Effect of milking frequency on milk somatic cell count characteristics and mammary secretory cell damage in cows

OBJECTIVES: To examine whether once daily milking (ODM) increases somatic cell count (SCC) and causes changes in the proportion of macrophages, lymphocytes, or neutrophils in milk, and to examine whether ODM causes mammary secretory cell damage. DESIGN: During 3 successive periods, cows were milked twice daily (days 1 through 4), once daily (days 5 through 10), and twice daily (days 11 through 14); thus, each cow served as its own control. Milk samples were collected during each milking for SCC, bovine serum albumin (BSA; except days 1 and 14), and N-acetyl-beta-D-glucosaminidase (NAGase) activity analyses. Differential cell counts were determined on days 2 (period 1), 5 and 8 (period 2), and 11 (period 3) in morning milk samples. ANIMALS: 8 multiparous Jersey cows. PROCEDURES: Cell counter (SCC), staining and counting (differential SCC), ELISA (BSA), and enzyme activity (NAGase). RESULTS: Once daily milking increased SCC (total and concentration), percentage of neutrophils, and BSA (content and total mass); during subsequent twice daily milking, SCC decreased, but BSA content and percentage of neutrophils remained increased. Activity of NAGase was not affected by ODM. CONCLUSIONS: ODM increases SCC, but this is not associated with damage to mammary secretory cells. The ODM-related sustained increase in BSA content coincided with an increase in neutrophil numbers, suggesting a causal relation.