Mammary growth in Holstein cows during the dry period: quantification of nucleic acids and histology.

Influence of the dry period on mammary growth was studied using multiparous Holstein cows. Sixty days before expected parturition, 13 cows were dried off, and another 13 cows were milked throughout the prepartum period. Lactating cows and dry cows were slaughtered at 53, 35, 20, and 7 d prepartum. Total mammary parenchymal DNA increased twofold from 53 to 7 d prepartum without influence of lactation status. However, overall rate of [3H]thymidine incorporation by mammary tissue was 80% greater in dry cows than in lactating cows, indicating that replacement of mammary cells was greater in dry cows. Of the mammary cells labeled with [3H]thymidine, the percentage of epithelial cells in dry cows was greater than that in lactating cows (96% vs. 86%). By 7 d prepartum, epithelial calls accounted for a greater percentage of total mammary cells in dry cows than in lactating cows (83% vs. 74%). Tissue area occupied by alveolar or ductular lumina decreased by 25 d into the dry period (35 d prepartum) and then increased to a maximum by 7 d prepartum. None of the mammary epithelial cells in dry cows were classified as secretory at 35 d prepartum, but 98% of the epithelial cells of dry cows were classified as secretory at 7 d prepartum. Results indicated that mammary involution did not occur during a typical dry period of dairy cows. Data suggest that a dry period is important for replacing senescent mammary epithelial cells and increasing the epithelial component of the gland prior to the next lactation.