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
However, fields under continuous no-till may have increasing weed issues if not managed correctly, including changes
weed species composition, densities, and pattern of weed emergence. Soybean growers often hear the advice, ‘start
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
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.
Figure 1. Problem weeds occurring in no-till soybean production systems in Canada
- 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
lower labeled rates of glyphosate,5,6 Dandelion roots are also able to generate new shoots, making
later in the season. Residual herbicides should be considered to manage germinating dandelion seeds. Seeds falling
wet soil in June can establish new plants; some seeds may remain dormant and maintain the seed bank from one year
- 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
management of this weed. Canada fleabane seeds lack a dormancy requirement and readily germinate in the fall,
and throughout the season during favourable conditions.7 Canada fleabane tolerates drought conditions
stressful to soybean, and scouting needs to occur throughout the season. Biotypes of Canada fleabane have been
resistant to both group 2 and 9 herbicides in Ontario. Managing herbicide resistant Canada fleabane early when
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
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
- 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
plant10 Seeds are dispersed in the fall when the plant matures. If left undisturbed, it becomes a
in long-distance seed dispersal. Kochia can also be transported by combines and other farm equipment. It is
drought and has high tolerance to saline soils. Its seeds are somewhat short-lived in the soil lasting only two to
years. Kochia also outcrosses producing large amounts of pollen as a source of significant genetic variability and
potential to create herbicide resistance.
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
in both the fall and spring.
- Correct weed identification - Knowing how to identify specific patterns of weed emergence is also critical early
the season. For example, distinguishing waterhemp from redroot pigweed at an early stage and tailoring control
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
250 thousand and a million seeds, making early identification and control critical to successful control in a
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
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
for weed management. Effective weed control is dependent on several factors, including climate, soil type, moisture
levels, and weed species, density, and size.
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
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
the soil and help reduce the seedbank during the growing season. In addition, applying an herbicide early and
the plants are growing vigorously, buys the grower time to minimize competition with weeds during a critical part of
crop development. With herbicides such as Xtendimax and Roundup Xtend, the key to success is early application at
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
and activity of post-emergence herbicides may be limited to suppression of perennial and winter annual weeds during
growing season. Spring control for perennial and winter annual weeds should begin before planting or emergence of the
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.
Table 1. Bayer herbicides used for control of major weed species in no-till
|Weed Species controlled
|XtendiMax® herbicide with VaporGrip® Technology
|Canada fleabane, Pigweed (redroot, Russian, smooth) Kochia, Palmer amaranth, Waterhemp and other broadleaf
|Sencor® 75 Herbicide
|Dandelion (seedling) Redroot pigweed, Common chickweed, plus additional broadleaf and grass species*
|Roundup Xtend® herbicide
|Canada fleabane Chickweed, Kochia, Dandelion, Pigweed (redroot, Russian, smooth)
*for a complete list of species controlled by this product please read the label
1 Brown, C. ed. 2017. Soybeans: Tillage Options. Ontario Ministry of Agriculture, Food, and Rural
Affairs. Agronomy Guide
for Field Crops. Pub. 811, p. 41.
2 2019. Eliminating early weed competition in soybeans. Top Crop Manager.
3 Alberta Pulse Growers. 2021. Soybean – Weed Control.
influences weed population and
community dynamics. Weed Science. Vol.54:47-58.
4 Swanton, et. al. 2006. Management in a modified no-tillage corn soybean-wheat rotation influences
weed population and
community dynamics. Weed Science. Vol.54:47-58.
5 Hein, T. 2010. Dandelions: on the rise. TopCrop Agannex. https://www.agannex.com.
6 Sikkema, P. 2014. Spring control of dandelion in no-till as influenced by Roundup rate. https://twitter.com.
7 Tozzi, E. and Van Acker, R.C. 2014. Effects of seedling emergence timing on the population dynamics
of horseweed (Conyza
canadensis var. canadensis). Weed Science: 62:451- 456.
8 Arnason, R. 2019. Palmer amaranth threat intensifies. The Western Producer. https://www.producer.com/news/palmer-amaranth-threat-intensifies/.
9 Soybean School. 2020. Waterhemp spreads to Western Canada. RealAgriculture. https://www.realagriculture.com/2020/04/soybean-school-waterhemp-spreads-to-western-canada/.
10 Weir, T. 2021. Kochia becomes major problem across Western Canada. The Western Producer. https://www.producer.com/crops/kochia-becomes-major-problem-across-western-canada/.
11 Halford, C., Hamill, A.S., Zhang, J., and Doucet, C. 2001. Critical period of weed control in
no-till soybean (Glycine
max) and corn (Zea mays). Weed Technology. Vol. 15:737-744.
Brown, C. ed. 2017. Weed Control. Ontario Ministry of Agriculture, Food, and Rural Affairs. Agronomy Guide for
Crops. Pub. 811, p. 277.
ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary from location to location
and from year to year,
as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and
whenever possible and should consider the impacts of these conditions on the grower’s fields.
Tank mixtures: The applicable labeling for each product must be in the possession of the user at
the time of
application. Follow applicable use instructions, including application rates, precautions and restrictions of each
product used in the tank mixture. Bayer has not tested all tank mix product formulations for compatibility or
performance other than specifically listed by brand name. Always predetermine the compatibility of tank mixtures
mixing small proportional quantities in advance. Bayer, Bayer Cross, Roundup Xtend®, Sencor®, VaporGrip® and
are registered trademarks of Bayer Group. Used under license. All other trademarks are the property of their
owners. Bayer CropScience Inc. is a member of CropLife Canada. ©2021 Bayer Group. All rights reserved. 2004_S7_CA