Is my crop rotation selecting for group 2 resistance? What s in my herbicide anyways? Jeff Stachler, Auglaize County, The Ohio State University, Wapakoneta, OH 45895 E-mail: stachler.1@osu.edu Resistance of weeds to herbicides continues to increase around the world. This is due to the near total reliance upon herbicides to manage weeds and in many cases a reduction in the number of crops grown in rotation. In order to talk about the impact of crop rotation on weed resistance we need to look at the current status of weed resistance in Manitoba and surrounding areas. In Manitoba, wild oat and green foxtail have been confirmed resistant to the ACCase inhibiting herbicides (Group 1). Wild oat, green foxtail, kochia, wild mustard, common hempnettle, stinkweed, cleavers, common chickweed, Powell amaranth, pale smartweed, redroot pigweed, and spiny annual sowthistle have been confirmed resistant to Acetolactate Synthase inhibiting herbicides (Group 2). The last case of Group 2 resistance was documented in 2009. Green foxtail has been selected resistant to Mitotic Inhibitor herbicides (Group 3). Wild mustard has been confirmed resistant to Growth Regulator herbicides (Group 4) including 2,4-D, dicamba, MCPA, mecoprop, and dichloroprop. Wild mustard has been confirmed resistant to Photosystem II inhibitors having binding site A (Group 5). Wild oat has been confirmed resistant to Lipid Synthesis Inhibitors (Group 8). Wild oat has been selected for resistance to Groups 1 and 2 herbicides, Groups 1 and 8 herbicides and Groups 1, 2 and 8 herbicides. Green foxtail has been selected for resistance to Groups 1 and 3 herbicides. Kochia has been confirmed resistant to Groups 2 and EPSP synthase inhibitor glyphosate (Group 9) in 2014. Weed species present in Manitoba but confirmed resistant in Alberta and Saskatchewan include Persain darnel resistant to Group 1 herbicides, ball mustard, wild buckwheat, Russian thistle, narrow-leaf hawksbeard, shepherd s-purse, cow cockle, and common lambsquarters resistant to Group 2 herbicides, common hempnettle resistant to Group 4 herbicides, and cleavers resistant to Groups 2 and 4. Weed species present in Manitoba but confirmed resistant in the United States is important to consider including weed species that may arrive in Manitoba. Marshelder, eastern black nightshade, waterhemp (a pigweed species) and common ragweed have been confirmed resistant to Group 2 herbicides in North Dakota, Minnesota, and/or other parts of the United States. Kochia has been confirmed resistant to Group 4 herbicides including 2,4-D, dicamba, and fluroxypyr in the eastern half of North Dakota. Kochia has been confirmed resistant to dicamba in Colorado, Kansas, and Nebraska. Kochia has been confirmed resistant to Group 5 herbicides in North Dakota and other states. Common ragweed, Canada fleabane, and waterhemp have been confirmed resistant to Group 9 herbicide in North Dakota and many other states. Canada fleabane has been confirmed Group 9 resistant in most states of the United States. All states that have waterhemp have confirmed resistance to Group 9 herbicide and waterhemp continues to move farther north in the Red River Valley to Grand Forks County, North Dakota and Polk County, MN and outside the valley near Cando in Towner County, North Dakota. There are some populations of wild oat and common lambsquarters that have been difficult to control with glyphosate in North Dakota. Green foxtail has been selected for resistance to Group 1 and 2 herbicides in the northern half of North Dakota and is continuing to increase in frequency. Kochia has been selected for resistance to Groups 2 and 4, Group 5 and Photosynthesis Inhibitors binding site A, but
different than Group 5 (Group 7), and Groups 2, 4, and 9 in North Dakota. Kochia has also been confirmed resistant to Groups 2, 4, 5, and 9 and Groups 4 and 9 in Kansas. Waterhemp has been selected resistant to Groups 2 and 9 in North Dakota and most states having waterhemp. Waterhemp has been confirmed resistant to Groups 2 and 9 and PPO inhibiting herbicides (Group 14) in Iowa, Illinois, and Missouri and Groups 2, 5, and 9, and HPPD inhibitors (Group 27) in Iowa and Groups 2, 5, 9, and 14 in Illinois. Canada fleabane has been selected resistant to Group 2 and 9 in multiple states. Common ragweed has been selected resistant to Groups 2 and 9 in North Dakota and other states and to Groups 2 and 14 in Minnesota and other states. Crop rotation is important to managing herbicide resistant weeds and other agronomic pests. The greater the number of crops in the rotation the better the long-term weed management. Crop rotation improves weed management by varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage reducing the chances of specific weed species becoming prevalent. Crop diversity can lead to the use of more sites of action of herbicides to manage weeds. However it is possible to apply herbicides having the same site of action across multiple crops. This is the case in Manitoba for Group 2 herbicides. Group 2 herbicides are relied upon to control weeds in pulse