Identification of temporal patterns of environmental heat stress of Holstein dairy heifers raised in Mediterranean climate during their in-utero and post-natal life periods and modelling their effects on age at first calving

A retrospective study was conducted to evaluate temporal patterns of environmental heat stress during the in-uterus period of development (IUP) and the 3-month post-natal (PN) period of dairy heifers, and to estimate their association with the age at first calving (AFC). Data from 30 dairy herds in Northern Greece including 9098 heifers were extracted from National Cattle Database. Data (2005-2019) regarding 230,100 farm-specific ambient daily temperature and relative humidity records, were obtained from ERA5-Land. Average monthly Temperature-Humidity-Index values (THI; low≤68, and high>68) were calculated and matched for each heifer to their IUP and PN. Subsequently, Cluster Analysis was used with monthly THIs as predictors to allocate heifers to THI clusters. The association of clusters with AFC was assessed with Generalized Linear Mixed Model analysis, an extended form of multiple linear regression. Finally, 8 Heat Stress Clusters (HSC; namely HSC-1 to HSC-8) were identified. Compared to HSC-8 (8th-9th IUP months and 1st PN month) heifers of HSC-5 (4th-7th IUP months) and HSC-6 (6th-8th IUP months) calved 13.8 and 17.8 days later, respectively (P < 0.01-0.001). Moreover, when AFC was treated as a binary variable, heifers of HSC-5 and HSC-6 had 1.15 and 1.34 (P < 0.01-0.001) higher risk of calving for the first time later than 787 days compared to HSC-8, respectively.