Can heat affect mating performance in sheep?

PRODUCTION ADVICE - NOVEMBER 2020 - ANIMAL BIOSECURITY & WELFARE

By Adrian Smith
Senior Land Services Officer - Mixed Farming Systems
P: 03 5881 9932 | M: 0447 778 515 | E: adrian.smith@lls.nsw.gov.au

High temperatures in an Australian summer are the norm. But how do these temperatures affect the performance of your sheep, what is it costing, and what can you do about it?

These were some of the questions that were asked at a recent Sheep Connect webinar, conducted by Dr Gordon Refshauge of NSW DPI. For those interested, the link to the full webinar is here - https://www.sheepconnectnsw.com.au/events/797/

Heat stress, through its various mechanisms, is estimated to cost the Australian sheep industry around $100-170m/year – which is expected to increase to over $250m/year as temperatures potentially rise.

There are several factors that contribute to the heat stress your livestock experience, including:

• External factors - ambient temperature, humidity (particularly in combination), radiating heat from surfaces such as soils
• Internal factors - digestion, maintenance (functions that keep the animal alive) and ‘physical’ activity such as walking.

Heat stress can lead to a range of behavioural changes such as seeking shade and shelter, consuming more water and diminished appetite (reduced feed intake).

Animals also show physiological changes when subjected to heat stress, including an increase in respiration (respiration accounts for around 60 per cent of heat loss in sheep), increased heart rate, increased sweating, higher blood flow around the body and a decrease in metabolism.

Sheep begin to experience heat stress at temperatures around 35⁰C with around 20 per cent humidity (typical in a Riverina summer). However, as humidity increases, the level of heat stress at the same ambient temperature increases – so that high heat stress can be seen at temperatures even in the high 20⁰Cs if humidity levels are high.

At temperatures above 38⁰C, extreme heat stress is likely.

What are the impacts of heat stress?

Apart from a decrease in appetite (and by extension a decrease in growth), there is a number of potential impacts heat stress has, particularly on a sheep’s reproductive performance. These include:

• a decrease in the production of antioxidants, which can impact sheep health
• a decrease in the quality (and often quantity) of sperm production
• an increase in production of stress hormones
• a decrease in mating activity
• detrimental impacts on the ewe’s ovum and eggs, embryo survival, and the placenta
• lower birth weights
• an increase in lamb mortality, and
• a decrease in milk production.

It is also known that as the number of days above 32⁰C during mating increases, the rate of fertility (of individual animals and within the flock generally) decreases. So, there are potentially significant consequences on the fertility and fecundity of your flock should you be trying to mate during the summer months, particularly as the data suggests, we are experiencing a greater number of days above this threshold.

As a producer, while we can’t control the weather, there are things we can do to minimise potential impacts of heat stress on the reproductive performance of your flock. These are divided into four categories:

1. Nutrition
   1. potential value in providing antioxidants and vitamin E supplements – but the jury is still out on this.
   2. providing higher quality feed under high heat stress conditions. Feed sources that are high in fibre generate more heat in the rumen and increase the heat load on the animal overall.
2. Environmental
   1. Provide shade
   2. Provide cool, fresh water
3. Breeding and breeds
   1. Some breeds are more tolerant of higher heat loads (such as dorpers). Merinos are reasonably tolerant.
   2. Increased animal performance is usually associated with higher metabolism rates. This higher metabolism often means increases in an animal’s latent heat production, which may make animals more susceptible to heat stress.
4. Management
   1. Time of shearing - some wool (staple length) is good, as it tends to protect (reflect) the animal from direct radiant heat. However, longer wool can lead to an increase in thermal insulation, and therefore raise the animal’s body temperature.
   2. Avoid summer mating – most of the impacts of heat stress on reproduction occur around ovulation – particularly three days either side. Heat stress events during mating can often lead to lower conception rates, and/or the need to extend the length of mating.
   3. Consider changing the location of your operation (extreme response)
   4. Minimise stock movement during heat events, particularly during mating
   5. Ensure your rams and ewes are in ideal condition before and during mating – including liveweight, body condition score and overall health.

Summary

Heat stress can have significant impacts on the performance of your sheep – particularly their reproductive performance.

If the number of days we experience above threshold values increases (ie. the number of days above 32⁰C), the potential impacts on fertility may become more pronounced.

Be aware of the risks, understand the things you can control and implement the appropriate management strategies to minimise the potential effects.

But remember, while heat stress may be an issue, don’t forget the fundamentals that drive flock fertility – nutrition, body condition, timing of joining, overall health and disease status. While it might be convenient to blame poor conception rates for example on ‘on-off’ events such as a heat wave, these critical elements are much more likely to have a greater effect on reproductive performance.

Acknowledgment: This article is based on a webinar provided by the Sheep Connect program, and delivered by Dr Gordon Refshauge of NSW DPI, Cowra. His contribution is gratefully and duly acknowledged.