Effect of forage particle length on rumen fermentation, sorting and chewing activity of late-lactation and non-lactating dairy cows

The objective of this study was to determine the effects of varying forage particle length on chewing activity, sorting behavior, rumen pH and rumen fill in late lactation and dry dairy cattle, fed rations with similar physically effective NDF but different mean particle length. Treatments consisted of three diets differing only in geometric mean length of forage: hay (5.40, 8.96 and 77.90 mm, for short (S), medium (M) and long (L) diets, respectively) for Experiment 1 (E1), and straw (10.16, 24.68 and 80.37 mm) for S, M and L diets, respectively, for Experiment 2 (E2). Hay or straw comprised the sole source of forage (50% and 75% of ration dry matter (DM) for E1 and E2, respectively). Both experiments used three rumen cannulated Holstein dairy cows, in late lactation for E1 and dry in E2, with 3 × 3 Latin square designs with 14 day periods. In E1, DM intake (DMI; 18.3 ± 2.1 kg/day; mean ± s.e.), pH (6.4 ± 0.1), time spent eating (280 ± 22.5 min/day), time spent ruminating (487 ± 17 min/day), and total time spent chewing (767 ± 34 min/day) were not different, whereas eating minutes per kilogram of DMI and NDF intake (NDFI) tended to increase linearly as forage length increased. Rumen digesta volume (l; 113.3 S, 117.8 M and 114.4 L ± 17.1) had a quadratic response, and rumen digesta weight tended to respond quadratically; however, differences were numerically small. In E2, DMI (8.3 ± 1.3 kg/day), pH (6.7 ± 0.1), time spent eating (236 ± 23.5 min/day), time spent ruminating (468 ± 45.2 min/day), total time spent chewing (704 ± 67.7 min/day) and minutes per kilogram NDFI were not different, whereas minutes per kilogram of DMI had a trend for a quadratic effect. Rumen digesta volume (111 ± 18.8 l) and weight (103 ± 17.4 kg) were not different. In both experiments, cows sorted against longer particles as determined by a particle length selection index; this behavior increased linearly as particle length increased. Greater forage particle length increased sorting behavior, but had no effect on rumen fermentation or chewing behavior.