GRAIN-YIELD IS REDUCED MORE BY WEED INTERFERENCE IN AN OLD THAN IN A NEW MAIZE HYBRID

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 interfer ence, and soil water deficit on dry matter accumulation and grain yiel d of an older and a more recent hybrid. Studies were carried out at th e flora Research Station in 1992, 1993, and 1994 on a London loam (Aqu ic Hapludalf that had been tile drained. The experiment was arranged i n a split-split-split-plot design with four replications. Treatments c onsisted of two soil-N levels (70 and 225 kg ha(-1)), weed-free all se ason 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 condu ctance, leaf chlorophyll fluorescence, and leaf chlorophyll content we re measured. Irrigation generally did not influence the measured param eters. Silking date and stomatal conductance during the grain-filling period were affected less by low soil N and weed interference in Pione er 3902 than in Pride 5. Aboveground dry matter at silking and maturit y was reduced about 20% by single stresses of low soil N and weed inte rference and about 55% by the combination of both stresses. Hybrid x s tress treatment interactions for dry matter were not significant. The reduction in grain yield due to weed interference was 21% greater in P ride 5 than in Pioneer 3902. The hybrid x N interaction for grain yiel d was not significant, because a smaller reduction in harvest index du e 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 d uring the silking period, as indicated by the greater reduction in har vest index and kernel number in the old than in the new hybrid due to stress.