Relationships between plasma hormone concentrations, udder development and the production of early mammary secretions in twin-bearing ewes on different planes of nutrition.

Ewes carrying twin fetuses were maintained during late pregnancy on a uniformly high plane (well fed), a uniformly low plane (underfed) or a low plane rising to a high plane (refed) of nutrition. The maternal plasma concentrations of hormones concerned with udder development, colostrum production and milk secretion were measured in samples obtained during the last 32 d before and the first 18 h after birth. Udder weights were derived from their linear dimensions, the yields of mammary secretions by hand milking during the first 18 h after birth and the transition from colostrum to milk was indicated by changes in the concentrations of lactose, lipid and immunoglobulin G. Underfeeding reduced prenatal udder development and colostrum accumulation and delayed the post-natal transition to milk secretion. Refeeding previously underfed ewes to a high plane during the last 5 d before birth had no effect on udder growth, but it did return the secretory functions of udder tissue to the levels observed in well-fed ewes. Nutritional effects on the plasma concentrations of insulin, placental lactogen, prolactin, cortisol and oestradiol-17 beta were small or absent. The relative changes in the plasma concentrations of growth hormone and insulin were such that the growth hormone: insulin ratio increased markedly during the last 11 d before birth in the underfed and refed ewes but not in the well-fed ewes. These differences in the growth hormone: insulin ratio were associated with greater increases in udder growth rate in the underfed and refed ewes than in the well-fed ewes. The plasma concentrations of progesterone decreased before birth in all ewes, but the decrease was delayed by underfeeding and refeeding rapidly overcame this effect. The slower progesterone withdrawal in the underfed ewes was associated with a delay in lactogenesis. It is concluded that the nutritionally induced changes in the growth hormone: insulin ratio and in the progesterone concentrations were the most likely endocrine determinants of the observed differences in the rates of mammogenesis and lactogenesis, respectively, but possible independent effects of substrate supply to the udder remain to be clarified.