This article appears in the June issue of Feed International. View all of the articles in the digital edition of this magazine.
As climate change becomes an omnipresent issue in global agriculture, cow heat stress will become an increasing issue in the usual areas, such as the United States, Asia and the Mediterranean, and in more temperate regions like the U.K., 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 National Research Council (NRC) has 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 United States where animals would have adapted to higher temperatures. Hence, animals from traditionally cooler climates may start to suffer physiological effects at a lower THI.
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. Additionally, drooling animals will lose saliva and one of the main buffers for the rumen: HCO3.
Heat stress mainly affects milk yield and milk fat percentage, but also lowers feed intake, reproductive performance and, often, body weight. 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). In this modern dairy industry, the trend is toward 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 maximizing body surface area exposed to air. This means that they will opt to stand rather than lie down. This has implications for claw and udder health.
Nutrition management strategies
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.
The main goal of nutritional management during heat stress is to maintain a healthy rumen function while providing optimal nutrient supply to limit the negative energy balance situation. This relies mainly on providing highly digestible feed while maintaining a safe forage to concentrate ratio.
As DMI drops during heat stress, energy and nutrient density often need to be increased. However, resist the temptation to simply add more energy as concentrates (starch) to avoid compromising rumen function further. Fat is a useful ingredient to use for increasing energy density without generating additional metabolic heat. No more than 7 to 8 percent of the total diet should be fat.
Encouraging feeding during cooler periods of the day and at night can often help offset the reduced DMI during the day. This also facilitates the shelf-life of ensiled forages that is reduced in higher THIs. Only high-quality forages should be used and protein digestibility should be addressed. Feeding protein and nitrogen sources that promote microbial activity and provide some bypass element without unnecessarily increasing blood or milk urea nitrogen levels may be used to increase overall protein digestibility.
Feed additives enhance ration
Requirements for certain minerals (potassium, sodium and magnesium) lost through sweating, panting and drooling increase and should be addressed through the diet. The dietary cation to anion balance of the diet should be closely monitored. Several feed additives as live yeast cultures, buffers, fat soluble vitamins (A, D, E), niacin and selenium can be considered as to improve rumen function, immune response, and to promote energy utilization and feed conversion efficiency.
Live yeasts have been shown to have beneficial effects on DMI, as well as subsequent milk production during periods of heat stress. In a study conducted during the Portuguese summer where temperatures ranged from 18C to 28C, adding yeast culture to the diet led to a numerical increase in DMI and milk production across the trial period of June and July.
Live yeasts can improve dairy cow performance when heat stress is a factor. This table highlights the increase in DMI and milk production with the introduction of a yeast culture. | Novais et al., 2008
Often neglected is probably the most important nutrient: water. Water requirements can increase significantly, so adequate provision of water is critical.
Effects of climate change
As climate change develops, heat stress is likely to become an increasing issue, including in more temperate climates.
Thresholds at which animals in the latter regions succumb to the effects of heat stress are likely to be lower in Europe (or more temperate climates) compared with published work derived from, for example, U.S. data.
Reduced DMI, milk production and fertility are some of the major physiological effects associated with heat stress, however, environmental and nutritional strategies exist to combat the effects of heat stress.