Macro- and Micro-Timing of Herbicide Application

L. T. Juras

Dow AgroSciences Saskatoon, SK

Currently, crop production without tillage relies predominantly on herbicides for weed control. Consequently, weeds which are poorly controlled will tend to increase (e.g. dandelion, scentless chamomile, foxtail barley, narrow-leaved hawk's-beard), and the prospect and development of herbicide resistance will also increase. If herbicide usage is to be optimized or reduced, it must be coordinated with properly planned crop rotations, and cropping practices that enhance crop growth over weeds will need to be implemented. A competitive crop can and should be used to supplement herbicide activity. With proper herbicide timing greater consistency and a higher degree of weed control can be obtained, which will translate into higher crop yields.

Macro or Systems Approach to Herbicide Timing

Herbicide application timing can be coordinated in two ways and should not be considered in isolation. The first approximation is to consider a macro or systems approach to weed management (Derksen, 1995). Within any one cropping year, there are four opportunities to use herbicides for weed control in annual crops: before seeding, within crop, pre-harvest, and post-harvest (Figure 1). By understanding or recognizing the interrelationship between crop rotations, seeding dates and other agronomic practices with weed population dynamics, herbicide use can be more efficient and used with greater success. For example, it is not surprising that winter annual species such as flixweed, stinkweed, or shepherd's-purse are associated with reduced tillage, since preseeding tillage was the most common means of controlling annual weeds in the past. Control of many winter annuals and biennial weeds can be easily obtained with 2,4-D &/or dicamba (Banvel) if applied in the fall or early spring. Once the weeds have overwintered and resumed growth, their control is more difficult (Table 1). Moreover, fall control of winter annual weeds will also reduce the risk of crop injury from 2,4-D or dicamba residues.

Table 1. Effect of 2,4-D application time on winter annual weed control in winter wheat.

Using all four application windows may not be necessary if crop rotations are planned with weed management in mind (Derksen and Juras, 1995). By controlling weeds in the previous fall, a pre-seed burnoff or tillage may not be necessary. Increased weed densities are associated with increased soil disturbance and delayed seeding. In zero-tillage crops seeded early, weed control measures will tend to be emphasized in-crop, whereas if seeding is delayed then the majority of weed control may occur with a pre-seeding treatment and an in-crop herbicide application may not be required. Other cases, for example perennial weeds, may require consecutive treatments to ensure some yield stability in-crop, as well as taking advantage of the weed's biology to maximize herbicidal effectiveness.

Perennial Weed Control

To fully benefit from the systems or macro approach, a multi-year plan incorporating crop rotations and integrated weed management practices is required for perennial weed control. For example, a four year wheat-canola-wheat-lentil sequence where only pre-seeding and in-crop treatments are applied, will select for weeds such as Canada thistle, dandelion, scentless chamomile, or winter annual weeds (Derksen and Juras, 1995). Essentially any weed species that has a variable germination rate and that grows under a maturing crop will benefit from this series of agronomic practices. Too often a single treatment during the "in-crop" application window is applied to control Canada thistle. This allows Canada thistle and other perennial weeds to recover and flourish during the latter part of the summer and throughout the fall.

Herbicide use and their application timings should be coordinated and planned. Pre-harvest applications with glyphosate (Roundup) work well. However, there is some variability in control with ratings ranging from 50 to 85% control one year after treatment (ADF Report, 1994). Research and field demonstration data (Agriculture and Agri-Food Canada and Saskatchewan Agriculture and Food) in northeast Saskatchewan (1990-93) showed that in cereal crops pre-harvest glyphosate reduced Canada thistle shoot densities by 72% one year following treatment and post-harvest applications of glyphosate reduced shoot numbers by 25-35% the year following treatment (Table 2). The addition of Banvel in-crop did not significantly reduce Canada thistle stand density over and above the glyphosate (Roundup) treatments. Crop yield response one year after treatment reflected the level of Canada thistle control (Table 2). The addition of dicamba (Banvel) used alone or in combination with glyphosate did not improve the yield response one year after treatment.

Table 2. Canada thistle control and cereal yield one year after treatment.

