MANURE, TILLAGE, AND CROP-ROTATION - EFFECTS ON RESIDUAL WEED INTERFERENCE IN SPRING BARLEY CROPPING SYSTEMS

Effects of management practices on crop growth and yields may or may n ot be mediated through crop-weed interactions. Our objective was to de tect and confirm a relationship between the influence of crop manageme nt factors on spring barley (Hordeum vulgare L.) and the potential occ urrence of crop-weed interference, A field study was established on a Normandin clay (fine, mixed, frigid Humic Cryaquept) at Normandin in Q uebec to investigate the effects of crop rotation, tillage (chisel plo w, CP; moldboard plow, MP), and nutrient source on midseason abovegrou nd dry weight and seed yield of barley and residual weed populations. Barley-weed interference was detected using ANOVA, principal component s analysis, and analysis of covariance, A reduction in midseason dry w eight (36%) and seed yield (59%) of barley in the CP relative to the M P treatment in 1994 was associated with interference from broadleaf pl antain (Plantago major L,) and dandelion (Taraxacum officinale Weber i n Wiggers), A 20% reduction in midseason dry weight and seed yield wit h CP tillage in 1995 was associated with interference from volunteer t imothy (Phleum pratense L.) and Kentucky bluegrass (Poa pratensis L,). Barley dry weight and seed yield were 29 and 26% greater in the barle y-forage rotation compared,vith the monoculture in all years except 19 95, despite greater weed pressure in the barley-forage rotation, confi rming the benefits of forages to subsequent crops in a rotation, Compa red with mineral fertilizer, application of manure resulted in lower d ry weight and seed yield for barley; however, this reduction was not a ssociated with weed interference, In contrast to tillage, crop rotatio n and nutrient source effects on crop variables appeared mediated thro ugh factors other than weed interference.