MESOTRIONE TO CONTROL TRIAZINE- AND ALS-RESISTANT AMARANTHUS IN GRAIN SORGHUM Cramer*, G.L. 1 Regehr, D.L. 2 1 Extension Agent, Kansas State University, Sedgwick County, Wichita, Kansas 67205 2 Professor, Weed Science, Kansas State University, Manhattan, Kansas 66506 Grain sorghum is an important crop for Kansas agriculture; however, weed control options are limited. Some sorghum weeds are resistant to many of the herbicides registered on sorghum. Triazine and/or ALS-resistant Palmer amaranth populations are common in Kansas. Palmer amaranth is a highly competitive weed that can greatly reduce sorghum grain yields. Mesotrione is registered for use in corn and effectively controls Palmer amaranth and many other weeds commonly found in grain sorghum. Kansas State University scientists have established that grain sorghum tolerates soil-applied mesotrione. Kansas State University scientists conducted research trials in 2006 with a focus on Amaranthus control. Herbicides were applied preemergence (PRE) and postemergence (POST). The trial was located at Colwich, KS - (Gruenbacher Farm) -Planted 15 June into tilled silt loam soil. -Grain Sorghum Hybrid: Pioneer 85G01 -Extremely high Palmer amaranth density (>2500 seedlings/m 2 ) -Applications: PRE (18 June) and POST (7 July) -Precipitation: 25 mm on 16 June; 12 mm on 19 June; 39 mm on 22 June -Yield: above-ground plant dry weight on 24 August These efficacy trials were conducted using PRE application. The Lumax ™ Section 18 label recommends application 7-14 days before planting, but the logistics of working in commercial fields made this difficult for small plot trials. Additionally, PRE applications are still being evaluated for efficacy and crop safety. Evaluate weed control with mesotrione plus S -metolachlor plus atrazine ( Lumax ™) in grain sorghum, with focus on control of triazine- and/or ALS-resistant weeds. Compare the mesotrione premixture with labeled treatments for control of Palmer amaranth and other broadleaf weeds. Lumax controlled ALS- and triazine-resistant Palmer amaranth and other broadleaf and grass weeds as well or better than other commercial herbicide treatments absent of mesotrione. Lumax provided good early-season Palmer amaranth control, but late-season control declined as weed biomass increased. Thus, Lumax may not be a stand alone treatment under extremely high weed populations. Integrated weed management practices are essential for effective Palmer amaranth control. Soil moisture and application timing appear to be important factors affecting Lumax performance. Future plans are to evaluate split applications of Lumax on grain sorghum (7-14 days preplant followed by preemergence). Mesotrione efficacy appeared to be dependent on timing, soil moisture, weed pressure and burndown of existing weeds (Table 1). Lumax provided good early-season control of Palmer amaranth (Figure 1), but late-season control declined with increasing weed biomass. Yields were greater in Lumax plots than in untreated check plots. The Palmer amaranth likely was triazine- and ALS-resistant. Lumax efficacy was far superior to other soil-applied (Figure 2) or POST herbicide treatments. The lack of weed control by atrazine and Peak suggest that the pigweeds in this trial were both triazine- and ALS-resistant. Table 1. Herbicidal control of Amaranthus palmeri in grain sorghum at Colwich, KS. Preemergence and postemergence treatments were applied 3 and 21 days after planting, respectively. 5.85 l/ha Lumax PRE Figure 1. Weed control 22 days after planting. Grain sorghum (Sorghum bicolor) acreage in Kansas has declined sharply in the past 25 years. In fact, not since 1952 has Kansas grain sorghum acreage been as low as it was in 2005. This trend is attributed largely to inferior weed control technology for sorghum. A poll of selected crop consultants and county agricultural extension agents in central Kansas indicates that on average, about 50 percent of the sorghum acres treated with acid amide and/or atrazine herbicides have Amaranthus weed species control failures. In some instances, these failures are due to inadequate precipitation to activate the soil-applied herbicides. More often, it is failure to control triazine resistant biotypes that is at fault. There is no doubt that triazine-resistant and ALS-resistant Amaranthus weed species are widespread across Kansas, and that the populations of resistant biotypes are increasing, especially in areas of intensive sorghum production. Kansas State University scientists conducted research trials in 2006 with a focus on Amaranthus control. Field tests on medium- to fine-textured soils have demonstrated that mesotrione plus s -metolachlor plus atrazine has adequate safety for use in grain sorghum, especially when applied 7-14 days prior to planting. Preemergence treatments, applied 3 days after planting, were compared with competitive postemergence treatments applied 21 days after planting. Weed control ratings taken 21, 35, and 63 days after planting showed that soil-applied mesotrione plus s -metolachlor plus atrazine provided good-to-excellent early-season control of Palmer amaranth, that was far superior to postemergence herbicide treatments. 1.57 kg/ha atrazine PRE Figure 2. Weed control 22 days after planting. Introduction Results and Discussion Objectives Summary and Implications 1215 16 21 15 LSD (0.05) 63 35 21 Preemergence / Postemergence 2285 16 34 58 1071/8 S-metolachlor fb carfentrazone + NIS 1 3007 33 48 45 1071/533 S-metolachlor fb 2,4-D amine 1239 14 20 45 1071/53 S-metolachlor fb prosulfuron + COC 1 3297 36 48 36 1071/280 S-metolachlor fb dicamba 1482 11 19 54 1071/280 S-metolachlor fb 1 bromoxynil 390 - - - - Untreated check 5753 - - - - Weed-free check Yield (kg/ha) Percent Control days after planting Rate (g/ha) 1 fb = followed by; crop oil concentrate and non-ionic surfactant, respectively. 727 3 3 13 1571 Atrazine 820 13 13 51 1071 S-metolachlor 2246 15 41 70 1124+1452 S-metolachlor + atrazine 3787 59 65 91 188+1879+ 701 Mesotrione + S-metolachlor + atrazine (5.85 l/ha Lumax) Preemergence treatments Abstract Materials and Methods

