Infradian rhythmicity in milk leukocyte activity together with plasma cortisol and prolactin levels throughout the lactation period in high-yielding crossbred cows

Infradian rhythmicity in milk leukocyte activity together with plasma cortisol and prolactin levels throughout the lactation period in high-yielding crossbred cows has been studied in 10 high-yielding (milk production: 5000l per lactation) Karan Fries crossbred (Holstein FriesianxTharparkar) cows. Milk and blood samples were collected from all the experimental animals. Isolation of milk phagocytes (neutrophils and macrophages) and lymphocytes were done by density gradient centrifugation. In vitro phagocytic index of milk neutrophils and macrophages was performed by colorimetric NBT reductive assay. Mitogen-induced milk lymphocyte blastogenic response was estimated by colorimetric MTT (tetrazolium) assay. Total plasma cortisol and prolactin were estimated by enzyme immune assay. Highest value of plasma cortisol and prolactin was observed at calving which decreased significantly (p<0.01) on 15th day postpartum for both prolactin and cortisol. Immune activity of milk leukocytes was highest on day 0 colostrum and decreased significantly (p<0.01) on 7th day postpartum. A significant (p<0.01) rise of plasma prolactin was observed around 135th and 225th days postpartum, whereas a peak level of plasma cortisol was observed at 105th, 180(th), and 270th days postpartum. Phagocytic index of milk neutrophils and macrophages remains almost in a steady state during mid-lactation period (between 100 and 200days postpartum). A decline in increasing trend of milk phagocytic activity was observed during late lactation. Mitogen-induced milk lymphocyte blastogenic response was highest on day 0 colostrum which decreased significantly (p<0.01) on 15th day postpartum. Con A-induced milk lymphocyte blastogenic response showed an increasing trend from 120th to 210th days postpartum. Upon correlation study, it showed that the plasma cortisol has a negative effect on milk leukocyte activity, while prolactin has a positive effect, though the effect is lactation stage specific.