Suitability of eight techniques for estimating digestibility of herbage from continuously grazed all-day pasture

The objective of this study was to estimate digestibility of herbage using eight different methods. Organic matter digestibility (OMD) was estimated with titanium (Ti) dioxide and acid-insoluble ash (AIA) as indigestible markers, four faecal nitrogen (N) equations, which use the same raw data, the pepsin-cellulase method (in vitro OMD) and digestibility trials with wethers (in vivo OMD). An all-day pasture with continuous stocking at 2.8 cows/ha was chosen for the comparison because it restricted selection during grazing and thus allowed comparison of in vitro- and stall feeding-based methods with methods used for pastures. A crossover experiment with eight lactating Simmental cows was conducted from May until July 2008, with two consecutive experimental periods of 28 days. The cows were divided in two similar experimental groups. Four cows were put into individual stalls and fed herbage clipped from the experimental pasture and feed intake was measured. The other four cows were put onto the fenced pasture. All cows were supplemented with 2 kg/d fresh matter of grain maize. After four weeks, the treatment groups were switched. ffered considerably between methods (by 20–110 g/kg). Applying the same method, OMD on pasture differed from OMD in-stall indicating that the grazing animal cannot be replaced by mowing even with high experimental effort and low opportunity for selection. It also differed over time. Only the faecal N methods were readily applicable on pasture at sufficiently high temporal resolution and – with one exception – produced similar results in-stall as the Ti method. They can in general be recommended for a large range of conditions including grazing studies but the variety of existing equations makes it difficult to select the appropriate one. This calls for the development of better defined and rigorously tested equations.