crops, wheat, barley, oats, potato, Clearfield crops of wheat, lentil, canola, and sunflower, and Express Sun sunflower. It is possible to use Group 2 herbicides in corn and soybean as well, although few are used at this time. In addition several Group 2 herbicides are used in pre-seed and post harvest applications in multiple crops. Let s investigate some of the possible crop rotations and herbicide choices. Wheat, dry bean, and ExpressSun sunflower are in a rotation with Simplicity, Solo and Express applied respectively. All three of these are Group 2 herbicides selecting for resistance in wild oat and broadleaf weed species. Wheat followed by canola is a common crop rotation sequence in Manitoba. However if wheat and Clearfield canola are applied with Harmony SG and Odyssey Ultra, respectively, only Group 2 herbicides are being used to control broadleaf weeds. In a wheat, Roundup Ready canola, wheat, and Clearfield lentil rotation the application of Broadband, glyphosate, Retain, and Solo, respectively may select for Group 2 resistance to common chickweed and hempnettle even though glyphosate, fluroxypyr,and 2,4-D are used in two of the crops in rotation. In a wheat, sunflower, wheat, dry bean rotation, the application of Barricade II, Muster, Stellar, and Solo, respectively allows for the selection of Group 2 resistant shepherd s-purse and stinkweed. A wheat, LibertyLink canola, wheat, and Clearfield sunflower rotation applied with Prestige, Liberty, Pixxaro, and Solo selects for Group 4 resistance in various broadleaf weeds, although limited due to use of Group 4 herbicides in only two crops in the rotation. A rotation of wheat, sunflower, wheat, and dry bean applied with Axial BIA, Assure II, Broadband, and Assure II, respectively will select for resistance in foxtails and wild oat. Roundup Ready (RR) soybean followed by RR soybean is not a crop rotation. If only glyphosate is applied selection for Group 9 resistant weeds will occur, especially kochia. Roundup Ready corn followed by RR soybean is a weak crop rotation and if only glyphosate is applied, then resistance to Group 9 herbicide is likely. A crop rotation of RR soybean, RR corn, RR soybean, and wheat applied with glyphosate, glyphosate, glyphosate, and a pre-seed glyphosate plus florasulam tank-mix followed by Harmony K followed by glyphosate as a preharvest application, respectively, selects for Group 9 resistant weeds.
As demonstrated a good crop rotation may not solve weed resistance management issues when herbicides having the same site of action are used in all or most of the crops in the rotation. The combination of two or more herbicides into a single product by the manufacturer is called a pre-mixture. Pre-mixtures are common in today s market place. Pre-mixtures are easy to use for growers and retailers because a single container may replace two or more containers. Herbicide pre-mixtures are used to broaden the spectrum of weeds controlled. Pre-mixtures allow for the mixing of more than a single site of action in a single product. However do the active ingredients in a pre-mixture provide overlapping control? Overlapping control is when two or more of the active ingredients have different sites of action and provide equally effective control of a weed. Herbicide pre-mixtures are used to maintain specific ratios of active ingredients to provide effective weed control. Sometimes a pre-mixture may not include a high enough rate of one of the active ingredients for the weeds present in a specific area and/or a specific environment since pre-mixtures are made for wide geographies. A few pre-mixtures have been developed to reduce antagonism. Some pre-mixtures include safeners and/or a specific adjuvant that are not contained in the single active ingredient products. Despite the advantages of pre-mixtures one negative aspect is that a new trade name is given to each new combination of herbicides and touted as a new product causing some growers to be confused. If growers do not know what the active ingredients are in a pre-mixture they may not realize what herbicide sites of action are contained in the product and whether a product provides overlapping weed control. Let s investigate some wheat herbicide pre-mixtures for overlapping weed control. Retain is a combination of thifensulfuron and tribenuron, Group 2 herbicides and fluroxypyr and 2,4-D, Group 4 herbicides. Common chickweed and hempnettle are only controlled with Group 2 herbicides in Retain and kochia and cleavers are only controlled by Group 4 herbicides with all other common species controlled by both groups of herbicides. Spectrum is a combination of florasulam, a Group 2 herbicide and clopyralid and MCPA, Group 4 herbicides. Common chickweed, cleavers, and hempnettle are only controlled with florasulam in Spectrum and kochia, flixweed, lambsquarters and Russian thistle are only controlled with Group 4 herbicides. Harmony K is a combination of thifensulfuron and tribenuron, Group 2 herbicides, dicamba, a Group 4 herbicide and clodinafop, a Group 1 herbicide. Common chickweed is only controlled by Group 2 