Influence of milking frequency and feeding level on plasmin activity and casein degradation in milk

摘要

A total of 14 identical twinsets of cows were used and fed on pasture (85%) and silage (15%). One group of 7 twinsets were milked once a day, whilst the other were milked twice a day as normal. Within each group, one twin received a daily intake of 15kgDM while the other was restricted to 8 kgDM/day. Treatments began around 30-35 weeks of lactation and continued for 4 weeks. The degree of casein degradation induced by plasmin was assessed by the ratio of γ-casein to B-casein in the milk after separation by anion exchange chromatography. Prior to treatment commencing there were no significant differences between animals in milk yield, ptasmin activity or γ B-casein ratio in milk. After 3 weeks of treatment, milk yield had decreased 24% relative topre-treatment yield for cows milked once daily, 38% on lower feed level and 44% for cows milked once daily. Plasmin activity in milk was not significantly affected by once a day milking, but was significantly greater (p<0.05) than pre-treatment level following 3 weeks of feed restriction. This increase was associated with an increased γ/β-kasein ratio in the milk (correlation coefficient of 0.51, p=0.01) indicating that degradation of casein occurs in milk whilst stored within the gland.