The effect of supplementing grass silage with barley on digestibility, in sacco degradability, rumen fermentation and methane production in sheep at two levels of intake

Abstract

Two experiments are reported which examined the digestibility, in sacco degradability, rumen fermentation and methane production of grass silage unsupplemented or supplemented with rolled barley at three ratios and two planes of nutrition. Grass silage was given with rolled barley to four mature wether sheep using forage:concentrate (F:C) ratios 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 d, with rumen fluid sampling on day 12. 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 ( P < 0.001) in both experiments with decreasing F:C ratio (0.74, 0.79, 0.83, 0.88 and 0.72, 0.75, 0.78, 0.82 for experiments 1 and 2 respectively). Decreasing F:C ratio depressed the effective degradability of silage OM from polyester fibre bags in both experiments (quadratic effect P < 0.05) and also for barley in experiment 1 (quadratic effect P < 0.05). Rumen pH was significantly decreased with decreasing F:C ratio in both experiments as was the total mean VFA concentration. The molar proportions of acetate and propionate tended to decrease and that of butyrate increased with decreasing F:C ratio. Over the range of barley inclusion in this study there were no associative effects on methane production (1 d −1) in experiment 1 (37.2, 41.5, 41.9, 44.0 1 d −1 for F:C ratio 1.00, 0.76, 0.51 and 0.26 respectively), but the opposite was true at the higher level of intake (methane 1 d −1, 54.1, 60.1, 58.0, 50.1 for F:C ratios 1.00, 0.78, 0.54, 0.28 respectively). Rumen stoichiometry could not be used to explain the change in methane production. Increased level of intake reduced methane production (1 kg −1 organic matter apparently digested) regardless of F:C ratio. Expressing methane relative to the efficiency of digestion, or the efficiency of animal production, enables the potential for reduction of methane generation by starch supplementation and/or level of intake to be realised.