Abstract: This study systematically investigated the effects of seasonal variation, parity, lactation stage, and production region on the plasmin (PL) and plasminogen (PG) activity in bovine raw milk. Raw milk samples were collected from an intensive dairy farm in Hohhot, Inner Mongolia to evaluate the impact of different seasons (summer and winter), lactation stages (initial, early and late) and parities (parities 1–4) on the plasmin activity and the transformation efficiency of PG. To examine the production region effect, another milk samples were collected from Ma’anshan, Anhui; Yinchuan, Ningxia; Harbin, Heilongjiang; and Changji, Xinjiang. Our results showed that season had no significant effect on the PL activity (P > 0.05) but significantly affected PG activation levels (P < 0.01). The PG activity of raw milk was 34.4% in summer than in winter, ((5.98 ± 0.25) versus (4.45 ± 0.43) U/L). Parity had no significant effect the activity of PL or PG (P > 0.05). Lactation stage had a significant impact on PG activation (P < 0.01); PG activity was 8.6% and 6.8% higher in the late stage than the initial and early stages, (5.67 ± 0.70) versus (5.22 ± 0.70) and (5.31 ± 0.67) U/L. Likewise, the geographical origin significantly impacted the conversion efficiency of PG (P < 0.05); PG activity was significantly higher in Harbin samples than in Yinchuan samples, (5.62 ± 0.88) versus (5.12 ± 1.19) U/L. Somatic cell count showed a weak positive correlation with both PL activity (r = 0.33) and PG activation (r = 0.21), suggesting that subclinical mastitis had a potential impact on them. This study revealed that both season and lactation stage affected milk stability by affecting the PG activation pathway, and the PL and PG activity in milk from high-latitude regions might be enhanced via the environment-physiology interaction. These findings provide a novel perspective for establishing a quality evaluation system for raw milk based on PL/PG regulation.

Key words: raw milk; plasmin; plasminogen; activity; dairy cows