A Common Sense Approach to Weed Management in Direct Seeding.

D. A. Derksen, Agriculture and Agri-Food Canada, Brandon, MB

(portions of the following paper were published in the proceedings from the 1995 Western Canada Agronomy Workshop held in Red Deer, AB)

Abstract: A new understanding of weed community dynamics and integrated weed management (IWM) strategies in zero-tillage systems is necessary to avoid adverse weed community changes and to reduce farmers' reliance on herbicides. Weed communities are the end result of the agronomic practices used to manage weeds. Negative weed community changes can be inhibited by varying management strategies. Reduced herbicide usage in zero tillage can be achieved by selectively using pre-seeding burn-off, in-crop, pre-harvest, and post-harvest herbicide options in concert with crop rotations and varied seeding dates. Data from a new management study at Indian Head are used to support the concepts presented and to show that low-input rotations are possible in zero-tillage productions systems.

Introduction

Figure 1: Agronomic factors that affect weed communities

The management of weeds in zero tillage and direct seeding is constantly raised as a major issue by producers adopting these conservation-tillage practices. Concerns relate to the potential for the increased cost of weed control and to the spectre of difficult to control weeds becoming predominant problems. Weed changes in reduced tillage have recently been reported by Blackshaw et al. (1994), Derksen et al. (1993 and 1994) and Moyer et al. (1994). The wise use of herbicides is important in zero tillage (Derksen et al. 1995). Reducing the amount of herbicide could increase net returns and reduce environmental concerns regarding this emerging production system. The objective of this paper is to present a new understanding of integrated weed management (IWM) in zero tillage that has application to direct seeding and other forms of conservation tillage.

Figure 3: Weed Management Options in an annual crop in conventional tillage.

Figure 2: Weed Management Options in an annual crop in zero tillage.

Weeds present within a field are the result of three factors that can be controlled by farmers, namely: tillage practices, herbicide practices, and cropping practices (Figure 1)(Lafond and Derksen 1995). These factors are interactive. For example, zero-tillage farmers eliminate tillage and therefore rely on herbicides and cropping practices to control weeds. If herbicide usage is to be reduced in zero tillage, then cropping practices that optimize crop growth at the expense of weeds must be employed. These cropping practices are components of integrated weed management (IWM) and include: crop rotation, optimizing nutrient levels, selective placement of fertilizer, varied dates of seeding, optimum seeding rates, the use of clean seed and equipment, etc. (Swanton et al. 1991).

Systems Approach to Understanding Weed Management in Zero-Tillage Systems

In conventional tillage, herbicide usage has been focused on the in-crop control of weeds using pre- or post-emergence herbicides and more recently on pre-harvest herbicide application (Figure 2). Consequently, most of our weed management strategies in conventional-tillage systems focus on broad spectrum herbicide mixtures that attempt to control as many weed species at as wide a range of leaf stages as possible. This has served agriculture well until the problem of weed resistance to herbicides occurred due to the over usage.

The approach of using broad ranging "one-shoot" herbicide mixtures for weed management was transplanted to zero-tillage with the addition of herbicide application pre- and post-seeding (Figure 3). Consequently, herbicide usage has gone up in zero tillage. To reduce herbicide usage and costs, the emphasis on in-crop treatments will have to decrease. For example, by using IWM strategies, such as the threshold concept, the broad ranging "one-shoot" herbicide mixture may not be required every year.

Figure 4: Typical Weed Management in zero tillage.

To reduce costs, the typical approach being used by zero-tillage farmers today is to apply a pre-seeding burn-off herbicide treatment, usually glyphosate (Roundup) or glyphosate mixtures followed by the same in-crop herbicide mixture used in conventional-tillage systems (Figure 4). In this situation, all of the weed control for one year occurs within four to six weeks. Therefore, weeds that germinate and establish after in-crop herbicide usage have become increasing problems. This is analogous to the development of herbicide resistance, in that, applying herbicides in the same pattern in zero tillage year after year has selected for late emerging weeds. Furthermore, these weeds are difficult to control with next spring's burn-off treatment. Canada thistle, dandelion, and night-flowering catchfly are examples. In order to stop the build up of these weeds, the occasional use of a pre- or post-harvest herbicide will be necessary.

