Heat stress has impact on ewe, ram reproduction

University of Adelaide third-year PhD candidate Bobbie Lewis Baida outlined some of the initial findings during a SA Drought Resilience and Innovation Hub field trip at the centre on Thursday last week, along with fellow researchers Alice Weaver (SARDI) and Jamee Daly (University of Adelaide).

The South Australian Sheep Industry-funded study aims to understand the variation in thermoregulation in rams and ewes during periods of heat and how this impacts reproductive performance to eventually develop mitigation strategies for sheep producers out on farms.

"There are multiple stages where heat stress can have an impact on reproduction - ovulation, conception, pregnancy loss, lamb quality and survivability," Ms Lewis Baida said.

"Previously, most of this research has been done in hot rooms with small mobs with heat stress at a targeted period of time.

"Whereas this work and future research is really going to focus on what's actually happening in the field, as opposed to what's going on intensively in hot rooms."

Initially, 15 Merino ewes had a temperature logger inserted in the centre's 'Animal House' to identify their differing core temperatures and temperature thresholds.

"We then selected 200 ewes with varying thermo-regulatory capabilities under field conditions," she said.

"We then conducted an AI program in early January and measured gestation length, ovulation rates, conception rates, twinning rates, and pregnancy losses through summer, while also trans-abdominally scanning their foetal growth and development.

"We are currently lambing, and the lambs are being monitored for survival and body morphology."

Ms Lewis Baida said there was evidence to suggest that heat-stressed ewes during gestation were not as fertile leading up to joining and had lambs with restrictive growth.

"Heat events do the most damage around mating," she said.

"While this year's summer was relatively mild, there were still some hot, humid temperatures and when those events last more than a week, the ewes don't get a chance to recover overnight and it can really take a toll."

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The study also used GPS collars during summer - a potential research-first - linking movement-tracking and temperature-sensing technologies together.

"We could see when the ewes made the decision to go in the shade and how this influenced their core temperature," Ms Lewis Baida said.

"It was interesting to see how individual sheep coped differently."

Ms Lewis Baida said next year would be spent analysing the data collected.

"We want to come up with strategies that can be adopted straight away on-farm, whether it is new management, infrastructure, nutrition or vitamins to help animals cope better with heat stress," she said.

Ms Lewis Baida also outlined heat stress research being undertaken on rams, funded by a science innovation award through Australian Wool Innovation.

"Using similar technology, we are looking at the effects of scrotal thermo-regulation on ram fertility and semen quality," she said.

"If we can identify rams with improved resilience, we may be able to select for those animals and have increased reproductive output during mating."

Ms Lewis Baida said an exciting part of the project was they were using ram lambs from the heat-stressed ewe project, "particularly the ones with superior thermo-regulatory capabilities".

"Because of this, we can look at any transgenerational patterns in their thermo-tolerance," she said.

When validating this technology on 15 ewes in the Turretfield Research Centre 'Animal House' in 2020, Ms Lewis Baida said that even in a small number of animals, a variation in temperature thresholds was witnessed.

She also said anything up to or beyond the 38.3-degree core temperature mark had reproductive implications.

"Sometimes it doesn't even have to be that high to have an effect, it's more mild than you may think, especially if they don't have that recovery time overnight," she said.

"As climate change progresses, this issue may only get worse."