Effect of twinning and stillbirth on the shape of lactation curve in Holstein dairy cows of Iran

Abstract. The aims of this study were to estimate the effects of twinning and stillbirth on the lactation curve traits in Holstein cows in Iran. Data from March 2000 to April 2009 comprising of 862 109 test-day milk records corresponding to 115 805 lactations of 58 756 cows in 27 herds were used. The lowest peak and total lactation yield but the highest days in milk (DIM) until peak yield, persistency and cumulative milk yield from calving up to DIM at peak production were found during the first lactation (P<0.05). Consequential effect of stillbirth and twinning on lactation curve traits, except for the factors associated with the downward slopes of the curve, were significant (P<0.05). Stillbirth affected the 100-d, 200-d and 305-d milk yields. Cows with stillborn calves had longer days to peak yield and higher persistency, but lower peak yield, than those with live calves (P<0.05). Number of days to peak yield, the persistency and total lactation yield from calving up to peak of lactation were higher for twin birth than for single birth calvings (P<0.05). The estimated correlation between peak yield and lactation yield was high (68%) indicating that peak yield is very important in determining the lactation yield.