Exogenous daily oxytocin injections given immediately before milking increase milk production. To investigate the mechanism by which oxytocin increases milk production, oxytocin injections were given before and after milking, and saline injection was given before milking as a control. The experimental design was a replicated Latin square; two complete trials were performed: one with 12 cows (45 d) and another with 15 cows (95 d). In the first trial, the least squares means of milk production were 29.2, 29.3, and 28.3 kg for oxytocin injection before milking, oxytocin injection after milking, and saline injection before milking, respectively. In the second trial, the least squares means of milk production were 33.3, 32.9, and 32.4 kg for oxytocin injection before milking, oxytocin injection after milking, and saline injection before milking, respectively. Oxytocin before and after milking significantly increased milk production by 3%. The results suggest that increases in milk production may not be caused by removal of residual milk but by increased gland output of milk. The effect on milk plasmin activity, fat, protein, SCC, and lactose was nonsignificant and may indicate that effect of oxytocin is not manifested through an effect on cell remodeling.

A longitudinal study was carried out on samples of bulk tank milk collected from 200 farms for 12 mo to evaluate SCC, plasmin and plasminogen activities, psychrotrophic bacteria count, and protein quality in milk and to examine the proteolytic effects of plasmin on milk proteins. Herds were selected randomly from client lists of two dairies to create four groups based on milk SCC of the month before the study; herds were reassigned monthly to one of four groups based on SCC for that month. Overall means were 3.73% fat, 3.13% protein by infrared analysis, 3.16% protein by Kjeldahl analysis, 2.42% casein percentage, 4.65% lactose, 5.43 cells/ml of log SCC, 1.13 U of plasmin, 45.6 U of plasminogen, and log 2.86 cfu/ml of psychrotrophic bacteria. The ANOVA showed a significant effect of month on all factors except SCC, which was fixed by the experimental design. Plasmin and plasminogen activities were high from December to May. Plasmin activities and psychrotroph counts were significantly higher for the high SCC group. Casein percentage and number were significantly higher for the low SCC group.