The onset of lactation in the Merino ewe and its modification by nutritional factors

Abstract

The results of five series of observations on the type of udder content's and rate of milk secretion in the Merino ewe at about the time of parturition are presented. In general the type of secretion underwent a sequence of changes from a slowly secreted, very viscous material to the rapidly secreted "normal colostrum" and normal milk. It is suggested that the very viscous material should not be described as "colostrum" even when it appears at parturition, and that "colostrum" is a mixture of normal milk with the pre-existing mammary secretions. In ewes fed to gain weight during pregnancy the viscous material had usually been replaced by later-appearing types at parturition; copious lactation usually commenced at parturition or some hours earlier. In ewes fed to lose weight during the last 2-6 weeks of pregnancy, the changes were delayed; prior to parturition the incidence of expressible secretion was lower than in well-fed ewes; the viscous material, or no secretion at all, was often present at parturition, and copious lactation was often not apparent until up to 12 hr after parturition. The delay in onset was not associated with the shorter gestation period of the poorly fed ewes. After the onset of copious lactation, the rate of secretion by poorly fed ewes was about half that of ewes fed to gain or maintain weight during pregnancy. That the Merino is a relatively poor producer of milk even when well fed is indicated by comparison with published data for other breeds. It is concluded that the delayed onset of lactation and reduced rate of production of milk by poorly fed ewes would substantially reduce the chance of survival of lambs in early life.