Effect of Gestation and Dry Period on Deoxyribonucleic Acid and Alveolar Characteristics of Bovine Mammary Glands

Udders of 21 lactating and 26 dry cows were analyzed for deoxyribonucleic acid (DNA) concentration and amount. Right and left halves of 21 cows were dried off at different times to give comparisons between dry periods of 15, 45, 60, and 75 days. Udders of five dry cows with dry periods of 8 and 48 days for right and left halves, respectively, were obtained about a week before calving. Histological sections of 26 udders were used to record alveolar density and surface area. The correlation coefficient between total DNA and milk yield per half udder (r = 0.34) was small but statistically significant (P < .05). Concentration of DNA and alveoli increased in dry udders compared to prepartum and lactating udders. Involution of udders of nonpregnant or early gestation cows was evidenced by reductions in fresh weight, dry weight, alveolar surface area, and DNA. In prepartum half udders, six weeks' difference in dry period did not significantly affect dry weight, DNA, or alveolar concentration. Lack of interference with either involution or alveolar development in the udder due to short dry periods indicates that subsequent subnormal lactation may be due to subnormal function rather than structure.