There's at least one big incentive for considering site-specific applications of insecticides in cotton — it's a good way to stop throwing good money after bad.
This is the theory behind a LSU AgCenter study in northeast Louisiana. Roger Leonard, research entomologist at the LSU AgCenter in Winnsboro, La., discussed the research during InfoAg Mid-South, a regional precision agriculture conference in Tunica, Miss.
The objective of the study was to use yield monitoring information to create cotton yield and profit-loss maps as a basis for site-specific prescriptions of Bidrin and Orthene for tarnished plant bugs. All of the site-specific applications were made by air.
The cost of insect control for tarnished plant bug is close to $40 an acre for Louisiana cotton producers, noted Leonard. “It's not likely that these costs will decrease. If we can reduce the frequency of insecticide applications in some areas, we have an opportunity to reduce input cost for this pest.”
Each crop input has a different set of principles guiding variable-rate prescriptions of pesticides, noted Leonard. For example, with defoliants and plant growth regulators, rates can be adjusted according to plant growth characteristics.
“One thing different for insect control is that it takes a minimum dose of insecticide to kill an insect. So variable-rate in its simplest form for controlling insect pests is either spraying an area or not spraying an area. There are not a lot of opportunities to vary the dose of insecticide because the minimum dose is what is required to kill that insect.
“Another point we need to remember is that you're not going to see a tremendous benefit to precision agriculture unless you have considerable yield variability. In fact, the value of the technology on any given field is dictated by the amount of variability across that field. The greater the level of yield variability in a field, the more opportunities that precision agricultural technologies can be used to manage variability and increase profits.”
The test was in northern Tensas Parish, on Hardwick Planting Co. near Newellton, La., on a farm operated by Jay Hardwick. The experiment compared spatially variable insecticide (SVI) treatments to a whole-field broadcast treatment. “Although our experiment only considered insecticides, it could be used for any variable input.
“In 2002, we based the application prescription on raw yield data captured with a cotton yield monitor. Data was combined from maps from the previous two to three years, and in some instances we were able to look at combinations of grain and cotton yield maps.”
In 2003 and 2004, researchers carried the experiment a step further, working with a crop profit and loss map created by Jane Niu and Ken Paxton in the LSU AgCenter Department of Agricultural Economics. “The maps indicated which areas of the field were not profitable based on the total input costs used on those areas.
“In an ideal world, the data should be used to detect yield-limiting problems and to develop a long-term solution to the problems. Of course, yield limitations may not be easily corrected or the solution may be cost-prohibitive.
“Our approach was to use the data to create a short-term management practice in the absence of solving the yield-limiting problem.”
Essentially, the researchers used variable-rate technology to not spray areas of the field that reached a certain threshold of low yield or profit loss.
Profit maps indicated there were several non-profitable areas of the fields. Profitability levels generated by models ranged from a loss of over $100 per acre to a profit of $200 per acre.
The plots were sprayed in long strips that ran all the way through the fields and were randomized and replicated. With variability running across the different treatment strips, “we were able to compare low-yielding and high-yielding areas within the same treatment as well as across treatments. That's an important consideration to determine if there is a benefit to using the technology in this manner.”
On most of the variable-rate fields, the researchers applied insecticide to 80 percent to 89 percent of the strips, while eliminating spraying on the remaining 11 percent to 20 percent.
When the plots were sampled after spraying, insects were found in the non-sprayed areas, whether they were high-yielding or low-yielding. However, the numbers of tarnished plant bugs recorded in those areas were always below a level that would have triggered an insecticide treatment.
“We also found high numbers of insects in the borders of fields. Many insect pests that must be controlled in cotton fields migrate from surrounding fields of crops or native hosts. It makes sense that boll-worms will lay eggs as soon as they get to the field. It makes sense with tarnished plant bugs because they move in from the edges of the fields. However, beyond the initial infestation on field borders, many cotton insect pests rapidly disperse across the entire field.
“It's also important to remember that insect pest problems are not always the limiting factor in low-yielding areas. So, regardless of which insecticide inputs are applied, we will never increase the yield beyond a certain point.”
Yields in the fields were collected for whole-field broadcast and variable-rate applications. “There was no significant difference among the prescription and broadcast treatments in any of the three years,” Leonard said. “We didn't anticipate increasing yield. We're just happy we didn't decrease yield, because we eliminated parts of the field from being sprayed with insecticide treatments.”
Overall, on fields treated with a variable-rate prescription, “we reduced spraying 10 percent to 20 percent of the total field area. Insecticide inputs were reduced $5 to $7 per acre. So there was a clear economic benefit from using the information generated by yield monitoring combined with site-specific pesticide applications.
“We hope other scientists will see this project as a model for which to evaluate other site-specific management decisions, such as applications of fertilizers, PGRs or fungicides,” Leonard said. “Precision agricultural technologies and the interpretation of site-specific data is constantly evolving. Future opportunities to use this information to improve farm efficiency are limited only by our own creativity.”
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