HORMONAL INDUCTION OF LACTATION IN NONPREGNANT EWES

Exogenous hormone treatments designed to induce nonpregnant ewes to la ctate at levels sufficient to rear orphan lambs were investigated in t hree breeds (Blackface, Dorset and Greyface) and in a total of 14 grou ps. The hormone treatments consisted of an induction phase lasting 1-6 weeks during which most ewes received daily subcutaneous injections o f 75 mg of progesterone and 100 mug of oestradiol-17beta, and a trigge r phase lasting 1 or 2 weeks during which 5 mg of oestradiol-17beta an d/or 10 mg of dexamethasone were given daily and the daily dose of pro gesterone was reduced to zero. From the end of the trigger phase for a t least 2 weeks, milk production was determined three times daily by h and milking after prior intravenous injections of 5 IU of oxytocin. Th e linear dimensions of the udder were measured in each ewe at 5-6 day intervals throughout the induction and trigger phases. Udder sizes inc reased in response to hormone treatment in all cases, and the rate of increase was usually 1.75-11.3 times greater during the trigger phase than during the induction phase. Accumulation of milk in the udder dur ing the trigger phase resulted in mean milk yields of 212-763 ml on th e first day of milking, which were higher than the mean yields of 130- 354 ml on the second day. Thereafter the mean daily milk yields increa sed progressively to reach 579-1301 ml after 14 days of milking. Group comparisons revealed the following: an induction phase of at least 4 weeks duration was required to ensure that all hormone-treated ewes pr oduced 800 ml or more of milk/day by 14 days of milking; during the tr igger phase, oestadiol-17beta alone was a more effective lactogenic ag ent than dexamethasone alone, and dexamethasone apparently hindered th e lactogenic actions of oestradiol-17beta when both hormones were give n together; extending the duration of the trigger phase from 1 to 2 we eks did not improve subsequent milk yields; and there were no signific ant breed differences in milk yield responses to similar hormone treat ments. It is concluded that a 4-6 week induction phase followed by a 1 -week trigger phase using the progesterone and oestradiol-17beta doses noted above but excluding dexamethasone would induce in most nonpregn ant ewes lactation at levels sufficient to rear orphan lambs. Compared to the compositions of normal colostrum and milk, the milk from some of the present ewes had lower dry matter contents, fat concentrations and immunoglobulin-G concentrations and generally similar lactose conc entrations. These differences were not considered to be sufficient to jeopardise the survival of lambs reared by such ewes.