Importance of humidity and temperature in breech strike of Merino sheep

Abstract

Context Breech strike is a serious disease particularly in wool sheep. Skin wrinkle and dags are known predisposing factors for breech strike, although a large part of the variation between sheep is unexplained. Aims We used sensor buttons to determine whether there were differences in humidity and temperature in the breech area of Merino sheep exposed to blowflies without traditional preventive measures being used in the flock. Methods Sixty-two Merino rams and 62 Merino ewes that had high or low breeding values for breech strike were each fitted with a sensor button to record hourly temperature and humidity in the breech over 49 days. The sensors were fitted before the onset of the blowfly season. The sensor was tied to a wool staple close to the skin in the breech and was removed when a sheep was struck. All remaining buttons were removed prior to shearing. Breech wrinkle, neck wrinkle, breech cover and faecal soiling (dags) were recorded on all the sheep. A total of 98 213 hourly temperature and humidity records were analysed with cubic smoothing splines to assess how humidity and temperature differed between struck and unstruck sheep, and whether the breech indicator traits affect temperature and humidity in the breech. Results Time of day and day-to-day variation prior to being struck explained most of the variation of temperature and humidity in the breech of the sheep. Humidity and temperature increased with an increase in breech cover. The humidity in the breech area was, on average, 2% higher in struck sheep prior to being struck. There was a sharp increase in humidity in struck sheep in the 8 days leading up to detection of breech strike relative to unstruck animals, most probably due to exudate from the wound. Conclusions Sheep prone to be struck have higher humidity in the breech than sheep less prone to be struck. Implications Temperature in the breech does not qualify as a potential indicator trait for breech strike in wool sheep, whereas humidity needs further research to determine its effectiveness as a potential indicator trait.