Figure 5: Weed Management Options for early and late seeding dates in zero tillage.

Figure 6: Weeds present at seeding in 1994 at Indian Head in the Direct Seeded Canola Study.

Seeding date and soil disturbance level can have an impact on the herbicides required to control weeds. In a three-year study at Indian Head, it was found that soil disturbance (tillage) increased weed densities and that early in the season weed densities were lower than at the end of the seeding season (Figure 5). Therefore, if soil disturbance is minimized by the use of zero-tillage, the pre-seeding burn-off treatment may be all that is required to control weeds at later dates of seeding. This is possible because a tillage operation that encourages a new flush of weed growth within the crop is eliminated. Furthermore, zero-tillage crops seeded early in the season may not require a pre-seeding burn-off or may require lower rates of burn-off treatments. The success of these approaches is dependent on environmental conditions, but illustrates the potential to reduced herbicide usage in zero-tillage systems (Figure 6).

To optimize the benefits of cropping practices on weed control, factors such as crop sequence, crop competitive ability, crop life cycle, seeding dates, and herbicide usage, must come together into a multi-year plan. Figure 7 illustrates a cereal-oilseed-cereal-pulse rotation that varies seeding dates and herbicide usage in zero-tillage. The early seeded pea crop does not receive a pre-seeding burn-off based on pre-harvest or fall weed control the year before. If a pre-seeding burn-off were required low rates of glyphosate may be all that was necessary. There is an option for either pre- or post-harvest herbicide application in the pea year depending on the weed species present. In this illustration, the seeding of the canola crop is delayed long enough for the burn-off treatment to control most of the weeds. Because weed emergence from a tillage operation is eliminated, a reduced in-crop herbicide application may be sufficient to suppress weed growth to the extent required to achieve optimum crop yield. Since the wheat crops are sown mid-season, some pre-seeding burn-off treatment and some in-crop treatments will likely be necessary. The figure illustrates several options for pre- and post-harvest weed control. Depending on the severity of perennial and winter annual weeds, these options may have to be exercised more frequently. Of greatest importance is the potential cumulative effect of managing weeds at different times of the season throughout the four years. This is a means of suppressing weed community changes.

Figure 7: A four year crop sequence illustrating different weed management options in zero tillage.

The previous figures illustrate that weed communities are the result of the selection pressure imposed on them. Varying the selection pressure is the primary tool at the disposal of the zero-tillage farmer to reduce herbicide usage and to reduce the likelihood of adverse weed community changes. Crop sequences could be expanded to include other cropping options that vary selection pressure on weeds, such as the production of winter cereals and forage crops. The use of surface applied granular herbicides in zero-tillage could also be added to a multi-year weed management program to control certain broadleaf weeds and to reduce the potential for weed resistance. Furthermore, using the threshold concept to potentially eliminate one application of a wild oat herbicide in four years would reduce herbicide usage, vary selection pressure, and reduce the changes of herbicide resistance occurring.

Specific Examples of Weed Management Options for each Herbicide Timing in Zero-Tillage Systems

Figure 8: The relative importance of herbicide usage as it varies with crop seeding date in zero tillage systems.

Pre-seeding Burn-off treatments: Tillage encourages weed germination throughout the spring. In zero tillage, weed emergence is slow in early spring, but the majority of weeds generally emerge by mid-season and a "second flush" of weeds does not occur because of the absence of tillage. For early seeded crops, the in-crop treatment is most important while for late seeded crops the burn-off treatment is the most important (Figure 8). This principle must be applied with flexibility. For example, if winter annual weeds were not controlled during the previous fall then a pre-seeding burn-off treatment may be necessary early in the season. Furthermore, if the season is initially dry followed by extensive rainfall, then weeds may germinate within the crop at later dates of seeding.

Figure 9: 1994 yield of wheat following canola (see text for details of rotations).

