As you sit back on a balmy summer’s evening with a cold one after a long, hot day, spare a thought for our bovine friends.
Of all the ruminants, dairy cattle are the most susceptible to heat stress, due to their high metabolic rate, poor ability to retain fluids via the kidneys and intestinal tract and underdeveloped sweating system.
Various factors play a role in how susceptible a herd is to heat stress.
These include temperament (“cool, calm and collected” versus “hot headed”), exercise (short distances of walking versus long, hilly walking), diet (highly fermentable feeds will increase internal heat production), water access, breed (Jerseys and Swiss Browns are more resilient than cross breeds, which in turn are more resilient than Holstein-Friesians), stage of lactation (lactating cows are less resilient than dry cows, high production cows are more susceptible than moderate production cows) and acclimatisation (up to three weeks of “preconditioning” is needed to enable cows to cope with heat better). Some of these factors we can control, others we can’t.
The bovine body likes to keep itself around 39 degrees C.
To do this the body generates heat via metabolism (food digestion) as well as gaining heat from the external environment (such as the sun).
Excess body heat (heat load) is then lost via convection, radiation, conduction or evaporation.
Once the environmental temperature exceeds the cow’s body temperature, heat loss can only occur by evaporation.
Evaporation is also the most efficient way cows can reduce their heat load, by sweating, panting (respiration) and salivation.
However if humidity increases, the cow’s ability to lose heat via evaporation decreases and effects of heat stress can be seen at temperatures as low as 26 degrees C if humidity is 40 per cent.
As the temperature and humidity rises, cows become lethargic and are less likely to walk more than 250m for water (think dehydration and gut impaction), they reduce their dry matter intake, often eating less roughage (leading to increase risks of LDAs later on).
Blood flow to internal organs is altered to enable better heat loss through the skin; this in combination with hormonal changes will decrease fertility days before ovulation, reduce libido, shorten oestrus cycles and increase the risk of early embryonic death in the first few weeks.
With reduced feed intake, there will be a drop in milk production and with losses of electrolytes through sweating, panting and salivating, milk quality will drop as well. As much as a 20 per cent drop in milk yield can occur.
These electrolyte imbalances also cause the rumen pH to drop, thus predisposing to ruminal acidosis and laminitis.
Ironically, as the temperature climbs, cows will tend to group together to create shade, respiration rates will increase to rates above 70 breaths per minute with eventual collapse and death occurring.
Much as we would love to, we can’t control the weather so what can we do?
- Water – enable close access to water, ideally a trough close to the dairy exit and water troughs in every paddock. Large volume concrete troughs will keep drinking water cooler.
Bury water pipes as black poly pipe sitting on the ground will heat the water..
A hot cow will drink up to 250l of water a day.
- Time – milk earlier, milk later.
It will probably be more comfortable for your milkers too.
Avoiding walking cows during the heat of the day (around 3pm) as this will increase their heat load.
- Feeding – Feed consumption can decrease by up to 35 per cent on a 35 degrees C day. Reduced intake means less milk.
Encourage an increase or maintenance of energy intake by several options.
Feed forage feeds during the cooler times of the day.
Or increase the energy density of the feed (such as increase grain or pellet intake or leafier forage over stalky forage).
Be aware that intake still needs to be balanced with high quality fibre, extra buffers if increasing grain/pellet rations, as well as electrolytes such as potassium, sodium and magnesium to replace those lost.
Feeding a slowing fermenting carbohydrate source such as corn/maize will also help reduce internal heat production.
- Shade – it’s the most obvious but possibly the one most overlooked.
Consider having “hot weather paddocks” – those which are close to the dairy, have adequate water supply and shade.
Whether it be in the form or natural shade (tree belts along northern and western borders and laneways, close to water sources); portable shade (cheap, portable structures covered with corrugated iron or shade cloth) or permanent shade (well ventilated, covered feed pads), shade is most effective method of increasing heat loss (and reducing heat absorption).
Consider covering the holding yard as well.
- Evaporative cooling – yard sprinklers are fine if there is air movement, otherwise they need to be combined with fans.
Water needs to be able to wet the cow’s skin for cooling to occur so cows shouldn’t be packed too tightly and droplet size and wetting time needs to be sufficient.
Wetting the yards will also help dissipate heat.
When considering your “hot weather paddock,” one which is under pivot or cannon irrigation is a bonus.
If in doubt, monitor your cows.
If we get a gradual increase in temperature then cows can adapt but sudden spikes in temperatures (and humidity) can cause problems.
Monitoring respiratory rates is a simple and helpful tool.
If your cows are breathing more than 60 times a minute then consider taking action. It’s a lot to consider but in the end it will result in cooler, happier, productive cows.