Dietary strategies for alleviating heat stress in dairy animals

Introduction

Thermal stress occurs in case of thermoregulation issues. When there is an imbalance between heat generation in the body and its dissipation through the body. Responses to heat stress are now regarded as a costly problem in the rearing of many species worldwide. In the dairy industry, dairy cattle are very sensitive to heat stress. Climate change, with a constant increase in the Earth temperature, negatively impact on the body temperature, health, and a variety of productivity traits of dairy cow, including the decreasing feed intake, milk production, milk quality, and reproductive performance. As global warming continues unabated, heat stress in dairy cows may become pervasive in the future. Changes in nutrition are necessary in warm weather to maintain nutrient intake, increase nutritional density, or to restore homeostasis. In addition, dairy cows may even capitulate during the hot summer, with the number of deaths increasing strongly with the increasing value of THI. Consequently, thermal stress poses a significant financial burden. Thus, methods effectively attenuate the negative effects of thermal stress which will improve production performance. At this time, advances in genetics, management and nutritional strategies have been applied to mitigate the adverse effects of heat stress in dairy cows.

1. Impact of thermal stress on the Indian economy

India is currently losing nearly 2 per cent of the total milk production, amounting to over ` 2,661 crore due to rise in stress among lactating cattle and buffaloes because of the stress and global warming. The estimated annual milk loss at present due to stress among cattle and buffaloes at all India level is 1.8 million tonnes, which is nearly 2% of the total milk production in the country.

2. Factors that affect the degree of heat loss or gain in the environment

There are several factors like – Ambient temperature, Relative humidity, Amount of solar radiation, Degree of night cooling, Ventilation and air flow and Length of the hot conditions

3. How should thermal stress be assessed?

An indication of thermal stress, such as increased water intake or reduced dietary intake. Cows are stressed when their respiration rate exceeds 75 to 90 breaths per minute and rectal temperature is over 39.4 °C. The normal body temperature ranges between 38.4 °C and 39.1 °C. The thermal comfort zone for dairy cattle is between 13 and 25 degrees Celsius.

There are some common effects on animal-

Livestock can tolerate higher temperatures when the relative humidity is less. THI makes it possible to evaluate the thermal environment around animals. This index takes into account the temperature and moisture of the surrounding air.

THI = 0.8 Ta + RH (Ta – 14.4) + 46.4

In which Ta = ambient air temperature (°C) and RH = relative air humidity (%).

The HSP70, HSP90 and HSP27 are the predominant HSPs and play a protective role during heat stress in livestock. Among them, HSP70 was identified as the ideal biological marker for quantifying thermal stress in animals.

4. Factors affecting the susceptibility of animal for heat stress

There are various factors, unrelated to climate, that affect the sensitivity of cows to heat stress. High yielding Holstein-Friesian cows in first, second or third parity at the first stage of lactation are the most susceptible: their heat production is twice that of low yielding or dry cows. The respiratory rate of heat stressed cows increases from 20/min under normal conditions to 100/min or more under heat stress conditions. When the ambient temperature exceeds 25°C, a 50/min rise can already be observed. One could argue that heifers, because of their relatively large body surface area and lower metabolism, produce less heat than adult cows. Heifers with a body weight of 600 kg generate more heat than heifers with a lower body weight and are therefore more prone to heat stress. The severity of the effects of thermal stress depends on the length of exposure and the severity of the thermal stress imposed. For example, a heat stress condition on one single day may have no effect at all because the cow still can adapt, while heat stress during days or weeks can have considerable negative effects, which will be aggravated by higher air humidity levels.

Some major factors affecting the dairy cow’s susceptibility to heat stress

• Breed (imported Holstein-Frisian versus local indigenous breed)
• Body condition score (high versus low)
• Parity (younger versus older)
• Behaviour (active/aggressive versus quiet)
• Level of milk production (high versus low)
• Housing conditions (well versus poorly ventilated)
• Lactation stage (early versus late or dry)
• Pasturing conditions (shady versus non-shady)
• Level of feed intake (high versus low)
• Ration composition (protein rich versus poor)

5. The Effects of Heat Stress on Dairy Animals

This can be divided into three categories:

(a) Milk yield and milk component

Reducing milk production is one of the main economic impacts of climatic stress on dairy cattle. During summer the feed intake reduces considerably than the actual need of the animal, which results in disturbing the energy balance of the animal thus affecting the activity of the mammary gland. about 40 to 50 per cent of the decline in milk production and 50 to 60 per cent can be explained by changes induced by hyperthermia. As well, the amount of protein and milk fat, and solids-not-fat (SNF) was reduced during thermal stress in dairy cattle. Generally, during the summer, the availability of green grasses or green forages declines, resulting in an increase in concentrated livestock feeding. As the proportion of concentrate in the diet increases between 50% and 60%, the percentage of milk fat tends to decrease. Thermal stress affects more productive cows compared to less productive ones. High-production cows eat and metabolize more feed, which generates more heat in these cows, thereby reducing their feed consumption and milk production.

