Perennial Weed Management in the Brown and Dark Brown Soil Zones of Western Canada

D.A. Derksen, R.E. Blackshaw, and A.G. Thomas

Agriculture and Agri-Food Canada, Brandon Research Centre, Brandon, MB. (204) 726 7650; Lethbridge Research Centre, Lethbridge, AB. (403) 327-4561; and

Saskatoon Research Centre, Saskatoon, SK. (306) 956 7200

The dark brown and brown soil zones have a lower yield potential due to higher evapotranspiration (drying power), frequently have fallow in rotations, and can have lower net returns than the black soil zone. These factors influence the weeds present in this area and the approach to weed management required to obtain optimal yields and net returns. Costly one-shot approaches to weed control often do not work well even in wet areas and are more risky in the brown soil zones, therefore, a multi-year integrated approach is required, especially for perennial weeds.

Perennial weed management consists of using a combination of best management strategies, such as pre-harvest glyphosate, and weed suppression at every opportunity. For example, if a choice exists between several herbicides for annual weed control within a field, the one that provides the greatest suppression of perennial weeds should be chosen. Suppressing or controlling perennial weeds in fallow, especially in reduced tillage, requires extra management skill. Without a crop to compete with weeds during the growing season, perennial weeds can proliferate. Weeds such as foxtail barley and dandelion are favoured by chem-fallow. Frequent herbicide application to control established and establishing perennials in fallow may be required. Given the high cost of pure chem-fallow and the fact that one or two stubble-retaining tillage operations improves moisture holding capacity, a combination of herbicides and tillage has been considered the best approach.

The frequency of perennial weeds in Saskatchewan decreased from the mid-1970s to the mid-1980s due primarily to dry conditions (Table 1). Since then, these weeds have increased in frequency in all soil zones due to wetter weather and a change in crop management towards greater diversification and reduced-tillage systems. Crop diversification has meant an increased acreage of less competitive crops in which few options to control or suppress perennial weeds occurs. Reduced tillage has provided a potential environment for perennial weeds to increase if they are not managed carefully.

Canada thistle, sow-thistle, and quackgrass have increased recently. Suppressing them with in-crop herbicides coupled with pre-harvest glyphosate can provide good weed management over time. Better long-term control of quackgrass from pre-seeding burn-off treatments can be expected compared to Canada thistle and sow-thistle. Since these weeds can be managed by the same means used in other soil zones, this paper will focus on perennial weeds that are a greater problem in the brown and dark brown soil zones, such as dandelion, foxtail barley, yellow toadflax, and the winter annual weed, downy brome.

Dandelion

Dandelion is a perennial weed arising from a deep fleshy tap root. Dandelion plants form a rosette. Leaves and flower stalks arise from the root crown without a stem. Dandelion plants have milky sap. They reproduce primarily from wind dispersed seed, but new shoots can grow from root crowns. Dandelion is a native of Europe and has become a weed in lawns, pastures (particularly overgrazed ones), and roadsides. Recently, dandelion has become a weed in annual cropping systems, especially in direct seeding.

Control - Pre-harvest glyphosate is an important component of dandelion management in direct seeding systems. This approach provides good control of dandelion for subsequent crops, but does not stop crop yield loss due to competition within the crop to which it is applied. Research conducted in a zero-tillage cropping at Indian Head, SK has shown that control one year after application from recommended rates of glyphosate was greater than 90% in both lentil and wheat crops (Figure 1). Dandelion control was enhanced by the application of fall 2,4-D at rates recommended for winter annual weed control, and kept control levels above 60% three years after pre-harvest treatment. The Aloss@ of control was due to the establishment of new plants, but remained high over time due to a combination of pre-seeding burn-off with glyphosate or glyphosate plus dicamba and suppression in crops where possible.

