Methods for combatting heat stress are not new or very scientific. Providing sufficient shade, as well as cooling and good ventilation (air flow) are the basic principles. In terms of outside shade, West (2003) suggests 4-6m2 per cow. Cooling is often in the form of spraying the animals with either a fine mist or heavier spray of water. Either of these require adequate ventilation in order to be effective, which can be provided via natural or forced (fans) ventilation. Reduced stocking density is also another simply method that can be employed.
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is a useful ingredient to use for increasing energy density without generating additional metabolic heat (Chamberlain, unpublished) no more than 7-8% of the total diet should be fat.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, such as Rumagen® (Alltech), without unnecessarily increasing blood or milk urea nitrogen levels may be used to increase overall protein digestibility. In a field trial in northern Italy, 200 dairy cows had their rations reformulated to include Rumagen during a heat stress situation. Improvements in both milk yield, as well as milk casein percentage were seen with a total return on investment of 3:1.
Improve rumen function
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: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, to promote energy utilisation and feed conversion efficiency. Live yeasts, such as Yea-Sacc® (Alltech) 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 18 to 28°C adding Yea-Sacc to the diet lead to a numerical increase in both DMI and milk production across the trial period (June/July) (Novais et al., 2008) (Table 1).
Combat increasing problems with heat stress
In conclusion, 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 compared with published work derived from, for example, US data. Reduced DMI, milk production and fertility are some of the major physiological effects associated with heat stress, however, both environmental and nutritional strategies exist to combat the effects of heat stress.