Figure 10: 1994 yield of wheat following lentil (see text for details of rotations).

In-crop herbicide usage: Several options exist for reducing in-crop herbicide usage including: reduced herbicide rates, using thresholds to avoid herbicide application where possible, or using cropping practices, such as delayed seeding, to avoid in-crop herbicide usage. The following example illustrates the use of delayed seeding and crop rotation to avoid in-crop herbicide usage. The Special Crop Management Study (SCMS) is a new study at the Indian Head Experimental Farm. There are six rotations in both zero and conventional tillage that follow a cereal-oilseed-cereal-pulse sequence similar to Figure 7. In this paper, only rotations one to three will be presented. All three rotations follow a spring wheat-canola-spring wheat-lentil sequence. Rotations one and two are high-input approaches where rotation one uses only post-emergence herbicides and rotation two uses pre-emergence herbicides in the broadleaf crop years and post-emergence herbicides in wheat. Rotation three is a low-input rotation where the wheat seeding is delayed by 7-14 days and no grassy weed herbicides are applied in-crop. A reduced broadleaf weed herbicide approach is used in all crops.

Figure 11: Total weed densities in 1994 prior to June in-crop herbicide application (see text for details of rotations).

Figure 12: Total weed densities in July of 1994 (see text for details of rotations).

In 1994, wheat yields were not reduced in the low-input rotation despite the lack of wild oat control (Figures 9 and 10). This translated into greater net returns. Yields were similar in both tillage systems and were greater following lentil than canola. The 1994 results are typical of those occurring in 1992 and 1993. Due to the rotational effect of controlling wild oats in the non-wheat years within canola and lentil crops and the effect of controlling wild oats with Roundup or tillage prior to seeding, weed densities were the same in the high and low input rotations (Figures 11 and 12).

Pre-harvest herbicide usage: Pre-harvest herbicide usage is a relatively new option for farmers in zero- and conventional-tillage systems. Excellent control of Canada thistle, quackgrass, and sow-thistle has been noted using Roundup. There may also be the potential to control or suppress many other weeds that are difficult to control even with Roundup. For example, in the study described above, pre-harvest Roundup applications were made to the lentil phase of the crop rotations. Seventy-five to eighty precent control of dandelion has occurred. Further research is required, but pre-harvest Roundup has also suppressed other difficult to control weeds, such as toad flax.

Fall post-harvest herbicide usage: The use of phenoxy type herbicides has long been recommended to control winter annual weeds, such as stinkweed and flixweed. Due to the reduced cost of Roundup and the availability of Rustler (glyphosate plus dicamba), fall control of winter annual weeds in zero-tillage systems is no longer practised on a regular basis. Some weeds that occur as summer annuals in conventional tillage can over winter in zero-tillage stubble and are difficult to control the following spring. Weeds such as night-flowering catchfly and blue bur have been confirmed to have a winter annual habit in zero tillage and cleavers has been reported to do so. Since these weeds are not controlled with the low rates of phenoxy herbicides used for winter annual weed control, further research is required with different herbicides or herbicide combinations. Moreover, some winter annuals and biennial weeds that generally occur in low densities may increase in zero tillage. Of particular concern are biennial wormwood, scentless chamomile, fleabane, pygmy flower, and American dragon's head.

Dandelion: A Common Sense Approach to Management

Dandelion has become an increasing weed problem in recent years due to the adoption of reduced tillage, especially reduced-tillage systems that rely solely on pre-seeding burnoff and in-crop herbicide treatments (Figure 4). Dandelion is an introduced perennial weed with a deep tap root that reproduces only from wind-dispersed seed and flowers from early spring to fall. Although it is relatively easy to control with tillage, it is difficult to kill with herbicides, especially when well established.

Table 1.0. Crop rotations. The rotations move from right to left (eg: in R5 spring wheat is grown after sunola).

^aPost-emergence herbicides used in the spring wheat phases and pre-emergence herbicides (trifluralin) used in canola and lentil.

^bIn this rotation grassy weeds were not controlled in the spring wheat phases and reduced herbicide levels were used for broadleaf weed control. Wheat seeding was delayed 10-14 days compared to other rotations.