Herbicide recommendations for in-crop control of dandelion in cereals and oilseeds are few and generally inadequate. Dicamba plus 2,4-D or MCPA, 2,4-D, MCPA, Prevail (tralkoxydim plus clopyralid plus MCPA), Poast FlaxMax (sethoxydim plus clopyralid plus MCPA), Attain (fluroxypr + 2,4-D), and CurtailM (clopyralid + MCPA) are recommended for in-crop weed control of dandelion (Table 2). Alberta weed guide provides more detail on formulations of phenoxy herbicides and information on some other products that suppress dandelion, such as linuron. Several problems exist for producers attempting to use these recommendations: i) control is limited to top growth or newly emerged spring seedlings, ii) rates to control dandelion are not listed for the phenoxy herbicides which have wide rate ranges available for use, and iii) no options exist for non-cereal crops other than flax.

Research in Saskatchewan and Alberta (L. Hall, AB Agriculture, personal communication) has shown that commercial rates of ExpressPak (tribenuron methyl plus 2,4-D ester) and Ally (metsulfuron methyl) plus 2,4-D are very effective in controlling/suppressing dandelion when compared to currently registered products. The residual nature of Ally limits the area to which this recommendation can apply. Herbicide-tolerant crops may provide new options for control or suppression of dandelion within cereal and oilseed crops. More research on in-crop control of dandelion is necessary.

Foxtail Barley

Foxtail barley is a shallow-rooted perennial bunch grass that is native to western North America. It is known for its ability to tolerate saline soils but is capable of productive growth on all soil types. It propagates mainly by seed which emerges from the top 3 cm of soil. Seed is spread by wind, machinery, and animals and germinates in the cooler temperatures of fall or spring. Seed germinating in the fall can overwinter and resume growth in early spring giving it a competitive advantage over many crops. Cultivation effectively controls seedlings and established plants. Reduced tillage allows foxtail barley to thrive.

Control - Glyphosate use appears critical to managing foxtail barley in reduced-tillage cropping systems. Glyphosate applied prior to seeding controlled small foxtail barley seedlings and increased crop yield (Table 3), but only stunted established plants. Fall-applied glyphosate was more effective in controlling established foxtail barley (Table 4), likely due to greater herbicide translocation to the root system. Glyphosate at 1 L/ac applied either pre-harvest or post-harvest gave equally good control of foxtail barley. In addition to glyphosate use, certain agronomic practices can help manage foxtail barley. Including a broadleaf crop such as flax allows some control of foxtail barley seedlings with selective grass herbicides (Table 5). Increasing the seeding rate of wheat by 50% reduced foxtail barley biomass by 20% and increased wheat yield up to 32%. Banded versus surface broadcast nitrogen fertilizer resulted in less foxtail barley biomass and increased wheat yield (Table 6). MON 375000, a new herbicide in the testing stage, can control foxtail barley in spring wheat.

Table 4. Foxtail barley biomass and crop yield response to post-harvest glyphosate.

Yellow Toadflax

Yellow toadflax is a perennial weed reproducing from seed and from shoots arising from extensively creeping roots. Yellow toadflax is distinguished by its yellow "snap dragon-like" flowers borne on leafy stems. It is a native plant of Eurasia and was introduced as an ornamental plant over one hundred years ago. Yellow toadflax is often found in waste places, pastures, roadsides, and more recently in direct-seeded fields. Its extensive root systems and tolerance to most herbicides makes it a difficult weed to control with or without tillage.

Control - Recent research conducted by Harker, O'Donovan, and Blackshaw (1996) in Alberta has found that pre-harvest glyphosate was effective in reducing yellow toadflax stands and increasing crop yields. Applications of pre-harvest glyphosate two years in a row increased control (Table 7). In another experiment, doubling crop seeding rates reduced toad flax, but to a lesser extent than treatments with glyphosate. Changing crop row spacing did not influence toadflax stands.

