Effects of nitrogen fertilizer, irrigation, and crop genotype on canopy relations and yields of an intercrop/weed mixture

Abstract

Two field experiments were conducted to investigate effects of soil conditions and crop genotypes on canopy relations and yield responses of two crops (barley and field pea) and a weed (white mustard) sown in trispecific mixtures. Treatments involved variations in nitrogen supply, water supply and pea genotype. Photosynthetic surface area and seed yield of pea were increased by increasing water supply, but were unaffected or diminished by increasing nitrogen supply. ‘Century’ pea produced about four times more surface area than ‘Alaska’ pea. Under irrigated, low-nitrogen conditions, seed yields of the two pea varieties were similar, but under high-nitrogen conditions, seed yield of the smaller variety declined significantly. Under high-nitrogen conditions, the larger ‘Century’ pea constituted a large proportion of the upper levels of the mixed-species canopies, while the smaller ‘Alaska’ pea was overtopped and shaded by mustard. Increasing nitrogen or water supply increased photosynthetic surface area of barley, but most of these increases occurred at canopy levels where only small amountse of photosynthetically active radiation ( par; or Q pa) were available. Increasing nitrogen supply had a much stronger positive effect on barley seed yield than did increasing water supply. Use of ‘Century’ pea rather than the smaller ‘Alaska’ pea decreased barley's photosynthetic surface area, access to Q pa, and seed yield; these effects were particularly pronounced under low-nitrogen conditions. In terms of canopy development, Q pa interception, and yield, nitrogen fertilizer benefitted mustard much more than it did either crop species. Averaged across irrigation regimes, pea genotypes, and years, application of N increased mustard biomass by 472%, but increased total crop seed yield only 31%. Increasing water supply had much less effect than nitrogen fertilizer on mustard's production of photosynthetic surface area and biomass. Use of ‘Century’ pea rather than the smaller ‘Alaska’ pea resulted in increased shading and decreased growth of mustard. Results of the experiments indicate that soil conditions, the relative values and desired yields of the different component crops, and the degree of priority placed on weed suppression should all bear on the choice of appropriate crop varieties for intercropping systems. When the snaller ‘ALaska’ pea was used in the intercrop/weed mixtures, a large total amount of crop seed (pea + barley) was produced under high-water, low-nitrogen conditions. Total crop seed yield was greatest, however, when the larger ‘Century’ pea was used in the mixtures and nitrogen fertilizer was applied. Mustard produced very little biomass under the former conditions, but produced a substantial amount of biomass under the latter conditions.