Alteration of the nutrient uptake by the udder over an extended milking interval in dairy cows

Little is known about modifications of the mammary utilization of nutrients circulating in blood plasma when milk yield is strongly decreased by once-daily milking. A trial was carried out to describe the mammary nutritional adjustments linked to the downregulation of milk synthesis as milk accumulated over an extended milking interval in the bovine udder. Three Holstein dairy cows yielding 34.0 kg/d of milk were fitted with an ultrasound flow probe around the left external pudic artery and with catheters inserted into the left carotid and milk vein to estimate mammary blood flow (MBF) and mammary uptake of acetate, β-hydroxybutyrate, nonesterified fatty acids, glycerol, glucose, O₂, and CO₂ release. The trial was carried out over 2 consecutive weeks, with wk 2 repeating wk 1. Cows were milked twice daily at 12-h milking intervals. On d 3, cows were milked at 0630 h and were not milked for 36 h until d 4 at 1830 h. Over the following days, twice-daily milking was resumed using 12-h milking intervals. Each half-udder was milked separately. Secretion rates of milk and milk proteins decreased 67% during the 12-to-36-h interval of milk accumulation, whereas that of milk fat fell 30%. Timing of changes in MBF and lactose levels in blood plasma was concomitant and significant after 19.5 and 21.5 h of milk accumulation in the udder, respectively. The MBF decreased, most likely because the usual increases in MBF no longer occurred when the udder was full of milk. After 24 h of milk accumulation, MBF did not increase further when cows lay down, and did not increase as usual 3 h after a meal, suggesting a possible physical effect of milk accumulated in the udder on MBF, complementing metabolic regulation. Mammary uptake or release of nutrients was lowered before 24 h for glucose, acetate, and β-hydroxybutyrate and after 24 h for total glycerol, O₂, and CO₂, mostly associated with the impaired MBF. However, these decreases ranged from 12 to 17%, and cannot entirely explain the −45 and −20% decreases in milk secretion rates observed during the entire 36 h of milk accumulation, thus confirming the primary role of intramammary metabolic regulation in the downregulation of milk secretion. The larger amount of nutrients taken up by the udder could explain the enhanced milk fat levels, involving a strongly modified metabolic fate of nutrients.