If you don't do anything to combat nematodes in your cotton field, you lose money. But if you do too much — such as making blanket applications of a fumigant — you can lose money, too.
A variable-rate application is one approach to finding an economical compromise to the problem, according to Charles Overstreet, an Extension nematologist at the LSU AgCenter, Baton Rouge, La.
Overstreet and fellow scientists Gene Burris, Boyd Padgett, David Sullivan and Maurice Wolcott have launched a project to explore the use of variable-rate technology to manage nematodes more economically in Louisiana cotton fields.
The scientists began by determining the relationship between nematode distribution and soil type, using a Veris machine to map the latter.
A Veris machine (which measures soil electrical conductivity) “can give us a good idea about soil texture, and nematodes are closely linked to texture,” Overstreet said. “Root-knot nematodes in particular occur only in the lighter, sandy soils. Reniform nematodes can occur in the lighter soils, but like the silty loam soils a lot better.”
The scientists divided several fields into 1-acre grids to determine the distribution of nematodes and ran the Veris rig to determine how soil texture was distributed.
The researchers immediately found that the 1-acre grids needed some adjusting. “So, we changed our sampling procedures a little bit. Instead of taking a sample from the middle of the field, we tried to make sure we were taking samples from the various zones (representing soil types) in the field.”
As suspected, the nematodes occupied the lighter and/or sandier parts of the field. “We also found that there were spots in fields where we didn't have a whole lot of nematodes, even within some sandy areas. This is what you would expect with root-knot nematodes. It's not exactly uniformly distributed.”
The scientists divided each field into seven management zones with Zone 1 being the lightest/sandiest soil and Zone 7 being the heaviest. Not all fields had the same range of soil type, meaning that Zone 1 in one field may not be the same soil type as Zone 1 in another field.
“The scientists created management zones for each field which were treated or not treated for nematodes. In one test field, about 30 percent of the field was not treated. Other fields indicated that nematodes could occur across entire fields.
Another important part of the study for growers was to determine if soil texture changed the response to nematicides. For example, “as soil texture gets heavier, do you still get the same results with a nematicide as you do in the lighter parts of the field?”
A trial, conducted in a producer's 71-acre field in Concordia Parish, indicated a better response to treatments at lower SEC readings (lighter soil) and decreasing response as soil becomes heavier.
Scientists broke down costs for treating Zone 1, Zones 1-2, Zones 1-3 and a blanket application. Most of the nematodes were present in Zones 1-3, noted Overstreet. Treating nematodes with Temik and Telone in only the Zone 1 cost around $230 an acre. Treating Zones 1-2 cost $550. Treating Zones 1-3 was $1,000. “If you treat the entire field, you're looking at nematicide costs of about $2,600.”
When the economics — yield lost or gained minus cost of application — is considered, only one of the treatment options clearly showed a return.
When only the nematodes in Zone 1 were treated (Telone and Temik), there was a loss of $380 because of additional problems with nematodes in some of the other zones. “Treating only Zones 1-2 indicated a profit of $320, even with the cost of the Telone. If you treated Zone 1-3, we didn't get any response in Zone 3, and the loss is $175. It's money wasted.”
Treating the entire field with Temik and Telone indicated a loss of $1,725. In addition, if the entire field were treated with Temik at the 3.5-pound to 5-pound rate — a standard farmer practice — the loss is $875.
“In at least four trials, as we move into heavier soils, we did not get nearly the response from nematicides,” Overstreet said. “This is not new to us. We've known this in the past, but we really haven't had the technology to do something about it until we used some of these Veris machines to get the SEC readings.