Farmers are nearly universally aware and actively prevent herbicide drift. Despite the best of intentions and appropriate use, environmental factors still influence off-target herbicide movement.

Weather-limited in-field days in growing seasons like 2024 are a tailor-made environment for crop injury, especially in corn and rice grown across turnrows in the Mississippi Delta.

“The years when the drift is the worst, in my opinion, is the years where we have a weather pattern like we've been in, where work is getting stacked up because the rain stacks up,” said MSU Extension specialist Jason Bond. “A lot of our planting dates are overlapping that may not overlap in a more favorable weather year. Our soybean planting wouldn't be as compressed as it is certainly and most of our corn would have been up and going before planting some of these other crops and then rice is right in there with it.”

Despite some challenging conditions, producers are cautious and aware of potential herbicide drift situations, he noted.

“Even though we've had some hiccups this year, we've had years that have been much worse and so I commend everybody for being aware of drift and making good efforts,” Bond said. “You know, even then accidents happen, but I think our awareness is good and we're cautious with it.”

Results of research

Mississippi State has contributed over 10 years of research trials to identifying and providing resources for off-target herbicide movement, especially as it pertains to rice, corn and cotton in the Delta region.

The MSU team primarily practices simulated drift research – where the crop is purposely exposed to a sub-lethal dose of the “drifting” herbicide.  In most commercial drift events, the application rate that impacts the target crop is typically between 1/10 and 1/100 of the original application rate. To complete research in line with those conditions, most of MSU’s research uses a 1/10 dose to simulate worst case scenario injury.

“The dose determines the poison, whether it's herbicides or antibiotics in humans, whatever the dose determines the poison,” Bond said. “What you never know in a commercial drift event is what the rate was that hit the non-target crop. If you could tell that, then you would have a whole lot better information to make an informed decision moving forward.”

An equally important often unknown aspect of off-target herbicide movement is the mixture of herbicides impacting the injured crop. For example, if metolachlor was included in the paraquat premix and then injured a nearby rice field, replant options could be significantly impacted.

Drift injury can present itself in a wide variety of symptoms and severities, with identifying the source of the drift and the type of spray being the most difficult, if not impossible factors to pin down. Perhaps more important, and what most of the MSU team’s research has focused on, is the opportunity to respond to drift situations in-field.

In-field response

“The stuff that has the worst reputation for drift is the stuff that tells on itself the worst when it gets where it's going,” Bond said. “That's paraquat, that's glyphosate, that's 2-4D, that's dicamba. The symptoms show up pretty quick, and they make a pretty showy symptom.”

When planting stacks up and applications occur on multiple fields in multiple crops in the same timing windows, it's not hard to find injury symptoms in proximity to an application that's been made, Bond said.

However, especially in paraquat drift onto corn or rice, which is one of the most common drift situations in the Delta region, the drift injury is often confined to a small percentage of the affected field.

“It's a sticky thing to navigate in that time between when the drift occurs, and when the crop takes off and starts growing,” Bond said. “Regardless of how close to the original yield potential that you're going to get, with yield potential the earlier the drift occurs, the closer you're going to get back to the original yield potential.”

“If we put paraquat on at one leaf in rice, we pretty much got back to the original yield potential,” Bond said. “We produced 60% injury at seven days at seven days after we exposed the rice to paraquat but then we got back to close to the original yield potential. The later in the growth stages we went, the less yield potential we were able to recover out.”

Regardless of the size or severity of the injury, drift is tough to see in field after a challenging start to the 2024 growing season.

“This has been a hard crop to get going,” Bond said. “It feels like we've been planting for three months and there's still a distance to go in some areas. The weather is the story of 2024.”