Grain Yield is Reduced More by Weed Interference in an Old than in a New Maize Hybrid

Abstract Grain yield improvement of maize ( Zea mays L.) hybrids in Ontario has been associated with increased stress tolerance of newer hybrids. This study was conducted to assess the effect of low soil N, weed interference, and soil water deficit on dry matter accumulation and grain yield of an older and a more recent hybrid. Studies were carried out at the Elora Research Station in 1992, 1993, and 1994 on a London loam (Aquic Hapludalf) that had been tile drained. The experiment was arranged in a split‐split‐split‐plot design with four replications. Treatments consisted of two soil‐N levels (70 and 225 kg ha −1 ), weedfree all season and weedy from the 3‐ to 4‐leaf stage of maize, irrigation and no irrigation, and the hybrids ‘Pioneer 3902’ (new) and ‘Pride 5’ (old). Maize phenology, dry matter accumulation, grain yield, stomatal conductance, leaf chlorophyll fluorescence, and leaf chlorophyll content were measured. Irrigation generally did not influence the measured parameters. Silking date and stomatal conductance during the grain‐filling period were affected less by low soil N and weed interference in Pioneer 3902 than in Pride 5. Aboveground dry matter at silking and maturity was reduced about 20% by single stresses of low soil N and weed interference and about 55% by the combination of both stresses. Hybrid × stress treatment interactions for dry matter were not significant. The reduction in grain yield due to weed interference was 21% greater in Pride 5 than in Pioneer 3902. The hybrid × N interaction for grain yield was not significant, because a smaller reduction in harvest index due to N stress in the new than in the old hybrid was offset by a larger reduction in dry matter accumulation during the grain‐filling period. The differential response of the two hybrids to stress was greatest during the silking period, as indicated by the greater reduction in harvest index and kernel number in the old than in the new hybrid due to stress.