Common Ragweed (Ambrosia artemisiifolia) Control in Soybeans

Common ragweed (Ambrosia artemisiifolia) is one of the most troublesome weeds in row crops of the mid-Atlantic region. Common ragweed can reach up to 6 feet in height with plants producing up to 62,000 seeds per plant and seed that can remain viable even after 39 years of burial in soil (Dickerson and Sweet 1971; Bassett and Crompton 1975). The majority of common ragweed emergence occurs in early spring, typically starting in mid-March and ending by June 1 (Meyers et al. 2004). Two common ragweed plants per 3 feet of row can result in soybean yield losses of 40-76% (Barnes et al. 2018). This publication will discuss common ragweed identification, resistance status as well as integrated weed management tactics and herbicide programs for effective, long term control.

Glyphosate (e.g. Roundup) and ALS-inhibiting (such as Classic, FirstRate, etc.) herbicides have traditionally been used by Virginia soybean growers to control common ragweed. Currently glyphosate (group 9)- and ALS (group 2)-resistant common ragweed occurs throughout the soybean growing regions of Virginia. If unknown, growers should assume all populations are resistant to herbicide groups 2 and 9. In soybeans without alternative herbicide tolerance traits, this leaves only PPO-inhibiting herbicides (Flexstar, Cobra, etc.; group 14) as an effective postemergence options. Reliance on these herbicides alone is not sustainable, as shown by the recent confirmation of PPO-resistant common ragweed populations in North Carolina, Maryland, and Delaware. For long-term, effective control of common ragweed, Integrated Weed Management tactics such as cultural and mechanical control must be utilized in conjunction with multiple, effective herbicide sites of action (SOA), which are easily recognized on product labels by group number.

Identification

Common ragweed possesses oval to spatulate shaped cotyledons (Figure 1) with a purple underside. Leaves are fernlike, hairy, and alternate although lower leaves may be opposite (Figure 2). Other characteristics can be seen in Figures 3 & 4.

Integrated Weed Management Tactics

Prevention

The prevention of weed infestations is essential to a cost-effective weed management program. Prevention tactics include equipment sanitation, such as thorough cleaning of harvest and tillage equipment. It is also vital to ensure weed seed is not introduced through other carriers such as manure and seed.

Cultural

Cultural weed management practices help to increase crop competitiveness against weeds. Such tactics include maintaining optimum fertility, utilizing narrow row spacing (15-inch rows or less), and ensuring a weed-free field at planting.

Cover crops are most effective for weed suppression when high biomass levels are achieved (Mirsky et al. 2011) (Figure 5). To ensure high biomass levels, early planting of fall cover crops is crucial. Delayed cover crop termination can also help to increase biomass levels. Research in Virginia indicates fall planted cereal rye alone can reduce common ragweed densities up to 33% when planted between mid- October and mid-November (Beam and Flessner 2019). Termination of cereal rye cover crop about May 1 typically maximizes biomass production.

Delayed planting date and crop rotation are also very effective strategies. Since common ragweed emergence is mostly complete by June 1, planting into a weed-free field after that time will greatly reduce common ragweed competition. Therefore, delayed planting, or better yet, double cropping soybeans after wheat or barley are effective cultural practices for common ragweed management. Research in Virginia indicates double cropping soybeans behind wheat can reduce common ragweed density 47% on average prior to planting compared to full season planting timing (Beam and Flessner 2019).

Mechanical

Mechanical tactics for weed control include tillage, harvest weed seed control, and hand pulling. Tillage can be extremely effective at reducing common ragweed densities prior to planting. A tillage pass utilizing a disc or chisel plow will control emerged common ragweed prior to planting (Barnes et al. 2017). Additionally, deep tillage with a moldboard plow once every 3-6 years can bury weed seeds to a depth where they are unable to germinate. Common ragweed germination is reduced when buried 1.5 inches and no germination was observed when buried at a depth of 3 inches or greater (Guillemin and Chauvel 2011). Frequent use of a moldboard plow is not as effective since previously buried weed seeds from earlier passes will be uncovered.

Harvest weed seed control has become increasingly popular in Australia where growers are dealing with multiple herbicide resistance issues. Methods of harvest weed seed control include windrow burning, chaff lining, chaff carting, and the Harrington Seed Destructor - an implement which destroys weed seeds exiting the rear of a combine (Walsh et al. 2012). Research in Virginia has shown a 20-28% reduction in common ragweed densities with harvest weed seed control when compared to conventional harvest (Beam et al. n.d.). For more information on harvest weed seed control, see Virginia Cooperative Extension Publication SPES-135NP.

