|Akers, R - VIRGINIA POLYTECHNIC & SU|
|Smith, J - DUKE UNIVERSITY|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 25, 1996
Publication Date: N/A
Interpretive Summary: For optimal milk production, dairy cows must have a nonlactating period of approximately 60 days prior to calving and the initiation of the next lactation. Despite the importance of this "dry period", little is known about processes of mammary growth during the nonlactating interval. Mammary growth was investigated by combining half-udder measures of total DNA (cell number) with histological evaluation and measures of rates of cell replication. Contrary to popular perception, results indicate that there is no appreciable loss of mammary secretory cells (mammary regression) during the early phase of the dry period. During a 60 day dry period, the number of mammary secretory cells doubles. Comparison with lactating controls indicated that cows enter the next lactation with the same number of mammary epithelial cells whether or not they have had a dry period. Nevertheless, during the dry period, turnover of mammary secretory cells is approximately 70% greater than that occurring in lactating cows during the same stage of gestation. Results suggest that the dry period is important for replacing senescent or damaged secretory cells prior to calving and establishment of the next lactation.
Technical Abstract: Influence of the dry period on mammary growth was studied in multiparous Holstein cows. Sixty days prior to expected parturition, 13 cows were dried off and an additional 13 cows were milked twice daily during the prepartum period. Lactating and dry cows were slaughtered at 53, 35, 20 and 7 d prepartum. Total mammary parenchymal DNA increased two-fold from 53 to 7 d prepartum, without influence of lactation status. However, overall rate of [3H]thymidine incorporation by mammary tissue was 50 to 80% greater in dry cows than in lactating cows, indicating that cell replacement was greater in mammary tissue of dry cows. Autoradiography indicated that 84% of cells incorporating [3H]thymidine were epithelial. Quantitative histology indicated that the epithelial content of tissue did not differ between dry and lactating cows. Tissue area occupied by alveolar or ductular lumena decreased by 25 d into the dry period (35 d prepartum) and then increased to a maximum by 7 d prepartum. Essentially all epithelial cells in mammary glands of dry cows were classified as nonsecretory at 35 d prepartum, but only 8% of epithelial cells in dry cows were classified as nonsecretory at 7 d prepartum, compared with 34% in lactating cows. Incorporation of [3H]uridine by mammary tissue of dry cows was maximal at 20 d prepartum. Results indicate that mammary involution does not occur during a typical dry period in dairy cows. Furthermore, data are consistent with the hypothesis that the dry period is important for replacement of senescent mammary epithelial cells prior to onset of the next lactation.