Palmer amaranth (Amaranthus palmeri) Control in Soybeans

Introduction

Palmer amaranth (Amaranthus palmeri) is one of the most troublesome weeds in row crops of the mid-Atlantic region. Palmer amaranth plants germinate throughout the growing season, can grow to over 6 feet in height, and produce up to 600,000 seeds per plant (Keeley et al. 1987, Sauer 1955). Eight Palmer amaranth plants per yard of row can result in soybean yield losses of 79% (Bensch et al. 2006). This publication will discuss Palmer amaranth 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, Pursuit, etc.) herbicides have traditionally been used by Virginia soybean growers to control Palmer amaranth. Currently glyphosate (group 9)- and ALS (group 2)-resistant Palmer amaranth occurs throughout the soybean growing regions of Virginia. If herbicide resistance status in a field is unknown, growers should assume resistance to herbicide groups 2 and 9. In soybeans without alternative herbicide tolerance traits, this leaves only PPO-inhibiting herbicides (Flexstar, Reflex, Ultra Blazer, Cobra, etc.; group 14) as an effective postemergence options. Reliance on these herbicides alone is not sustainable, as shown by the 2015 confirmation of PPO-resistant Palmer amaranth populations in North Carolina and Tennessee, among other states. For long-term, effective control of Palmer amaranth, integrated weed management tactics such as cultural and mechanical control must be utilized in conjunction with multiple, effective herbicide sites of action (SOA).

Identification

Palmer amaranth cotyledons are linear to lanceolate in shape (Figure 1). Palmer amaranth can be differentiated from other pigweed species, such as redroot pigweed and spiny amaranth, by its lack of hair and long petiole length (Figures 2 & 3). Palmer amaranth bears separate female (Figure 4) and male flowers (Figure 5). Female flowers can be distinguished by their sharp bracts.

Integrated Weed Management Tactics

Prevention

The prevention of weed infestations is essential to a cost-effective weed management program. Equipment sanitation such as thorough cleaning of harvest and tillage equipment should always be practiced, especially when moving from weed-infested to non-infested fields. It is best to hand pull Palmer amaranth rather than driving a combine through it. If that is not possible, harvest weedy fields last to reduce spread of seed from field to field. In addition, farmers should be cautious when purchasing used combines that have previously harvested Palmer amaranth infested fields. Lastly, 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 crop rotation, maintaining optimum fertility, utilizing narrow row spacing (15-inch rows or less), recommended seeding rates, and ensuring a weed-free field at planting.

Cover crops are most effective for weed suppression (Figure 6) when high biomass levels (>7500 lbs dry biomass per acre) are achieved (Mirsky et al. 2011). To ensure high biomass levels, early planting of fall cover crops is crucial. Delayed cover crop termination can also help to increase biomass levels. Termination of cereal rye cover crop about May 1 typically maximizes biomass production. Greenhouse studies in Virginia have shown a 77% Palmer amaranth stand reduction from 9000 pounds per acre of cereal rye biomass (Rector and Flessner 2018).

Mechanical

Mechanical tactics for weed control include tillage, harvest weed seed control, and hand pulling.

Deep tillage with a moldboard plow once every 3-4 years can bury Palmer amaranth seeds to a depth where they are unable to germinate, resulting in up to a 73% reduction in emergence of pigweed (Amaranthus) species when compared to no-till production systems (Farmer et al. 2017). Annual use of a moldboard plow reduces tillage’s effectiveness since previously buried weed seeds from earlier passes will be returned to the soil surface. Immediate establishment of a cover crop after fall plowing will provide soil erosion control as well as additional cultural control of seed that were not buried.

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 Arkansas has shown up to a 72% reduction in Palmer amaranth using harvest weed seed control alone when compared to conventional harvest (Norsworthy et al. 2016). For more information on harvest weed seed control, see Virginia Cooperative Extension Publication SPES-135NP, Harvest Weed Seed Control.

Hand pulling can be an effective method to remove Palmer amaranth escapes in a field. Do not ignore escapes. When hand pulling, pulled weeds should be removed from the field. Pulled plants left in the field can reestablish roots and still produce seed.