Herbicide Programs

The basis of any weed control program is to start clean and stay clean. This begins with an effective burndown application as close as possible to planting. Burndown applications should include at least two effective sites of action for herbicide resistance management.

To stay clean, a pre-emergence application, either applied at planting or with burndown, should be followed with a post-emergence application before weeds reach 4 inches in height. These applications should also include multiple, effective sites of action on common ragweed whenever possible.

All herbicide applications should follow label directions and utilize adjuvants, nozzles, and spray volume listed on the label. Herbicides must be applied at the full labeled rate to ensure control and reduce the possibility of escapes and resistance. Care must be taken with respect to rotation restrictions for future crops. The following herbicide programs were developed assuming glyphosate and ALS-resistant common ragweed, but these herbicides may be utilized for control of other weed species in an herbicide program. Weed control ratings are based on data from the 2019 Field Crops Pest Management Guide (Virginia Cooperative Extension Publication 456-016). Please consult this publication for more information.

Pesticide Precautions

Select and use all pesticides carefully. Before using any pesticide, read the instructions printed on the label of its container; follow those instructions, heed all cautions and warnings, and note precautions about residues. Store pesticides in their original containers. Store them where children and animals cannot get to them — away from food, feed, seed, and other materials that could become harmful if contaminated. Dispose of empty pesticide containers in the manner specified on their labels. See your doctor if symptoms of illness occur during or after use of any pesticide.

Commercial products and/or services are named in this publication for information purposes only. Virginia Cooperative Extension, Virginia Tech, and Virginia State University do not endorse or warrant these products and/or services, and they do not intend or imply discrimination against other products and/or services that also may be suitable. 

References

Barnes, E.R., Jhala, A.J., Knezevic, S.Z., Sikkema, P.H., Lindquist, J.L. 2018. “Common Ragweed (Ambrosia artemisiifolia L.) Interference with Soybean in Nebraska.” Agronomy Journal 110:646-653.

Barnes, E.R., Werle, R., Sandell, L.D., Lindquist, J.L., Knezevic, S.Z., Sikkema, P.H., Jhala, A.J. 2017. “Influence of Tillage on Common Ragweed (Ambrosia artemisiifolia) Emergence Pattern in Nebraska. Weed Technology 31:623-631.

Bassett, I.J., Crompton, C.W. 1975. “The Biology of Canadian Weeds. 11. Ambrosia artemisiifolia L. and A. psilostachya DC.” Canadian Journal of Plant Science 55:463- 476.

Beam, S.C., Flessner, M. 2019. Integrated weed management systems to control common ragweed in Virginia soybean. Proceedings of the Weed Science Society of America Annual Meeting 59:241.

Beam, S.C., Mirsky, S., Cahoon, C., Haak, D., Flessner, M.L. n.d. Harvest Weed Seed Control of Italian Ryegrass (Lolium perenne ssp multiflorum), Common Ragweed (Ambrosia artemisiifolia), and Palmer Amaranth (Amaranthus palmeri). Weed Technology In Press.

Dickerson, C. T., Sweet, R.D. 1971. “Common Ragweed Ecotypes.” Weed Science 19:64- 66.

Guillemin, J.P., Chauvel, B. 2011. Effects of the Seed Weight and Burial Depth on the Seed Behavior of Common Ragweed (Ambrosia aretemisiifolia). Weed Biol and Manag 11:217-223.

Flessner, M.L., Chandran, R., VanGessel, M., Lingenfelter, D., Wallace, J., Johnson, Q., Vollmer, K., Hines, T. 2019. “Weed Control in Field Crops.” In Pest Management Guide: Field Crops. Virginia Cooperative Extension Publication 456-016.

Meyers, M.W., Curran, W.S., VanGessel, M.J., Calvin, D.D., Mortensen, D.A., Majek, B.A., Karsten, H.D., Roth, G.W. 2004. “Predicting Weed Emergence for Eight Annual Species in the Northeastern United States.” Weed Science 52:913-919.

Mirsky, S.B., Curran, W.S., Mortensen, D.M., Ryan, M.R., Shumway, D.L. 2011. “Timing of Cover-Crop Management Effects on Weed Suppression in No-Till Planted Soybean using a Roller-Crimper.” Weed Science 59:380-389.

Walsh, M.J., Harrington, R.B., Powles, S.B. 2012. “Harrington Seed Destructor: A New Nonchemical Weed Control Tool for Global Grain Crops.” Crop Science 52:1343-1347.