Source: AAFC, Melfort; SAF, Tisdale; Head & Associates. 1994.

In canola, herbicide application reduced Canada thistle shoot densities one year after treatment by 65-70 % with clopyralid, 50% with pre-harvest glyphosate and post-harvest glyphosate applications had no significant effect on Canada thistle stand density (Table 3). The best Canada thistle control in this project was a combination of an in-crop clopyralid (Lontrel) application followed by a pre-harvest application of glyphosate. This combination reduced Canada thistle density by 80-85% one year after treatment. Crop yield in the following year's crop reflected the second year control of Canada thistle (Table 3). The highest crop yield was obtained in treatments containing clopyralid (Lontrel), either applied alone (high rate) or in combination with Roundup, applied either pre- or post-harvest.

Table 3. Canada thistle control and cereal yield one year after treatment.

Source: AAFC, Melfort; SAF, Tisdale; Head & Associates. 1994.

Similarly, the use of clopyralid/MCPA (Curtail M) in-crop in combination with glyphosate (Roundup) pre-harvest provided the greatest control of Canada thistle and perennial sow thistle one year after treatment, and subsequently the highest crop yield in the following year (Table 4).

Table 4. Canada thistle control and barley yield one year after treatment.

Source: Dow AgroSciences, 1996-97.

Micro-Timing of Herbicide Application

Within the systems or macro approach, the actual timing of herbicide application can significantly impact on the weed control achieved, and more importantly, on crop yield. In many cases herbicides are effective over a wide range of weed growth stages. This allows the producer to delay application until most weeds have emerged. However, this delay in application will cost the producer significant yield losses. For example, early removal of wild oats (2-3 leaf stage) with Achieve or Triumph Plus resulted in a 3.5-fold increase in wheat yield compared to the untreated check (Table 5). If wild oat control was delayed until the 5-6 leaf stage, the yield increase was reduced to 1.9-times the untreated check - a 45% yield loss penalty.

Table 5. Effect of stage of application on wild oat control in wheat.

Source: Kirkland - AAFC, Scott, 1994.

In canola, delaying wild oat removal until the 2-leaf stage produced an irreversible yield loss of 10-15% (Table 6). Conversely, an eight day delay in controlling wild mustard and volunteer canola in wheat resulted in a 6 bu/ac yield loss (Kirkland, 1988).

Table 6. Duration of wild oat competition and canola yield loss.

Source: Agriculture and Agri-Food Canada Research Trials - western Canada.

The yield penalty for delaying herbicide application will depend on weed density and crop stand, but perhaps more importantly, on the relative time of emergence between the crop and the weeds. Many studies have shown that crops which emerge before weeds compete better and suffer less yield losses (O'Donovan et al., 1985; O'Donovan, 1992). Approximately 3 % yield loss is observed for every day that wild oat plants emerge before the crop (O'Donovan et al., 1985). Volunteer barley extracts a higher yield penalty. A light infestation of 10 barley plants/m² in canola resulted in yield losses of 18% if it emerges 3 days before the crop. Conversely, volunteer barley that emerged after canola had little effect on canola yield even at high volunteer barley densities (O'Donovan, 1992).

Multiple weed flushes are commonly used as reasons for delaying herbicide application. This line of reasoning over-emphasizes the yield impact of later-emerging weeds and under-emphasizes the yield loss arising from weeds emerging before the crop. Research conducted at Agriculture and Agri-Food Canada at Brandon shows the effect of wild mustard competition and time of removal on canola yield and oil content (Table 7). Even if wild mustard is removed as early as the 1-leaf stage of canola and the field remained weedy to harvest, there was no significant yield loss or reduction in oil content.

Table 7. Critical period of wild mustard control in canola.

Source: Agriculture & Agri-Food Canada, Brandon.

Crop density can strongly modify the competitive effects of weeds, and consequent yield losses. For example, with a light crop stand (50 plants/m²) and with relatively light weed pressure (e.g. 20 volunteer wheat plants/m²) yield loss is greater but proportional to higher canola stand densities (100-200 plants/m²) (Fig. 1). However, under higher weed pressures pressure (e.g. 40 volunteer wheat plants/m²) canola stands with 50 plants/m² could not compete and suffered yield losses disproportionately greater than canola stands with 100-200 plants/m², i.e. nearly three-fold yield loss rather than a two-fold yield loss.

