Effect of gender, gonadectomy and oestradiol-17 beta on growth in lambs under grazing conditions

To identify separate effects of gender, castration and exogenous oestrogen on growth, castrated lambs of both sexes and entire male lambs (n = 8) were implanted subcutaneously with three sizes of oestradiol-17 beta implants, or not implanted, and grazed on ryegrass and white clover pasture for 180 d ays. A group of non-implanted entire female lambs (n = 8) was run together with the others. Non-implanted entire male lambs grew faster, had heavier h eads, less internal, non-carcass fat and more protein and less fat and wate r in the carcass than non-implanted entire females. In addition, they had h igher 12th vertebral spine, thicker tibia, and heavier and larger humerus t han entire female lambs. Castration of male lambs reduced live-weight gain, weight of head and content of protein in the carcass whereas it increased carcass fat content. In addition, it caused lengthening of cannon bones and reduced height of 12th vertebral spine and length of tibia. In females, go nadectomy increased height of 12th vertebral spine and diameter to length r atio of the radius. Oestradiol treatment increased live-weight gain, reduce d total internal and carcass fat, and increased water and protein content o f the carcass in gonadectomized animals of either sex, and increased weight of carcass and head in spayed ewe lambs. Oestradiol treatment inhibited lo ngitudinal growth of cannon bones and stimulated that of vertebral column a nd ribs, but had little effect on the dimensions of limb bones apart from i ncreasing their diameter. Oestradiol treatment had no effect on muscle leng th but increased muscle girth and weight, except for m. splenius in ram lam bs where muscle weight was reduced. Effects of oestradiol on skeletal measu rements in most cases were linearly related to dose of oestradiol. It was c oncluded that the variable effects of sex steroids on the skeleton were rel ated to the differential pattern of skeletal maturation. In early maturing bones acceleration of the growth process by an exogenous sex steroid caused elongation to cease prematurely, whereas in late-maturing bones the accele ration effect on elongation did not result in premature cessation. This obs ervation may explain the often contradictory reports in the literature on t he effects of sex steroids on linear growth of bone.