Growth of the Udders of Dairy Heifers as Measured by Milk Yield and Desoxyribonucleic Acid

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Abstract
In support of the theory that lobule-alveolar mammary gland growth was stimulated to varying degrees by recurring estrous cycles, yearling, 2-yr, and 3-yr-old nulliparous heifers were brought into lactation by the daily injection of 0.3 mg of estradiol benzoate/100 lb body weight for 14 days. The daily milk yield at the maximum indicates the extent of gland development present in these animals. Three yearlings produced a mean of 5.2 lb of milk; five 2-yr-olds, a mean of 7.7 lb of milk; and seven 3-yr-olds a mean of 15.0 lb of milk. A group of 3-yr-olds ovariectomized before lactation was induced to produce a mean of 5.8 lb of milk. Decreasing levels of progesterone and increasing levels of estradiol benzoate were then administered for 9 wk and lactation reinitiated. The yearlings then yielded a mean maximum of 12.8 lb per day, a 146% increase, the 2-yr-olds yielded 19.5 lb per day, a 153% increase, and the ovariectomized heifers yielded 18.2 lb, a 198% increase. The 3-yr-old heifers were sacrificed and the two halves of the udders were dried and fat extracted by two methods before determining the total DNA by halves. The right half of a normal and of two ovariectomized heifers were surgically removed and total DNA determined. Six months later the left halves were removed and total DNA determined. The DNA of the left half of the normal animal increased 21%, that of the ovariectomized animal without treatment, 4%, and the animal injected with ovarian hormones 222% over the right halves.