Heat Stress In Dairy Cattle

Having experienced our first few days of summer, now may be a good time for you to consider the effect that heat stress is having on your cows and whether it is feasible for you to take preventative measures to minimise its consequences.

Cows are adversely affected any time the ambient temperature is outside their thermoneutral zone, which ranges from 16 to 25ºC. The effect of humidity is just as important as that of temperature. Recent work has combined the effects of both temperature and humidity into a single index, called the temperature-humidity index or THI. The effect of various THI values are explained in the following table:

 

There is a table available here which gives the THI for a given temperature and relative humidity. As an example, at a temperature of 32ºC and relative humidity of 70% (similar conditions to the hot days we have had already had this season) the THI is 85. So these days have had a significant effect on your cows' production.

The most significant effects of heat stress on cattle are illustrated in the above diagram. From a clinical perspective, the signs of hyperthermia in order of increasing severity are as follows:

  • Seek shade, refuse to lie down, increased water intake
  • decreased food intake, reduced or absent rumination (chewing of cud)
  • Crowding around water sources, increased respiratory rate
  • Body splashing
  • Open mouthed and laboured breathing
  • Excessive salivation
  • Incoordination, inability to move
  • Collapse, convulsions, coma
  • Organ failure and death
Limiting the effects of heat stress

In this region it is not economically feasible to spend large sums of money to minimise the effects of HS, given the number of days where the effect is severe. But with a limited budget and by adjusting some simple management routines, the economic losses associated with HS can be reduced.

    Options to consider include:
  1. Adequate shade - in the form of trees, shade cloth, canvas coverings or shed cover (in order of increasing cost and effectiveness). If trees are chosen, consideration must be given to the orientation of the plantation, and the species of tree and the shade it provides. Shade cloth and canvases are often appropriate for dairy yards during the time of maximum stress (corresponding to the PM milking). Shed cover is far superior if roof height, ventilation, orientation to the sun, and insulation are all appropriate, but costs considerably more. Some farmers in northern Victoria have build sheds which can be also used as both shade and as cover for a calving pad in winter - which helps justify the expense.

     

  2. Fans - large industrial fans are used further north but are unlikely to be feasible here.

     

  3. Sprinklers - are a very effective means of lowering cow temperature if used correctly. They must wet the skin of the cow to be effective, rather than just applying a mist to the cows coat. The latter will occur if droplet size is too small. Mists will lower air temperature near by, but will actually further retard heat dissipation from the cow by acting as a form of insulation. Continuous or 2 minute on / 5 minute off cycles are preferred, with the latter saving water and allowing warm air to escape from beneath the herd between spraying times. The duration required is usually a minimum of 30 to 60 minutes. There is little expense involved in sprinkler systems, they are very effective and there is no reason why they cannot be installed even temporarily on the top rail of a dairy yard.

     

  4. Cow management - having a shade paddock close to the dairy is the ideal situation so movement and hence further stress during the worst time of day can be minimised. When yarding, cows should not be rushed and water points should be numerous and sufficiently large so enough cows can get access at the same time. For larger herds it may be wise to yard and milk the herd in 2 or 3 groups so cows can remain under shade for longer rather than waiting unnecessarily on yard concrete.

     

  5. Nutrition - the ideal situation is to feed lower quantities or higher quality feed. On hot days cows will eat concentrates in preference to other sources of feed, so to avoid acidosis you must ensure they still get adequate roughage. You can feed slightly more concentrate but avoid large abrupt changes. There is evidence emerging which also supports feeding your cows the majority of their daily allocation (of all feeds) during the cool of night. This makes sense given few of you cows will graze at all as the temperature approaches the mid thirties. So allotting more pasture/crop for the evening grazing may be of benefit.

     

  6. Water - Obviously, your trough water supply should be capable of supplying adequate volumes (up to 120L/cow/day) at sufficient volume flow rate.

     

  7. Breed selection - White cows do cope better during the heat of day, but are more prone to developing various types of skin cancer. Black cows do feel the effects of HS earlier but radiate heat better at night and hence recover more rapidly.

     

Treatment of individual cows

If an individual cow shows signs of severe heat stress as described earlier, then immediate action should involve hosing the cow down with large volumes of cold water, minimising stress/physical exertion, placing her in the shade and provide air movement. Some cows may also benefit from large volumes of intravenous fluids, rectal enemas and stomach tubing with water.

So next time you begin feeling the effects of a hot day, take time to consider your cows level of comfort and the money you may be loosing. Simple strategies such as sprinklers, changing paddock selection and delaying the afternoon milking for even a couple of hours may make a huge difference.

The Queensland DPI has a publication and CD-ROM package titled Managing hot cows in Australia, which is an excellent source of information for those interested.

 



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