HEAT STRESSED HOLSTEIN HEIFERS - THRESHOLD DETERMINATION IN CENTRAL CROATIA
AbstractIn the light of increasingly rapid climate change worldwide, and with the purpose of reduction of financial losses of dairy farmers and enabling the sustainable farming, the necessity of implementation of breeding values for heat resistance in breeding strategies, have become more and more pronounced. Estimation of breeding values requires determination of THI threshold value. Therefore, the objective of this research was to determine the THI threshold value for Holstein heifers in environmental conditions in Central Croatia. With that purpose individual test-day records of Holstein heifers with records of ambient temperature and relative humidity in the barns collected in regular milk recording from January 2006 to December 2012 were analysed. The THI threshold values for daily milk yield were determined by least square analyses of variance for each given THI value (from 65 to 76) using the PROC MIXED (SAS). The THI ≥ 65 cause significant change in Holstein heifers’ daily production. Significant decrease of daily milk yield was observed at THI = 65 with estimated drop from 0.087 till 0.254 kg milk/day (THI from 65 – 76). The THI = 65, as the lowest value at which significant decrease in daily milk yield was determined was taken as the threshold value for Holstein heifers in Central Croatia.