Competition between Wimmera ryegrass and narrow-leafed lupins

Abstract

The ways in which Wimmera ryegrass (Lolium rigidum) affects the growth and yield of narrow-leafed lupins (Lupinus angustifolius) were studied in two experiments at Baker's Hill, W.A. (average annual rainfall 650 mm). In the first experiment, Uniharvest lupins and Wimmera ryegrass were grown alone and together starting at two dates. The three rates of ryegrass produced similar biomasses (total dry matter) at maturity (about 8000 kg/ha) but lupin grain yield decreased significantly with ryegrass sowing rate. One effect of ryegrass at the highest sowing rate was to alter the distribution of dry matter in the lupin canopy so that, at the end of the growing season, there was a lower percentage of leaf in the lower part of the canopy and this was associated with less light being received. Water stress in lupins at the end of the growing season was higher at the later planting with ryegrass. A multiple regression analysis showed that 74% of the variation in lupin grain yield between plots was due to variation in lupin height and biomass, and ryegrass tiller numbers in October. Lupins had little effect on ryegrass yield; although there were fewer tillers in the presence of lupins, weights of tillers were higher. In the second experiment, the effects of 38 ryegrass plants/m2 on growth of both Uniharvest and Unicrop lupins grown at a density of 40 plants/m2 were studied. Lupin biomass was unaffected until October but was then reduced significantly by the ryegrass, which yielded 4000 kg/ha dry matter. Grain yield was reduced because fewer pods were produced on lateral branches in both varieties, and on main stems in Uniharvest. The experiments showed that in good growing seasons, in high rainfall areas, lupins will yield grain even when grown with very high densities of ryegrass.