Alternative Weed Control Options

Introduction

Farmers must continuously deal with weeds in crops and their importance is reflected in the amount of manual labor, tillage, and herbicides used for their control. Herbicides are typically 20 to 30% of input costs in North American cropping systems. In Canada, over 80% of total pesticide sales are herbicides and they account for approximately $1.1 billion in annual sales.

Successful weed management will require a shift away from simply controlling problem weeds to systems that prevent weed reproduction, reduce weed emergence, and minimize weed competition with crops. This paper will explore the merits of diverse cropping practices in terms of managing weeds in a cost-effective and environmentally sustainable manner.

Diverse Cropping Practices and Weed Management

Crop rotations
Changes in weed populations are the result of selection pressures imposed by agronomic practices in conjunction with the modifying effects of the prevailing environmental conditions. Weed density data from 56 site-years of western Canadian studies indicated that the overall ranking of effects on weed populations was climate > crop rotation > tillage (Blackshaw et al. 2003). This indicates that crop rotation is indeed one of the most important factors that farmers can alter to better manage their weed infestations. Indeed, a summary of rotation studies found that weed densities in rotation were less than in monoculture in 19 of 25 cases.

Monoculture cropping facilitates an increase in weed species that are able to effectively compete with that crop or that overcome competition through some avoidance mechanism. A long-term rotation study that compared continuous winter wheat production with winter wheat grown in rotation with canola, flax, or fallow found that the major difference in weed populations was between monoculture winter wheat and the more diverse rotations (Blackshaw et al. 2001a). Mean weed densities in May in the winter wheat phase of the rotation were 49 plants m-2 in continuous winter wheat but less than 10 plants m-2 in the other rotations.

Crop diversification provides more control opportunities and disrupts the life cycle of weeds and thus their reproductive potential. Crop rotations including cereal, oilseed, and pulse crops allow for greater herbicide choice over years. Annual and perennial forages, especially when grown in rotation with annual grain crops, can be an effective strategy to reduce weed populations. Different crops are naturally planted at different times of the year and this can significantly affect weed populations. Systematically changing planting dates and crop species prevents any one weed species from developing into a major problem (Derksen et al. 2002).

Cover crops, green manures and silage crops
The use of cover crops and their residues has the potential to reduce pests including weeds through competition, physical suppression and allelopathic effects. Winter rye can be a useful cover crop to suppress weeds in fall and early spring. Undersown biennial sweetclover effectively reduced weed establishment after harvest and in early spring and its decaying residues provided excellent weed control throughout the remaining portion of the fallow year (Table 1). Higher wheat yields the following year were due to much lower weed populations and to nitrogen fixation by sweetclover. The use of green manure and cover crops such as sweetclover, red clover, and fall rye for weed management has been widely adopted by organic farmers on the Canadian Prairies but their use also has good potential in conventional farming systems.

Table 1. Effect of undersown sweetclover in 1993 on weed growth during fallow in 1994 and spring wheat yield in 1995.

Harker et al. (2003b) demonstrated that early cut cereal silage effectively lowers weed populations by terminating weed growth before viable seed production and thus weed seedbanks were reduced over time.

Intercropping and underseeding
A three-year study conducted at two locations in Alberta found that intercropping has potential as a weed management tool. Weed biomass in a barley-canola mixture was less than in monocultures of either crop (Table 2). Weed biomass in a barley-pea mixture was similar to monoculture barley but much less than in monoculture peas.

Table 2. Weed biomass response to monocultures of barley, canola, and peas and various mixtures of these crops.


Underseeded forages in annual crops add diversity to cropping systems. They contribute to weed suppression and can be harvested as hay or grazed directly by livestock. Livestock consume many weeds when grazing and thus contribute to diversified weed control.

Crop species, cultivars, seed rates and row spacing
Crop species and crop cultivars vary considerably in their competitive ability with weeds. Increased competitive ability of crops has been associated with early emergence, rapid leaf expansion forming a dense canopy, increased plant height, early vigorous root growth, and increased root size.

Spring wheat cultivars that were taller and tillered more profusely caused the greatest reductions in seed production of the simulated weeds oat and mustard (Hucl 1998). These wheat cultivars also yielded 25 to 30% more than the less competitive cultivars in the presence of competing species. There are theories that ‘competitive’ cultivars may yield less under weed-free conditions as they may put more resources into vegetative growth at the expense of grain production. However, Results of this study did not support that concept. A Danish study similarly reported that there was no correlation between competitiveness with weeds and yield potential of seven barley cultivars. This has important implications for plant breeders; they need not sacrifice yield potential when selecting for competitive genotypes.

