Effect of Heat Stress in Dairy Cows and its Management

MANOJ SHARMA*Krishi Vigyan Kendra, Kapurthala.

The main limitation with crossbred dairy cattle in India is its susceptibility to

diseases and not adjusting with hot and humid climate resulting in the reduced milk

production to even much less than the half of their potential. The heat stress in cattle

occurs when the cow’s heat load is greater than her capacity to dissipate heat. The

cow’s heat load is determined by her own body heat production plus environmental

heat that includes air temperature and humidity and both are combined into a

temperature humidity index (THI). The THI above 72 is usually considered the point

at which stress occurs. Cows react to heat stress in different ways, such as, seek out

shade, reduce feed intake, increase water intake and usually stand rather than lying

down. In addition to this, blood flow to the skin is increased while blood flow to the

internal organs is decreased, respiration rate increases, rectal temperature and

sweating increases. It is important to mention that the lactating cows are more

affected by increased rate of respiration and rectal temperature in summer than that

of dry cows and heifer, because in lactating animals large amount of heat is

produced in order to sustain milk production.

Likewise, Water intake increases by nearly one-third times more in hot

weather. Potassium requirement of these cattle increases due to profuse sweating.

These animals have a lower rate of feed passage and reduced gut motility. Heat

stress affects rumen fermentation adversely and the total volatile fatty acid (VFA)

production is decreased even when the feed intake is same.

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* Deputy Director ( Training)

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Effect on Reproductive Efficiency

As the quality and quantity of the feed declines during hot months, some

cows may become anoestrus and those cows or heifers that continue to cycle, the

duration of estrus and expression of estral signs are reduced. Similarly, irregular

intervals may be noticed. Studies have shown that exposure of dairy cows to heat

stress from a few days before estrus, throughout estrus, during insemination and

ovulation and the first 2 to 3 week of pregnancy, may affect embryo development

adversely to a great extent. During this period a series of events take place at time

interval of hours or days, and interference with any one of them can severely affect

normal progress of pregnancy. Therefore, lower pregnancy rates occur either due to

higher rate of fertilization failure and/or early embryonic death. In addition to this,

heat stress during the dry period may alter the development of the placenta.

Further, dairy farmers using natural service bulls should know that high

temperature severely affects a bull’s ability to maintain optimal scrotal and testicular

temperature. Thus, higher temperature results in low as well as abnormal sperm cells

production. Low sperm output and poor semen quality results in poor pregnancy

rates. Moreover, spermatogenesis is such a long process that adverse effects of

thermal stress continues for several weeks or a month even though temperature

begin to moderate. If cows are still cycling normally and in good condition but bull’s

fertility is suspected, then under these conditions dairy farmers must go for artificial

insemination rather than using natural service of heat stressed bulls.

Effect on Feed Intake

As summer temperatures and humidity increases, cows experience a

“summer slump,” whereas on the other hand, cow comfort is essential to get

maximum production. As and when the temperature is more than 75 degrees

Fahrenheit and humidity is high (85%), the cow is under stress. If the air movement is

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negligible around the animal’s body then high ambient temperature plus high

humidity plus body heat generation results in lower feed intake, low milk production,

and reduced reproductive performance (Table 1 ). It is important to note that the rate

of feed intake begins to decline when environmental temperature reach about

78ºFand starts declining rapidly if temperature rises above 78ºF. In order to

overcome the impact of heat stress, there is a change in the physiological activities

by the cow, such as, increased respiration rate and increased sweating. If these

processes are not successful, then body temperature increases and results in

reduced feed intake and an increase in the maintenance requirement etc. Further,

offering more forage to animals during hot and humid climate will cause more heat

production in animal body, increasing heat stress problem more quickly. Hence under

such situations, farmers should increase the amount of concentrate in the daily

feeding schedule of dairy farmers. This will surely help in sustenance of milk

production due to the fact that reduction in the dry matter intake will be compensated

to some extent.

Effect on Health of Dairy cows

The cow’s comfort and performance decline quickly at temperature

above 78ºF with relative humidity of 80 per cent. As a result of this, there is an

increase in the somatic cell counts (SCC) and a higher incidence of mastitis.

