Net Mixture Effects on Nutritive Value in Lucerne-Grass Swards of Varied Composition and Diversity

Climate change and associated risks of extreme weather has led to growing interest in drought-tolerant species such as lucerne (Medicago sativa L.), tall fescue (Festuca arundinacea Schreb.) and cocksfoot (Dactylis glomerata L.) in mixed grass-legume forage production in North-Western Europe. Lucerne and grasses have distinct nutritive value that can be combined when grown in mixtures. The extent of a ‘net mixture effect’ (NE) on the nutritive value, that is the deviations of the mixture quality from the expected one derived from the pure stands, has not been studied in any depth and requires further investigation in the context of climate change. The present study was conducted at four sites during two main cropping years with the aim of comparing potentially drought-tolerant mixtures against pure stands. With significant sward type × site × year interactions, nutritive value of the mixed swards often differed significantly from pure stands, ranging intermediate between the component pure stand nutritive value. The concentration of water-soluble carbohydrate (WSC) did not differ between mixtures and lucerne pure stands. Significant NE were found, with larger measured concentrations in mixtures compared to those predicted, for neutral and acid detergent fibre and the crude protein:WSC ratio. The concentrations of WSC and metabolizable energy were smaller than expected. The NE was not influenced by sward type or species number, except for WSC at a few sites. The results show that improved knowledge of positive mixing effects could be used to specifically enhance the nutritive value in grasses-lucerne mixtures irrespective of sward diversity.