Vencill, W.K.; T.L. Grey; Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of Further Herbicide Resistance in Georgia; Project Type: Working Group; This project primarily addresses the 2013 Special Priority
Project Title: Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of Further Herbicide Resistance Risk in Georgia
William Vencill, University of Georgia
Summary: The widespread use of herbicides such as glyphosate has resulted in the development of herbicide-resistant weeds. GR Palmer amaranth is the most significant species of concern in cotton production; more than 2 million ha of agricultural land in the Midsouth and SE US are estimated to be infested. However, with the imminent introduction of other herbicide-resistant crops such as auxinic-resistant cotton and soybean, weed scientists need to better be able to predict herbicide-resistant weeds. To preserve other herbicide mechanisms of action, we need to know more about the potential for other cases of glyphosate as well as other herbicide resistance and multiple-resistant weed populations. The immediate goal of this project is to 1) conduct a survey in 20 fields geographically dispersed in Georgia and collect mature weed seeds from as many species as possible and determine sensitivity to seven herbicide mechanisms of action (EPSP (WSSA Group 9), GS (WSSA Group 10), PSII (WSSA Group 5), PPO (WSSA Group 14), ALS (WSSA Group 2), HPPD (WSSA Group 27), and Auxinic (WSSA Group 4)) and if any multiple herbicide resistance is present and 2) determine a risk of major agronomic weeds in Georgia to further herbicide resistance development. In Fall 2013, 28 fields were sampled in southwest, central, southeast, and northeast Georgia (Figure 1).
Problem and Justification: In 2009, growers in the United States harvested 3 million ha of upland cotton with an estimated production value of $3.5 billion. Worldwide, the US ranks third in cotton lint production (2.8 million MT) behind China (7.5 million MT) and India (3.8 million MT). The profitability of US cotton has been greatly improved since the commercial introduction of GM HT cultivars; between 1997 and 2008, the adoption of GM HT cotton provided a cumulative net farm income benefit of $799 million. The majority of savings are directly related to the replacement of expensive, soil-applied chemicals required for early season weed control with the low cost, broad spectrum herbicide, glyphosate (Roundup). The use of HT cotton cultivars has provided growers with supplementary returns beyond an increase in farm profits (Norsworthy et al. 2012, Vencill et al. 2012). Studies evaluating the implementation of GM crops state that the adoption of CT systems is one of the most significant advantages of HT technology. Following the release of GM glyphosate-tolerant cotton varieties (1997), the proportion of cotton acres managed to allow for >30% crop residue remaining on the soil surface increased 109%. Proposed benefits of CT include: reduced soil erosion, improved soil structure, increased carbon sequestration, enhanced moisture holding capacity, and decreased surface water runoff. Studies also suggest that CT can reduce certain grower input costs, such as those for fuel, labor, andmachinery repair. The adoption of GM HT cotton has also resulted in a decrease in herbicide use. Since 1997, the US, Australia, Argentina, and South Africa have reduced the amounts of herbicide active ingredients applied in cotton by 3.4% (-6.3 million kg). Concurrently, the associated environmental impact quotient (EIQ) fell by 5.5%. The cumulative (1997 to 2008) value of non-pecuniary benefits derived from GM HT cotton, such as the adoption of CT and reduced herbicide use, was an estimated $461 million for US growers.
The rapid and widespread adoption of GM HT crop technology has not come without problems; the unprecedented use of glyphosate over space and time has facilitated the development of glyphosate resistance in 21 weed species worldwide, including Palmer amaranth. In 2004, the existence of GR Palmer amaranth was confirmed at a 250 ha field site in Macon County, GA. Production at this site had been a monoculture of glyphosate-tolerant cotton where glyphosate, often applied at reduced rates, was employed as the sole means of weed control for at least seven years. Currently, GR Palmer amaranth infests more than 2 million ha in 10 states (Alabama, Arkansas, Georgia, Louisiana, Missouri, Mississippi, North Carolina, New Mexico, South Carolina and Tennessee). Postemergence herbicide options for the control of Palmer amaranth in cotton are extremely limited; Pyrithiobac has been, historically, an effective means of controlling escaped Palmer amaranth however, there is now widespread resistance to this herbicide throughout the SE US. Glufosinate is an additional tool for controlling in-field Palmer amaranth populations that are resistant to glyphosate, pyrithiobac or both, but only if it is applied to extremely small (<10 cm) weed seedlings.
