Effect of nonstructural to structural carbohydrate ratio on rate and extent of nutrient utilization in situ

The objective was to relate differences in the chemical composition of the cell soluble and cell wall fractions and relate this to rate and extent of nutrient utilization by rumen microorganisms when diets were formulated to vary in nonstructural carbohydrate (NSC) to structural carbohydrate (SC) ratio. Nine diets were formulated to be similar in crude protein, acid detergent fiber, net energy for lactation, soluble protein, calcium and phosphorous. The diets, formulated from 16 different forage and grain sources, varied in NSC:SC ratio from 0.7 to 2.5, resulting in forage:concentrate ratios varying from 40:60 to 80:20. Diets were weighed into duplicate nylon bags and incubated in the rumen of one of two lactating cows for 2, 4, 8, 12, 18, 24, 48 and 100 h. No relationship was observed between NSC:SC ratio and rate or extent of dry matter, neutral detergent fiber or total nonstructural carbohydrate disappearance. However, in all cases > 95% of the variation in rate and extent of nutrient disappearance could be explained by the proportion of sugars (arabinose, xylose, glucose) in diet dry matter or cell wall. Composition of the sugars in the carbohydrate fraction of individual feedstuffs or total mixed diets may provide useful information when predicting nutrient utilization by ruminal microorganisms.