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Soil Applied Residual Herbicide Affect on Conventional and Imidazolinone Resistant Canola in the Southeastern US. Timothy Grey 1 , Paul Raymer 2 , and G. David Buntin 2

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Soil Applied Residual Herbicide Affect on Conventional and Imidazolinone Resistant Canola

in the Southeastern US

Timothy Grey1, Paul Raymer2, and G. David Buntin2

1The University of Georgia, 115 Coastal Way, Tifton, Georgia, 31793-0748,, 21109 Experiment St. Griffin Georgia 30223


Interest in canola quality oilseed rape, Brassica naupus L., in the southeastern US has recently increased (Anonymous 2002) . There are several potential markets for canola in this region including: 1) whole grain exports of non-genetically modified canola, 2) newly developed specialty oil lines, 3) improved commodity prices (ERS-USDA 2002) and 4) production uses for biodiesel. Agronomic production of a profitable canola crop in the southeastern U.S. will depend on reliable information about rotational restrictions to cotton, peanut, and other agronomic herbicides. Agronomic row crop production in Georgia, Florida, Alabama, and South Carolina represent 1,005,000 hectares of cotton, 323,300 hectares of corn, 329,820 hectares of peanut, 348,030 hectares of soybean, and 25,500 hectares of tobacco (NASS, 2002). Wheat, oats, and rye, planted in winter rotations with these crops represent 434,230 hectares (NASS, 2002). Canola integrated as part of a winter rotation, October planted and May harvested, could potentially increase farmer profits, as an alternative oil seed crop.

Herbicides used in row crop production in the southeastern U.S. vary by crop. In the south, imazapic and diclosulam are applied to approximately 85% of the peanut hectares. Pyrithiobac is applied to greater than 30% of the cotton crop. Imazapic, diclosulam, and pyrithiobac mode of action is through acetolactate synthase (ALS) inhibition. Other ALS herbicides used in these crops include chlorimuron and trifloxysulfuron. Herbicides having different modes of action that are used in the southeastern US in cotton and peanut include sulfentrazone, fomesafen, and flumioxazin. However, rotational data for these herbicides for canola have not been established in the southeastern U.S. One reason to evaluate canola for herbicide carryover in this region is the moderate temperature and moisture regimes that can perpetuate the degradation of pesticides.

Clearfield canola cultivars have ALS resistance, specifically to imidazolinone herbicides. Clearfield-resistant canola was marketed commercially in Canada during 1998 and is now available for evaluation in the United States. The use of Clearfield canola could be very beneficial for canola growers in the southeastern U.S., allowing the use of imazamox herbicide that has selective control of wild radish (Raphanus raphanistrum L.), many other dicot weeds, and Italian ryegrass (Lolium multiflorum Lam.) (Grey et al. 2003). Additionally, Clearfield canola potentially could be grown with reduced risk of injury, in rotation with cotton, peanuts, and other row crops that have had ALS mode of action herbicides (imazapic, diclosulam, pyrithiobac, etc.) applied for weed control. However, research for tolerance to these herbicide has not been established.


Experiments were conducted in Georgia during 2002-2003 at the Bledsoe Research Farm near Williamson and in 2003-2004 and 2004-2005 at the Southwest Georgia Branch Experiment Station located near Plains in separate areas of the same field. Soils were a Cecil sandy clay loam (clayey, kaolinitic, thermic, Typic Hapludult) at Williamson and a Faceville sandy loam (clayey, kaolinitic, thermic, Typic Kandiudults) at Plains. Organic matter ranged from 1.0 to 2.0% with pH from 6.0 to 6.5 at both locations. In the fall of each season, seedbeds were conventionally tilled using a moldboard plow and smoothing with a rotary tiller. Canola was seeded with a Great Plains drill with a 19 cm row width on October 11, 2002, November 11, 2003, and October 18, 2004 at a rate of 28 seed per m of row. Each experiment was arranged as a split plot with treatments replicated four times. Plot size was 3.66 m wide by 7.6 m long (Figure 1). Herbicides were PRE applied with a CO2-pressurized backpack sprayer calibrated to deliver 187 L/ha at 210 kPa. Emergence dates for all trials was approximately one week after planting. Split plots were seeded on 1.83 m bed with imidazolinone-tolerant ‘Pioneer 45A71’ and on the other 1.83 m bed to the to conventional canola ‘Flint’. The intent was to determine tolerance to ALS herbicides.

