Milk yield and mammary growth effects due to increased milking frequency during early lactation.

Increased milking frequency (IMF) at the beginning of lactation has been shown to increase milk yield not only during IMF but also after its cessation. The objectives of this experiment evaluated the effects of increased milking frequency initiated during early lactation on mammary growth and effects on milk yield (MY). Thirty-one cows were divided into treatment groups: 1) 2X: cows milked twice daily (2X) beginning at parturition (d 1), 2) IMF1: cows milked four times daily (4X) from d 1 to 21 postpartum (PP) and 3) IMF4: cows milked 2X d 1 to 3 and 4X d 4 to 21 PP. The 4X cows were milked immediately before 2X cows and again approximately 3 h later, at the end of the normal milking routine. All cows were milked 2X from d 21 to 305 postpartum. Milk yields were 34.5, 37.8 and 37.6 kg/d during wk 1 to 44 for 2X, IMF1 and IMF4, respectively. Mammary biopsies from four cows per treatment were obtained on d 7 and 14 PP to evaluate mammary cell proliferation. Tritiated-thymidine incorporation tended to increase on d 7 in IMF1 cows, and arithmetic means of the percentage of cells expressing Ki-67 proliferation antigen were consistent with a proliferative response to IMF though not significant. Blood was sampled three times per wk during the first 2 wk and then once per wk during wk 3, 4, 5, 6, 8, and 10. Plasma insulin-like growth factor-1 (IGF-1) averaged 20.1 ng/ml in IMF cows vs. 24.2 in 2X but was not accompanied by a change in bST. Prolactin was also not affected by treatment. Neither milk yield nor potential effects on mammary cell proliferation were correlated with systemic IGF-1. Implementing an IMF routine increases MY during treatment and elicits a carryover effect on the remainder of lactation. Milk yield responses after an IMF routine may be the result of increased mammary cell proliferation.