Herbicide Programs

The basis of any weed control program is to start clean and stay clean. This begins with an effective burndown herbicide application. 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 is needed. Application as close to planting as possible is best so that herbicide activity persists for longer into the soybean crop. Postemergence herbicides should be applied with a residual herbicide before weeds reach 4 inches in height. Timing of application is extremely critical to the success of the herbicide. All applications should include multiple, effective sites of action on Palmer amaranth 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. The following herbicide programs were developed assuming glyphosate (group 9) and ALS (group 2)- resistant Palmer amaranth, but these herbicides may be utilized for control of other weed species in an herbicide program. Despite a decline in postemergence efficacy, ALS (group 2) herbicides are still useful, preemergence, in controlling Palmer amaranth. Efficacy ratings are based on data from the 2019 Mid-Atlantic Field Crop Weed Management Guide (456-016).

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.

1. Follow label instructions for rates, adjuvants, timing, soybean size, and rotational restrictions.
2. For full season plantings: Planting intervals for 2,4-D are 1.0 pt/A: 15 days, 1 qt/A: 30 days.
3. For full season plantings: There is no planting interval for Xtend soybeans following applications of new dicamba formulations. For non-Xtend soybeans and other dicamba formulations (Clarity, etc.) planting restriction is 1 inch of rain/irrigation and 14 days at 0.5 pt/A of Clarity.
4. Liberty should be applied 2 hours after sunrise and prior to 2 hours before sunset on hot, humid days. Therefore, Liberty is not typically recommended for use in April.
5. Make no more than two applications of a PPO-inhibiting (group 14) herbicide per season. Do not tank-mix with other group 14 herbicides. Allow 14 days or more between Sharpen and other group 14 herbicide applications.

1. Follow label instructions for rates, adjuvants, timing, soybean size, and rotational restrictions. If weeds were not completely controlled or have emerged since preplant burndown, add Gramoxone (22) or Liberty (10) to the options listed.
2. Treflan must be incorporated.
3. Only one application of an ALS-inhibiting (2) herbicide is allowed per year.
4. Only two applications of PPO-inhibiting (14) herbicides are allowed per year.
5. Product contains metribuzin; some soybean varieties are sensitive to metribuzin.
6. Fomesafen is an active ingredient in Prefix and Reflex. Only one application of fomesafen is allowed every other year.
7. This product should not be applied if Sharpen was applied as a burndown.
8. The combination of Lorox/Linex + Metribuzin increases the potential for soybean injury and is discouraged.

1. Follow label instructions for rates, adjuvants, timing, soybean size, and rotational restrictions. Make only one application of ALS-inhibiting (2) herbicide and no more than two applications of a PPO-inhibiting (14) herbicide per season.
2. Fomesafen is an active ingredient in Flexstar GT and Reflex. Only one application of fomesafen is allowed every other year. Only two applications of a PPO-inhibiting (14) herbicide are allowed per year.
3. Enlist Duo cannot be tank-mixed with Liberty.

References

Bensch CN, Horak MJ, Peterson D (2006) Interference of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp (A. rudis) in soybean. Weed Sci 51:37–43

Farmer JA, Bradley KW, Young BG, Steckel LE, Johnson WG, Norsworthy JK, Davis VM, Loux MM (2017) Influence of Tillage Method on Management of Amaranthus Species in Soybean. Weed Technol 31:10– 20

Flessner ML, 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.

Keeley PE, Carter CH, Thullen RJ (1987) Influence of Planting Date on Growth of Palmer Amaranth (Amaranthus palmeri). Weed Sci 35:199–204

Mirsky SB, Curran WS, Mortensen DM, Ryan MR, Shumway DL (2011) Timing of Cover-Crop Management Effects on Weed Suppression in No-Till Planted Soybean using a Roller-Crimper. Weed Sci 59:380– 389

Norsworthy JK, Korres NE, Walsh MJ, Powles SB (2016) Integrating Herbicide Programs with Harvest Weed Seed Control and Other Fall Management Practices for the Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri). Weed Sci 64:540– 550

Rector LS, Flessner ML (2018) Effects of Cereal Rye Biomass on Germination and Growth of Palmer Amaranth (Amaranthus palmeri) and Common Ragweed (Ambrosia artemisiifolia). Page 42 in Northeastern Plant, Pest, and Soils Conference

Sauer J (1955) Revision of the dioecious amaranths. Madrono 13:5–46

Walsh MJ, Harrington RB, Powles SB (2012) Harrington Seed Destructor: A new nonchemical weed control tool for global grain crops. Crop Sci 52:1343–1347

Originally published by Scott Hagood.