The objectives of this study were to analyze the effects of some environmental factors on lactation curve traits. A total of 65757 lactations from 40672 cows collected from 1999 to 2002 by the Iranian animal breeding center were used. In order to estimate lactation curve parameters, incomplete gamma function which was proposed by wood was used. Lactation curve traits which were analyzed included a scaling factor associated with logarithm yield at the beginning, the inclining and declining slopes before and after peak yield, DIM to reach peak yield, peak yield, persistency and 305-d milk yield. The results showed that the log-transformed of incomplete gamma function explained 70.8% of variation in daily milk yield. Environmental factors such as calving age, interaction of herd-year of calving, season of calving, parity and days open had significant effects on lactation curve traits. The highest persistency and DIM until peak yield but the lowest peak yield and 305-d milk yield were for 1st parity cows. Peak yield happened later and was lower in those cows calving in spring, compared with those calving in other seasons as well as 305-d milk yield and persistency. The highest correlation was between persistency and DIM until peak yield (r = 0.86) and the lowest correlation was between logarithm yield at the beginning of lactation and factor associated with the inclining slope of the lactation curves. 305-d milk yield was highly correlated with peak yield (r = 0.80) but correlation between 305-d milk yield and persistency was relatively low and mostly positive (r = 0.055).
کلید واژگان :Incomplete gamma function, Persistency, 305-d milk yield
ارزش ریالی : 600000 ریال
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