Associative effects between orchardgrass and red clover silages on voluntary intake and digestion in sheep: Evidence of a synergy on digestible dry matter intake

The aim of this study was to assess the associative effects between orchardgrass (Dactylis glomerata) and red clover (Trifolium pratense) silages as a model of preserved grass-legume mixture on voluntary intake parameters and digestive efficiency in sheep. Ten sheep were assigned to a repeated 5 x 5 Latin square design, in which 5 proportions of orchardgrass and red clover silages were tested (0:100, 25:75, 50:50, 75:25, and 0:100, on a DM basis). Measurements were performed simultaneously on intake, feeding behavior (eating time and chewing activity), digestive parameters (nutrient digestibility and rumen parameters), nitrogen balance, and enteric methane (CH4) emissions using the SF6 tracer technique. Positive associative effects were observed on daily voluntary DMI (P < 0.001), which was greater when sheep were fed with at least 50% red clover (1.56-1.59 kg/d) compared with those fed with 0 or 25% red clover (1.29 and 1.45 kg/d, respectively; P < 0.001). This synergy between forages was not observed on nutrient digestibility, as these parameters linearly decreased with increasing proportions of red clover (P < 0.001). As a result, positive associative effects were observed on daily voluntary intake of digestible DM (P < 0.001). Taken together, the results indicate that the synergy on voluntary intake was due to a greater motivation of animals to eat mixtures (seen in increased intake rates) rather than a positive associative effect on digestive efficiency. Methane yield decreased linearly with increasing proportion of red clover (P < 0.001) and ranged from 20.0 g/kg DMI for sheep fed 100% orchardgrass to 16.1 g/kg DMI for sheep fed 100% red clover. Mixtures of orchardgrass and red clover quadratically (P = 0.03) decreased urinary nitrogen losses and tended (P = 0.099) to quadratically increase nitrogen retention. This synergy between orchardgrass and red clover silages could improve animal performances in addition to the known agronomic benefits of grass-legume mixtures.