Competition between lupin (Lupinus angustifolius L.) and great brome (Bromus diandrus Roth.): development of leaf area, light interception and yields

Abstract

Great brome (Bromus diandrus) is a common weed in wheat and can be expected as a weed in lupin crops (Lupinus angustifolius L.), particularly those in rotation with wheat. Although chemical control of brome in lupin is available, those plants escaping control compete with the lupin. This competition has been investigated by comparing the growth of lupin (cv. Yandee) in monoculture with its growth in the presence of 2 densities of brome, and with the growth of brome in monoculture. Each of the 4 treatments was grown with high and low fertiliser applications, to see how the competition was modified by nutrient availability. Water was not limiting. Daily measurements were made of lupin growth rates and the development of leaf area, and light interception and radiation use efficiencies were calculated (g DM assimilated per MJ intercepted photosynthetically active radiation). Radiation use efficiency changed with the phase of development of lupin, increasing to over 3 g/MJ at 95 days after sowing. It was higher in the high fertiliser treatments and in the presence of brome, particularly before 95 days after sowing. Changes in unit leaf rate (i.e. net assimilation rate) of lupin due to brome were small, but these, and changes in radiation use efficiencies, may have played a minor role in the depression of crop growth rates, more important during early lupin growth than later when leaf area indices had reached >15% of their maximum values. Depressions of the crop growth rate of lupin caused by the presence of brome were linearly related to changes in leaf area indices of lupin. The decrease in lupin seed yield per plant was due to a decrease in pod number per plant, not to a change in the number of seeds per pod or single seed weight. The number of pods was linearly related to the number of axes per plant (i.e. branches plus main stem).