(b) Reproduction

A poor diet delays puberty, reduces conception rate and increases pregnancy loss in heifers. In animals under nutritional restrictions, blood flow is diverted from internal organs to peripheral tissues in an effort to reduce body temperature by increasing heat loss. This mechanism leads to a reduction of blood flow, aimed at internal organs such as the uterus, Fallopian tubes and ovaries, decreased blood supply to these organs also implies low nutrient availability and therefore low functional capacity.

(c) Growth performance

The effects of heat stress conditions on growth performance are the products of the decrease in anabolic activity and the increase of tissue catabolism mainly in fat depots and/or lean body mass. The decrease of anabolism is essentially caused by the decrease in voluntary feed intake of essential nutrients; particularly metabolizable energy for both maintenance and gain weight and decrease in feed digestibility and feed utilization and this causes loss of production per unit of food (feed efficiency). In addition, the decrease in thyroid hormone levels during summer may be attributed to the decrease in thyroid stimulating hormone and / or the increase in glucocorticoid hormone or the interaction between the thyroid, and the adrenaline and noradrenalin released in response to temperature may contribute to depression of body weight gain.

6. Strategies for limiting thermal stress

• Physical modification of the environment
• Genetic development of heat tolerant breeds
• Improved nutritional management practices

Management of heat stress by nutritional modification

Among all nutritional interventions could be one of the easiest and most promising options for minimizing the effect of heat stress. Therefore, nutritional interventions should be attempting to improve the following attributes in a synchronized way:

1. Maintenance of modified nutrition requirement

• Dietary fibre
• Protein
• Dietary Fat
• Water

2. Maintaining the mineral balance

3. Maintenance of the oxidative stress and healthy rumen

• Feed additives (Buffers, Niacine, Antioxidant, Fungal culture)

1. Maintenance of modified nutrition requirement

Fibre

High fibre diets can increase heat production. Acetate metabolism (associated with a high-fibre diet) produces more endogenous heat than propionate heat (associated with a high-concentration diet). Consequently, the increased intake of dietary fibre can increase the thermal load and then thermal stress occur. Dietary fibre is needed in a proper supply to ensure proper rumen activity. In dairy cattle, some of the milk output lost (35-50%) during HS could be recovered through nutritional management. In ruminants, diets formulated for low metabolic heat increments can help to improve feed intake and performance under HS conditions. A high fermentable carbohydrate diets can be used under hot conditions to stimulate energy intake, but this positive effect must be balanced with the potential for rumen acidosis associated with high-grain diets. To avoid this disorder is important to maintain the optimal rumen function, with a level of ADF and NDF that should not be lower than 18% and 28% on dry matter basis of the diet, respectively.

Protein

The increase in protein content with the help of highly degradable materials seems to increase the thermal load of the cows. The excess nitrogen supplied by the proteins must be detoxified into liver urea via a metabolic pathway, which is a very high energy demand (1 g urea = 7.3 Kcal). Protein content and feed degradation in ruminants can cause a reduction in feed consumption, resulting in a reduction in milk production. Feeding low level of less degradable protein also encourages animal for DMI. Feeding excess of rumen degradable protein during HS was responsible for decreased DMI and milk production. Crude protein in the diet should not exceed 18%, while the level of rumen -degradable protein should not exceed 61% of crude protein summarized that dairy cows fed diet containing 16.1% CP with low degradability (60% of total CP) had greater milk yield, than cows fed diet characterized by high protein content (18.5% CP) with medium degradability (65% of total CP). Essential nutritional amino acids may help prevent the risk of HS. During HS, transcription and translation of RNA are inhibited, with a decrease in milk protein synthesis. Methionine is one of the most important limiting amino acids for dairy cattle. Methionine supplementation enhances milk production and antioxidant capacity, decreases lymphocytic apoptosis.

