Evaluation of physical structure 1. Grass silage

Physical structure of 19 grass silages was investigated by measuring chewing activity with a group of eight cows for 4 d. Eating, ruminating, and total chewing indexes varied considerably among cows. Silages were made from perennial ryegrass different in growth stage at harvest, chopping lengths, and preservation methods. Eating index varied from 17.2 to 35.0 min/kg, ruminating index from 39.7 to 61.9 min/kg, and total chewing index from 58.5 to 96.9 min/kg of DMI. Chewing indexes increased from the vegetative to the generative growth stage of grass silages and decreased, not always significantly, when chopped to 24 mm. Compared with wilted silages, direct-cut silages had higher eating and total chewing indexes because of lower. palatability and higher water content, but ruminating index was only slightly affected by preservation method. Chemical (DM, crude fiber, NDF, ADF, lignin, sugars) and physical parameters (median particle size, milling resistance, density) and in vitro digestibility were used to derive equations to predict ruminating and total chewing index. Crude fiber explained 82 and 75% of the variation, respectively, in ruminating and total chewing indexes. The supplemental contribution of physical characteristics was only marginal.