Relationships between nutrition, puberty and mammary development in cattle

Production of colostrum and milk in sufficient amounts is essential for survival of the young in most mammals.Thus, lactation is an integral part of the reproductive cycle and, in line with its role in reproduction, mammary development occurs in distinct phases related to reproductive development during fetal life, puberty, pregnancy, and lactation.The main developments, quantitatively as well as qualitatively, occur during late pregnancy, when mammary ducts develop into lobulo-alveolar tissue with differentiated cells capable of producing milk with all its components.It is the number of mammary cells in the mammary glands that largely determines the lactation potential of the mother.In cattle, lactation has an added significance in view of the important role of cow's milk in human nutrition.Mammary development in cattle largely follows the same pattern as in other mammals, but mammary cell growth is believed to end at parturition.Therefore the milk yield potential of cows is determined, to a large extent, by the growth of the mammary glands around puberty and during pregnancy.The extent of mammary gland development during puberty is much less pronounced than during pregnancy, but it has become clear that mammary development in this period is critical for the future milk yield potential of heifers (Sejrsen, 1978; Foldager & Sejrsen, 1987; Johnsson, 1988; Troccon & Petit, 1989; Waldo et af.1989).One of the important factors influencing pubertal mammary growth is nutrition.Given this background it is the objective of the present paper to provide an overview of the relationships between nutrition, puberty and mammary development in heifers. PUBERTAL MAMMARY DEVELOPMENTThe basic structures of the mammary glands are formed in fetal life, but the epithelial tissue is still rudimentary at birth.There are a few mammary duct cells present adjacent to the gland cistern, but there are no alveoli.In contrast, the non-epithelial tissues, i.e. the stroma and the circulatory system, are almost fully developed at birth; so is the outer shape of the glands.In the first few months after birth the glands grow isometrically at the same rate as the rest of the body; only the non-epithelial tissues grow in this period.At 2-3 months of age, well in advance of the onset of puberty, the glands start to grow at a faster rate than the rest of the body.The growth becomes allometric (Sinha & Tucker, 1969).In this phase there is rapid growth of the fat pad, and of the ducts that branch into it.Alveoli are still unformed.The ducts need the fat pad for their growth (Faulkin & De Ome, 1960), and in some species ductular growth may be limited by the size of the fat pad.This is unlikely to be the case in heifers, since the tissue with the branching ducts, the parenchyma, only constitutes about one-third of the total gland at this stage of development (Sejrsen & Foldager, 1992).In contrast to the situation in rodents, the ducts in heifers always seem to be surrounded by connective tissue (see Akers, 1990).