Effects of decreased availability of glucose for dairy cows.

After a preliminary trial to establish dose responses to phlorizin, five Holstein cows at 6 wk postpartum were used to test the response of cows in negative energy balance to a sudden decrease in availability of glucose caused by phlorizin. Cows were fed equal amounts of feed twice daily to supply 100% of NRC recommendations for protein and 90% of NRC recommendations for NEL and were in negative energy balance throughout the experiment. Phlorizin at 0, 2, and 4 g/d was injected subcutaneously in equal amounts every 6 h for 48 h and caused excretion of 0, 225, and 337 g/d of glucose in urine. Milk production was not decreased, but percentage of milk fat increased linearly (3.34, 3.56, and 3.70%) with increasing phlorizin. Concentrations of glucose (64.2, 62.6, and 59.4 mg/dl) and insulin (518, 432, and 329 pg/ml) in blood plasma decreased linearly, whereas beta-hydroxybutyrate (6.11, 8.88, and 9.98 mg/dl) and NEFA (181, 220, and 271 mu eq/L) increased linearly. The changes were most dramatic during the final 12 h of the 48-h injection interval. Healthy, early lactation cows in negative energy balance seem to have the capacity to make metabolic adjustments to provide adequate substrates for maintenance and milk synthesis and to compensate for short-term increased needs for glucose.