Fertility following synchronization of oestrus in the sheep with intravaginal sponges. III. Effects of supplementary oestrogen during treatment, duration of treatment, and number of inseminations

Abstract

A factorial test is described, involving 1104 ewes, in which was studied the effects on lambing performance of (a) the inclusion of oestradiol-17G (0, 10, or 50 ¦g) in Cronolone-impregnated sponges, (b) two doses of Cronolone, (c) treatment for 12 or 16 days, (d) insemination on detection of oestrus or on a fixed time basis, and (e) one or two inseminations. Both dose levels of oestradiol delayed the time of onset of oestrus (P < 0.05) but did not affect its incidence. The higher dose resulted in a 7.4% reduction in the lambing percentage (P < 0.05). Dose of Cronolone (P < 0.01) and duration of insertion of sponges (P < 0.10) affected the time of onset of oestrus but did not affect the percentage of ewes which lambed. Two inseminations half a day apart increased the lambing percentage by 11 .4 %. The optimum treatment - use of sponges impregnated with 30 or 50 mg Cronolone alone (i.e. without oestrogen) followed by two inseminations of all ewes at fixed times relative to withdrawal of sponges - resulted in lambing of 61 .8 % of ewes which received such treatment. It is concluded that this is the best available estimate of the potential of this treatment when used on a large scale.