Heat stress in Swifter sheep in the Netherlands: panting score and salivary cortisol as a measuring tool for predicting heat stress and the effect of a shadow creating trailer

Abstract

Heat stress in sheep has become more important over the years due to rising global temperatures. The Dutch Food Safety Authority (NVWA) has therefore recently enforced a new directive stating that any flock in the Netherlands where at least 10% of the sheep is exhibiting a panting score of 2 or higher is in serious offence, provided that no protection measures have been taken. However, this directive is based on data of research of which the set-up is not comparable with the Netherlands. This research project therefore focused on determining the suitability of panting score and salivary cortisol as measuring tools to predict heat stress in Dutch circumstances. Furthermore, it was studied whether access to a shadow creating trailer (SCT) affected these predictions. This was done by measuring the rectal temperature, temperature-humidity index (THI) and panting score of 40 Swifter sheep twice a day, five days a week at 11:00 and 16:00 for the duration of three weeks. Salivary cortisol samples were taken once a day at 16:00 two days a week. In addition, two cameras were installed to observe their behaviour daily during the entire three weeks. Using a generalized linear mixed model fit by maximum likelihood (Laplace Approximation) and Akaike's information criterion (AIC) for model reduction showed salivary cortisol should not be included in the model and could therefore not be used to predict heat stress. Panting score, THI and access to the SCT were included in the model and can therefore be used to predict heat stress in Dutch circumstances. The odds of sheep experiencing heat stress with no access to the SCT were 3.3 times greater than the odds of sheep with access to the SCT. The activity of the sheep in the sun group seemed to be higher than the sheep in the shade group, regardless of THI. Scrum formation, the aggregation of sheep in a tight group, was generally only seen in the sun group during a clear blue sky, thin layer of clouds or when the sun broke through the clouds. In conclusion, this study did not prove that salivary cortisol can be used to predict heat stress in Dutch circumstances. However, panting score, THI and access to an SCT are suitable to predict heat stress and should preferably all be taken into account as the model included all three parameters simultaneously. The SCT was effective in preventing heat stress in sheep. Scrum formation occurred regularly and was mostly seen in the sun group. It is therefore adaptive behaviour to having no access to shade, possibly even adaptive behaviour to (strong) radiation from the sun. Sheep with no access to the SCT were likely to exhibit higher activity than sheep with no access the SCT. Future studies should focus on how possible differences in activity affect the weight of sheep with and sheep without access to shade.