Evaluation of in-farm versus weather station data for use as heat stress indicator in dairy sheep

Abstract

Heat stress is a limiting factor in dairy production in hot climates impairing growth, milk production and reproduction. The most widely investigated climatic factors related with heat stress are: air temperature and relative humidity. Previously dairy sheep studies of heat tolerance depended on measurements of physiological functions on individual animals such as rectal temperatures, respiration rates or volumes of air inhaled; unfortunately, such measurements are costly and not feasible on a large scale. This study aims to evaluate in-farm (IF) versus weather station (WS) data to be used as heat stress indicator in dairy sheep. Data were collected in three farms in November 2002 till July 2003. Maximum temperature (T) and relative humidity (RH) were monitored by means of thermo-hygrographs placed in the farms at a height of 1.5m above the ground. Both IF and WS data were taken 24 h before milk recording. The data contained 1,059 test-day records belonging to 275 Valle del Belice ewes. The correlation of WS-T with IF-T was 0.83 and with IF-RH was -0.70. The correlation of WS-RH with IF-T was -0.77 and with IF-RH was 0.78. The correlation of milk production with WS-T was -0.49, with WS-RH 0.46, with IF-T -0.50 and with IF-RH 0.30. GLM analyses undertaken were based on models that included fixed effects of flock, DIM, and T RH or Temperature-humidity index (THI). This resulted in a decrease of milk production of -49.7 g per unit increase of T RH if IF data were used versus a decrease of -36.6 g per unit increase T RH if WS information were used. However when using the THI with IF climatic information there was a decrease of -35.0 g per unit increase of THI versus -44.8 g using WS data. By comparing the models, using the R2 and root MSE, these were always slightly better when using WS rather than IF information, especially with THI. Therefore it seems that the use of weather stations might replace the IF collection