Fat

Fat supplementation increases the net energy input of dairy cow subject to thermal stresses due to its higher energy density and lower metabolic heat compared to fibre or starch. In thermo-neutral conditions, cows fed with protected tallow showed a more efficient use of metabolizable energy for lactation than those not receiving additional tallow. Diets with a fat supplement of less than 5% do not have adverse toxic effects on ruminal microflora. Rumen-protected fats in the diet greatly reduce the increase in metabolic heat, improving the role of fats during the period of heat stress. The high fat content has led to a measurable decrease in heat generation also under thermos-neutral conditions. The best alternative is the use of processed fats, which bypass the intact rumen environment, not affecting the microbial growth of the rumen.

Water

The first step in reducing heat stress is to provide cold water for all lactating and dry cows. Water metabolism in thermal stress conditions is closely related to the thermoregulatory requirements of ruminants. High-producing dairy cows have higher metabolic rates than lower-producing ones; this implies they experience more difficulties to dissipate body heat during the hot season, and this is also reflected in the higher water requirement for thermoregulatory purpose. The cow may drink up to 50% more water when the temperature/humidity value is greater than 80%. Acute water restriction induces the kidney to slow glomerular filtration and increase urea reabsorption; this induces the increase of plasma levels of creatinine and urea, whereas chronic water restriction induces a reduction of these plasma metabolites. An important characteristic of drinking water, that is, under certain geographical conditions, is its mineral content, especially in terms of salinity and total dissolved salts (TDS).

2. Maintaining the mineral balance

The negative effects heat stresses are associated with changes in mineral metabolism.

There are some minerals –

Sodium & Potassium

Mineral electrolytes, sodium and potassium are important for maintaining the water balance, ion balance and acid base of heat-exposed cows. Urinary excretion in a hot environment compared to excrete in a cooler environment can increase by as much as 80% and 18%, respectively. When the cow is stressed by heat, it loses a considerable quantity of K. Increasing food K concentrations to 1.2% or higher results in an increase of 3% to 9% in milk yield. Alkaline diets are more preferable. Thus, sodium bicarbonate and magnesium oxide were found to be very effective in relieving heat stress. A diet with high chloride content depressed DMI and was associated with low blood pH and reduced blood buffering. Thus the level of dietary chloride should not exceed 0.35% of DM. The Na & Cl interaction leads to the greatest DMI and 4% fat corrected milk yield when the concentration of both minerals was increased.

Dietary cation-anion difference (DCAD)

DCAD =(Na⁺+k⁺) – (Cl^–+SO^4-) meq/100g DM

An increase in DCAD in the diet was observed to increase DMI in heat-stressed cows. The calculation of Na+, K+, and Cl concentrations has a significant impact on productivity and health status by influencing the equilibrium of the acid base. A DCAD of 50 mEq/100 g of dry matter in the diet of Holstein dairy cows during heat stress would improve AA availability for protein synthesis, essential AA concentrations, and the ratio of essential AA: total AA were all higher because additional AA becomes available that would otherwise be used for the maintenance of acid base balance. A higher DCAD also reduced blood urea nitrogen in heat stressed Holstein dairy cows, suggesting the possibility that it enhanced microbial ammonia utilization for protein synthesis and ruminal N metabolism or utilization. A DCAD of +25 to +30 mEq/100 g DM is considered to be optimal for milk production in heat-stressed cows.

Important mineral level in diet of dairy animals during summers are:

Manganese

Manganese minimized oxidative stress during heat stress. The metabolism of carbohydrates, amino acids and cholesterol in the animal body is regulated and controlled by many known manganese-activated enzymes. Manganese acts as an antioxidant in the body to be part of the enzyme manganese superoxide dismutase (MnSOD) which is a major antioxidant enzyme found in mitochondria. MnSOD catalysis the conversion of superoxide radicals into hydrogen peroxide that can be further reduced to a non-toxic product, water, by other antioxidant enzymes like catalase. Mn is required for the synthesis of steroid hormones (oestrogen, progesterone and testosterone).

Zinc

Zinc plays major biological roles in the living system, which includes cell growth, development, differentiation, homeostasis, connective tissue growth and maintenance, DNA synthesis, RNA transcription, cell division, cell activation and the immune system. Carbon dioxide and carboxypeptidase are the two important examples of zinc-containing enzymes, which are crucial to the regulation of carbon dioxide (CO2) and protein digestion, respectively, in the animal organism. Zinc is considered essential to sexual maturity, reproductive effectiveness, regulation and the development of oestrus.

