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Endocrine Factors Influencing the Intensity of Milk Secretion. A. Estrogen, Thyroxine, and Growth Hormone

1957; Elsevier BV; Volume: 40; Issue: 1 Linguagem: Inglês

10.3168/jds.s0022-0302(57)94438-7

1529-9066

C. W. Turner, H. Yamamoto, H.L. Ruppert,

Milk Quality and Mastitis in Dairy Cows

The total yield of a dairy cow during a lactation period is dependent on the total number of epithelial cells that are stimulated to growth during the first two-thirds of pregnancy and the initiation and maintenance of the intensity of milk synthesis in each of these cells.The role of three hormones-estrogen, thyroxine, and a growth hormone--in the regulation •of the intensity of secretion during the normal declining segment of the lactation is described.Editor.After parturition, the secretion of milk increases for a period of 1 to 2 months, reaches a peak or maximum rate of production, then declines gradually.The rate of decline in individual cows varies greatly.Some cows are said to be very persistent, whereas others decline rather rapidly.Gaines (12) defined persistency as the degree with which the rate of secretion is maintained as lactation ad,,•ances.An exponential eqnation has been found to accuratel~' describe the rate of decline of milk secretion of nonpregnant cows (3).It was shown further that a superabundance of feed is stored as depot fat and does not appreciably affect the characteristic course of the declining curve (17).On the other hand, underfeeding and several environmental, managerial and physiological factors, including pregnancy (4), may decrease persistency.Under uniform conditions, it is believed that the control of persistency of milk secretion is primarily dependent upon the rate of secretion of certain hormones that can influence the intensity of milk secretion in the individual cells of the udder.A second factor of importance would then be the gradual involution and loss of epithelial cells with the advance of lactation.The total yield of milk of a dairy cow during a lactation period is dependent upon the total number of epithelial cells, which are stimulated to growth during the first two-thirds of pregnancy, and the initiation and maintenance of the intensity of mill;: synthesis in each of these cells.In the preceding paper were described the endocrine factors which induced the growth of the mammary gland of sterile heifers and initiated the secretion of