Mammary cell population dynamics. Percentage of A. epithelial or B. stromal cells in mammary tissue of cows (n = 6) assigned to unilateral frequent milking (UFM; twice daily milking (2×) of the left udder half, four times daily milking (4×) of the right udder half) for days 1 to 21 of lactation. Mammary biopsies from both udder halves were obtained on d 21, 23, and 40 of lactation. Each bar represents Least Squares mean ± pooled standard error. The proportion of mammary epithelial and stromal cells was not affected by 4× milking or by day of lactation (P ≥ 0.40).