Early Spring Herbicide Timing for No-Till Soybeans



Nearly two-thirds of the soybean hectares in Ontario have been converted to a no-till or reduced tillage system.¹ Fields that have been continuously no-tilled may have a shift in weed species composition, densities, and pattern of emergence. Dandelion and glyphosate-resistant (GR) Canada fleabane are two weeds that are increasingly important to control early.

The reduction of tillage and interrow cultivation in combination with herbicides used as the primary weed management tactic has led to an increased relative abundance of dandelion in soybean fields.² The life cycle and potential for a biotype to be glyphosate resistant make Canada fleabane (CF) especially important to control early in the season. Growers need to take an aggressive approach to managing both weeds in no-till soybean systems.


Early Scouting


Close monitoring of fields for perennial and winter annual weeds is critical to season-long weed management. Dandelions are early-emerging perennial weeds that may be regrowing from last year or starting from seed. Growers with gaps in crop stands last year may have weeds with large dandelion root systems from the end of last season. Canada fleabane emerges in both the fall and spring. Because of the early presence and continued emergence throughout the year, CF may be one of the most serious weeds to control. The low-growing, lobed leaves of dandelion and CF may be difficult to see as their growth initiates in early spring.


Dandelion taproots have carbohydrates moving to aboveground plant parts during rapid spring growth.³ For this reason, dandelion may be difficult to control with lower labeled rates of glyphosate.⁴ Dandelion roots are also able to generate new shoots and control may be reduced later in the season. Residual herbicides should also 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.


Canada fleabane has a relatively short-lived seed. Seeds may live one to three years under laboratory conditions.⁵ Controlling CF before it produces seed is key to long-term management of this weed. Seeds do not require a period of dormancy and readily germinate in the fall, spring, and throughout the season during favourable conditions.⁹ Canada fleabane tolerates drought conditions that are stressful to corn and soybean and will need to be scouted 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 in the season is critical to maximize yield potential.


Monsanto Herbicide Treatment Options for Spring Control of Dandelion and GR Canada Fleabane in Soybeans

Pre-plant or pre-emergence


  • Roundup WeatherMAX® + Classic®
  • Roundup WeatherMAX® + Guardian® Plus
  • Roundup WeatherMAX® + Eragon (suppression)


GR Canada fleabane:

  • Roundup Transorb® HC + Eragon®
  • Roundup Transorb® HC + Optill®
  • Roundup Transorb® HC + Integrity®
  • Roundup Transorb® HC + Sencor®


Post Emergence


  • Roundup WeatherMAX® + Classic*


Spring Control


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. Soybean yields can be reduced by 16% from early weed competition.6


Dandelion populations that are left uncontrolled can become established and negatively affect soybean production. Herbicide applications that control top growth without complete control of root growth lead to this perennial weed becoming a robust plant. Roundup WeatherMAX® applied at 0.67 L/acre and 1.0 L/acre in the spring provided 80 and 92% dandelion control, respectively. However, Roundup WeatherMAX applied at 1.33 L/acre provided up to 95% control of dandelion. 4 This higher labeled rate of Roundup WeatherMAX provides more consistent control of dandelion, and is recommended when targeting dandelion in the spring.


Critical Period Of Weed Control


The critical period of weed control is a period in the crop life cycle when weed competition causes crop yield loss. The beginning of this period has been reported as the V1 or V2 soybean stage depending on timing of weed emergence.8 A delay in weed emergence in no-till is expected due to cooler soils in the spring.


To get a customized tank-mix recommendation for your farm, check out our Chemistry Recommendation tool. 


Remember to always read and follow pesticide label directions.



1. OMAFRA Staff. 2009. Soybeans>tillage options. Agronomy Guide for Field Crops. Pub. 811.

2. 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.

3. Hein, T. 2010. Dandelions: on the rise. TopCrop Agannex. www.agannex.com.

4. Sikkema, P. 2014. Spring control of dandelion in no-till as influenced by Roundup rate. https://twitter.com.

5. Shrestha, A., Hembree, K., and Wright, S. 2008. Biology and management of horseweed and hairy fleabane in California. University of California. Publication 8314.

6. Sprague, C. 2009. Weeds not controlled prior to soybean emergence can reduce yield. Michigan State University.

7. Davis, V.M., Kruger, G.R., Young, B.G., and Johnson, W.G. 2010. Fall and spring preplant herbicide applications influence spring emergence of glyphosate-resistant horseweed (Conyza canadensis). Weed Technology. 24:11-19.

8. 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.

9. 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. Nandula, V.K., Eubank, T.W., Poston, D.H., Koger, C.H., and Reddy, K.N. 2006. Factors affecting germination of horseweed (Conyza canadensis). Weed Science vol. 54: 898-902. Loux, M., Stachler, J., Johnson, B., Nice, G., Davis, V., and Nordby, D. 2006. Biology and management of horseweed. Purdue Extension. GWC-9.