For implementation of simple yield loss models into threshold-based weed management systems, © ]996 European Weed Research Society a thorough validation is needed over a great diversity of sites. Yield losses by eompetition wsth Sinapis alba L. {white mustard) as a model weed, were studied in 12 experiments in sugar beet (Beta vutgarLs L.) and in 11 experiments in spring wheat (Triiicum aestivunt L.). Mosl data sets were bet ler described by a model based on the relative leaf area of the weed than by a hyperbolic model based on weed deasity. This leaf area mode] accounted lor (part of) the effect of different emerging times of the S. alba. whereas the density modd did not. A parameter that allows the maximum yield loss to be smaller than K)()% was mostly not needed to describe the effects of weed competition. The parameter that denotes the competitiveness of the weed species wiih respect to the crop decreased the later the relative leaf area of the rnustard was determined. This decrease could be estimated from the differences in relative growth rate of the leaf area of crop and..?. atba. However, the accuracy of this estimation was poor. The parameter value of the leaf area model varied considerably between sites and years. The results strongly suggest that the predictive ability ofthe !eaf area mode! needs to be improved before ii can he applied in weed management systems. Such improvement would require additional information about effects ot abiotic factors on piant development and morphology and ihe definition of a lime window for predictions with an acceptable level of error.

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This chapter describes the three main techniques of mechanical weed management, i.e. (1) the use of tillage, (2) cutting weeds, and (3) pulling weeds. The economics of mechanical weed management is discussed.

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The rising costs of pesticides, soil erosion, and environmental pollution associated with corn (Zea mays L.) production have led to consideration of alternative production methods. Growing cover crops with corn could address these problems. Field experiments were conducted in 1993 and 1994 at l'Assomption and Macdonald in Quebec to determine the effects of interseeding 12 cover crops on corn yield and yield components. Fall rye (Secale cereale L.), hairy vetch (Vicia villosa Roth), a mixture of red clover (Trifolium pratense L.) and ryegrass (Lolium multiflorum Lam.), a mixture of white clover (T. repens L.) and ryegrass, subterranean clover (T. subterraneum L.), yellow sweetclover (Melilotus officinalis Lam.), black medic (Medicago lupulina L.), Persian clover (T. resupinatum L.), strawberry clover (T. fragiferum L.), crimson clover (T. incarnatum L.), alfalfa (Med. sativa L.), and berseem clover (T. alexandrinum L.) were seeded 10 and 20 d after corn emergence. The experimental design was a splitplot, randomized complete block with four replications at each site. The mainplots were the cover crop planting dates; the subplots were the 12 cover crop treatments and 3 controls (hand weeding, chemical weeding, and no weeding). Seeding the cover crops either 10 or 20 d after corn planting did not affect corn grain yield. Chlorophyll fluorescence measurements (Fv/Fm) indicated that corn plants were stressed when interseeded with crimson clover (P < 0.05). When there was competition for moisture, crimson clover was found to be too competitive with corn at the seeding rates used in this experiment. Corn yield was less affected by the interseeded cover crops under conditions of adequate rainfall. No consistent differences in corn grain yield components were found for cover crop treatments.

Interseeded cover crops can minimize soil erosion, increase organic matter and nutrient levels and the yield of subsequent crops. However, their performance is very sensitive to local conditions. Field experitnents were conducted at two Quebec locations in 1993 and 1994 in order to evaluate the potential use of forage legumes and grasses as interseeds in corn in eastern Canada. Twelve forage species were evaluated. Fall rye {Secale cereal L.), hairy vetch (Vicia villosa Roth), a mixture of red clover (Trifolium pratense L.) and ryegrass (Lolium multiftorum Lam), a mixture of white clover (Trifolium repens L.) and ryegrass, subterranean clover {Trifolium suhterraneum L.), yellow sweet clover (Melilotus ojficinalis Lam.), black medic (Medicago lupulina L.), Persian clover (Trifolium resupinatum L.), strawberry clover (Trifolium fragiferum L.), crimson clover (Trifolium incarnatum L.), annual alfalfa (Medicago sativa L.) and berseem clover (Trifolium alexandrinum L.) were seeded at two planting dates (10 and 20 days after corn emergence). The control treatments were: hand weeding, chemical weeding and nonweeded. Early seeded forages established better and had higher biomass accumulation than the late seeded ones. In the presence of larger weed populations, the interseeded forages did not develop well due to competition with the weeds. At Macdonald crimson clover provided good soil cover while Persian clover, fall rye and alfalfa provided relatively little cover. Strawberry clover and hairy vetch did not provide early ground cover due to their late development. Forage mixtures of red or white clover and rye grass established well and achieved high populations at the end of the growing season. Fall rye provided good early ground cover but senesced by the middle of the season. The better establishment and early germination of crimson clover caused a 19% reduction in corn grain yield in 1993. In 1994 none of the cover crops caused a reduction in corn yield.

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