Early Spring Herbicide Timing for No-Till Soybean

Start Clean, Stay Clean

In Canada, soybeans have the potential to yield well under a variety of tillage systems, including no-till. Roughly two-thirds of the soybean hectares in Ontario, for example, have been converted to no-till or reduced tillage systems.1 However, fields under continuous no-till may have increasing weed issues if not managed correctly, including changes in weed species composition, densities, and pattern of weed emergence. Soybean growers often hear the advice, ‘start clean, stay clean.’ Controlling weeds early, during the critical weed-free period for soybeans, is important for minimizing yield losses from weed competition. Soybeans do not compete well with weeds and can be adversely affected especially right after germination. Yield loss estimates to weed competition ranges from 20-40 percent.^2,3 Soybeans need a leg-up until large enough to crowd out their weed competitors. Effective burndown and soil-applied residual herbicides should be applied prior to crop emergence.

Know Your Weeds

Growers need to take an aggressive approach to managing weeds in no-till soybean systems. Knowing which weeds are present will help growers develop an appropriate control strategy. Among the weeds that are increasingly important in no-till systems in Canada are dandelion, Canada fleabane, Palmer amaranth, waterhemp, and kochia.

• Dandelion - The reduction of tillage and interrow cultivation, in combination with herbicides used as the primary weed management tactic, has resulted in increased relative abundance of dandelion in soybean fields.⁴ Dandelion taproots have carbohydrates moving to aboveground plant parts during rapid spring growth, making dandelion difficult to control with lower labeled rates of glyphosate^,5,6 Dandelion roots are also able to generate new shoots, making control difficult later in the season. Residual herbicides should be considered to manage germinating dandelion seeds. Seeds falling on wet soil in June can establish new plants; some seeds may remain dormant and maintain the seed bank from one year to the next.
• Canada fleabane (CF) - The life cycle and documented potential for biotypes to be glyphosate-resistant make Canada fleabane especially important to control early in the season. Controlling CF before it produces seed is key to long-term management of this weed. Canada fleabane seeds lack a dormancy requirement and readily germinate in the fall, spring, and throughout the season during favourable conditions.⁷ Canada fleabane tolerates drought conditions typically stressful to soybean, and scouting needs to occur throughout the season. Biotypes of Canada fleabane have been confirmed resistant to both group 2 and 9 herbicides in Ontario. Managing herbicide resistant Canada fleabane early when weed seedlings are small enough to be killed is critical to maximizing crop yields.
• Palmer amaranth -- Palmer amaranth is an aggressive weed – it can grow five to seven centimetres a day, and a single plant can produce a million seeds. It has a prolonged emergence period, rapid growth rate, prolific seed production, and propensity to evolve herbicide resistance quickly.⁸
• Waterhemp -- Contaminated combines, other farm equipment and water can transport waterhemp and Palmer amaranth seeds from field to field. Learning the slight differences appearances between waterhemp and other pigweed species is important.⁹
• Kochia – Unlike most weeds, kochia was introduced as an ornamental planting by European immigrants and became widely established. It has prolific seed production reproducing from seeds and typically producing around 15,000 seeds per plant¹⁰ Seeds are dispersed in the fall when the plant matures. If left undisturbed, it becomes a tumbleweed resulting in long-distance seed dispersal. Kochia can also be transported by combines and other farm equipment. It is tolerant to drought and has high tolerance to saline soils. Its seeds are somewhat short-lived in the soil lasting only two to three years. Kochia also outcrosses producing large amounts of pollen as a source of significant genetic variability and high potential to create herbicide resistance.

Early Scouting

To determine which weeds are present, and because key characteristics of these weed species make them so prolific and difficult to control, early scouting is critical to successful management of weeds in a no-till scenario. Four main considerations for effective weed control are:

• Weeds overwintering - early emerging perennial weeds that have regrown from last year or started from seed are a challenge to growers.
• Season long weed emergence - weeds like Canada fleabane need to be monitored early and continuously since they emerge in both the fall and spring.
• Correct weed identification - Knowing how to identify specific patterns of weed emergence is also critical early in the season. For example, distinguishing waterhemp from redroot pigweed at an early stage and tailoring control methods and chemical application around the predominant weeds emerging in no-till fields (See Table 1).
• Seed Banks - Several weed species are prolific, creating huge seed banks. A single waterhemp plant can produce between 250 thousand and a million seeds, making early identification and control critical to successful control in a no-till situation.

Critical Period for Weed Control

The critical period of weed control is a period in the crop life cycle when weed competition causes crop yield loss. Yield loss is minimized when weeds are controlled during this period. For soybeans, the critical weed-free period corresponds to the first-third trifoliate leaf stage (V1-V3). The beginning of this period has been reported as the V1 or V2 soybean stage depending on timing of weed emergence.¹¹ A delay in weed emergence in no-till is expected due to cooler soils in the spring; therefore, scouting fields to identify optimal timing of herbicide application is critical for weed management. Effective weed control is dependent on several factors, including climate, soil type, moisture levels, and weed species, density, and size.

Herbicide Options

For soybean growers with dandelion, Canada fleabane, pigweed, waterhemp and other broadleaf weed species present in their fields, the use of pre-plant and pre-emergence residual herbicides offers the best opportunity for management. In-season management options for these weeds are limited and may not provide effective control once the soybean crop has emerged. Table 1 shows available herbicide options from Bayer.

Burndown and Pre-Emergence Herbicides

Burndown and pre-emergence residual herbicides provide effective control of weeds as seedlings germinate and emerge from the soil and help reduce the seedbank during the growing season. In addition, applying an herbicide early and ensuring the plants are growing vigorously, buys the grower time to minimize competition with weeds during a critical part of the crop development. With herbicides such as Xtendimax and Roundup Xtend, the key to success is early application at first pass with a high rate of the herbicide. Including pre-emergence herbicides in a spray program can reduce the post-emergence herbicide applications by ensuring a healthy crop that can compete for resources and crowd out weeds.

Herbicide Application Timing

Key to effective weed control is timing of herbicide applications. Warm spring temperatures initiate weed growth sooner, and activity of post-emergence herbicides may be limited to suppression of perennial and winter annual weeds during the growing season. Spring control for perennial and winter annual weeds should begin before planting or emergence of the crop.

Table 1 lists possible management options for controlling weeds pre- and post-emergence. Preplant and preemergence residual herbicide applications help manage weed resistance and early season weed competition.

*for a complete list of species controlled by this product please read the label