Downy Brome

Downy brome is a winter annual grass that is becoming increasingly troublesome in southern Alberta and Saskatchewan. Its rapid increase in density and distribution on cropland is associated with adoption of conservation tillage practices and continuous cropping. It causes the greatest problem in winter annual crops such as winter wheat and fall rye because the growth habits of the crop and weed are similar. Downy brome emerges over wide ranging soil temperature and moisture conditions and establishes well on the soil surface. It resumes growth in the spring at soil temperatures of 3 to 5 ^oC, thus attaining a competitive advantage over many other plant species. Downy brome heads out in late-May and produces mature seed by early July. Seed has little dormancy and is capable of germinating that fall. Most seed only will remain viable for 2 to 4 years. Downy brome at moderate densities emerging within 3 weeks of the crop can reduce winter wheat yield by 30 to 40%. Downy brome emerging in the spring in winter wheat is much less competitive. If well controlled before seeding, downy brome is usually less of a problem in vigorous spring-seeded crops. Poor crop stands will allow downy brome to establish and be more competitive with crops.

Control - Glyphosate is effective on small and large plants. A rate of 0.5 L/ac will kill small plants; higher rates are needed for larger plants. Plants need to be sprayed before seed heads are visible to prevent viable seed production. Gramoxone is best applied to small downy brome plants because it relies on contact action for activity. Control with cultivation on fallow or before seeding can be poor especially under moist soil conditions. A burn-down herbicide treatment will usually provide more dependable control than cultivation. Many growers find a glyphosate application in October is the best way to keep downy brome in check. Metribuzin (Sencor) at 300 g/ac applied post-emergence in the fall will give in-crop control of downy brome in winter wheat. Sencor cannot be used in fall rye. Wild oat herbicides commonly used in cereal crops are ineffective on downy brome. In oilseed and pulse crops, Treflan and Edge products will provide some control. Assure and Poast will provide complete control if applied at the 2- to 5-leaf stage. Crop rotation is one of the main ways that winter wheat growers manage downy brome. Producing winter wheat only once every 3 to 4 years keeps downy brome at low levels.

Conclusions

Pre-harvest glyphosate is the cornerstone to managing perennial weeds in direct-seeding systems when coupled with IWM practices, such as good crop agronomy to enhance crop competition and suppression by in-crop herbicides applied early in the season. The frequency of pre-harvest weed management should be determined by the abundance of perennial weeds within a field. In cycles of wet growing seasons, pre-harvest glyphosate may have to be applied more frequently (one year in two or three). During dry cycles, pre-harvest weed management may be required less frequently, but weed management in fallow will become more critical for long-term perennial weed management.

References

Derksen, D.A. and A.G. Thomas. 1996. Dandelion Control in Cereal and Oilseed Crops. Proceedings from the Expert Committee on Weeds. Victoria, BC.

Blackshaw, R.E., J.T. O'Donovan, and K.N. Harker. 1997. Integrated Management of Foxtail Barley in Conservation Cropping Systems. PARI Factbook eds. G.P. Lafond, H.M. Plas, and C.K. Pavlik. AAFC Swift Current, SK.

Harker K.N., J.T. O'Donovan, and R.E. Blackshaw. 1996. Integrated Management of Yellow Toadflax in a Zero Tillage Regime. PARI Factbook eds. G.P. Lafond, H.M. Plas, and C.K. Pavlik. AAFC Swift Current, SK.

Thomas, A.G. and R.F. Wise. 1983. Weed surveys of Saskatchewan cereal and oilseed crops from 1976 to 1979. Agric. Can. Weed Survey Series Publ. 83-6. 260 pp.

Thomas, A.G. and R.F. Wise. 1987. Weed survey of Saskatchewan cereal and oilseed crops (1986). Agric. Can. Weed Survey Series Publ. 87-1. Regina, SK. 251 pp.

Thomas, A.G., R.F. Wise, B.L. Frick, and L.T. Juras. 1996. Saskatchewan weed survey of cereal, oilseed and pulse crops (1995). Agriculture and Agri-Food Canada Weed Survey Series Publ. 96-1. Saskatoon, SK. 419 pp.