Heat stress is a natural phenomenon that affects dairy cows and other domestic animals in tropical, sub-tropical and often in temperate regions of the world during the summer months.
Heat and humidity during the summer months combine to make avery uncomfortable environment for dairy cows.
The thermo-neutral zone of dairy cows ranges from just above zero to 22ºC, above this critical temperature (combined with humidity) cows begin to alter their basal metabolism and metabolic rate.
To be able to estimate occurrence and severity of heat stress and to evaluate environmental effects on lactating dairy cows, scientists developed an index that combined measures of both ambient temperature and relative humidity. This index is called Temperature Humidity Index (THI) and is calculated as:
THI = (1.8xT+32)– ((0.55-0.0055xRH) x (1.8xT-26))
Where: T=Temperature in ºC, and RH=Relative Humidity in percentage.
When the THI index ranges from 72 to 79, cows begin to suffer from mild to moderate heat stress. At THI of more than 80 cows become heat stressed.
It is important to realise that in many temperate regions of the world where summers are mild and temperature rarely exceeds 30ºC, moderate to severe episodes of heat stress can occur due to high humidity.
THI index of 75 or above can occur when temperature is 27ºC, if humidity is above 80 per cent. Furthermore, new data analysis from on-farm studies concluded that milk production in high producing dairy cows began to decline at an average THI of 68.
Heat stress negatively impacts a variety of dairy parameters including milk yield and reproduction and therefore is a significant financial burden.
The economic losses to the dairy industry in the USA alone due to heat stress were estimated to be in the range of 900 to 1,500 million dollars a year.
This figure represents a loss in the range of 110 to 190 dollars per cow per year. Hence implementing measures to alleviate heat stress and restore cow’s health and production efficiency will generate an interesting return on investment.
In an attempt to dissipate body heat, dairy cows react to heat stress by reducing feed intake and rumination time, increasing respiration rate, standing time and water intake, excessive salivation, drooling and panting.
This leads to less favourable rumen environment and function, lower rumen pH and lower VFA and microbial protein production and nutrients digestibility. Moreover, maintenance requirements increases as the cows attempts to lose body heat.
Heat stress can decrease feed intake by more than 30 per cent, even on well-managed and well-cooled dairies heat stress decreases feed intake by 10 to 15 per cent.
As a consequence, heat stress leads to significant reduction in milk yield - even in well-cooled dairies, heat stress typically decreases milk yield by 10-15 per cent, and in non-cooled management systems milk yield can decrease by 40-50 per cent during severe heat stress conditions.
The reduced feed intake and rumen function and increased maintenance requirements caused by heat stress has traditionally been assumed to be primarily responsible for the decrease in milk yield. However, recently it was demonstrated that reduced nutrient intake accounts for only about 40 to 50 per cent of the heat stress-induced decrease in milk synthesis.
A large portion of the direct effects of heat stress may be a consequence of changes in endocrine function and nutrient partitioning.
A few authors demonstrated, using the model of thermo-neutral pair fed cows, that heat stressed cows show increased insulin effectiveness and sensitivity. Insulin is a potent anti-lipolytic signal (blocks fat break down) and the primary driver of cellular glucose entry.
Heat stressed cows become hypersensitive to insulin, and will reduce or block adipose mobilisation and increasing glucose burning in an attempt to minimise metabolic heat production.
This diverts glucose from mammary tissue to other body tissue (primarily skeletal muscle) and reduces glucose supply to the mammary gland for lactose production, leading to reduced milk yield. This may be the primary mechanism which accounts for the additional reductions in milk yield that cannot be explained by decreased feed intake and rumen function.
It also has been shown recently that heat stress during the dry period impairs mammary gland development and alters metabolism in dry and transition cow, which in turn, reduces milk yield in subsequent lactation even after cows have returned to more comfortable environmental conditions after calving.
In addition to its negative effect on intake, rumen function and health, digestibility and milk production and composition, heat stress also negatively affects fertility and reproduction in dairy cows. Heat stressed cows show a reduced plasma estradiol and LH concentrations and reduced numbers of FSH and LH receptors on granulosa cells. Some data indicates that only 10 to 20 per cent of inseminations in “heat stressed“ cows result in pregnancies.
