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seeding among weeds

Blackshaw, R.E., L.J. Molnar, H.-H. Muendel, G. Saindon and X. Li. 2000. Intergration of cropping practices and herbicides improves weed management in dry bean ( Phaseolus vulgaris ). Weed Technol. 14: 327-336.

There is considerable evidence that narrower row spacing combined with a higher seeding rate can work together to reduce overall weed competition. Some studies have shown that very narrow row spacings (4 inches) or planting in a uniform grid can maximize yield of cereal crops at higher seeding rates (Kirkland, 1993; Weiner et al., 2001). That said, seeding rate appears to be a more important factor than row spacing. Thus, it should be possible to continue to use low disturbance direct seeding without adding more openers. Since disturbance may increase the numbers of weeds and increase the persistence of weed seeds in the seedbank, some openers may be counterproductive (see also Tillage System). Seeding in paired rows may be a way of increasing the seedbed utilization without requiring more openers on seeding equipment; however, the impact of paired rows has not yet been determined.

The same trend has also been observed in wheat, even though wheat is not as competitive with weeds as barley. Research in Montana showed that increasing spring wheat seeding rate from 175 plants/m 2 to 280 plants/m 2 increased wheat yield by 12% (Stougaard and Xue, 2004), reduced wild oat biomass and seed production by 20% (Xue and Stougaard, 2002) and increased economic returns (Stougaard and Xue, 2005).

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Manipulating row spacing in crops that are generally planted as row crops (corn, soybeans, dry beans) also has potential to affect weed control. Research on dry bean in Alberta has shown that reducing row spacing to 8 inches and increasing plant density to 50 pl/m 2 (from 20) increased bean yield and resulted in better weed control, allowing for the use of reduced herbicide rates (Blackshaw et al., 2000). However, these narrow rows have been shown to more susceptible to white mould.

O’Donovan, J.T. J.C. Newman, K.N. Harker, R.E. Blackshaw and D.W. McAndrew. 1999. Effect of barley plant density on wild oat interference, shoot biomass and seed yield under zero tillage. Can. J. Plant Sci. 79:655-662.

The picture was very different when wild oats were present. The lowest seeding rate (14 plants/ft 2 ) resulted in a yield of 84 bu/ac. Increasing the target plant density to 21 plants/ft 2 increased oat yield to 97 bu/ac. However, increasing seeding rate even further (28, 35 and 42 plants/ft 2 ) did not result in corresponding yield increases (see graph at left). Yield peaked at around 100 bu/ac, well below the yield obtained where there was no wild oat pressure (Bill May, personal communication).

Stougaard, R.N. and Q. Xue. 2005. Quality versus quantity: spring wheat seed size and seeding rate effects on Avena fatua interference, economic returns and economic thresholds. Weed Research 45:351-360.

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Of 1800 species reported as weeds of rice, those of the Cyperaceae and Poaceae are predominant. The adoption of direct‐seeding has resulted in a change in the relative abundance of weed species in rice crops. In particular, Echinochloa spp., Ischaemum rugosum, Cyperus difformis, and Fimbristylis miliacea are widely adapted to conditions of DSR. Species exhibit variability in germination and establishment response to the water regime postsowing, which is a major factor in interspecifically selecting constituents of the weed flora. The relatively rapid emergence of “weedy” (red) rice, rice phenotypically similar to cultivars but exhibiting undesirable agronomic traits, has been observed in several Asian countries practicing DSR, and this poses a severe threat to the sustainability of the production system.

Rice (Oryza sativa L.) is a principal source of food for more than half of the world population, especially in South and Southeast Asia and Latin America. Elsewhere, it represents a high‐value commodity crop. Change in the method of crop establishment from traditional manual transplanting of seedlings to direct‐seeding has occurred in many Asian countries in the last two decades in response to rising production costs, especially for labor and water. Direct‐seeding of rice (DSR) may involve sowing pregerminated seed onto a puddled soil surface (wet‐seeding) or into shallow standing water (water‐seeding), or dry seed into a prepared seedbed (dry‐seeding). In Europe, Australia, and the United States, direct‐seeding is highly mechanized. The risk of crop yield loss due to competition from weeds by all seeding methods is higher than for transplanted rice because of the absence of the size differential between the crop and weeds and the suppressive effect of standing water on weed growth at crop establishment.

Stale seedbeds, tillage practices for land leveling, choice of competitive rice cultivars, mechanical weeders, herbicides, and associated water management are component technologies essential to the control of weeds in DSR. Herbicides in particular are an important tool of weed management, but hand weeding is either partially or extensively practiced in countries of Asia, Africa, and Latin America. Though yet to be globally commercialized, transgenic rice varieties engineered for herbicide resistance are a potential means of weed control. The release of herbicide‐resistant rice for red rice control in the United States has indicated the need to critically examine mitigation methods for the control of gene flow. Integrating preventive and interventional methods of weed control remains essential in managing weed communities in DSR, both to prohibit the evolution of herbicide resistance and to maximize the relative contributions of individual components where herbicides are not widely used. There remains a need to further develop understanding of the mechanisms and dynamics of rice weed competition and of the community dynamics of weed populations in DSR to underpin sustainable weed management practices.