Jasmine Kneebone ensures cows are in paddocks with good groundcover and shade, along with ready access to water troughs.
Photo by
Jeanette Severs
Where the management strategy is to dry cows off during summer, Dairy Australia research indicates cows that experience heat stress during late pregnancy have smaller placentas with reduced blood flow through the uterus and umbilical cord.
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The calves tend to be born earlier and lighter than the calves of cows that are kept cool during their dry period.
These cows also experience a higher incidence of mastitis and retained foetal membranes, because their immune function is already distressed.
Calves born in hot weather also experience lower growth rates and are less productive and fertile in their first lactation.
They are more susceptible to illness and have higher mortality rates.
Participants at the field day learned observation strategies to monitor heat stress in cows, including interpreting panting.
Photo by
Jeanette Severs
In heifers and cows, heat stress decreases oestradiol production, a major female sex hormone that regulates oestrous, leading to ovarian inactivity.
Hormone imbalance impairs oocyte development, resulting in lower conception rates.
In the womb, endometrial blood flow is reduced leading to a reduced ability to retain a pregnancy, and increasing embryo mortality.
The heat stressed dairy herd is increasingly likely to produce a decreased six week and 100-day in-calf rate and higher not-in-calf rates.
Veterinarian Lauren Foster said farmers who relied on collars to indicate oestrus need to more aware of failures during heat stress periods.
“Cows ruminate less when it’s hot,” Dr Foster said.
“That’s also an indication of a cow being on heat.
“So you need to know how to understand the data you’re receiving from the collars, before wasting time and money on assuming the cow isn’t in calf.
“In northern Victoria, we’ve also seen cows cycle even though they’re in-calf because you increase the protein percentage of their feed after the weather cools down, then they experience a weight gain.
“This has a flow-on effect to hormone production.
“In northern Victoria, a couple of days after the hot spell has cooled down, we’ve seen groups of cows cycling as if you’ve been running around with a heap of PG.”
The aftermath of heat stress could see farmers thinking their cows are in-cycle when they are still in-calf because of erroneous data.
Photo by
Jeanette Severs
In the longer term, changes in breeding strategies in the herd to improve conception rates can include joining more heifers in summer and joining cows in cooler months; and looking at including genetics that support heat tolerance ABVs and production in the herd.
Lauren warned farmers against relying solely on heat tolerance ABVs to improve herd production.
She said relying on one ABV risked losing improvements in other necessary traits in the herd.
The reliability of the heat tolerance ABV was 38 per cent, lower than conventional production traits.
Heat tolerance is also favourably linked with fertility and unfavourably with production.
As more data is gathered, in time it should be easier to incorporate heat tolerance ABVs with other, positive, production ABVs.
Care in hot weather should be extended to calves, whose long-term welfare can be affected for up to two lactations.
Photo by
Jeanette Severs
The latest science is identifying the short and long-term implications of heat stress in cows.
Photo by
Jeanette Severs
Using a centre pivot or other sprinklers to provide a spray of water during hot days in the paddock, will help cool the environment for cows.
Photo by
Jeanette Severs
