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 ge notypes with restricted branching are being investigated. Since agronomic r ecommendations for lupins have been developed with conventional, normal bra nching cultivars, agronomy, particularly population density, may need adjus tment for restricted branching types. Here we report a study of population density requirements of 3 restricted branching genotypes that cover the ran ge of variation available in degree of branching restriction, and a normal branching cultivar, Merrit, which is widely grown and has good general adap tation. The experiments were performed during 3 growing seasons at a number of sites throughout Western Australia. Data were analysed using recently d eveloped statistical techniques, including spatial analysis to reduce error variation and a smoothing spline to minimise assumptions made about the fo rm of the response to density. Generally, responsiveness to density was sim ilar for all genotypes, as were optimal densities, which, with only one exc eption, 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 wer e 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 ind ex. Shoot dry mass tended to be lower in restricted branching genotypes, bu t harvest indices were similar. However, for yield, shoot mass, and especia lly harvest index, interactions between genotype and environment were gener ally highly significant. These results indicate that the restricted branchi ng genotypes currently available do not have wide suitability, but in envir onments where harvest index in normal branching lupins is poor or unstable and yield is not as strongly limited by growth, they could provide higher o r more stable yields. Further work is required to identify 'niche' environm ents. Grain from restricted branching genotypes was generally less variable in si ze and had higher concentrations of P, K, S, Mg, and Mn, whereas some genot ypes also had higher Ca, Cu, Na, and protein. Some of these changes in grai n quality might be related to grain filling earlier in the season, under le ss stressful conditions and with less competition from concurrent vegetativ e growth than in normal branching genotypes.