Effects of feeding beets and ensiled beets on rumen volatile fatty acid and ethanol metabolism in dairy cows

Ensiling beets allows for year-round feeding, and the high sugar content results in the production of ethanol-rich silage. This study aimed to investigate the effects of feeding sugar- and ethanol-rich beet diets to dairy cows on rumen ethanol metabolism and absorption. Beets were washed, crushed, and stored in daily portions either frozen at -20°C or ensiled in barrels. Four rumen-cannulated Danish Holstein cows were used in a partially replicated 3 × 3 Latin square design with 21-d periods. The control diet (CON) was based on grass-clover silage, corn silage, barley, soybean meal, dried beet pulp, urea, and minerals. For the sugar-rich (SUG) and ethanol-rich (ETH) diets, 20% of DM from CON was replaced with either beets or ensiled beets. To assess the transfer of carbon between ethanol and VFA, including acetate, propionate, butyrate, isobutyrate, and isovalerate, [¹³C]-labeled isotopes of these compounds were individually infused into the rumen for 5 h each. A dynamic 6-pool rumen model was fit to measured isotopic enrichments to estimate synthesis, absorption, and interconversions for ethanol and VFA. The basal ethanol synthesis level for CON was 325 mmol/h. Feeding ETH maintained this level, whereas SUG resulted in a slight decrease in ruminal ethanol synthesis. From the total influx (intake + production) of ethanol, 63.1% was metabolized to acetate and 22.2% was absorbed for CON, 57.2% was metabolized to acetate and 37.5% was absorbed for SUG, and 43.7% was metabolized to acetate and 45.5% was absorbed for ETH. Furthermore, 85.3% of the additional ethanol from the beet silage diet was absorbed. Especially due to the high basal ethanol synthesis, the addition of the rumen pools of ethanol, isobutyrate, and isovalerate influenced model estimates for acetate, propionate, and butyrate. For CON, excluding these pools led to overestimations of synthesis for the included pools by 7.8%, 10.4%, and 12.8%, respectively.