The effect of supplementing grass silage with soya bean meal on digestibility, in sacco degradability, rumen fermentation and methane production in sheep

Abstract

Two experiments are reported which examined the digestibility, in sacco degradability, rumen fermentation and methane production of grass silage (GS) unsupplemented or supplemented with soya bean meal (SBM) at three ratios and two planes of nutrition. GS was given with SBM to four mature wether sheep using proportions of GS in the diet of 1.00, 0.75, 0.50, and 0.25 (dry matter (DM) basis) at near to maintenance (experiment 1) and 1.5× maintenance (experiment 2) in two consecutive Latin square designs. All diets within plane of nutrition were designed to be isoenergetic and all animals received a mineral/vitamin supplement. The diets were fed for 28 days, with rumen fluid sampling on day 11. Apparent digestibility and rumen degradability (in sacco) of diet organic matter (OM) were measured and methane produced was measured in open-circuit respiration chambers for four consecutive 24 h periods. Apparent digestibility of organic matter (OMD) increased linearly in both experiments with decreasing proportion of GS (0.75, 0.81, 0.86, 0.91, P<0.001 and 0.74, 0.80, 0.81, 0.88, P<0.001 for experiments 1 and 2 respectively). Decreasing proportion of GS depressed the effective degradability of silage OM from polyester fibre bags in both experiments (quadratic effect P<0.05). The molar proportions of propionate and butyrate tended to decrease with decreasing proportion of GS. Over the range of SBM inclusion in this study there were associative effects on methane production (l per day) in experiment 1 (35.8, 36.4, 35.8, 33.7 for proportion of GS 1.00, 0.76, 0.51 and 0.26, respectively), and experiment 2 (35.7, 42.4, 45.2, 43.1 for proportion of GS 1.00, 0.73, 0.52, 0.28, respectively) though the lower intake of silage in experiment 2 may impinge on this. Rumen stoichiometry could not be used to explain the change in methane production. Increased level of intake reduced methane production (l kg −1 organic matter apparently digested) regardless of proportion of GS. The use of the by difference method to determine the metabolisable energy of SBM gave an over-estimation of methane energy loss as a percent of gross energy intake of 22%. Expressing methane relative to digestibility, shows the potential for reduction of methane generation by protein supplementation (225 g kg −1 DM crude protein in the diet).