^cThis rotation used the same herbicide approach as R3 and the rates of fertilizer used for all crops were half that applied in the other rotations (ie: one half soil test recommendations)

Note: Each of these rotations was present in zero- and conventional-tillage systems with all phases of the rotation present each year, for a total of 192 plots.

Figure 13. Visual Control of Dandelion rated in the spring of 1995 in the high-input (fall 2,4-D) and low-input (no fall 2,4-D) plots. Pre-harvest application of glyphosate.

Control of dandelion was observed in the Special Crops Management Study described earlier. The study has six crop rotations (Table 1). The following additional experimental information is required to understand the results illustrated in Figures 13 and 14. Pre-harvest glyphosate (Roundup) was applied at 890 g ai/ha (1.0 L/ac) to the lentil phase of all six crop rotations, therefore, the effect of pre-harvest glyphosate can be evaluated one, two, and three seasons after treatment (Figure 13). Given the high density of Canada thistle present in 1994, pre-harvest glyphosate was also applied to the wheat plots that followed the oilseed phase of the rotations, therefore a comparison in dandelion control could be made between wheat and lentil in 1995. The high-input rotations (1,2,5, and 6) all were treated with 2,4-D at 420 g ai/ha (0.34L/ac of the 500 formulation) in the fall following of each year, while the low-input rotations (3 and 4) receive no fall herbicide treatments. The wheat crops in the high-input rotations received fenoxaprop-p-ethyl + MCPA + thifensulfuron methyl (Truimph-Plus) while the low-input rotations were treated with tribenuron methyl + 2,4-D ester (Express Pak). The high-input canola crops were treated with clethodim (Select) plus ethametsulfuron methyl (Muster) while the low-input rotation received only clethodim. The high-input lentils were treated with metribuzin (Sencor) at full rate and the low-input lentil and peas were treated with metribuzin at 2/3 full rate.

Figure 14: Dandelion density in wheat plots in high-input (fall 2,4-D) and low-input (no fall 2,4-D) rotations. Pre-harvest glyphosate applied one and three seasons prior to data collected in wheat plots.

Dandelion control was visually rated in the spring of 1995 (Figure 13). One season after pre-harvest glyphosate, dandelion control was greater than 95%. In general, the high-input rotations had better control ratings than the low-input rotations with the difference between the rotations increasing with time. This was due to the annual post-harvest application of 2,4-D in the high-input plots which suppressed dandelion even though it was applied at a low rate usually used for winter annual weeds . Dandelion control when pre-harvest glyphosate was applied to wheat was only slightly lower than when applied to lentil. This occurred despite the greater biomass of the standing wheat crop which could have potentially intercepted more glyphosate than the lentil canopy. Three seasons after treatment dandelion control was greater than 75% in the high-input rotations, but had dropped below 50% in the low-input treatments.

Figure 14 compares dandelion densities in the high-input and low-input rotations in July of 1993, prior to in-crop spraying in 1994, and in July of 1994. Although there were more dandelion plants in the low-input wheat following oilseed plots, control of dandelion was better due to the use of tribenuron plus 2,4-D ester compared to thifensulfuron plus MCPA.

Crop rotation, pre-harvest glyphosate, post-harvest 2,4-D, and the use of herbicides that suppress dandelion within crop are important components of an integrated management system for dandelion. Further research is required on the effectiveness of post-harvest glyphosate and the suppressant effect of other in-crop herbicides on dandelion.

Conclusions

Weed communities change in response to changes in the environment and changing agronomic practices. Varying agronomic practices, such as rotating crops and seeding dates, can be used to suppress weed community changes and reduce herbicide usage. Determining which agronomic practice has lead to a specific weed control problem may be the first step toward solving the problem. The continued development of IWM strategies for conservation tillage systems is essential to enhance the sustainability of the system.

Acknowledgements

The financial support for this research from Agriculture and Agri-Food Canada and the Parkland Agricultural Research Initiative are gratefully acknowledged.

References

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