Breeding talks heat up

Renewed focus on the effects of heat stress on reproduction

NSW DPI livestock researcher Dr Gordon Refshauge says lower reproductive success rates occur when temperatures are greater than 32 degrees.

NSW DPI livestock researcher Dr Gordon Refshauge says lower reproductive success rates occur when temperatures are greater than 32 degrees.


Heat stress hots up as an impact on sheep reproduction


With the Australian sheep flock reaching an all-time low of less than 63 million head, there is a renewed focus on the effects of heat stress on reproduction.

Lamb survivability has never been more important and ensuring producers are aware of the impacts to reproduction is a firm focus in the industry.

NSW Department of Primary Industries livestock researcher Gordon Refshauge will discuss his findings and future directions on the topic of heat stress during an online livestock forum presented by the Graham Centre for Agricultural Innovation on July 31.

Dr Reshauge said evidence in field research trials showed lower reproductive success rates occurred when temperatures during mating were greater than 32 degrees Celsius.

While it was a surprisingly low figure, evidence was supported by studies undertaken 20 years apart.

"Producers and pregnancy scanners across NSW have reported lower than expected pregnancy rates in summer-mated flocks as hot, dry conditions in recent years has led to failures due to the poor conditions and low nutritional status of ewes, with some failures due to summer heat waves," Dr Refshauge said.

Dr Refshauge said heat stress affected reproduction through the negative impacts of excess oxygen radicals produced by the stressed cells of the body, the reduction of sex hormone production following reduced feed intake and increased stress hormones.

"Reproduction can be negatively affected by heat stress at any stage," he said.

"Impairments can occur to the developing sperm in the ram or egg in the ewe.

"The amount of blood flowing to the pregnancy can be permanently reduced, affecting milk production and increasing the risk of hyperthermia in newborn lambs.

"Heat stress affects sperm quality and concentration in rams and chronic heat stress from long term heat exposure can cause infertility, but variation in these impacts between flocks and rams depends on several factors.

"Heat stress has a greater impact on egg quality and embryo survival, because extreme heat stress conditions need only last for two to three days to have an effect."

Solutions to reduce joining pressure

Success at lambing from a successful joining.

Success at lambing from a successful joining.

Scientific studies have shown ewes exposed to heat stress can lose newly conceived embryos and embryos up the three days old with losses up to 100 per Anchorcent. Fewer losses occur beyond five days of age.

Heat stress can also affect blood flow to the placenta during heat waves leading to low birth weight which affects their survival and subsequent growth and performance, including wool quality and fleece production.

While providing shade can make significant improvements to birth weight and lamb survival, aspect, shade, water quality, body condition, forage quality, the animal's antioxidant capacity and the timing of the heat wave coinciding with the time of mating, all impact on the flocks' exposure to heat stress.

Resilience too is improved by acclimatisation, where sheep make physiological adjustments to high heat loads, the sheep 'get used to the heat' and can develop tolerance to high heat loads.

Dr Refshauge said to better protect livestock systems sheep producers needs nutritional, environmental, genetic and management modifications to occur, with widespread and rapid industry adoption.

"Recommendations on how to minimise the impact of heat stress on reproduction include not moving stock in the heat of the day, providing shade and quality water, which are important tips to be mindful of, yet they are extremely simplistic and may not provide the benefits needed," he said.

"Research is critical now to uncover solutions, and that requires the research sector to work closely with producers to ensure their findings are adoptable, easy to apply and cost effective."

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