Increasing the physically effective fiber content of dairy cow diets may lower efficiency of feed use.

Barley silages varying in theoretical chop length were used to evaluate the effects of physically effective (pe) neutral detergent fiber (NDF) content of dairy cow diets on nutrient intakes, site and extent of digestion, microbial protein synthesis, and milk production. The experiment was designed as a replicated 3 x 3 Latin square using 6 lactating dairy cows with ruminal and duodenal cannulas. During each of 3 periods, cows were offered 1 of 3 diets (low, medium, and high peNDF) obtained using barley silage that varied in particle length: fine (theoretical chop length of 4.8 mm), medium (equal proportions of long and fine silages), and long (theoretical chop length of 9.5 mm). The peNDF contents were determined by multiplying the proportion (dry matter basis) of feed retained on the 2 screens (8 and 19 mm) of the Penn State Particle Separator by the NDF content of the diet, and were 10.5, 11.8, and 13.8% for the low, medium, and high diets, respectively. Increased forage particle length linearly increased intake of peNDF but intakes of dry matter, organic matter, starch, and N were highest for cows fed the medium peNDF diet. Digestibilities of organic matter, NDF, and acid detergent fiber in the total tract were linearly decreased with increasing dietary peNDF, although total digestibility of starch and N was not affected by the treatments. Nevertheless, decreased digestibility due to increased dietary peNDF did not reduce milk production or milk composition because the cows were in mid to late lactation. Ruminal microbial protein synthesis and microbial efficiency were numerically higher with the low peNDF than with the medium or high peNDF diets. These results indicate that increasing the peNDF content of a diet containing barley silage decreases fiber digestibility in the total tract and lowers microbial efficiency. Therefore, the benefits of increasing dietary particle size, expressed as peNDF, on reducing the risk of ruminal acidosis should be weighed against potentially negative effects on efficiency of feed use.