Contribution of Propionate to Glucose Synthesis in the Lactating and Nonlactating Cow

Abstract

Ruminal volatile fatty acid production rates and blood glucose entry rate were measured in two cows, nonlactating and again lactating, with a constant infusion isotope dilution technique. Incorporation of ruminal acetate, propionate, and buty rate carbon into blood glucose was determined, and the contribution of propionate to glucose synthesis calculated.When the cows were nonlactating and consuming 9.5kg dry matter of a high grain ration, acetate and butyrate production averaged 42 and 8 moles per day. Because of an infusion error, propionate production could not be calculated, but was estimated to be 12 moles per day. When cows were lactating and consuming 17.0kg dry matter of the same ration, acetate and butyrate production averaged 77 and 6 moles per day. Propionate production was estimated at 25 moles per day. Total ruminal volatile acid production per kilogram ration dry matter, including the estimated propionate production, was 6.4 and 6.7 moles during the lactating and nonlactating phases. Average blood glucose entry rates during the lactating and nonlactating phases were 14.0 and 7.6 moles per day (∼0.81 and 0.44g/hr/kg body weight3/4).Propionate contributed to a minimum of 32% of the blood glucose entry rate in the nonlactating phase and 45% in the lactating phase. These estimates are minimum values, based upon the transfer of propionate-2-14C carbon to glucose. A maximum estimate, based upon the extent of carbon cycling in the citric acid cycle, suggested that propionate contributed to not more than 60% of the glucose formed during the lactating phase. The amount of cycling in the citric acid cycle was calculated from the distribution of radio-activity in glucose derived from propionate-2-14C.