Heat stress occurs when cows are unable to control their body temperature at high temperatures, which leads to reduced feed intake, rumination, rumen function, milk yield and milk fat and protein levels, and increased respiration rate and risk of acidosis. The higher the relative humidity (RH) the lower the temperature at which heat stress occurs, which Dairy NZ has indicated occurs at a Temperature Humidity Index (THI) of 68 for Friesians and 75 for Jerseys, which equates to 21 and 25.5oC at RH of 75%.
Famers can help reduce heat stress by ensuring cows have access to plenty of clean drinking water (>100 litres/cow/day) and shade, reducing walking distances, speed and time in the collecting yard. Cows could be cooled using sprinklers, but this may require using fans to allow for evaporative cooling through air movement.
Diets high in fibre result in higher heat production from digestion, so an option may be to ensure cows have access to lusher pastures and/or lower fibre supplements, particularly protected fats. Another strategy is to improve feed degradation and efficiency of utilisation, which can be done by feeding rumen specific strains of live yeast.
Starch degrading bacteria numbers increase at the expense of fibre degrading bacteria with increasing severity of heat stress. Rumen specific strains of live yeast are able to reduce this effect, resulting in better degradation of forage feeds. Trials have demonstrated improved neutral detergent fibre (NDF) degradation, resulting in increases in efficiency of feed utilisation as measured by increases in milk and protein yields. The improvement in milk production was greater with heat stressed than non-stressed cows. Cows fed these specific strains of live yeast also maintained higher rumen pH levels.
Heat stress increases oxidative stress, which can adversely affect immune function resulting in higher somatic cell counts and mastitis, and lower conception rates. Specific antioxidants and organic selenium have been shown in trials to help alleviate these issues.
Heat stress may also occur at lower temperatures than indicated above with cows consuming mycotoxins, particularly on some types of pasture. Cows or heifers clumping, and looking for shade is often an indication of heat stress at these lower temperatures. Management options are to move stock to alternative pastures, and/or to feed a suitable dual action mycotoxin binder. Mycotoxin binders should ideally be fed twice a day, as they need to be in the gut at the same time as the mycotoxins to which they bind.
As featured in NZ Dairy Farmer