There are many managerial actions that can be taken at farm level to alleviate heat stress and reduce its impact on production and reproduction efficiency.
Breed and animal selection is one measure; Jersey cows and light colour coated cows can cope better with heat stress than Holstein and dark colour coated cows. Shaving the cows can help increase evaporative heat loss.
Some studies showed that evaporative heat loss is 6 to 8 per cent higher in shaved cows compared to unshaved cows under the same environmental conditions. Providing shade, improving air movement and ventilation in the farm, supplying cold water in abundance can help the cow lose heat.
The most effective measure, by far, is cooling the cow using a combination of fans and sprinklers. Many studies showed the effectiveness of this combination compared to either fans or sprinkles separately. Moreover, a 5-minute cycle (1 minute sprinkling followed by 4 minutes fanning) proved to be more effective than other longer or shorter cycles in reducing respiration rate and alleviating symptoms and consequences of heat stress.
There are few feeding and nutrition measures that can be employed to reduce the effect of heat stress on animal performance. Timing and frequency of feeding, i.e. providing most of the ration (80 per cent) during the cooler periods of the day (early morning from 4:00 to 6:00 a.m., and late evening from 9:00 to 11:00 p.m).
Adding water to dry rations and thoroughly mixing the ration can help reduce sorting and alleviate some of the negative consequences of heat stress on intake and rumen function. Adding fat (saturated fatty acids), extra minerals (especially Na, k, and Mg to compensate for higher losses from the body due to increased sweating) and extra vitamins (especially vit. A, D, E to compensate decline consumption due to decreasing intake during hot, humid weather) and buffers (especially KHCO3).
Reducing levels of NPN and increasing RUP in ration can help compensate the decline in microbial protein flow to the small intestine (Nutri-Meth, Nutriad’s coated by-pass methionine is very effective in increasing the supply of metabolisable methionine to high producing dairy cows). Adding preservatives and anti-oxidants (such as Nutriad’s Bacti-Nil and Oxy-Nil) to the TMR prevents heating, and increases freshness and bunk life during hot weather and can preserve TMR nutritional quality and improve feed intake.
Other effective nutritional strategies to reduce the impact of heat stress on production and reproduction efficiency, include the use of smart feed additives. Because heat stress affects rumen, endocrine and metabolic function, additives that act at both levels (rumen and metabolism) can be employed to alleviate heat stress.
Nutri-Ferm Prime, a direct fed microbial product based on two different strains of fungal extracts and two strains of yeast cultures, has shown to be effective in stabilising rumen function and improving microbial growth and fermentation capacity.
Nutri-Ferm Prime has been rigorously tested in vitro and in vivo and found to be very effective in improving growth rate of major bacterial and fungal species in the rumen (on average 20 per cent increase in growth rate and 17 per cent decline in doubling time).
Addition of Nutri-Ferm Prime to the ration improves rumen microbial fermentation and increases microbial protein synthesis and fibre digestion, leading to 4 – 8 per cent improvement in milk yield.
Niacin is another additive that can be used to alleviate heat stress in dairy cows. Niacin is involved in most energy-yielding pathways within the animal and its cells, therefore is important for energy metabolism and milk production.
Recent research demonstrated that Niacin plays a role in the transfer of core body heat to skin via its effect on vasodilatation and possibly sweating rate. In those studies heat stressed cows supplemented with Niacin managed to maintain lower core body temperature (measured as vaginal and rectal temperature) compared to controls, by increasing evaporative heat loss by 23 per cent.
Nutriad’s Nutri-PP is a protected sources of Niacin based on our patented micro-encapsulation technology, assures that Niacin will escape rumen fermentation and be absorbed from the small intestine. Nutri-PP has shown to be effective in lowering body core temperature and increasing evaporative heat loss in high productive dairy cows subjected to heat stress conditions.
Heat stress affects dairy cows in many regions of the world and leads to substantial economic losses through its detrimental effect on cow’s rumen health, metabolism, production and reproduction.
We are just starting to understand the physiological and metabolic consequences of heat stress. Seasonal heat stress is now also recognised as a problem in temperate areas. There are many measures one can take to alleviate heat stress or reduce its negative consequences on production and reproduction.