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Stay on guard for heat stressed herds this summer

30 January 2014

Stay on guard for heat stressed herds this summer

Higher summer temperatures can evaporate record dairy returns unless farmers stay on guard for heat stress in their herds.

SealesWinslow Science Extension Manager, James Hague says if farmers are feeling the heat, their herds will be too and heat stress can hit production, milk quality, earnings and overall herd health.

“Cows are not terribly good at regulating their body temperatures as their ability to sweat is limited. The digestive system also creates heat and as a result core body temperature increases. When cows are heat stressed, dry matter intake reduces, and demand for water greatly increases.

“Losing just one kilo of dry matter intake is worth around two litres of milk (0.16 kgMS) which at an $8.30 milk price equates to $1.33 in lost income per cow per day. The financial loss quickly adds up and there are the animal welfare concerns including the risk of health and fertility issues cause by heat stress.”

Heat stressed cows tend to crowd around any available shade and as their water demand is high, they also crowd troughs. Stressed animals will also pant. Adequate shelter and ample water, as well as misting or spray systems can alleviate heat stress, but James Hague says attention to the herd’s diet is also crucial.

“Loss of dry matter intake accounts for around half the loss of milk yield. The other half is potentially due to inefficient digestion and damage to the animal, so it is important the diet is well balanced to maintain rumen conditions. Nutritionally there are a number of practical and very cost effective options.”

Levucell yeast, fed at a rate of 0.5 g per cow per day controls rumen pH and improves fibre digestion under heat stress conditions. This means more Feed Conversion Efficiency (FCE) at a cost of less than four cents a day. Rice bran feed, which does not have to undergo extensive fermentation to provide the cow with energy, keeps the heat of digestion low. It can be fed at rates of up to 2 kg a day. Salt, potassium, and sodium bicarbonate can also aid with heat stress as stressed animals lose electrolytes.

“Keeping the electrolyte balance with sodium and potassium is crucial as animals can lose around 13% of their daily potassium requirement through sweating. Damage can occur in the liver, which reduces the animals’ ability to process glucose, which impacts on milk production. Additives such as betaine and chromium assist with maintaining liver function and have been found to be valuable under heat stress conditions.”

Both high temperature and high humidity combined result in the worst effects on stock. A Temperature Humidity Index (THI), combining temperature and humidity, can be used as indicator of heat stress risks. SealesWinslow provides an online calculator which enables farmers to log temperature and humidity and calculate potential milk losses due to heat stress.

The higher the THI, the more severe the impact. As the accompanying graphic shows, when the THI gets above 68, signs of heat stress will set in. Above 80 causes moderate to severe stress and above 90 is severe. There is an almost linear drop off in the relative amount of milk produced as the THI rises.

The calculator and additional nutrition advice is available at


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