herbicides in Harmony K, kochia, cleavers, perennial sowthistle, and nightflowering catchfly are only controlled with dicamba and barnyardgrass, foxtail, and wild oat are only controlled with clodinafop. Altitude is a combination of imazamox, a Group 2 herbicide and furoxypyr and MCPA, Group 4 herbicides. Redroot pigweed, smartweed, foxtail, and wild oat are only controlled with imazamox in Altitude and common chickweed, kochia, and flixweed are only controlled with Group 4 herbicides. The soybean herbicide Flexstar GT combines fomesafen, a Group 14 herbicide and glyphosate a Group 9 herbicide. Glyphosate-resistant kochia and waterhemp will only be controlled by fomesafen in Flexstar GT and Canada fleabane, lambsquarters, smartweed, wild buckwheat, cleavers, common chickweed, hempnettle, and all grass species are only controlled with glyphosate. Viper ADV is a combination of imazamox, a Group 2 herbicide and bentazon, a Group 6 herbicide. Redroot pigweed, barnyardgrass, foxtail, and wild oat are only controlled with imazamox in Viper ADV and common chickweed, cleavers, cocklebur, and hairy nightshade are only controlled with bentazon. These examples clearly show that most pre-mixtures do not provide overlapping control allowing for the selection of herbicide-resistant weeds.
Careful weed management is necessary today to reduce the risk of selecting for additional herbicide resistant weed species. I have developed what I call the non-negotiables of weed management. The first non-negotiable is field perimeter weed management. Weeds, especially annual weeds are commonly found on the outside perimeter of a field. These weeds are sprayed with a reduced rate of herbicide(s) because there is no overlap on the outside nozzle. This allows for the selection of herbicide resistant weeds. During harvest these surviving weeds can be gathered into the combine and spread farther out into the field. Mowing the outside perimeter of all fields will drastically reduce weed seed production of the surviving plants and eliminate them from being moved further into the field by the combine. Weed densities are usually higher on the inside field perimeter than other areas of the field. Managing this inside field perimeter differently than the rest of the field should reduce weed densities over time. Some specific inside field perimeter practices include applying a preemergence herbicide to the field perimeter, applying a higher rate of a preemergence herbicide to the field perimeter, apply a postemergence herbicide to the field perimeter before the remainder of the field, make a planned second postemergence application to the field perimeter and/or remove all weeds by hand. The second non-negotiable is to scout fields often. The more times a field is scouted the more timely herbicide application(s) can be made. Scouting late in the season is helpful for evaluating the weed control program for the season and whether additional control strategies are still needed. The third non-negotiable is zero tolerance. Zero tolerance, credited to Jason Norsworthy is the elimination of any surviving weed in a field after herbicide application(s) and prior to seed production. Zero tolerance attempts to eliminate resistant seeds from being added to the soil seed bank and becoming future problems. The fourth non-negotiable is the diversification of weed management strategies. These include applying soil residual herbicides, tank-mixing two or more herbicides with overlapping sites of action, rotating herbicide sites of action, rotating herbicide resistant crops, and cultural practices such as crop rotation, increased crop competition, and improved plant health to name a few. The fifth non-negotiable is to plant into a field without weeds. In a no-tillage field that means applying herbicides to control weeds prior to planting the crop. In tilled fields that means controlling all weeds present at the time of tillage. The sixth non-negotiable is to maximize herbicide activity. These include choosing the most effective herbicides for the weeds present, applying postemergence herbicides to small (< 7.5 cm) annual weeds, apply full labeled rate of postemergence herbicides, apply the most effective adjuvant with postemergence herbicides, apply the correct spray volume for the type of herbicide(s) used, obtain the best droplet size for the herbicides being applied, use the narrowest nozzle spacing, reduce sprayer travel speed, and apply during the best weather conditions. The seventh non-negotiable is understanding weed biology. Knowing how many seeds a plant produces or when seeds become mature or at what stage is the most effective time to control a weed or how long will a seed survive in the soil are things to consider. The final non-negotiable is to react quickly to a changing weed population. Don t allow weeds to produce seeds in a field after herbicide application(s). When just a single plant is in a field after
herbicide application(s) the weed control strategy for that season needs to be adapted instead of waiting for next season or believing one weed is not a problem.