Reproductive Performance of Lactating Holstein Cows as Influenced by Season of Calving and Parity Under Subtropical Conditions

Normal lactation records of 1717 Holstein cows belong to commercial dairy farm covered a period of 11 years from 1995 to 2005 were used in this study to evaluate the effects of calving season and parity on reproductive performance under subtropical desert conditions. The reproductive traits studied were age at first calving (AFC, month), first service period (FSP, day), number of services per conception (NSC, number), days open (DO, day), calving interval (CI, month), projected minimum calving interval (PMCI, month), breeding interval (BI, day) and conception rate (CR, %). NSC, DO, CI, and PMCI were enhanced significantly in winter compared to summer. Also, PMCI value in winter was lower compared to summer. The overall means of FSP, DO, PMCI, and BI were lower in multiparous cows compared to those in primiparous. No influences of interaction between season of calving and parity were detected on the reproductive traits of FSP, NSC, DO, CI and PMCI. CR had been influenced by season of calving as cows calved in winter exhibited higher CR (44%) than those calved in summer (39%). Heritability estimate of NSC, DO, CI and PMCI heritabilities were low and ranged from 0.07 to 0.19. In contrast, FSP and BI had medium heritabilities. Genetic correlations were low and positive between each of NSC, FSP or BI, between CI and each of NSC or FSP. In contrast, Genetic correlations were medium to high positive among other traits. The phenotypic correlations between NSC and each of FSP or BI were negatively low. Other phenotypic correlations were mostly similar in value and direction as the corresponding genetic correlations. Genetic trends of FSP, DO, CI and PMCI exhibited clear deterioration by time. They tended to decrease throughout the years of study. In contrast NSC and BI breeding values tended to increase during the same period.