Dairy cows are more susceptible to heat load and stress than most farm animals due to metabolic heat production, which is strongly correlated with rumen fermentation, in combination with their low surface area:mass ratio.
Heat stress occurs when the amount of metabolic heat generated and gained from the environmental surrounds is greater that a cows capacity to offload heat. Dairy cows are more susceptible to heat load and stress than most farm animals due to metabolic heat production, which is strongly correlated with rumen fermentation, in combination with their low surface area:mass ratio.
Affected animals evoke a series of physiological and behavioural responses in an attempt to reduce their heat load. These include increased respiration and panting, sweating, increased water intake, seeking shade, grazing during the cooler times of day, decreased movement and reduced rumination.
Increasing heat load affects the way nutrients are metabolised. For example, blood glucose expenditure is prioritised for core body temperature maintenance and to support immune function at the expense of milk synthesis. During heat stress events, dairy cows tend to secrete less lactose in milk and we often observe increased protein (muscle) breakdown.
As a result, we often see significant reductions in milk volume, protein and fat percentages in lactating cows, primarily via reduced feed intake. Indeed, up to 10% of the variability in milk production has been attributed to the effect of temperature.
Heat stress also has major impacts on reproductive performance, particularly in young cows, due to the reduced supply of nutrients which contribute to poor conception rates and increased embryonic losses. Research has shown that heat stress in the last trimester of gestation can result in foetal programming, which can reduce the lifetime production of the yet unborn calf.
Heat stress alters mineral requirements of dairy cow to maintain body balance (homeostasis), temperature regulation and production. Electrolyte concentrations of sodium (Na+) and potassium (K+) are reduced in rumen fluid due to the increased loss of Na+ in urine and K+ in sweat.
Heat stress inhibits rumination and in turn, rumen function. Saliva loss during panting and reduced saliva production can reduce buffering capacity inside the rumen, increasing the risk of acidosis. Heat stress can suppress immune function, increasing the incidence of some diseases.
The risk of heat stress is measured using the temperature-humidity index (THI). The Dairy Australia Cool Cows Action Plan provides the following THI thresholds for heat stress: Low (<75), Moderate (75–77), High (78–82) and Severe (>82).
On-farm monitoring of cow respiration rate is a critical measure of heat load. Respiration rates above 70 breaths per minute indicate that your cows are battling to offload heat and extra mitigation steps should be implemented immediately. If they approach 80 breaths or above, urgent action should be taken.
A number of practical, cost-effective strategies to minimise the impact of heat stress upon production and cow welfare are available:
Ensuring all cows have adequate access to fresh, high quality and clean drinking water. If your reticulation system and troughs per paddock are challenged, large ‘coffin type’ water troughs at the exit of the dairy are an excellent option.
Providing the highest quality fresh or conserved forages (i.e. high NDF digestibility) will help minimise heat generation in the rumen.
Adjusting the ingredients of supplementary feed provided in the dairy or PMR/TMR during high periods of increasing THI.
The Dairy Australia “Cool Cows” website outlines a number of ways to reduce heat stress. These include planting shade trees; constructing portable or permanent shade structures; and the installation of sprinklers, fans and shade cloth in the dairy and yards.
Even though water is not regarded as a nutrient, water metabolism is critical during periods of heat stress. It has a ‘cooling effect’ inside the reticulorumen and increases the passage of feed through the rumen. Water also influences the metabolism of other nutrients.
A well-considered nutritional plan should be developed well in advance of heat stress events. This plan should include adequate stocks of high quality silage (e.g. maize) with low NDF and high digestibility and adjustments can be made to concentrates fed in the dairy.
Supplementary feed should include a slow-fermenting starch (e.g. finely ground maize), dietary fat (including rumen-protected fat) and a rebalancing of rumen degradable to rumen bypass protein.
Additional sodium, potassium and chloride ions, rumen buffers and other specific additives, such as a yeast culture or yeast-based additives, an osmolyte as per natural betaine and niacin (Vitamin B3), can help to minimise the negative effects of heat stress via metabolic manipulations.
The addition of the trace element, chromium, can help to energise the function of insulin hormone for improved energy metabolism and the efficient removal of cortisol hormones released in blood as an adaptive mechanism of heat stress.