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Figure 1. The Interaction between Canola Density and Volunteer Wheat Density on Yield Loss.

Unfortunately, recommended crop seeding rates were established under optimum management conditions on hand-weeded plots. Under normal conditions and in most areas, the highest recommended canola seeding rate (8 kg/ha) would provide a plant population of between 80-120 plants/m². Given the weed pressures that exist and the greater use of post-emergence herbicides, current recommended seeding rates are likely too low for maximum yield response. In deciding which fields to treat first, crop and weed density plus their relative times of emergence should weigh heavily in the decision-making process.

With the introduction of herbicide-tolerant canola varieties, many producers have the option of seeding canola in weedy fields where previously it was not possible. Heavy weed pressures emphasize the need for earlier weed removal and high crop densities to ensure reasonable crop yields. In a Special Crop Management Study established at Indian Head, SK (1992-95), a crop sequence of cereal-oilseed-cereal-lentil was established with various levels of herbicide and fertilizer inputs as treatment rotations (Derksen and Juras, 1995). Weeds present included volunteer wheat, wild oats, wild mustard, stinkweed and others. Due to heavy weed pressures, rotations that included pre-emergent herbicides as part of the rotation reduced pre-and post-spray weed densities and increased canola yields (Table 8).

Table 8. Weed dynamics and canola yield in a high input crop rotation (1992-95).

Source: Derksen AAFC, Brandon, SCMS study, Indian Head, SK 1992-95.

Similarly, increased canola yields were observed with herbicide-tolerant canola varieties when pre-emergent herbicides were used in combination with post-emergent herbicides (Tables 9, 10 and 11).

Table 9. The use of ethalfluralin (Edge) and glufosinate (Liberty) in Liberty-Link Canola.

Source: Conservation & Development Branch of Alberta, Agriculture and Agri-Food Canada, University of Alberta, Ag-Quest, Dow AgroSciences.

Table 10. The use of ethalfluralin (Edge) and imazethapyr (Pursuit) in Smart-Canola.

Source: Conservation & Development Branch of Alberta, Agriculture and Agri-Food Canada, University of Alberta, Ag-Quest, Dow AgroSciences.

Table 11. The use of ethalfluralin (Edge) and glyphosate (Roundup) in Roundup-Ready Canola.

Source: Conservation & Development Branch of Alberta, Agriculture and Agri-Food Canada, University of Alberta, Ag-Quest, Dow AgroSciences.

In conclusion, if herbicide usage is to be optimized or reduced, it must be coordinated with properly planned crop rotations, and cropping practices that enhance crop growth over weeds will need to be implemented. A competitive crop can and should be used to supplement herbicide activity. With proper herbicide timing greater consistency and a higher degree of weed control can be obtained, which will translate into higher crop yields.

References

Button, R., Head, K. Townly-Smith, L. 1994. Canada thistle control in northeast Saskatchewan. Final report, Agriculture Development Fund, 30 pp.

Derksen, D. A. 1995. Towards a new understanding of weed management in zero-tillage systems. Proceedings: Western Canada Agronomy Workshop. Red Deer, AB. p. 192-199.

Derksen D. A. and L. T. Juras. 1996. Integrated Weed Management (IWM) and Crop Sequencing. Proceedings: Soils and Crops Workshop. Saskatoon, SK. p. 102-117.

Kirkland, K. J. 1988. Maximum economic yields as related to weed populations and herbicide use. Proceedings: Soils and Crops volume 1. p. 23-48.

O' Donovan, J. T. 1992. Seed yields of canola and volunteer barley as influenced by their relative times of emergence. Can. J. Plant Sci. 72: 263-267.

O'Donovan, J. T., E.A. de St. Remy, P. A. O'Sullivan, D. A. Dew and A. K. Sharma. 1985. Influence of the relative time of emergence of wild oat (Avena fatua) on yield loss in barley (Hordeum vulgare) and wheat (Triticum aestivum). Weed Sci. 33: 48-53.