A Note on Some Genetic and Non-Genetic Factors Affecting Clotting Time of Ayrshire Milk

A total of 731 milk samples from ayrshire cows in their 1st to 6th parity, daughters of 20 sires were analysed. Clotting time was determined at 35°C using Hansen's Standard rennet. The complete statistical model accounted for ca. 30% of the total variation in clotting time. Parity and lactation stage did not significantly affect the clotting time, although there was a tendency for the older cows and those in late lactation to have longer coagulation times. Mastitis incidence and lactose content influenced the clotting time significantly. Milk with a viscometric reading >20 (severe mastitis) had 1.6 times longer clotting times than milk with a reading <15. Heritability of clotting time was 0.23 to 0.27±0.12. Strong negative genetic correlations between clotting time on one hand and protein and fat% on the other were found. There was a genetic correlation of 0.6 between the protein and the lactose %.