Restricted branching narrow-leafed lupin. I. Population density

Narrow-leafed lupin is a major grain crop in southern Australia. Its freely branching growth habit is often seen as a limitation to grain yield, so genotypes with restricted branching are being investigated. Since agronomic recommendations for lupins have been developed with conventional, normal branching cultivars, agronomy, particularly population density, may need adjustment for restricted branching types. Here we report a study of population density requirements of 3 restricted branching genotypes that cover the range of variation available in degree of branching restriction, and a normal branching cultivar, Merrit, which is widely grown and has good general adaptation. The experiments were performed during 3 growing seasons at a number of sites throughout Western Australia. Data were analysed using recently developed statistical techniques, including spatial analysis to reduce error variation and a smoothing spline to minimise assumptions made about the form of the response to density. Generally, responsiveness to density was similar for all genotypes, as were optimal densities, which, with only one exception, were achieved at 35–57 plants/m 2 . At higher densities there were fewer pods per plant and seeds per pod but heavier seeds; these effects were similar for all genotypes except in the case of pods per plant, which was reduced more by high density in the restricted branching types. Apart from yield, other advantages of high density were taller plants, larger grain, and slightly higher grain protein concentration. Yields were highly correlated with shoot dry mass, but not with harvest index. Shoot dry mass tended to be lower in restricted branching genotypes, but harvest indices were similar. However, for yield, shoot mass, and especially harvest index, interactions between genotype and environment were generally highly significant. These results indicate that the restricted branching genotypes currently available do not have wide suitability, but in environments where harvest index in normal branching lupins is poor or unstable and yield is not as strongly limited by growth, they could provide higher or more stable yields. Further work is required to identify ‘niche’ environments. Grain from restricted branching genotypes was generally less variable in size and had higher concentrations of P, K, S, Mg, and Mn, whereas some genotypes also had higher Ca, Cu, Na, and protein. Some of these changes in grain quality might be related to grain filling e