GR Palmer amaranth has quickly become the single greatest threat to the economic sustainability of cotton production. GA cotton growers spent in excess of $24/ha ($60/A or $100 million statewide) for herbicides to control GR Palmer amaranth; this represents a cost increase of at least 200%, relative to the expenditures for controlling the susceptible biotype of the species. Concurrently, the amount of herbicide active ingredient applied increased by 150% (4.25 lb ai to 10.625 lb ai). Even with this level of herbicide input, 92% of these growers hand weeded 54% of the statewide crop at an average cost of $10/ha ($24/A). The number of growers returning to tillage and in-field cultivation for the management of GR Palmer amaranth is steadily increasing; in 2010 50% of the cotton acreage in GA received tillage prior to planting (including deep tillage and/or herbicide incorporation). Widespread GR Palmer amaranth has promoted the development of other herbicide-resistant crop cultivars such as glufosinate-resistant and auxinic-resistant cotton and soybean. We need to know the risk of herbicide resistance development to these herbicides.
Objectives:
· Survey weed populations in 20 fields in five geographic regions of Georgia and determine resistance to six herbicide mechanisms of action
· Determine potential risk of most common weed species in Georgia agricultural production to develop herbicide resistance
Procedures:
1) A field survey was carried out in Fall 2013 (Table 1) once mature seed were ready to be harvested. In each field, a transect was run and mature plants were collected, bulked in a cotton bag, and air-dried. Plants from each site was hand-threshed to avoid sample cross-contamination. After a chilling period at 4 C, seeds were planted in 52 by 26 by 5 cm flats containing a potting mixture. A minimum total of 100 F1 seedlings (3 to 5 cm tall) per population (three flats or replicates per experiment run and repeated) were be sprayed with glyphosate, imazapic , fomesafen, glufosinate, atrazine, 2,4-D and sethoxydim for grass weeds according using moving-nozzle cabinet sprayer equipped with a flat-fan nozzle tip (TeeJet 8002VS, Spraying Systems Co., Wheaton, IL) calibrated to deliver 200 L ha-1143 of spray solution at 200 kPa in a single pass over the foliage. A discriminatory dose (1x and 2x the labeled rate) was applied when plants were at 2-4 leaf stage. Plants were visually rated for survival 2 WAT using a scale of 0-100 with 0 as no injury and 100 complete kill. For those populations surviving a discriminatory dose, a dose response study was carried out to further analyze the level of herbicide resistance.
Table 1. Location of sites, crop, tillage, and weeds collected.
SW001 / Terrell / 31.69333 / -84.4225 / peanut / convtl / Palmer amaranth,Texas millet,barnyardgrass, cutleaf groundcherry, coffee senna
SW002 / Terrell / 31.655 / -84.46583 / cotton / convtl / Palmer amaranth, crowfootgrass
SW003 / Berrien / 31.34917 / -83.38806 / cotton / convtll / Palmer amaranth, Florida beggarweed
SW004 / Colquitt / 31.16556 / -83.63917 / peanut / convtl / Florida beggarweed
SW005 / Colquitt / 31.07889 / -83.81944 / cotton / strip-till / Palmer amaranth
SW006 / Berrien / 31.14567 / -83.17087 / Corn / Convtl / Palmer amaranth
M001 / Bleckley / 32.4925 / -83.34889 / peanut / strip-till / Palmer amaranth, Texas millet, crowfootgrass, sicklepod, cutleaf groundcherry, coffee senna
M002 / Bleckley / 32.45972 / -83.36722 / cotton / strip-till / Palmer amaranth
M003 / Peach / 32.56583 / -83.91167 / soybean / strip-till / Palmer amaranth, sicklepod
M004 / Taylor / 32.55583 / -84.07139 / cotton / strip-till / Palmer amaranth, sicklepod, coffee senna
M005 / Macon / 32.49028 / -84.05472 / cotton / strip-till / Palmer amaranth, sicklepod, coffee senna
M006 / Dooly / 32.21944 / -83.93944 / cotton / strip-till / Palmer amaranth, sicklepod, coffee senna
SE01 / Burke / 33.08727 / -82.0483 / peanut / convtl / Palmer amaranth, goosegrass, Texas millet
SE02 / Burke / 33.08657 / -82.0461 / peanut / convtl / Palmer amaranth, Texas millet., crowfootgrass
SE03 / Burke / 33.09179 / -82.06795 / cotton / convtl / Palmer amaranth
SE04 / Burke / 33.08806 / -82.02694 / peanut / convtl / large crabgrass, goosegrass
SE05 / Burke / 33.09884 / -82.10721 / peanut / convtl / hairy fleabane, yellow nutsedge, annual sedge
