The Harrison team participates in outreach, field days, and other activities described below to spread the word about resistance and encourage local farmers to make resistance management planning part of their annual process.
Herbicide Resistance Screening
To confirm presence of resistant weed populations in Harrison County, the Harrison team collected seeds from suspected resistant populations. Seeds were grown in greenhouses at Iowa State University and treated with Roundup (glyphosate), Cobra (lactofen), and Callisto (mesotrione). Though we used Cobra and Callisto in these studies, these results can be assumed to apply to other PPO- and HPPD-inhibiting herbicides, respectively.
Populations of common weeds exhibited resistance to all three herbicides tested. All waterhemp populations sampled exhibited some degree of resistance to Roundup (HG 9) in this study, with survival between 36 and 75% 21 days after treatment (Table 1). Waterhemp response to Cobra (HG 14) was variable, with plant survival between 10 and 100%. Palmer amaranth showed moderate to high level of resistance to Roundup and Callisto (HG 27), but were completely controlled by Cobra. Survival of giant ragweed to Roundup was between 21 and 78%, while one giant ragweed sample was highly resistant to Callisto. All giant ragweed samples in this study were susceptible to Cobra. See the table for details.
In 2018, samples were once again collected from locations across Harrison County. Four waterhemp samples were submitted for molecular analysis to the University of Illinois Plant Clinic. All four samples tested positive for resistance to glyphosate, while two of four samples tested positive for resistance to PPO-inhibitors. Additional samples were collected for testing at Iowa State University greenhouses. Results will be posted when they become available.
These results confirm the presence of resistance to many of the most commonly used herbicides in corn and soybean production in weed populations in Harrison County. To reduce the spread and development of herbicide resistance, the use of diverse weed management programs are encouraged, including timely applications of herbicide mixtures containing multiple, effective mechanisms of action in addition to cultural and mechanical techniques where feasible. Due to high seed production in many weed species, all efforts to reduce and prevent seed production by resistant individuals will be highly beneficial.
| 2017 Samples | Weed Survival (%)* | |||
| Species | Location | Roundup (9) | Cobra (14) | Callisto (27) |
| Waterhemp | NE of Logan | 60 | 35 | Not tested |
| Waterhemp | NW of Logan | 36 | 75 | Not tested |
| Waterhemp | SE of Logan | 67 | 10 | Not tested |
| Waterhemp | SE of Logan | 75 | 10 | Not tested |
| Waterhemp | S of Modale | 59 | 100 | Not tested |
| Palmer amaranth | SW of Modale | 60 | 0 | 100 |
| Palmer amaranth | SW of Modale | 20 | 0 | 30 |
| Giant ragweed | W of Logan | 23 | 0 | Not tested |
| Giant ragweed | W of Missouri Valley | 21 | Not tested | 100 |
| Giant ragweed | NW of Mondamin | 78 | Not tested | Not tested |
Field Trials
Field trials were conducted in 2018 to allow side-by-side comparisons of 10 comprehensive weed management programs in four cooperator’s fields. Trials were conducted in 3 soybean fields (two no-till, one with tillage) and one cornfield. Herbicide programs were designed to demonstrate ‘Good, ‘Better’, and ‘Best’ herbicide programs, and their effectiveness in managing weeds and resistance. All programs involved two passes, and ranged from tillage followed by Liberty (glufosinate), to a pre-mix consisting of three herbicides modes of action (MOAs) followed by Liberty plus Outlook (dimethenamid), for a total of four unique MOAs for the season. Below is a set of photographs that includes highlights from the 2018 field trials.
Dicamba 1 was treated with both pre and post planting treatments:
Pre-planting:
2,4-d LV4
Glyphosate
AMS
Post-planting:
Glyphosate
AMS
Clethodim
HSCOC
Dicamba 2 was treated with both pre and post planting treatments:
Pre-planting:
Xtendimax
glyphosate
Adjuvant
DRA
Post-planting:
Xtendimax
Warrant
Glyphosate
Clethodim
Adjuvant
DRA
Dicamba 3 was treated with both pre and post planting treatments:
Pre-planting:
Xtendimax
Glyphosate
Sonic
Adjuvant
DRA
Post-planting:
Xtendimax
Warrant
Glyphosate
Clethodim
Adjuvant
DRA
Dicamba 4 was treated with both pre and post planting treatments:
Pre-planting:
2,4-d LV4
Glyphosate
AMS
Post-planting:
Glyphosate
AMS
Clethodim
HSCOC
Dicamba 5 was treated with both pre and post planting treatments:
Pre-planting:
Engenia
Glyphosate
Outlook
Adjuvant
DRA
Post-planting:
Engenia
Outlook
Glyphosate
Clethodim
Adjuvant
DRA
Dicamba 6 was treated with both pre and post planting treatments:
Pre-planting:
Engenia
Glyphosate
Zidua PRO
Adjuvant
DRA
Post-planting:
Engenia
Outlook
Glyphosate
Clethodim
Adjuvant
DRA
Liberty 1 was tilled and then received post-planting treatments:
Pre-planting:
Tillage
Post-planting:
Liberty
AMS
Outlook
Clethodim
HSCOC
Liberty 2 used a boundary and and post planting treatments:
Pre-planting:
Boundary
Post-planting:
Liberty
AMS
Dual
Clethodim
HSCOC
Liberty 3 was treated with both pre and post planting treatments:
Pre-planting:
Zidua Pro
Post-planting:
Liberty
AMS
Outlook
Clethodim
HSCOC
Disclaimer: Always Read and Follow Label Instructions. Roundup is a registered trademark of Bayer Group. Cobra is a registered trademark of Valent USA LLC. Callisto is a registered trademark of Syngenta Crop Protection. Liberty and Outlook are registered trademarks of BASF Corporation. Products were chosen for demonstration only; no endorsement of any product is intended.