No silver bullet, no magic potion, no miracle medicine exists to eliminate aflatoxin as a threat to Southwest corn production.
But growers do have options to increase their odds of producing grain with levels low enough to avoid dockage or, at worst, being refused at the elevator.
“When we plant in a high risk aflatoxin environment, we want to get levels low or to even zero,” says Seth Murray, Texas AgriLife Experiment Station corn breeder, who spoke at the recent Blackland Income Growth Conference at Waco.“It takes an integrated approach, a combination of practices.”
Murray and Texas AgriLife Extension Pathologist Tom Isakeit discussed ways to minimize the contamination threat of the devastating fungus, Aspergillus flavus.
Murray is looking at varieties with improved tolerance to aflatoxin contamination. “No resistance exists,” he notes, but factors such as heat resistance, insect tolerance and certain physical aspects of the corn ear and kernel may play a role in reducing infection. “Insect control and cultural practices, along with biological controls, are other possibilities.”
Reducing insect pressure may be a factor in some areas. “In some locations, insect vectors are a major factor in aflatoxin contamination,” he says. “But in College Station, we see little effect from insects as a vector.”
Kernel hardness and stress tolerance in corn hybrids are important — hybrids with open husks are more prone to aflatoxin. “We see more fungus and more toxins in these hybrids; they are geared to Midwest conditions because they dry down better.”
A harder endosperm resists contamination better. Ear droop also plays a role. Many newer hybrids produce ears that stand straight. Those that droop are less likely to collect moisture that creates a favorable environment for the fungus, Murray says. The fungus is much more likely to enter injured seed.
Commercial breeders have not done a lot of work on developing aflatoxin-resistant hybrids. With support from the Texas Corn Producers Board, three new inbred lines less susceptible to aflatoxin contamination were recently released by the Texas AgriLife Experiment Station.
“So far, the best resistance we’ve found does not yield well. Improving yields in these strains takes time.” It will be a few years before growers find these traits in commercial hybrids.
In the meantime, Isakeit says, growers should select hybrids best adapted to their specific growing areas. “Look at performance trials for those specific areas and conditions.”
“Don’t rely on transgenic hybrids to control mycotoxins,” Murray says. “We’ve seen a small effect from Bt corn in reducing insect damage, but no significant reduction in aflatoxin levels. We do see significant differences in aflatoxin contamination from hybrid parent lines.”
Cultural practices are also a management option , Isakeit says, but their beneficial effects may be limited. Key practices include using the optimal amount of nitrogen fertilizer, crop rotation and tillage.
“The fungus that produces aflatoxin doesn’t grow on crops such as wheat or sorghum, so rotation could be helpful. Also, since the fungus produces spores on cob residue and only spores from surface residue can get to the corn ear, tilling residue into soil may be helpful.”
Biocontrol agents are relatively new tools for aflatoxin management. Two primary products, Afla-Guard and AF-36, are Aspergillus flavus strains that do not produce aflatoxin but, if applied in a timely manner and under proper growing conditions, will produce vast numbers of spores that out-compete the native toxic strains for limited space in the corn kernels.
“Both need moisture, either moist soil or dew, to activate and produce spores,” Isakeit says. The wind picks up spores and carries them to the corn ear. The fungus, which is applied on barley or wheat seed, takes two to three days to sporulate. The grain carrier turns fuzzy green with spores.
Isakeit says biocontrols have reduced aflatoxin in his tests, but are not slam dunk certainties. “They can fail to sporulate because of drought. A heavy rain can cover them with soil. If the product is applied too early, the fungus produces spores before they are needed. If it’s applied too late, it doesn’t activate in time to compete with the toxic strains of Aspergillus.
“Atoxigenic strains work in small, experiment plots, but we still need to run replicated tests on a larger scale to quantify the economic impact of atoxigenic treatments under commercial conditions.”
Afla-Guard application rates run from 10 pounds to 20 pounds per acre and Isakeit says the 10-pound rate has worked well in his trials. Efficacy at a lower rate is uncertain. A liquid formulation, which could help farmers with ease of application, has been tested but is still experimental. So far, applying the material on grain seed is the only option.
Early application also would help farmers get the material out efficiently, but Isakeit says some questions remain about proper timing. “At-planting is too early, but we know we can’t wait too late, and we know that uniform application is very important.”
Those are areas he and others are still evaluating. “Overall, we need more research into the different approaches for managing aflatoxin, but there are several things that can be done now.”
More information on this topic can be found at: http://aflatoxin.tamu.edu/aflatoxin.html