تأثیر فصل زایش بر برخی صفات تولیدی و تولید مثلی در گاوهای هلشتاین استان تهران

     The aim of this research was to investigate the calving season effect on some of the productive traits including milk production, fat percentage and some of reproductive traits including pregnancy period and calf birth weight and also estimation of genetic parameters of studied traits too. In this study the data of 22360 Holestein dairy cows form Tehran province’s 65 herds of between 2012 to 2016 was used that were collected by the country animal breeding center. The study of fixed effects (HYS) and comparison of means was carried out with GLM Procedure of SAS 9.2 software. Estimation of genetic parameters was done by ASReml 3.0 software. The effect of calving season on all traits was significant (p<0.05). The highest milk production was calculated in autumn (8765.93±43.41 kg). The highest and lowest amount of fat percentage was in autumn (3.20±0.01 %) and winter (3.18±0.01 %) respectively. The longest and shortest pregnancy period was observed in summer (289.93±1.65 days) and spring (291.33±1.66 days) respectively (p<0.05). The highest and lowest birth weight of calf was in winter (42.86±0.37 kg) and summer (42.31±0.34 kg) respectively (p<0.05). Heritability of milk production, fat percentage, pregnancy period and calf birth weight was 0.17 ± 0.01, 0.02 ± 0.01, 0.03 ± 0.008 and 0.30 ± 0.11 respectively. The genetic and phenotypic correlation between milk production and fat percentage was calculated 0.29 ± 0.12 and -0.08 ± 0.01 respectively. The genetic and phenotypic correlation between calf birth weight and pregnancy period was 0.20 ± 0.18 and -0.05 ± 0.03. Using management programs like determination of suitable time for insemination of ready cows for pregnancy can result in improvement of milk production and fat percentage in autumn and winter. In regard to positive correlation between calf birth weight and pregnancy period, accurate management and appropriate nutrition can be effective in reducing of dystocia rate in cold seasons.