THE YIELD OF WHEAT FOLLOWING LUPINS - EFFECTS OF DIFFERENT LUPIN GENOTYPES AND MANAGEMENT

More nitrogen is fixed by grain lupins (Lupinus sp.) than is removed in the harvested grain. Differences in residual N after different lupin species and genotypes (L. albus, L. cosentinii, L. angustifolius) and different agronomic treatments (harvesting, sowing dates, sowing rates and fertilizer treatments) were estimated using a simple nitrogen (N) balance. For six experiments, the relationship between the estimated residual N value and the yield of the following wheat crop was also examined. L. albus grew poorly on the infertile sandy soils and had the lowest estimated residual N value, whereas L. cosentinii had a high estimated residual N value and L. angustifolius was intermediate. Across sites and seasons, the estimated residual N value of L. angustifolius varied from 37 to 165 kg N/ha. For an individual experiment the greatest range for L. angustifolius was from 59 to 114 kg N/ha. Despite the wide range in residual N across genotypes, there was a poor relationship between estimated residual N value and yield of the following wheat crop. Except for lupin crops which grew poorly, the impact of agronomic treatment on the residual N from lupins had little effect on yield of the following wheat. On the basis of these results there seems to be no reason to manage the lupin crop for high residual N value, and any improvements to the nitrogen harvest index of lupins through breeding or management axe not expected to reduce the residual effect.