Herbicide tolerant crops have revolutionized weed control for many Southwestern corn, cotton and soybean farmers, but with innovation comes an increased need for product stewardship, says a Texas A&M Extension weed specialist.
Paul Baumann, speaking at the recent Agricultural Technology Conference at the Texas A&M-Commerce campus, said farmers' weed management options have expanded with herbicide-resistant crops. But he cautioned growers about jumping into new weed control systems without evaluating the economic and environmental repercussions.
“These new crop options provide additional weed control opportunities beyond traditional herbicides,” Baumann said. “Farmers now have more options at critical (crop and weed growth) stages. Roundup, for example, gives them more flexibility with timing compared to pre-emergence or postemergence applications or at specific weed or crop sizes.”
Baumann said products like Roundup also may reduce the overall pesticide load on the environment.
“But the value of herbicide tolerant crops depends on the weed control system and the herbicides used. Growers must consider the added cost of herbicide tolerant crops compared to the flexibility and performance of the available herbicides.”
Herbicide tolerance, he said, allows farmers to manage tough weed problems. “But don't sacrifice yield for just a little more weed control. Ask if a herbicide tolerant program offers better economic or environmental advantages. And not all crops exhibit the same degree of tolerance. Current soybean varieties have a greater tolerance for Roundup, than RR cotton.
He suggests farmers evaluate equipment on hand. “They may need a hooded sprayer for applications of Roundup in cotton after it reaches the four true-leaf stage. Proper calibration also plays a crucial role in any herbicide program.”
Options vary with each crop.
For corn, growers have Liberty Link and Roundup Ready tolerance programs. They also have IT corn tolerant to Lightning herbicide.
“The Roundup Ready program offers exceptional performance against most weeds at reasonable size,” he said. “It gets most broadleaf and grass weeds when applied during the growing season and may be used as a remedial treatment.”
Baumann cautions against relying on Roundup for a total program, however. “Corn farmers may need a pre-emergence treatment to get through the first three weeks. If they wait for weeds to emerge, they may get some competition at a critical growth period for the corn.”
He said weed competition within the first ten weeks after emergence may affect yield. “We see little influence on yield at 13 or 24 weeks,” he said.
He also cautions growers to be aware of potential weed shifts and herbicide resistance. “We face the potential for resistance to develop if we use the same product year after year.”
He said Lightning is a good knockdown product, with a reasonably broad spectrum of activity. It offers less flexibility than Roundup or Liberty with regard to weed size at treatment but does provide residual control of pigweed species.
Cotton farmers have Roundup Ready cotton varieties and will soon have Liberty Link cotton tolerant to glufosinate (Ignite) herbicide.
“Again, Roundup offers excellent control of many troublesome weeds, but we need to educate growers to develop a program approach. They also need to pay attention to treatment restrictions, such as the four-leaf stage cut-off.”
He also warned growers to be alert to weed shifts and herbicide resistant weeds.
“Also, if growers are not certain they'll be able to make a timely application with Roundup, they may want to include a pre-emergence or at-planting treatment in their weed control strategy.
“Timing is also important, especially for weeds such as the morningglory species. At 10 to 14 days after emergence, if weeds are not dead, a second application may be necessary. Don't wait until weeds recover from an initial application.”
Baumann said the generic glyphosate products also work well in most situations. “We have at least 20 or 30 generic products registered for use in Texas cotton,” he said. “Some of these may cause some speckling of leaves early but the cotton grows out of it and suffers no effect on growth, yield or quality. There have been some reports of cotton terminal growth damage, but we have seen none of this in our trials.”
Baumann recommends farmers put a pencil to the generic products to determine if they get as good a buy weighing the lower price against potential for less customer service.
“Most of these products do a good job, but be careful of untested products. And remember that not every manufacturer worldwide maintains the same quality control as U.S. companies.”
He said Liberty also offers a broad spectrum of control on grasses and broadleaf weeds. “However, like Roundup, it provides no residual activity. Envoke, from Syngenta, also offers broad-spectrum control applied postemergence after the five-leaf stage and can be used on non-herbicide tolerant cotton varieties. And we still have the traditional ppi, pre and postemergence products available.”
Soybean farmers continue to rely mostly on Roundup Ready varieties, Baumann said. “Roundup gives good control and allows the canopy to cover the row and reduce weed competition. Row spacing also plays a role.”
He said morningglory may be one of the toughest weeds in Roundup Ready soybeans and cotton. “Farmers may need repeat applications to take it out. Also, weed shifts are possible.”
Baumann said farmers who have switched to herbicide tolerant crops should be aware of the potential for weeds to develop resistance to herbicides. “Resistance is a plant's inherited ability to survive a herbicide application that it would have been susceptible to before.
“Weeds produce billions of seed and very few are different genetically, but a very small percentage may be tolerant to a particular herbicide. When a farmer uses a highly effective herbicide, he takes out the susceptible weeds and the one that's tolerant will flourish.”
He said the herbicide actually helps since it takes away all the natural competition. The resistant one grows, sets seed and the progeny survives with a higher percentage of resistant weeds that continue to expand. “It creates a mess,” Baumann says.
“We do have herbicide resistance in Texas. Even though glyphosate (i.e. Roundup) is a highly effective herbicide on a broad weed spectrum, use does eliminate competition for resistant weeds. And used a number of times in resistant crops, resistance is more likely to develop.”
Baumann says the problem is exacerbated because alternative chemical development has slowed in recent years.
“One advantage with Roundup is that it provides no soil residual. Also, rotating to a non-Roundup Ready crop will help delay resistance. There is also something to be said for the fact that we have used Roundup for several years now and have not documented the presence of any Roundup resistant weeds in Texas.” Baumann also recommends using effective alternative herbicides.
He says farmers need to avoid herbicide resistance to maintain their arsenal of products. “We can develop cross resistance, weeds resistance to a number of herbicides.” He said high registration costs for chemical companies also impede new products to combat resistance.
“It pays for farmers to check fields to see if several species normally controlled by a particular herbicide are present. It's unlikely that two species will develop resistance. Re-growth from the base of a perennial plant not completely controlled by a Roundup application is indicative that resistance is not evident.”
Baumann said growers should use herbicides only when necessary and then rotate herbicides, using different modes of action.
“Clean tillage and harvest equipment to prevent moving weed seeds and perennial plant parts. Consider cultivation and rotate crops.”