Effects of type of carbohydrate supplementation to lush pasture on microbial fermentation in continuous culture

Eight single-flow continuous culture fermenters were used to study the effe cts of the type of energy source on ruminal N utilization from high quality pasture. The four dietary treatments included high quality grass and legum e pasture alone (50:50; wt/wt), pasture plus soybean hulls, pasture plus be et pulp, and pasture plus corn. Diets supplemented with additional sources of energy (soybean hulls, beet pulp, and corn) were isocaloric but differed in the type and rate of carbohydrate fermentation. Energy supplements cons tituted 45% of the total dietary dry matter and were fed twice daily at 12- h intervals in place of pasture, which is characteristic of grain feeding a t milking when animals are in a grazing situation. Energy supplementation r educed pH, NH3 N now, and NH3 N concentration and increased bacterial N flo w (as a percentage of N intake). The supplementation of corn and soybean hu lls resulted in the highest microbial N flow (as a percentage of N intake). Corn had a tendency to reduce fiber digestion because of excessively low N H3 N concentrations. Beet pulp was similar to corn in that it decreased NH3 N concentrations. Supplementation of soybean hulls resulted in a more sync hronized fermentation, greater volatile fatty acid production, and greater fiber digestion. Nitrogen utilization by microbes was maximized by suppleme ntation with soybean hulls or corn twice a day. With diets based on pasture , it may be more important to improve bacterial N flow and bacterial utiliz ation of N than to maximize the efficiency of bacterial protein synthesis b ecause better utilization of N by ruminal microorganisms results in higher bacterial N flow and higher fiber digestion.