• By Vicky Boyd,
Each year, University of Arkansas weed scientist Jason Norsworthy screens barnyardgrass seed samples sent in by growers and consultants who suspect the weed is resistant to one or more herbicides. This season will be no exception, and Northworthy offered some tips to help him provide meaningful results in a timely manner.
Based on recent surveys, he said, between 74% and 92% of all Arkansas rice acres are infested with barnyardgrass, and many populations have become resistant to one or more herbicides.
Growers spend an average of $114.40 per acre on weed control, with $87.40 of that specifically targeting barnyardgrass.
Slightly more than half the time — 52% —if growers were not successful obtaining adequate control of the weed with their first herbicide application, they will need three additional treatments to manage the weed. And 20% of the time they’ll never be able to obtain successful control if they didn’t do so with their first application.
Norsworthy said 81% of the state’s rice fields received a noneffective herbicide application, citing information from 2019. One herbicide may cost a grower $15-$30 per acre with another $7-$8 per acre for application. As a result, growers are wasting millions of dollars annually using products to which resistance has been confirmed.
Resistance to multiple MOAs
Based on the screening program as well as surveys, he said the state has barnyardgrass populations that are resistant to one or more of the following Weed Science Society of America herbicide groups:
- Group 1 — Clincher, Ricestar and Provisia.
- Group 2 — Regiment, Newpath, Beyond and FullPage.
- Group 4 — Facet (quinclorac) and Loyant
- Group 7 — Propanil.
- Group 13 — Clomazone or Command.
Each year between 2018 and 2020, he has received more than 100 samples for screening. The service is free to the state’s growers and consultants, thanks to funding from the Arkansas Rice Promotion Board.
This winter, Norsworthy will screen samples against seven different herbicide modes of action. They are glyphosate, clomazone, propanil, quinclorac, imazethapyr, cyhalofop and florpyrauxifen-benzyl.
Tips for submitting samples
✔ Obtain a screening submission form from your county agent, complete it and include it with your sample. The submission form will ask for field GPS coordinates and location, among other information.
You also can obtain a form by emailing Norsworthy at firstname.lastname@example.org.
✔ Collect 25-30 seed heads and place them in a paper bag. Make sure the seeds are mature. “The biggest problem is harvesting the seeds too early,” Norsworthy said. “If I can’t get the plants to germinate, I can’t evaluate them.”
In fact, he said, he’s not able to screen about 20% of the samples because the seeds were too immature to germinate.
✔ Send the samples by the Nov. 1 deadline. This provides Norsworthy enough time to screen them in the greenhouse and return results to county agents by late February so growers can use them to make informed decision about their upcoming herbicide program.