Hybrid cultivars have been widely developed to increase yield of some crops but competitive ability with weeds may also be improved. Early vigor is often greater in hybrid than in open-pollinated canola cultivars and these hybrid cultivars have been shown to be more competitive with weeds (Harker et al. 2003a).

The establishment of a crop with a more uniform and dense plant distribution can increase its ability to suppress weeds. This is due to more rapid canopy closure that better shades weeds and to better root distribution that improves access to soil nutrients and water. A study where treatments were applied in four consecutive years found that an increase in spring wheat seed rate from 50 to 300 kg ha-1 reduced stork’s-bill (Erodium cicutarium (L.) L’Her. ex Ait.) biomass by 53 to 95% and increased wheat yield by 56 to 498%. Additionally, stork’s-bill in the soil seedbank for future weed infestations was reduced by 79% (Figure 1). The greatest weed suppression may occur when higher seed rates were combined with planting of large wheat seed.

Figure 1. Effect of increasing wheat seed rates on stork’s-bill (redstem filaree) seed in the soil seedbank at the Conclusion of a four-year study.

Some of the greatest agronomic benefits are realized when crops are grown both in narrow rows and at higher densities. Blackshaw et al. (2000a) documented lower weed biomass and greater dry bean yields when row spacing was decreased from 69 to 23 cm and when dry bean density was simultaneously increased from 20 to 50 plants m-2.

Tillage diversity
As farmers have adopted zero tillage there have been concurrent increases in numbers of certain weed species such as foxtail barley (Hordeum jubatum L.) and dandelion (Taxacum officinale Weber in Wiggers). Periodic wide-blade tillage within a zero-till system has been shown to effectively control foxtail barley (Blackshaw et al. 2000b). Tillage diversity is not something that most farmers consider but it may have merit in some situations.

Conclusion

Diverse cropping systems have many advantages in terms of increased soil health, decreased pest pressure, and more stable and potentially higher net returns. In terms of weeds, diverse cropping systems prevent any one or two weed species from becoming major weed problems and can result in reduced weed densities over time. Diverse cropping systems not only include various cereal, oilseed, pulse, and forage crops in rotation but also make use of intercropping, green manure or cover crops, and silage or hay production. Weed management can be further improved by growing competitive crop cultivars, using higher seed rates of good quality seed, planting in narrow rows, and alternating between spring-and winter-sown crops. Adoption of diverse cropping systems is required to ensure the economic and environmental health of future farming operations.

References

• Blackshaw, R.E., Larney, F.J., Lindwall, C.W., Watson, P.R., and Derksen, D.A. 2001a. Tillage intensity and crop rotation affect weed community dynamics in a winter wheat cropping system. Can. J. Plant Sci. 81:805-813.
• Blackshaw, R.E., Molnar, L.J., Muendel, H.-H. Saindon, G., and Li, X. 2000a. Integration of cropping practices and herbicides improves weed management in dry bean (Phaseolus vulgaris). Weed Technol. 14:327-336.
• Blackshaw, R.E., G. Semach, G., Li, X., O’Donovan, J.T. and Harker, K.N. 2000b. Tillage, fertiliser and glyphosate timing effects on foxtail barley (Hordeum jubatum) management in wheat. Can. J. Plant Sci. 80:655-660.
• Blackshaw, R.E., Thomas, A.G., Derksen, D.A., Moyer, J.R., Watson, P.R., Legere, A., and Turnbull, G.C. 2003. Examining tillage and crop rotation effects on weed populations on the Canadian prairies. In Kohli, R.K. and Singh, H.P., eds. Handbook of Sustainable Weed Management, Haworth Press, Binghampton, NY. (In press)
• Derksen, D.A., Anderson, R.L., Blackshaw, R.E., and Maxwell, B. 2002. Weed dynamics and management strategies for cropping systems in the northern Great Plains. Agron. J. 94:174-185.
• Harker, K.N., Clayton, G.W., Blackshaw, R.E., O’Donovan, J.T., and Stevenson, F.C. 2003a. Seeding rate, herbicide timing and competitive hybrids contribute to integrated weed management in canola (Brassica napus). Can. J. Plant Sci. 83:433-440.
• Harker, K.N., Kirkland, K.J., Baron, V.S., and Clayton, G.W. 2003b. Early-harvest barley (Hordeum vulgare) silage reduces wild oat (Avena fatua) densities under zero tillage. Weed Technol. 17:102-110.
• Hucl, P. 1998. Response to weed control by four spring wheat genotypes differing in competitive ability. Can. J. Plant Sci. 78:171-173.