Abstract
Three experiments were conducted to better understand why milking-induced prolactin (PRL) release decreases as lactation advances. Experiment 1 compared the effects of milking, 2-min manual stimulation of the mammary gland (without milking), or injection of 1 IU of oxytocin (without milking) on hormonal release in early lactation cows, late-lactation and nongestating cows, and late-lactation and gestating cows (n = 6 per physiological status). Blood samples were collected from 20 min before the start of the treatments to 60 min after. During milking, PRL release (area under the curve above the baseline) was greater in the early lactation cows than in the late-lactation cows but was unaffected by gestation. Lactation stage and gestation did not affect PRL release by manual stimulation. Oxytocin did not induce a significant release of PRL or cortisol. Cortisol release was unaffected by physiological status and was similar for milking and mammary stimulation. Milking-induced β-endorphin release was not affected by physiological status. Experiment 2 compared the effects of milking, 2-min manual stimulation, or 10-min manual stimulation in cows in early (n = 6) and late (n = 6) lactation. Prolactin release was greater in the early lactation cows than in the late-lactation cows for all 3 treatments. A 10-min manual stimulation induced greater PRL release than a 2-min stimulation did. Cortisol release was greater in the early lactation cows but was similar among the 3 treatments. Experiment 3 compared the effects of a 5-min manual stimulation and the injection of domperidone (a dopamine antagonist) in cows in early (n = 6) and late (n = 6) lactation. Manually induced PRL release was greater in the early lactation cows than in the late-lactation cows. Prolactin release was greater with domperidone injection than with manual stimulation and was not affected by lactation stage. Thus, the reduction of milking-induced PRL release in late lactation is not a consequence of the lower sensitivity of the mammary gland to stimulation, a shorter milking time, the gestation stage, or the reduced capacity of the pituitary gland to secrete PRL.