Prolactin in normal and residual milk of Holstein cows and its relationship to milk yield

Prolactin concentrations were measured in blood serum, normal milk, and residual milk samples collected at the evening milking from 12 Holstein cows which were divided into a high, medium, or low group according to milk yield. The milk samples were also analyzed for fat content. Prolactin analysis was by means of a double antibody radio- immunoassay which was shown in this study to be able to measure bovine prolactin with some reliability.

Average daily yield of normal milk was 58.1, 43.8, and 25.8 pounds in the high, medium, and low groups, respectively. Average yield of residual milk, measured only at the evening milking, was 5.2, 4.6, and 3.1 pounds and represented 18.9, 20.2, and 24.4% of the total evening milking in the three groups. The percentage of fat in the normal milk ranged from 2.7 to 3.7% while that of the residual milk ranged from 6.2 to 15.0%. Serum prolactin averaged 22.5, 18.8, and 15.9 ng/ml in the high, medium, and low groups, respectively. Normal milk prolactin was 42.7, 38.2, and 49.7 ng/ml while residual milk prolactin averaged 28.5, 30.0, and 34.1 ng/ml.

There was a lack of significant correlation between either normal milk fat and normal milk prolactin or residual milk fat and residual milk prolactin. Daily milk yield was correlated positively (P < .01) with post-milking serum prolactin and negatively (P < .01) with residual milk pro- lactin. Simple correlation of post-milking serum prolactin and days in milk was not significant (P < .01). Normal milk prolactin was correlated positively (P < .05) with residual milk prolactin but was not significantly correlated with serum prolactin. Simple linear regressions of serum prolactin or residual milk prolactin on daily milk yield were significant (P < .01). The results failed to support the theory that concentrations of residual milk prolactin would be higher in high-producing cows than in low-producing cows. Greater prolactin utilization by high-producing cows, possibly resulting from an increase in total numbers of prolactin receptors or decreased binding efficiency in low producers, was suggested as a possible explanation of the results.