Annual ryegrass (Lolium rigidum) reduces the uptake and utilisation of fertiliser-nitrogen by wheat

The effect of timing of annual ryegrass (Lolium rigidum) emergence on the uptake and utilisation of N by wheat was investigated in a field trial on a duplex soil at Katanning, Western Australia, and in a glasshouse study in which N-15-fertiliser was applied. Three treatments were used to investigate the effect of timing of annual ryegrass emergence on the uptake and utilisation of N by wheat: simultaneous sowing of wheat and annual ryegrass, sowing of annual ryegrass 1 week before wheat, and sowing of the annual ryegrass 1 week after wheat. A control treatment, consisting of wheat sown alone, was also included. Plant densities during the field trial were 105 and 140 plants/m(2) for wheat and annual ryegrass, respectively, whereas in the glasshouse they were 105 plants/m(2) for wheat and 155 plants/m(2) for annual ryegrass. Fertiliser-N was applied at seeding of wheat at 50 kg N/ha in the field trial and 60 kg N/ha in the glasshouse. The introduction of annual ryegrass into the wheat system reduced the production of biomass and the grain yield of wheat. The earlier the annual ryegrass was introduced into the system, the greater the reduction in the biomass and grain yield of wheat. Poor tillering and slow rates of growth were accountable for the reduction in biomass, whilst the reduction in wheat grain yield was caused by the reductions in ear number, kernels per ear, and kernel size. Grain N content and hence grain protein was also reduced by the introduction of annual ryegrass into the wheat system. Irrespective of the timing of introduction of annual ryegrass, the low N uptake of wheat resulted from a reduction in the uptake of both soil and fertiliser-N. This indicates that annual ryegrass competed with wheat not only for the fertiliser-N that was applied at seeding of wheat, but also for mineralised soil N. The competition for N reduced the total recoveries of fertiliser-N in the wheat plant. Total recoveries of fertiliser-N in the wheat plant suggest that 59% of the fertiliser-N was not taken up by wheat when annual ryegrass was sown 1 week earlier than wheat or at the same time as wheat, whereas only 32% was not taken up by the wheat when annual ryegrass was sown 1 week later than wheat. More competitive wheat genotypes would be those with better efficiency in the uptake of N and its utilisation in maintaining yield and grain protein under infestations of annual ryegrass.