hot climates. The most common approach to ameliorate HS is to alter the cow’s environment through provision of house or shade (along with feed and drinking water), evaporative cooling system with water in the form of fog, mist or sprinkling with natural or forced air movement, and possibly cooling ponds. Modification of microenvironment to enhance heat dissipation mechanism to relieve HS is one of the most important measures to be considered in hot environment. Cooling ponds and sprinklers can also be used to cool the environment. Buffalo prefers to do wallowing in ponds during summer season to mitigate summer/heat stress. Cooling can also improve reproductive performance in cows and heifers, and probably, the most effective cooling systems currently in use are those that couple evaporative cooling with tunnel ventilation or cross ventilation. Nutritional modifications could help animals to maintain homeostasis or prevent nutrient deficiencies that result from HS. Lower Dry Matter Intake (DMI) during hot weather reduces nutrients available for absorption, and absorbed nutrients are used less efficiently. Rations should be >18% protein on a dry basis as overfeeding requires more energy to excrete excess nitrogen as urea. DMI and milk yield increased for cows fed with diets containing 14% versus 17 or 21% acid detergent fiber (ADF). However, milk yield was less sensitive to change in dairy temperature for cows fed with 14% ADF diet. Increasing dietary fat content enhanced milk production efficiency and yield in the warm season. HS causes oxidative damage which could be minimized through supplementation of vitamins C, E and A also mineral such as zinc.

Conclusions

Extended periods of high air temperature coupled with high relative humidity compromise the ability of dairy animal to dissipate excess body heat which affects feed intake, milk production, and reproductive efficiency and ultimately reducing profitability for dairy farmers.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4823286/

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