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ARTICLES >> Livestock Articles

Heat and ewe body temperature do influence joining

Posted by Bestprac on May 31 2012

By Greg Curran
Veterinary Officer and Technical Specialist
NSW DPI Broken Hill

Flock scanning in the far north-west by Cousins Merino Services has shown, for the first time, that high daytime temperatures during joining combined with high ewe body temperatures reduces fertility. 

In two flocks joined during January and February and into March, ewes showed generally good fertility (70% to 90% of ewes were pregnant), consistent with excellent pasture and a good summer.

It is known that, for both sheep and cattle, any animal’s body temperature is repeatable. In other words, some always have relatively high temperatures, some low, and most stay about average. On this basis, a body temperature taken at one stage of an animal’s life, or at one time of the year, will reflect its body temperature at other times, relative to those of other animals in its mob. This was the reasoning behind taking the body temperatures of pregnant and non-pregnant ewes when the ewes were scanned, as an indicator of their body temperatures during joining.

In research terms, the hypothesis being tested was that ewes with higher body temperatures would not conceive as well as ewes with lower body temperatures, when both were exposed to high summer temperatures. Experimentally, it has been known for decades that when Merino ewes are subjected to high temperatures for short periods just before, and at the time of, ovulation, they usually don’t conceive, or they lose the pregnancy very early. This work had not examined whether individual ewe body temperatures made any difference to fertility under field conditions, although in cattle this had been shown to be the case.
In both of the flocks tested, the difference in fertility was statistically highly significant. The distributions of body temperatures in non-pregnant and pregnant ewes are shown here in the graphs. In flock 2, the ewes were shorn shortly before scanning, reducing insulation and altering body temperatures compared with those in flock 1. The average body temperature of non-pregnant ewes in flock 1 was 0.32°C higher than that of pregnant ewes; the difference between the two groups in flock 2 was +0.13°C. Allowance was made for the sequence in which ewes presented in the race, and body temperatures were measured with simple digital thermometers.

The rams were taken out at scanning in both flocks, so some ewes would have been classed as ‘non-pregnant’ because their pregnancies were very early. If those undetected ewes with very early pregnancies also had generally lower body temperatures, then this would strengthen the already strong evidence that ewes with lower body temperatures have higher fertility under summer heat than those with higher body temperatures.

This work opens up the possibility that selecting and breeding sheep with lower body temperatures and sheep that withstand the heat better may improve fertility. It has been shown in cattle that the ability to withstand heat stress (‘thermotolerance’) is strongly heritable and leads to higher conception rates and better calf survival.

These histograms show the distribution of ewe body temperatures in non-pregnant (top histogram in each panel, ‘0’) and pregnant (bottom histogram, ‘1’) ewes. The body temperatures of the ewes in the top histogram in each panel are higher (and the ewes are thus less fertile) than those of the ewes in the lower histogram, particularly in flock 1. The ewes in flock 2 were shorn before scanning, so the differences in body temperature between the pregnant and non-pregnant sheep are not the same as those in flock 1.

 

Last changed: Jun 01 2012

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