EFFECT OF HEAT STRESS ON LACTATING DAIRY CATTLE THEIR MANAGEMENT AND CONTROL

Satya Prakash Yadav*, Himalaya Bhardwaj

Bihar Veterinary College, Bihar Animal Sciences University, Patna, Bihar.

*Corresponding author: drsatya202118@gmail.com

INTRODUCTION  

In the summer season, one of the key problems in animal husbandry is Heat Stress (HS) condition in animals, Heat stress is a slight increase in the internal body temperature above its normal range in high-yielding cows, which generate more heat due to the larger volume of feed consumed, associated with increased heat released during metabolism. they normally maintain a relatively constant body temperature (i.e. 38.4°C to 39.0°C).

Multiple techniques have been employed to describe the environmental heat load. However, because air temperature and relative humidity have a significant impact on cattle’s ability to regulate their body temperature, the temperature humidity index (THI), incorporates both factors into one of the most widely utilized.

In this assessment, it was previously thought that the lowest THI limit at which cows would develop HS was 72, covering a temperature range of 23.9 to 32.2 °C and relative humidity of 65% to zero. However, today, as milk production per cow continues to increase, the lowest THI cut-off point has shifted to 68, including a temperature range of 22.2 to 26.7 °C and relative humidity of 45% to zero. Whereas at 22 °C and 10% RH (THI 65), most cows will show no signs of stress at all. Raising the temperature to 27 °C and the relative humidity to 35% (THI 73) will double the animal’s respiratory rate to 75 per minute.

Clinical signs:

Early signs of mild heat stress include an increased respiratory rate, elevated rectal temperature, a tendency to seek shade, refusal to lie down, increased water intake, and reduced feed intake.

As heat load increases cows will tend to crowd around water troughs, lie down in any wet areas (including any muddy areas around troughs), become agitated and restless, and have laboured open-mouth breathing with excessive salivation.

In severe cases, frothy discharge from the nose can be indicative of pulmonary oedema. At this stage, the animal’s core body temperature commonly exceeds 41°C. With very severe heat stress cows become ataxic, refuse to move, collapse with convulsions, coma, and eventually experience physiological failure, resulting in death.

Effects on production:

Under the influence of short-term HS in dairy cows, The effect of reduced appetite and feed intake in heat-stressed cows will be an immediate fall in production with reduced weight gain or decreased milk yield (10-20% or more). Similar effects on milk fat and protein yield are observed. Milk quality parameters are also affected, with SCC and bacterial counts commonly increased during periods of hot, humid weather. Higher-producing animals that have the highest feed intake typically are the most severely affected. The effects of excessive heat load on production usually build up over several days, so severe stress is generally associated with heat wave conditions that persist and where there is little relief from the high temperature and humidity at night. To maintain milk production in heat-stressed cows that have a decreased dry matter intake (DMI), the nutrient density of the diet should be increased.

Effects on reproduction:

Environmental heat has a major impact on reproductive performance in cattle. Prolonged times to conception associated with hot weather are mediated through both reduced conception rates and submission rates. The reduction in conception rates in heat-stressed cows appears to be related to the level of production, with larger declines occurring in higher-producing cows.

The hyperthermia resulting from maternal heat stress has some (detrimental effects on the physiological processes that are important for the establishment and maintenance of pregnancy following fertilization and may lead to an increased loss of early-stage embryos. While older embryos (>7 days) appear to be more tolerant of heat load, there is some evidence that heat load in early gestation increases the risk of foetal loss out to Day 90 of gestation.

There is also some loss in fertility in the bull, particularly involving spermatogenesis. In addition, there can be interactions with reduced nutrient intakes that are superimposed on the direct effects of high temperatures on reproduction.

Effects on health:

To relieve heat stress, cows wallow in mud. The necessity to wash muddy udders increases the risk of milking wet udders, which puts animals at risk of developing new intramural infections. Heat stress can contribute to lameness, increased respiratory rates, and excessive loss of saliva from drooling all contribute to the overall reduction in the natural buffering of the rumen contents, which is believed to be a significant factor of subacute rumen acidosis (SARA).

Heat stress affects calf viability by impeding foetal growth in the last trimester of pregnancy and by depressing colostral quality and immunoglobulin transfer. Uterine blood flow and placental mass are reduced and endocrine profiles are altered when cows are heat-stressed during the last 90 days of gestation.

Management and control:

• Ensure good airflow in barns or shelters to allow heat to dissipate. Natural ventilation, fans, or misters can help improve airflow and reduce the temperature inside buildings.
• Ensure constant access to clean, cool water. Cattle may drink more during hot weather to stay hydrated and regulate body temperature.
• Provide shaded areas where cattle can escape direct sunlight. Natural shade from trees or artificial structures such as shade cloths can help reduce heat load.
• Adjust feeding practices to reduce metabolic heat production. Provide high-quality forage and consider feeding during cooler parts of the day to minimize heat stress.
• Install sprinkler systems or misters in holding areas or feeding areas to cool cattle. Water droplets can help cool the air and the cattle’s skin.
• Minimize handling and transportation of cattle during the hottest parts of the day. If unavoidable, handle them gently to reduce stress.
• Carefully observed the body condition of cattle during hot weather. Thin cattle are more susceptible to heat stress, so adjust feeding if necessary.
• Consider milking during cooler parts of the day, such as early morning or late evening, to reduce stress on the cattle.
• Use heat stress index tools to monitor conditions and take preventive measures when thresholds are reached.
• Monitor cattle for signs of heat stress, such as increased respiration rate, reduced feed intake, or lethargy. Early detection can prevent more serious health issues.
• Select cattle breeds A longer-term strategy to manage heat stress is the use of crossbreeding. Jerseys and Brown’s Swiss cattle are more tolerant to hot conditions than are Holsteins and could be considered in a future cross-breeding program.
• If cattle are on pasture, where pastures only are available the highest quality (lowest percentage neutral detergent fiber) pastures should be fed at night.

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