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Soil profile could be hiding nematode variability in cotton

If you're considering variable-rate applications of a high-end nematicide for root-knot nematode in cotton, don't forget that, according to new research, your soil profile could be hiding variability.

Maurice Wolcott, plant pathologist and precision ag GIS specialist, LSU AgCenter, speaking at the Conservation Systems Cotton and Rice Conference in Tunica, Miss., noted that research has proven that root-knot nematodes cause more damage to cotton in lighter soils. Conversely, their impact is much less severe in heavier soils.

Researchers have also recognized that this could be the basis for making variable-rate applications of a nematicide. In developing a prescription for a variable-rate application, sandier parts of the field could be considered responsive to a fumigant application; the heavier soils could be considered non-responsive.

“Field research using yield monitors indicate that we can get a much higher yield increase in the responsive areas (where nematode damage is the highest) of 200 pounds to 400 pounds of lint per acre,” Wolcott said. “Dow now has a label for a 3-gallon rate application of Telone II with a band underneath the bed, so you're talking about $40 to $45 in total costs. If you can make the first 100 pounds, you make a profit.”

Savings could also come with a better understanding of soil variability beneath the surface, Wolcott and LSU AgCenter Extension nematologist Charles Overstreet have found.

From 2004 through 2006, field research was conducted in several commercial cotton fields that had previously been mapped using an electrical conductivity mapping cart to generate a soil map of the fields. The map detailed variability in the top 12 inches (shallow EC), as well as in the soil profile down to 36 inches (deep EC).

Researchers applied alternating strips of Telone II, a fumigant nematicide, to remove as much of the impact as possible of root-knot nematodes. The study showed significant benefits could be achieved from a variable-rate application.

But there was an anomaly in the data. At harvest, when they analyzed yield data and compared it to maps of nematode populations, they found areas where optimum yields were obtained in spite of the presence of nematode populations previously considered to be extremely damaging.

In other words, in some areas where sandy topsoil was present, optimum yields were achieved with no fumigant application.

Researchers re-sampled those areas down to a depth of 24 inches and discovered that in the top 12-inches of soil, “we had high levels of root-knot nematode. But when we look at the entire soil profile down to 24 inches in the soils, we found our sand was decreasing and our clay content was increasing at deeper soil depths. In the responsive soils, the sand texture and high root-knot populations went much deeper.

Wolcott concluded, “To get a look at what your whole soil root zone is doing, the deeper values collected from EC mapping carts make more sense than the shallow values.

“That cotton root has the potential to go down 24 to 36 inches. We've always sampled the top 6 inches and based our management recommendations on what we've found. But in this case, what's going on past that 6- to 8-inch layer can explain things a whole lot better.”

Getting fertility right is important, too, noted Wolcott. If left untreated, nematodes “have the potential to compromise the entire root system. If we take the nematodes out down to 12 inches, that's the portion that the cotton plant is going to depend on for nutrients. That makes it even more important to make sure the nutrients are there at those depths.”

Wolcott provided some tips on the variable-rate application of fumigants for root-knot nematode. He stressed that “the odds of hitting a home run on your first trip to the plate are virtually nil. So be prepared to modify it through the years and refine it.”

  • Develop soil texture zones using the best data you can. It might be your personal knowledge of the field. It might be some NRCS soil data, or bare-ground aerial imagery.

  • Collect a composite zone sample from the lighter textured soils.

  • Focus on low-yielding, low vigor areas and light-textured soils more than the high-yielding parts of the field.

  • Find out what nematode species is in the field.

  • Supplement the management plan with yield monitoring or crop imagery data.

  • Apply a fumigant, Telone or K-Pam, in verification strips through the field, 12-rows in small fields and 24 rows in larger fields, to evaluate the crop response.

  • Leave an untreated strip through the entire length of the field and a treated strip the entire length of the field to evaluate the accuracy of the first prescription, so that in the second year, you can either expand or shrink the treated area.

  • When sampling, collect a sample large enough to send to both the nematode lab and the soil lab. “You want to take care of your nutrient issues. You can't make silk purse yields off sow-ear fertility, even if you take out the nematodes.”

Wolcott characterized nematode injury to yield and profit as either “armed robbery” where there is almost total devastation, to embezzlement, where cotton looks good, but check strips in fields indicate losses of 150 pounds to 175 pounds of lint per acre.

“Remember that nematodes are somewhat of a daunting challenge. You can see an insect, see the spray going on the insects and find dead bugs. If you're working with weeds, you can see the weed, see the spray going on the weed and a couple of days later you can find dead weeds.

“With nematodes, there is something out there you can't see, you can't see the chemical being applied and after you apply it, you don't know if anything is dead, and it might be mid-season before you know any results.”

“Treatments for nematodes fall into the so-called Cadillac treatments, fumigants such as Telone II or K-Pam, where you can get a very good response. What's more widely used in the Cotton Belt are the Chevrolet nematicides such as Temik or seed treatments, which are marketed for lighter to moderate pressure situations, in areas of light to moderate yield risk.”

One big problem with applying fumigants — that they require deep placement 10 to 14 inches deep — has been solved with the advent of a modified coulter 30-inches in diameter, which works real well in reduced or no-till situations.

“Equipment is now on the market that is capable of treating only the areas that show a high potential for response. The big ‘if’ is creating a prescription that doesn't cost more than the yield increase, basically it's the cost of intensive sampling. However, this process has become less expensive with the advent of soil electrical conductivity mapping carts, which can divide a field into its respective soil textures.”

In some cases, a blanket treatment of a fumigant can be feasible. In treatments of a fumigant on four Louisiana research fields, researchers reported an average response of 233 pounds of lint per acre. “That's a pretty healthy yield increase,” Wolcott said. “If cotton is 50 cents a pound, you can make money doing this. If it goes to 70 cents to 80 cents a pound, it becomes even more profitable.”

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