Lactation Biology II

The risk of acquiring a new intramammary infection during the dry period increases with milk production at drying-off and decreases as mammary gland involution progresses. A method commonly used to reduce milk production is a drastic reduction in feed supply in the days that precede drying-off. Our recent work has shown that milk production can also be reduced by an inhibition of the lactogenic signal driven by prolactin (PRL). To compare these 2 drying-off procedures, 24 Holstein cows in late lactation were assigned to 3 treatments based on milk yield, SCC and parity. They were fed 1) a lactation diet until drying-off (Control), 2) only dry hay the last 5 d before drying-off (Hay), or 3) as Control cows, but received twice daily i.m. injection of quinagolide (4 mg per injection), a specific inhibitor of PRL-release, from 5 d before drying-off to 13 d after (Quin). Cows from Control and Hay treatments received water injections. Blood and mammary secretion samples were collected on the last 7 d before and 1, 3, 5, 7, 10, and 14 d after the last milking. Quinagolide induced a sharp decrease (P < 0.001) in basal serum PRL concentrations on all the injection days. Interestingly, PRL was also depressed in Hay cows before drying-off. Both Quin and Hay treatments induced a drop (P < 0.001) in milk production, which averaged 17.9 and 10.1 kg/d, respectively, at drying-off compared with 24.8 kg/d for Control cows. Feeding dry hay decreased blood concentrations of glucose (P < 0.001) and most aminoacids (12 / 19; P < 0.05), and increased blood concentrations of β-hydoxybutyrate and Nonesterified fatty acids significantly (P < 0.001), whereas quinagolide did not affect these metabolites. Mammary secretion SCC was greater at d 1 of the dry period in the Hay (P < 0.001) and at d 5 in the Quin (P < 0.05) than in the Control cows. The BSA concentration increased faster (P < 0.05) in the mammary secretion of both Hay and Quin than that of Control cows, whereas the citrate:lactoferrin ratio, another indicator of involution rate, decreased faster (P < 0.001). In conclusion, this experiment shows that PRL-release inhibition could be a new alternative to reduce milk production before the drying-off and to hasten mammary gland involution without disturbing the metabolism of the cow. This strategy may reduced the incidence of new intramammary infection at drying-off.