In the social sciences, they call it unintended consequences, in the military, collateral damage — good intentions gone awry.
In row-crop farming, there is no technical term for it, but the effects always work out the same way: higher input costs, coupled with added work and worry for producers.
Whatever one chooses to call it — unintended consequences or collateral damage — horseweed is a case of a good intention gone awry.
In the decades when conventional tillage practices were the norm rather than the exception, horseweed was never a problem. A winter annual, it germinated in fall, grew during the winter and produced seed during the spring and early summer.
The windblown seed that nature had equipped to germinate on the soil surface was easily eliminated through conventional-tillage.
The problem started when minimal-tillage cropping systems were introduced roughly a generation ago both as cost-saving measure and as a strategy for building up organic residue. Minimal-tillage provided the optimal conditions in which horseweed seed could germinate and grow.
Michael Patterson, an Alabama Cooperative Extension System weed scientist and Auburn University professor of agronomy and soils, remembers the first time he encountered horseweed problems in 2003 on cotton acreage located a few miles west of Decatur.
He was accompanied by his colleague Charles Burmester, a Tennessee Valley-based agronomist who had first inspected the field and invited Patterson up from Auburn University to take a closer look.
“It was a cotton field just covered with horseweed,” Patterson recalls. “The cotton was planted and was up about 3 or 4 inches, while the horseweed was standing at between 6 and 8 inches.”
Fortunately for the producer, extensive cultivation, reinforced with a few herbicide applications, brought the horseweed under control.
“He got a handle on it, but it caused him some grief,” Patterson says.
This marked the beginning of a recurrent theme throughout north Alabama, Patterson says. Alongside the far more virulent Palmer pigweed, horseweed rates as a comparative lightweight.
A few herbicide sprayings, and in worst-case scenarios, cultivation, usually stops these weeds in their tracks before they can pose serious threats.
Not helping with cost-cutting measures
However, in an era when cost-cutting is a paramount concern among producers, horseweed is causing plenty of grief, especially among the unfortunate handful of Tennessee Valley producers who are dealing with horseweed and pigweed.
“It may not be as big a problem as pigweed, but it’s still a problem,” Patterson says. “You’re not going to have the same sized populations with horseweed as you do pigweed because it doesn’t produce as many seed, but it’s still a problem considering that it can grow as tall as 6 feet.”
The part that concerns Patterson and Burmester alike is that the weed is undergoing a change in growth patterns.
“We’re talking about a weed that has historically been considered a winter annual that germinated in the fall and grew in the spring and that could be taken out by tillage,” Patterson says. “Now this weed is turning up during the summer in cotton and soybeans, especially those that are planted within minimal-tillage, Roundup Ready cropping systems.”
Several herbicides, particularly gramoxone and paraquat, mixed with diuron or valor and used in burn-down applications ahead of cotton, soybean and corn plantings are still effective.
So is the old stand-by, 2,4-D, as well as dicamba. However, timing is critical.
With 2,4-D applications for example, growers must wait a month to plant cotton to avoid crop damage. The wait for soybeans is considerably less — only a week, if the grower opts to apply a pint-per-acre of 2,4-D ester, but 2 weeks if the grower applies a pint of 2,4-D amine.
Growers should delay corn planting for a week following 2,4-D application.
Patterson says the use of glufosinate, with LibertyLink cropping systems, is gaining favor among some growers.
The critical issue, especially with cotton, Burmester says, is taking out the weed early — before it reaches sufficient size — for example, in the case of cotton, “kill ‘em before the crop emerges.”
“With cotton, for example, once the weeds are up, you don’t have many options,” Burmester says. “There’s nothing other than Liberty herbicide, which is only available with the LibertyLink variety.”
Burmester says he’s noticed many growers using a combination of dicamba and Roundup, though they’re reporting more problems compared with last year.
He says these problems have encouraged many growers to return to Liberty cropping systems, which, in addition to burning down the horseweed, carries fewer restrictions between spraying and planting.
Many growers still find 2,4-D effective, which is a good thing, Burmester says, because it ensures that more Liberty herbicide is held in reserve to deal with pigweed outbreaks.
“We just don’t want to put too much pressure on Liberty herbicide because of what we’re encountering with pigweed,” he says. “We don’t want to put too much pressure on it with over-use.”
(For reference, you might also want to read Management of horseweed escapes prior to planting).