The impacts hot weather can have on milking herds are well-known and dairy farmers throughout mainland Australia employ shade and evaporative cooling, with supporting herd management strategies, to prevent heat stress and keep their milking cows both fully productive and comfortable.
The effects of heat stress on dry cows – those in the last two months of pregnancy – have been less well understood, but recent research has brought to light new findings, according to Dr Steve Little, Capacity+ Ag Consulting, for Dairy Australia.
“The research clearly shows that if cows suffer heat stress through the dry period it affects the development of their udder and placenta and suppresses their immune system,” Steve said.
“Heat stress on dry cows has a dramatic effect on the development of mammary tissue in the udder and that leads to decreased milk production in the following lactation. Researchers have shown that this decrease can be up to 5 litres per day for up to 30 weeks,” he said.
In addition to lost production, heat stress on dry cows also affects placental development leading to smaller, lighter calves being born, and a greater risk of health problems around calving such as mastitis and retained membranes.
When a dry cow experiences heat stress, it increases the body temperature of her foetal calf. This alters the calf’s metabolism and gene expression. These changes are long-lasting, affecting the calf’s health and performance well beyond birth into adulthood. Calves heat stressed in utero are more susceptible to infection pre-weaning, are less fertile as maiden heifers and go on to have decreased milk production in their first lactation.
“The effects of heat-stress on dry cows are serious and there are negative long-term impacts on the overall productivity of the herd and consequently on business profits,” Steve said.
To minimise the threat of heat stress and its effects on dry cows and their unborn calves, Steve recommends that farmers who dry cows off over the hot months of the year ensure their early dry cows and transition cows have access to adequate shade and cool drinking water at all times.
“As per the milking herd, the aim should be to protect dry cows from direct sunlight, particularly during the hottest part of the day. Four square metres of shade per head at midday is recommended.
“If existing natural shade from trees in paddocks used for dry cows is inadequate, then alternative paddocks should be sought. Portable or permanent shade structures are also options. Longer term, the aim should be to establish more tree belts along dry cow paddocks and springer paddocks,” Steve said.
This story was first published in Leading Agriculture magazine.