SE06 / Burke / 33.03575 / -82.07956 / noncrop / Palmer amaranth
SE07 / Burke / 32.93642 / -82.16745 / cotton / convtl / Palmer amaranth
SE08 / Burke / 32.92808 / -82.14848 / cotton / convtl / Palmer amaranth
SE09 / Burke / 32.93078 / -82.15701 / cotton / convtl / Palmer amaranth
SE10 / Jefferson / 33.14567 / -82.45687 / soybean / convtl / Palmer amaranth
SE11 / Glascock / 33.30519 / -82.52494 / soybean / convtl / Palmer amaranth
NE01 / Elbert / 34.2198 / -82.99521 / soybean / convtl / Sicklepod, large crabgrass
NE02 / Hart / 34.26438 / -82.89561 / soybean / convtl / Palmer amaranth, Cocklebur, sicklepod, large crabgrass, crowfootgrass
NE03 / Madison / 34.02975 / -82.24683 / soybean / convtl / Palmer amaranth
NE04 / Madison / 34.02975 / -82.24683 / soybean / convtl / Palmer amaranth
NE05 / Elbert / 34.20672 / -82.91484 / soybean / convtl / Palmer amaranth
Results
As expected, Palmer amaranth was the dominant weed (86% sampled fields had Palmer amaranth present) found in fields during the sampling process (Table 2). This data agrees with recent Southern Weed Science Society surveys that show Palmer amaranth to be the most common as well as troublesome weed in Georgia annual crops (Webster 2009). In addition, glyphosate-resistant Palmer amaranth was the most predominate herbicide resistance type found in this survey with 69% of the Palmer amaranth populations sampled exhibiting glyphosate resistance. No other weeds examined were found to glyphosate resistant.
Table 2. Results of screening of collected weed response to six herbicide MOA. Weeds were considered resistant if <50% control at 4x labeled field use rate.
Weed / Resistance (% samples collected)% Fields collected / Group 2 (ACCase) / Group 4 (auxins) / Group 5 (PS inhib) / Group 9 (EPSP) / Group 10 (GS) / Group 14 (PPO)
Palmer amaranth / 86 / NA / 0 / 3 / 69 / 0 / 0
Sicklepod / 24 / NA / 0 / 3 / 0 / 0 / 0
Coffee senna / 21 / NA / 0 / 0 / 0 / 0 / 0
Cutleaf groundcherry / 7 / NA / 0 / 0 / 0 / 0 / 0
Florida beggarweed / 3 / NA / 0 / 0 / 0 / 0 / 0
Annual fleabane / 3 / NA / 0 / 0 / 0 / 0 / 0
Texas millet / 14 / 0 / NA / 0 / 0 / 0 / 0
Crowfootgrass / 10 / 0 / NA / 0 / 0 / 0 / 0
Barnyardgrass / 3 / 0 / NA / 0 / 0 / 0 / 0
Goosegrass / 3 / 0 / NA / 0 / 0 / 0 / 0
Large crabgrass / 3 / 0 / NA / 0 / 0 / 0 / 0
Annual sedge / 3 / 0 / NA / 0 / 0 / 0 / 0
Atrazine resistance was found in two weed species. A population of Palmer amaranth was found to be atrazine resistant (ED50 = 3.57 lb ai/A atrazine). Laboratory tests indicated the population has metabolic resistance (enhanced glutathione transferase) rather than target-site resistance. This is the first case of atrazine-resistant Palmer amaranth outside a dairy operation in Georgia. A sicklepod population from Taylor Co. was found to be atrazine-resistant (ED50 = 3.0 lb ai/A) and also appears to have metabolic atrazine resistance. This is the first case of reported herbicide resistance in sicklepod (Heap 2014). As a result of these herbicide resistance cases, we will conduct a survey of fields in cooperation with the extension specialist for corn (Dr. Eric Prostko) and county extension agents in southwest Georgia this year to screen for other cases of atrazine-resistant Palmer amaranth and sicklepod.
Fortunately, no resistance was found to Group 2, 4, 10, and 14 herbicides in any of the weeds collected. This will be helpful information as Group 10 (glufosinate) herbicides are widely being used in the state and the introduction of auxinic-resistant crops will increase the selection pressure in Group 4 herbicides.
Potential Outcomes and Impacts
As a result of this work, the first case of herbicide resistance in sicklepod has been discovered and atrazine resistance has been discovered in Palmer amaranth in a situation where crops are rotated. These results have been shared with extension personnel in the state so that they and growers can be on the lookout for other cases of atrazine resistance in these weeds and have been shared through popular media (Prostko, E.P. 2014. Palmer amaranth resistant to atrazine confirmed in Georgia. Southeast Farm Press, Jan. 29, 2014). In addition, we will be working with local extension personnel this year to screen for other cases of herbicide-resistance in these weeds.