Studies of flock mating of sheep. 6. Influence of age, hormone treatment, shearing and diet on the libido of Merino rams

Abstract

Libido, as assessed from the number of services performed in a series of pen libido tests, was not significantly increased in 2 1/2 -3-year-old Merino rams by shearing, by testosterone treatment (3-1 0 mg day-1 for 15 days or 50 mg day-1 for 18 days), by Human Chorionic Gonadotrophin (500 i.u. day-1 for 18 days) or a high protein diet fed for 5 1/2 weeks. In contrast, undernutrition, which led to a bodyweight loss of 0.5-0.6 kg per week, caused a decline in libido within five weeks. Injections of p-chlorophenylalanine (0.5-1 g day-1 for seven days) did not induce sexual activity in two 2-year-old rams that had shown no sexual interest in oestrous ewes in a series of tests during the previous seven months. In each of two groups of rams (n = 27; n = 30), the libido of a ram relative to that of his contemporaries changed very little over long periods (up to two years). Mating dexterity, as assessed from the ratio of mounts to services, was more variable. The behaviour and mating performance of individual rams within 11 pairs of rams joined (in the same combinations) with flocks of ewes in two successive years was investigated. In 4 of 9 pairs in which the dominance order was clearly defined in both years, the order was reversed between years. However, there was little change between years in either the relative service activity or the number of ewes marked by an individual ram relative to the total number of ewes marked.