Chair of the Department of Animal Sciences at the University of Florida Geoff Dahl, who has been presenting his findings at workshops in Victoria and NSW in the past couple of weeks, said while the initial effects of heat stress on dry cows was “bad” it also had a “significant carryover” to their next lactation.
“Dry cows can produce five litres of milk per day less during their next lactation if they have been heat stressed, compared to their herd mate, who was cooled,” Dr Dahl said.
As well as that, Dr Dahl said the cows’ immune system was compromised.
Dr Dahl said traditionally in the US, and similarly in Australia, during a cow’s dry period they weren’t managed closely, but that has turned out to be a “real problem”.
“We also looked at the impact (of heat stress) on the calf in utero and found the calves were born lighter and had a lower immune status,” he said.
Dr Dahl said when that calf matured it also produced less milk, about five litres a day less, than its herd mate and then passed that lower production on to its offspring.
That essentially means one heat stress event can have an effect over three generations of dairy cows.
“Heat stress in a cow’s dry period can affect a mature cow for the next lactation … but the offspring is programmed for lower productivity for life,” Dr Dahl said.
He said on his travels through Queensland and down to Victorian dairy regions all areas had the potential to be affected by heat stress.
“Higher humidity, like in Florida, can cause heat stress … but even with lower humidity, cows can definitely be subjected to environmental extremes causing heat stress.”
The direct effect of sunlight was also a factor.
Dr Dahl said the most effective and practical way to measure heat stress was to measure respiration rate.
“If a cow is taking above 60 breaths per minute, they are probably heat stressed.”
He said to avoid heat stress, the first thing was to have shade available for cows and if possible have the ability to soak them to cool off. Economic analysis had found the cost of putting in a facility to cool cows off was worthwhile.
Dr Dahl said there weren’t really any dietary supplements that could reverse the effects of heat stress and the best approach was to manage dry cows and “cool them off” to reduce their body temperature to normal.