Retention of Cr-labelled grass hay and silage in different segments of the gastrointestinal tract of dairy cows

Abstract

Two experiments were conducted to study solids retention in different segments of the digestive tract of dairy cows. In Exp. 1 three lactating cows with rumen and duodenal cannulae were fed grass silage and lib, and 0, 3 or 6 kg of concentrates per day in a Latin square design. Retention in the total tract (MRT tot), stomachs (MRT st) and, by difference, intestines (MRT int), was measured from the changes in Cr concentration in duodenal and faecal samples after a single dose of Cr-mordanted fibre in the rumen. A 3-pool kinetics model with lag was fitted 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 concentrates at 16–17 kg DM/d and the dry cows were fed 8.0 kg hay alone. Hay and faeces were Cr-mordanted and given as a pulse dose in the rumen or through the reticulo-omasal canal, respectively. Retention in total tract, post-rumen (MRT pt ) and by difference, rumen-reticulum (MRT L ), was 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 decrease in retention time with intake was also more marked. A very small effect of intake on MRT int and MRT pr, was seen in both experiments. Lag as a proportion of MRT for individual parts of the gastrointestinal tract was used as an indicator of incomplete mixing. The proportion 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 was 0.88 and 0.67, respectively. Retention in the rumen was characterized by both a lag phase and multiple pools.