Copper

Cu also acts as a part of enzyme superoxide dismutase for protecting cells against the toxic effects of superoxide radical by converting them to hydrogen peroxide in the cytosol. Therefore, lactating cows were recommended for Cu supplementation at 11 mg/kg dry matter (DM). Cu plays an important role in the formation of haemoglobin, and for normal functioning of thyroid gland. Deficiency of copper may be the cause of delayed onset of puberty, repeat breeding, low conception, early embryonic mortality and increased incidence of retention of placenta. Reproduction is hampered due to depressed oestrus associated with anaemia and longer service period due to inactive ovaries. The important sign related to reproduction in cattle is decline in fertility.

Chromium

Chromium is a trace element that facilitates the effect of insulin on glucose, lipid and protein metabolism. Chromium supplements have the potential to reduce the negative effects of heat stress. Dairy cows in early lactation supplemented with chromium under hot conditions have shown a reduction of weight loss, an improvement of milk production, a reduction of plasma NEFA concentrations, and an improvement of breeding rates.

Selenium

Among the trace minerals, Se is probably the most interesting to support the antioxidant defense of the cow in HS. The most efficient form of Se feed for cattle seems to be Se yeast. Selenium is involved in the structure of the enzyme glutathione peroxydase. The biochemical function of glutathione peroxidase is to reduce lipid hydroperoxides to their corresponding alcohols and to reduce free hydrogen peroxide to water inhibiting the formation of other highly reactive oxygen free radical and protecting cell membrane against lipid peroxidation or oxidative damage. Selenium is also associated with thyroxin, a thyroid hormonal agent that regulates metabolism, reproduction, circulation and muscle function. With vitamin E, it plays an important role in the immunity and health of the mammary glands, particularly under stressful conditions.

3. Management of the oxidative stress and maintenance of healthy rumen

Feed additives

Buffers

Na- bicarbonate is the most prevalent and important ruminal buffer used to prevent acidosis, which is common during summers due to higher intake of concentrates.

Niacine

Niacin is a subcutaneous vasodilator in many species and has been studied to achieve a decrease in body temperature. However, niacin is rapidly metabolized in the rumen, leading to poorly administered niacin in the small intestine. Niacin helps mitigate HS by increasing heat loss by evaporating the body and reducing heat effects at the cellular level. Amongst the vitamins, the role of niacin in lipid metabolism has been studied.

Antioxidant

Thermal stress generates an imbalance in the oxidative balance, with major consequences for the vital function, life and death of the affected cells. Decreased dietary intake under heat conditions also impacts vitamin intake, which plays a significant role in immune function and performance. To reduce the negative impacts of environmental stress, vitamins A, C and E are generally used due to their anti-stress effects. Antioxidants diminish cortisol secretion and oxidative stress. Supplementation of Vitamin C and vitamin E have a negative effect on cortisol level during heat stress. L-Ascorbic acid and B- complex vitamin can be synthesized by ruminants and a supplementation in the diet is not required under normal conditions.

Vitamin E plays an important role in maintaining the integrity of the membrane in nearly all cells of the body against oxygen free radical’s toxic against their per oxidation. Because HS could cause increased free oxygen radicals and might induce oxidative stress. observed the reduction of milk linear somatic cell score in dairy cows given vitamin E ruminal bolus under HS conditions had positive effects on reproduction and lactation performance of high producing dairy cows kept under HS condition. Vitamin A and vitamin E play an important role in the immunity and health of the mammary gland, particularly under stressful conditions.

Fungal culture

Fungal culture and plant extracts may have a positive effect on rumen metabolism and regulating body temperature. Yeast supplementation increases DMI in transition cows, which increases milk yield and reduces weight loss during thermal loads. Yeast (S.cerevisiae) maintains rumen health by-Improving rumen pH, reduce acidosis risk, Improved fibre digestion and nitrogen utilization and Rumen microflora stabilization. However, Aspergillus oryzae decreased rectal temperature during thermal stress.

Conclusion

Nutritional stress has negative impact on productive and reproductive performance of livestock’s which results in increased economic burden to farmers. CP in diet should not exceed 18% while the level of rumen –degradable protein should not exceed 61% of crude protein. Maintenance of proper mineral balance with special care of K and Na and positive DCAD. Feed additives like niacin, fungal culture, buffers and antioxidant may be beneficial.