Competition and facilitation in hairy vetch-barley intercrops

Intercrops between legumes and non-legumes are widely used for fodder production and as cover crops, but little quantitative data are available on competition between species in the mixture. The objective of the present study was to assess the interaction between hairy vetch (Vicia villosa Roth.) and barley (Hordeum vulgare L.) grown as pure crops or intercrops with different proportions of seed rates at sowing. A 4-year field study was conducted using hairy vetch and barley as pure stands at full sowing density and as intercrops at different proportions of their own full seed rate according to the replacement principle. Interaction between species was evaluated on the basis of Land Equivalent Ratio (LER), Relative Neighbour Effect (RNE) and Aggressivity (A) calculated on biomass and nitrogen (N) accumulation. The N accumulation of the mixed crops increased linearly with the legume proportion in the mixture. The mixtures were more efficient than the pure crops in terms of N use (LER > 1). Partial LER values indicated that the barley component benefited from the presence of the legume, while the hairy vetch partial LER decreased with increasing barley proportion in the mixture. The competitive response in terms of biomass accumulation was high for both species when their density in the mixture was high. Concerning N accumulation, barley benefited from an asymmetric interspecific facilitation while the vetch behaviour was similar to that observed for biomass accumulation. Barley dominance progressively increased reaching a maximum just before the last sampling date. At the last sampling date the competitive ability of hairy vetch showed a considerable increase in all mixtures (A ≈ 0). These findings indicate that the use of mixtures between hairy vetch and barley allows an increase in the use efficiency of N resource with respect to pure crops. Barley is the dominant component of the mixture and the hairy vetch is able to cope with the cereal competitive effect only at the end of the shooting phase of barley. Moreover when the soil N supply is high, the recovering ability of the vetch in the mixture could be strongly reduced. In such conditions, the proportion of legumes at sowing should be increased in order to counteract barley dominance and allow a late recovery of the legume component.