MILKING FREQUENCY ALTERS THE MILK-YIELD AND MAMMARY BLOOD-FLOW RESPONSE TO INTRAMAMMARY INFUSION OF INSULIN-LIKE GROWTH FACTOR-I IN THE GOAT

The milk yield and mammary blood flow responses to close-arterial, intra-mammary infusion of IGF-I were investigated in five Saanen goats milked frequently or normally the day before. Animals were infused for 6 h with recombinant human IGF-I (1.3 nmol/min) and milked hourly following i.v. injection of oxytocin beginning 2 h before infusion and then every 2 h. On one occasion animals were milked five times (after i.v. injection of oxytocin) on the day before infusion and on the other they were milked twice, without oxytocin. The ratio of milk yield from the infused to that from non-infused gland increased by 17 +/- 4% (mean +/- S.F.M.) in goats milked twice the day before infusion and by 6 +/- 2% when the infusion was preceded by frequent milking. Maximal responses were obtained 4 h after the start of the infusion and differed significantly (P < 0.05), according to pretreatment milking. Blood flow through the infused gland rose in parallel to the milk yield response. At 5 h, when maximal levels were achieved, blood flow was 182 +/- 23% of the pre-infusion flow rate following twice-daily milking and 139 +/- 3% of the pre-infusion flow rate following more frequent milk removal. Thus, more frequent milk removal on the day before close-arterial infusion of IGF-I attenuated both the milk yield and mammary blood-flow response to the infusion of IGF-I.