Effect of once-daily milking and bovine somatotropin administration in Friesian cows on milk protease activity during late lactation

摘要

Plasmin (PN) is the major protease in milk (Fox, 1981). Increased PN activity in milk is generally considered undesirable, because it hydrolyzes α- and β-caseins, thereby adversely affecting milk processing characteristics (Fox, 1981). Moreover, PNactivity in milk increases with advancing lactation (Politis et al., 1989). This means that in countries such as Ireland and New Zealand, where a seasonal dairy industry is predominant, and as a result all cows are at the same stage of lactation, high levels of PN activity can be expected in milk collected for processing towards the end of lactation. In addition, once daily milking (ODM) of cows, a common management tool onNew Zealand dairy farm, during the latter part of lactation, causes a significantreduction in milk yield (Carruthers et al.,1993;Stelwagen et al.,1994a). This practice is likely to increase PN activity in milk even further during late lactation. The first objective of this study was to assess changes in the activity of PN (active enzyme), plasminogen (PGN, inactive enzyme), and plasminogen activator (PA) in milk during twice (TDM) and ODM in late lactation. The second objective was to investigate if the administration of bovine somatotropin (bST) reduces PN activity in milk. Data from Politis et al. (1990) suggest an inverse relationship between PN activity in milk and bST, in bST-treated dairy cows on TDM.