Effects of monensin on in vivo rumen propionate production and blood glucose kinetics in cattle.

Four rumen-fistulated steers (154 to 253 kg), fed two different diets in succession, were used to determine effects of monensin on rumen propionate production rates and blood glucose kinetics as determined by single-injection isotope-dilution techniques. A high-roughage and a high-grain diet, with and without 150 mg of monensin daily, were fed isoenergetically at 2-hour intervals. Monensin increased rumen propionate pool sizes from 32 to 57 g for the high-roughage diet and from 37 to 66 g for the high-grain diet and increased rumen propionate production rates from 441 to 659 g/day for the high-roughage diet and from 510 to 899 g/day for the high-grain diet. Molar percentages of rumen propionate were increased significantly by monensin in the high-grain diet. Blood glucose pool sizes were not changed significantly by either monensin or isoenergetic diets. Monensin increased irreversible losses of glucose from 582 to 677 g/day for the high-grain diet. Monensin tended to increase glucose total entry rates for both diets and to increase irreversible loss of glucose for the high-roughage diet but the differences were not significant. Thus, increases in glucose kinetics are minor in contrast to major increases of rumen propionate production caused by monensin.

Rumen propionate and blood glucose kinetics in growing cattle fed isoenergetic diets.

The relationship between rumen propionate production and blood glucose kinetics was examined in four rumen-fistulated Holstein steers fed isoenergetic amounts of 80/20 (G) and 30/70 (R) grain/chopped alfalfa hay diets at 2-hour intervals. Single-injection rumen propionate and blood glucose kinetics were determined in consecutive 4-hour periods by using [6-3H]glucose intravenously then [1-14C]propionate intraruminally. Rumen propionate specific activity was determined after isolation and quantitation by high-pressure liquid chromatography. Average rumen propionate production rates and pool sizes were 441 g/day and 32.0 g when diet R was fed but increased to 510 g/day (P less than 0.05) and 36.5 g (P less than 0.10), respectively, when diet G was fed. Propionate production as related to digestible energy (DE) intake, averaged 0.56 mole/Mcal DE for R and 0.64 mole/Mcal DE for G. Rumen propionate turnover times were similar for both diets. Despite differences in propionate availability, there were no significant dietary differences in glucose kinetic parameters. Average glucose pool sizes and irreversible losses were 27.0 g and 585 g/day for R and 27.0 g and 582 g/day for G.