Local regulation of increased milk yield due to early lactation increased milking frequency

Increased milking frequency (IMF) during early lactation increases milk yield not only during the IMF period, but also after the cow is returned to twice daily milking (2X). The increase in yield is locally regulated within the gland; however the mechanism for the increase in yield is unknown. The objective of this study was to demonstrate a difference in milk and component yield, both during the IMF period and throughout the remainder of lactation, and examine potential local mechanisms driving the increase in production. Eight multiparous dairy cows were assigned to unilateral frequent milking [UFM ; 2X left udder half and 4-times-daily milking (4X) right udder half] for the first 21 days of lactation. Both udder halves were milked 2X for the remainder of lactation. Early lactation IMF significantly increased milk, fat, and protein yields in the right udder halves by 2.27 kg/d, 73.5 g/d, and 68 g/d respectively through the first 210 DIM (P < 0.001). At d 21, the right udder halves had a significant increase in activated signal transducer and activator of transcription 5 (STAT5), as well as a reduction in activated Akt (P ≤ 0.05). There was no difference in STAT3 expression at d 21. There was no significant difference in gene expression of prolactin, insulin-like growth factor 1 (IGF-1), insulin-like growth factor binding protein 5 (IGFPB5), or chitinase 3-like-1 (CHI3L1) in mammary tissue at d 21 or 60; and no difference in protein expression of STAT5, Akt, or STAT3 in mammary tissue at d 60.