Three studies were conducted to investigate the main effects and interactions of forage particle size (PS) and the level of neutral detergent soluble fiber (NDSF) in the diet on performance, digestive processes, and chewing activity in lactating dairy cows and on feed degradability and ruminal variables in sheep. In experiment 1, eight multiparous Holstein cows in early lactation (66 ± 5 days in milk), and in experiment 2, eight multiparous Holstein cows in mid-lactation (146 ± 6 days in milk) were used. Each experiment was a replicated 4 × 4 Latin square design including 2 levels of NDSF each combined with 2 PS (short = 20 mm or long = 40 mm) of alfalfa hay. In experiment 1, short forage PS, alone or in combination with low level of NDSF, increased DM intake, and increased intake of physically effective neural detergent fiber (peNDF) 1.18 mm (P 0.05). Eating time and eating per unit of DM, NDF and peNDF 1.18 mm decreased in short-PS diets (P 0.05). Feeding diets with high NDSF increased digestibility of DM and NDF (P 0.05), decreased the proportion of propionate and concentration of ruminal ammonia and increased the proportion of butyrate (P 0.05). Feeding diets with short PS decreased molar proportion of acetate and increased molar proportion of propionate (P 0.05). Runimal pH was similar among treatments. Milk production and composition were not affected by dietary treatments. In experiment 2, forage PS alone, or in combination with NDSF inclusion, had no effect on daily DM intake. Eating time per kg of NDF ingested was higher in long versus short alfalfa hay-based diets (P 0.05). Feeding diets high in NDSF lowered DM intake (P 0.05), but increased apparent digestibility of all nutrients including NDF (P 0.05). Ruminal pH and concentrations of VFA were unaffected; however, the proportion of butyrate was higher in high NDSF-diets (P 0.05). Changes in milk composition included lower milk protein content in high NDSF-diets and higher lactose content for short hay-based diets (P 0.05). Results of experiments 1 and 2 suggest that NDSF sources can be successfully included to partly replace starchy grains in diets exceeding minimum fiber recommendations. The in situ experiment was conducted using four male sheep with the two carbohydrate sources and the two alfalfa PS. Degradation rate constant of alfalfa dry matter was higher (P 0.05), and disappearance of alfalfa NDF at 4 h of incubation (P = 0.06) and mean ruminal pH (5.76 versus 5.86, P 0.01) were lower in diets Key words: Forage particle size, Neutral detergent soluble fiber, Ruminant, Performance, Digestibility, Chewing activity
