RETENTION OF CR-LABELED GRASS HAY AND SILAGE IN DIFFERENT SEGMENTS OFTHE GASTROINTESTINAL-TRACT OF DAIRY-COWS

Two experiments were conducted to study solids retention in different segments of the digestive tract of dairy cows. In Exp. 1 three lactati ng cows with rumen and duodenal cannulae were fed grass silage ad lib. and 0, 3 or 6 kg of concentrates per day in a Latin square design. Re tention in the total tract (MRT(tot)), stomachs (MRT(st)) and, by diff erence, intestines (MRT(int)), was measured from the changes in Cr con centration in duodenal and faecal samples after a single dose of Cr-mo rdanted fibre in the rumen. A 3-pool kinetics model with lag was fitte d to the data. In Exp. 2, 3 lactating and 2 dry rumen cannulated cows were used. The lactating cows were fed hay, fresh grass and concentrat es at 16-17 kg DM/d and the dry cows were fed 8.0 kg hay alone. Hay an d faeces were Cr-mordanted and given as a pulse dose in the rumen or t hrough the reticulo-omasal canal, respectively. Retention in total tra ct, post-lumen (MRT(pr)) and by difference, rumen-reticulum (MRT(r))wa s calculated as in Exp. 1. The silage based diets in Exp. 1 tended to give higher MRT(tot) than the hay and grass diets in Exp. 2 but the de crease in retention time with intake was also more marked. A very smal l effect of intake on MRT(int) and MRT(pr) was seen in both experiment s. Lag as a proportion of MRT for individual parts of the gastrointest inal tract was used as an indicator of incomplete mixing. The proporti on decreased in the order of: intestines > post-rumen,stomachs > rumen . The proportion of MRT(st)) (Exp. 1) and MRT(r) (Exp. 2) in MRT(tot) and 0.67, respectively. Retention in the rumen was characterized by bo th a lag phase and multiple pools.