Using fistulated sheep to compare in sacco and in vitro rumen degradation of selected feeds

Abstract

Despite its beneficial role in almost all feed evaluation systems, the in sacco method is inconsistent and labour intensive in estimating the degradation of ruminant feeds. This study compared a rumen fluid-based in vitro method with the in sacco method to estimate degradation of 12 feeds comprising six concentrates and six grasses representing three fields during two seasons of winter and spring. Three feed groups (n = 4) were formed by using two grasses (one per season) and two concentrates. These feeds were incubated alongside blanks, in duplicate, for various hours either in sacco in three fistulated sheep or in vitro by using rumen fluid from the same sheep over three periods using two separate 3 × 3 Latin square designs. The in sacco and in vitro data were statistically compared for the effects of sheep, feed group and period on degradation at each incubation time within each method. The degradation data were fitted exponentially to derive water soluble (a), insoluble (b) and degradation rate (c) constants to estimate effective degradability at rumen outflow rate of 0.02 (P0.02) for each feed in each method. These values were statistically compared for variations within and between feed types for each method. The sheep, group and period did not differ significantly (P > 0.05) for feed degradation at most incubation times within any of these two methods. The in vitro method was able to mimic the in sacco degradation of feeds, although the absolute in vitro values were always lower than the in sacco values. The higher in sacco degradation for almost all feeds were partly attributed to the much greater losses of these feeds during their washing with water at 0 h. The in vitro method did identify significant differences (P < 0.05) in dry matter or crude protein degradations between and within different concentrates and grasses in parallel to the in sacco method. The ranking of feeds was comparable for these methods. The in vitro method can be used in parallel to the in sacco method to estimate degradation of various feeds. However, there is a need for coordinated studies to improve the prediction ability of the in vitro method for its adoption to routinely estimate the degradation of ruminant feeds.