Heat stress effect assessment on milk yield in dairy herds of the Tunisian northern west area

Dairy herds are raised under dry climate in Tunisia which make them subject to heat stress which is known to considerably decline their milk yield. This paper aimed to evaluate heat stress impact on the dairy performances of the Tunisian North West cows. We used 4789 test-day recorded between 2008 and 2011 on 585 Holstein cows in their first three parities, calved between 2007 and 2011 and belong to 10 herds from the Tunisian North-West area. Temperature and humidity data were associated to each test-day and Thermal Humidity Indexes (THI) were calculated. First, the effect of temperature and humidity factors were assessed separately, then the effect of their interaction and finally the effect of their THI using three statistical models. For model 2 and 3, all fixed factors were significant (P<0.001) while for model 1 the humidity didn’t show a significant on the milk performances. The highest milk yield was 51 kg and was found to be reached during the lowest daily temperature and the highest humidity; corresponding to the lowest THI index. The lowest milk yield was 1 kg and was found to be recorded during the hottest temperature and the lowest humidity; resulting in the highest THI; which have significantly exceeded the THIThreshold. Our analysis has showed that Tunisian dairy cows in the North West area undergo moderate heat stress during one third of the year and that the average milk yield decline is about 5 kg/day.