Therefore, during hot and humid weather conditions, the animals become

more susceptible to the attack of diseases. Further, it is worth to mention that

the flies are instrumental in the establishment of coagulase – negative

staphylococcal teat canal colonization in dairy heifers. Such colonization leads

to intramammary infections (IMI), which persists throughout the lactation

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period. Therefore, fly control is especially important during hot and humid

when conditions are optimal for them to multiply at a very fast rate.

Table 1 . Effects of heat stress on maintenance requirement, feed intake and milk

production.

Temperature

(F)

Maintenance

Requirement

(%)

Projected

Intake required

to maintain milk

(DM, pounds)

Actual feed

Consumed

(DM, pounds)

Milk

Produced

(pounds)

68

77

86

95

104

100

104

111

120

132

40.1

40.6

41.7

42.8

44.5

40.1

39.0

37.3

36.8

22.5

59.5

55.1

50.7

39.7

26.5

McDowell et. Al., 1976. Journal of Dairy Science, 59:965.

Effect on the Milk yield

During hot and humid weather conditions, the dairy cow reduce the intake of

the nutrients which are necessary for producing milk as well as for body

maintenance. The lactating cows are affected more with heat load due to increased

metabolically derived heat associated with milk production and whenever the heat

gain is more than the heat loss (as occurs during hot and humid weather),

hyperthermia results and milk production is reduced proportionately.

Similarly, higher environmental temperature during last three months of

gestation alters blood flow and maternal–fetal hormone concentrations, resulting in

lower birth weight of calves and reduced milk yield in the subsequent lactation.

Likewise, prolonged hyperthermia during late gestation interferes with normal

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placental growth and endocrine function, which results in lower calf birth weight and

hormonal alterations affect mammary development and lactogenesis. Reduced

hormonal activity particularly T4 during pregnancy affects metabolic state of the dam

at parturition and thus reduce mammary development prior to the initiation of

lactation. It has been established that reduction in milk yield during heat stress is

mainly, due to less feed intake on one hand and increased maintenance requirement,

which reduce feed efficiency on the other hand.

Strategies for Managing Heat stress

There are many tools to combat heat stress, but management must be excellent if

high performance is to be maintained. Some of the points are given as under:

1. Always protect cows from direct sunlight by providing shelter with additional

cooling device such as, fans and sprinklers near feed manger. Preferably

keep east – west orientation of the sheds.

2. Feed as much during morning and evening hours and throughout the day

more often to encourage more consumption of dry matter.

3. Increase the energy content of the diet by adding an energy-rich feedstuff,

such as maize, or other cereal, and reduce the amount of fiber in the diet.

4. Provide fresh and cooled water all the time. This is the single most important

consideration. If cows are not consuming all the cool water they can, then dry

matter intake will not be maximized. Remember that the key to feeding the

heat-stressed cow is managing her to increase dry matter intake.

5. Use clean water for animal washing. Also remove the organic matter from the

teats, as it is excellent transport medium for bacteria and may cause an

infection.

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6. Minimize drastic ration changes, which force cows off-fed. Blended ration

must contain both the forage and grains. Vitamin A, Vitamin E and selenium

should be supplemented in diet during these months.

7. Make an increase in the levels of sodium (0.4 to 0.5%), potassium (1.5%) and

magnesium (0.3 to 0.35%) due to the reason that increased urination and

sweating during hot and humid conditions may result in deficiency of these

elements.

8. Teat dipping with germicidal dips is recommended. Similarly, keep check on

the flies’ population during hot and humid conditions.

Summary

The effect of hot, humid weather is costly to the dairy farmers in many ways.

In addition to reduced milk yield (15 to 40%), cows have lower milk fat content,

impaired reproductive performance and greater susceptibility to health problems

during hot weather. Feed intake declines and the energy, which is consumed by the

cow, is used less efficiently for milk production.

Heat stress during the summer is a fact of life for dairy farmers in India. The

best strategy for the dairy farmers is to employ management tools to prevent the

animals from direct Sunlight responsible for causing heat stress in their animals.

Making use of a combination of the tools discussed in this article can reduce the heat

stress to some extent.

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