Association between lactation number and quality traits of milk from individual cows with a focus on plasmin activity

This study investigated the hypothesis that milk quality traits and milk proteolytic activity vary with lactation number in dairy cows. Milk was collected from 109 individual cows, 20 Swedish Holstein and 89 Swedish Red, from 12 commercial Swedish farms. Samples were analysed individually and grouped into three lactation categories: lactations 1-2 (N = 56), lactations 3-4 (N = 32) and lactations ≥5 (N = 21). The principal finding was that plasmin (PL) activity increased markedly with advancing lactation number, with significantly higher activity observed in cows in lactations 3-4 and ≥5 compared with those in lactations 1-2 (P < 0.001). Although plasminogen (PG) concentrations tended to decrease with increasing lactation number, this trend did not reach statistical significance (P = 0.298). As a result, the PG-to-PL ratio decreased progressively with advancing lactation (P < 0.001), indicating increased proteolytic potential in milk from older cows. Lactation number also had a clear influence on milk protein composition. The proportion of total casein decreased steadily across lactation groups, while the proportion of whey proteins increased (P ≤ 0.004). Within the casein fractions, total β-casein declined significantly from lactations 1-2 to ≥5 (P < 0.001), whereas the relative amount of β-casein B increased with lactation number (P = 0.003). The proportions of αs2-casein and κ-casein were highest in cows with lactations ≥5 (P ≤ 0.019). Among whey proteins, α-lactalbumin increased markedly with lactation number (P < 0.001), and β-lactoglobulin showed an increasing, although statistically non-significant, tendency (P = 0.071). Gross milk composition was unaffected by lactation number; however, somatic cell count (SCC) increased significantly in cows in lactations 3-4 and ≥5 compared with lactations 1-2 (P = 0.002). Overall, advancing stage of lactation was associated with enhanced PL-mediated proteolysis, shifts in milk protein profile and higher SCCs, whereas the basic milk composition remained stable.