Effect of forage particle size and intake level on fermentation patterns and sites and extent of digestion in dairy cows fed mixed diets

The aim of this study was to examine the effects and possible interactions between forage particle size and intake level on the site and extent of digestion and on ruminal fermentations in dairy cows fed mixed diets. Two forage particle sizes (chopped [C] or ground and pelleted alfalfa [G]) and two intake levels (ad libitum [H] and 80% of ad libitum [L]) were combined in a 2x2 factorial arrangement. Four mid-lactating cows equipped with ruminal and duodenal cannulas were used in a 4x4 Latin square design. On a dry matter (DM) basis, the four diets consisted of 18% maize silage, 25% dehydrated pelleted whole-crop maize, 25% dehydrated alfalfa and 32% concentrate. They were distributed twice daily with three hours access after each distribution. Dry matter intake averaged 21.9 kg day(-1) for the H level. Feeding G rather than C diets reduced chewing time (22.1 vs. 27.7 min kg(-1) DM). Feeding G diets sharply reduced organic matter (OM) and cell-wall constituents digestibilities (P<0.01) with these effects being greater at H than at L intake (P<0.10). Starch digestibility and the site of digestion of OM and carbohydrate were unaffected, but non ammonia nitrogen flow into the duodenum was increased (P<0.05). Feeding G diets did not affect ruminal fermentation at the L intake level but the molar proportion of acetic acid was decreased and that of propionic acid was increased at the H intake level. When feeding GH diet, these changes in ruminal digestibility and fermentation patterns were associated with a reduced rumen retention time and lower cellulolytic activity which, in turn, were related to modifications in the ruminal environment. The range of pH during the day and hours below 5.6 were increased, whereas ruminal NH3 was decreased compared to the three others diets. When cows were fed at H level the digestibilities of OM and cell-wall constituents were depressed (P<0.01), along with the percentage of digestible materials disappeared in the rumen. Efficiency of microbial synthesis remained unaffected but the ruminal degradability of feed nitrogen tended to be lowered (P<0.10). The H level increased ruminal content but did not change ruminal retention time. Lower ruminal digestibilities were mainly related to a depressed rate of digestion at H level. It is concluded that reducing the forage particle size can strongly affect the quantity and the nature of the digestive endproducts by altering ruminal conditions. However, the magnitude of these digestive effects is dependent on the amount of fermentable OM ingested. (C) 1998 Elsevier Science B.V.