Screening of herbicide tolerance to two classes of herbicides was investigated. Modes of action included acetolactate synthase inhibitors (ALS) and protoporphyrinogen oxidase inhibitors (PPO). Five ALS herbicides and three PPO herbicides were evaluated. Factors included cultivars (IMI-tolerant canola and traditional canola) and herbicide rate (1x, ½x, ¼x) that resulted in 25 treatments. Treatments included a nontreated check, the ALS herbicides diclosulam, imazapic, chloriuron, pyrithiobac and trifloxysulfuron, and the PPO herbicides flumioxazin, fomesafen, and sulfentrazone. (Table 1).

Crop injury was visually estimated on a scale of 0 (no injury) to 100% (death). Crop injury ratings were taken at 140, 70, and 58 days after planting (DAP) for the 2002, 2003, and 2004 plantings, respectively. Canola stand counts were taken 58 DAP for the 2004 planting. Plots were harvested (2004 only) with conventional plot combine.


Pioneer 45A71 and Flint canola exhibited extensive injury and nearly complete stand failure (Table 1) and crop loss with the PPO herbicides flumioxazin, fomesafen, and sulfentrazone (Figure 4). This susceptibility resulted in in no or significantly reduced yields (data not shown). For the ALS herbicides imazapic, chlorimuron, pyrithiobac, and trifloxysulfuron, Pioneer 45A71 canola injury was 17% or less for all rates (Figure 5) and yields were not different from the nontreated control (Data not shown). Pioneer 45A71 exhibited excellent tolerance to imazapic at ¼, ½, and 1x rates (Figure 2). In contrast, Flint canola exhibited 57% injury or greater (Figures 3 & 5) and yield reductions for imazapic, chlorimuron, pyrithiobac, diclosulam, and trifloxysulfuron at all rates (Data not shown).

Pioneer 45A71 exhibited dose response injury of 21, 43, and 63% for the ¼, ½, and 1x rates of diclosulam, respectively (Figure 5). Additionally, stand was reduced from 26 to 14 plants/m row for the nontreated check to diclosulam at 82 g/ha, respectively. This indicates that imi-resistant canola is not resistant to all ALS herbicides which should be considered when planning potential rotational options for canola.

Figure 4. PPO inhibitor injury on Pioneer 45A71 and Flint canola.

Herbicide rate

% injury

Herbicide/canola combination

Imazapic rate

Figure 2 (Left). Imazapic carryover simulation to Pioneer 45A71 (left bed of each picture) and Flint (right bed of each picture) at ¼ x (top), ½ x (middle), and 1x rates (bottom). Note dose response for Flint from ¼x to ½x rates but no difference for Pioneer 45A71.

¼ X

Figure 5. ALS inhibitor injury on Pioneer 45A71 and Flint canola.


Anonymous. 2002. Oilseed plant going to Claxton in Georgia Faces. Univ. Georgia College of Agric. & Env. Sci. at

Economic Research Service - U.S. Department of Agriculture (ERS-USDA). 2002. Title I, Commodity Programs in Farm Policy, the 2002 farm bill: provisions and economic implications. at

National Agricultural Statistics Service (NASS). 2002. National Agricultural Statistics Service U.S. Dept. of Agri.. 2000 Annual summary. NASS-USDA, Washington, DC.

Figure 1. Pioneer 45A71 (staked bed, left) and Flint (nonstaked bed, right) canola planted in Plains, GA.

% injury

Herbicide rate

½ X

Figure 3 (Top). Imazapic carryover simulation to Flint canola from ¼x application rate.

Herbicide/canola combination

1 X