Plasminogen activation system in milk from Friesian and Jersey cows: relationships with milk production and composition

To evaluate the effect of breed and stage of lactation on plasminogen activation system in milk from Friesian and Jersey cows and its relationships between the enzymatic system and milk production and composition, 8 Friesian and 8 Jersey cows kept in the same herd were used to provide milk samples throughout lactation (from calving to 9 months in milk). Individual milk yield was recorded monthly and individual milk samples were collected and analysed for fat, protein, casein, somatic cell count, plasmin (PL), plasminogen (PG) and plasminogen activator (PA) activities. Parameters of the function y(n)=anbecn which describes the lactation curve were estimated for each animal. Breed did not affect PL, PG and PA activities but PG/PL ratio resulted significantly lower in milk from Friesian cows than that from Jersey cows (3.97 vs 4.91). PL and PG activities increased significantly with advancing lactation. A significant negative correlation was observed between PA activity and casein/total protein ratio for Friesian cows (r=-0.32). The latter correlation for Jersey cows (r=-0.28) was not significant. The overall correlation coefficients between PA activity and the parameter c of lactation curve (descending phase to the dry period) (r=0.21) reached the significant threshold confirming the involvement of PA on the involution of the mammary gland.