Grain sorghum is an important crop for Kansas agriculture; however, weed control options are limited.

Some sorghum weeds are resistant to many of the herbicides registered on sorghum. Triazine and/or ALS-resistant Palmer amaranth populations are common in Kansas.

Palmer amaranth is a highly competitive weed that can greatly reduce sorghum grain yields.

Mesotrione is registered for use in corn and effectively controls Palmer amaranth and many other weeds commonly found in grain sorghum.

Kansas State University scientists have established that grain sorghum tolerates soil-applied mesotrione.

Kansas State University scientists conducted research trials in 2006 with a focus on Amaranthus control.

Herbicides were applied preemergence (PRE) and postemergence (POST).

The trial was located at Colwich, KS - (Gruenbacher Farm)

-Planted 15 June into tilled silt loam soil.

-Grain Sorghum Hybrid: Pioneer 85G01

-Extremely high Palmer amaranth density (>2500 seedlings/m 2 )

-Applications: PRE (18 June) and POST (7 July)

-Precipitation: 25 mm on 16 June; 12 mm on 19 June; 39 mm on 22 June

-Yield: above-ground plant dry weight on 24 August

These efficacy trials were conducted using PRE application. The Lumax ™ Section 18 label recommends application 7-14 days before planting, but the logistics of working in commercial fields made this difficult for small plot trials. Additionally, PRE applications are still being evaluated for efficacy and crop safety.

Evaluate weed control with mesotrione plus S -metolachlor plus atrazine ( Lumax ™) in grain sorghum, with focus on control of triazine- and/or ALS-resistant weeds.

Compare the mesotrione premixture with labeled treatments for control of Palmer amaranth and other broadleaf weeds.

Lumax controlled ALS- and triazine-resistant Palmer amaranth and other broadleaf and grass weeds as well or better than other commercial herbicide treatments absent of mesotrione.

Lumax provided good early-season Palmer amaranth control, but late-season control declined as weed biomass increased. Thus, Lumax may not be a stand alone treatment under extremely high weed populations. Integrated weed management practices are essential for effective Palmer amaranth control.

Soil moisture and application timing appear to be important factors affecting Lumax performance.

Future plans are to evaluate split applications of Lumax on grain sorghum (7-14 days preplant followed by preemergence).

Mesotrione efficacy appeared to be dependent on timing, soil moisture, weed pressure and burndown of existing weeds (Table 1).

Lumax provided good early-season control of Palmer amaranth (Figure 1), but late-season control declined with increasing weed biomass.

Yields were greater in Lumax plots than in untreated check plots. The Palmer amaranth likely was triazine- and ALS-resistant.

Lumax efficacy was far superior to other soil-applied (Figure 2) or POST herbicide treatments.

The lack of weed control by atrazine and Peak suggest that the pigweeds in this trial were both triazine- and ALS-resistant.