As climate change becomes an omnipresent issue in global agriculture, heat stress in Europe will become an increasing issue in, not only the usual areas, such as the Mediterranean and Central Europe but also in more temperate countries like UK, Germany and France.
It is accepted that heat stress results from a combination of temperature and humidity over and above the thermoneutral zone of the cow. The NRC (2007) have defined a temperature and humidity index (THI) that correlates with rectal temperature and shows thresholds above which the animal will suffer heat stress. However, much of the early heat stress work was carried out in the USA where animals would have adapted to higher temperatures to start with (Chamberlain, unpublished). Hence, animals from traditionally cooler climates may start to suffer physiological effects at a lower THI.
Biological effects of heat stress in cows
Heat stress results from the combined effect of elevated ambient temperature and humidity, which compromises the cow’s ability to dissipate body heat. Cows are relatively adept at coping with short-term increases in temperature and humidity as long as they return to lower values relatively soon. Heat stress mainly affects performances through a reduced feed intake, associated with a reduced rumination and buffer content in saliva. These effects are cumulative and result in negative energy balance with potential for ruminal sub-acute acidosis (SARA). Cows initially rely on non-evaporative cooling methods, such as convection. However, in increasingly hot and humid environments these are less and less effective meaning the cow must turn to methods, such as panting and even these methods become less effective as humidity increases (West, 2003). Additionally, drooling animals will lose saliva and one of the main buffers for the rumen: HCO3(Chamberlain, unpublished).
Effect on milk yield
Heat stress mainly affects milk yield and milk fat percentage, but also lowers feed intake, reproductive performance and, often, bodyweight. If heat stress occurs prior to insemination, it is associated with decreased fertility in cattle which can continue well after temperatures have cooled down. High-yielding animals are more susceptible due to higher milk production and dry matter intake (DMI) (West, 2003). In this modern dairy industry the trend is towards larger farms with more cows producing more milk – this situation lends itself to greater negative impact from heat stress. Affected animals will try to lose heat by maximising body surface area exposed to air. This means that they will opt to stand rather than lie down. This has implications for both claw and udder health.
Environmental and nutritional modifications can be implemented to address the challenges of heat stress. Genetic selection for more effective thermoregulation and more efficient heat loss has also been cited as a potential mechanism for reducing impact on farm (West, 2003).