Battling herbicide-resistant weeds
Both Culpepper and Norsworthy agree that the strongest strategy is knowing the plant's biology and understanding its strengths and weaknesses.
"We have found the few weaknesses that glyphosate-resistant Palmer amaranth has," Culpepper explained. "By using its biological weaknesses, we can devise a complex management system that understands that the life of this seed is short lived. We know that it has a shallow emergence depth. If the seed is deeper than three inches, it won't germinate. And the seed requires lots of sunlight to germinate. So, if we can use residue to block the sunlight from reaching the soil, then Palmer amaranth emergence is immensely reduced."
Norsworthy explained the benefit of integrating multiple strategies. "In regards to barnyardgrass and Palmer amaranth, rotating from cotton to drill-seeded/double crop soybean is an effective management strategy. The presence of the wheat crop in double crop soybean suppresses early season emergence of summer annual weeds. Secondly, the delayed planting of soybean means that fewer weeds will emerge in the soybean crop and in turn be exposed to herbicides such as glyphosate in Roundup Ready soybean. Thirdly, drill seeding soybean will ensure that crop canopy formation occurs four to five weeks sooner than the conventional practice of seeding in rows, which will in turn suppress weeds that emerge within the crop and also result in little to no weed emergence once canopy formation occurs (i.e. few weeds in soybean that will need to be controlled or exposed to selection by a herbicide)."
In Norsworthy's example, at least four non-chemical tactics are employed (crop rotation, delayed planting, physical suppression from the wheat and increased competitiveness by drill-seeding soybean).
"The goal here is preventing escape weeds (those few individuals that may have evolved resistance) from producing seed," Norsworthy said. "Once a resistant individual produces seed, it becomes quite difficult to regain effective use of the herbicide to which resistance has evolved."
Rising Costs Will Force Change
Culpepper explained that alternative strategies will continue to grow in popularity due to the rising cost of controlling herbicide-resistant weeds.
"Glyphosate-resistant Palmer amaranth has already forced us to change in Georgia," Culpepper said. "In Georgia, it's gone from $20 per acre for herbicides to $63 per acre now plus additional costs in hand weeding. In fact, in 2010, 92 percent of our cotton growers hand weeded 54 percent of our cotton crop at a cost of nearly $16 million. This is following an expense of $15 million for hand weeding during 2009. Although these programs are very effective, that input level is not sustainable long term for our farmers. They will have to find more economically sustainable alternatives."
The search for these alternatives by growers are underway and being pursued aggressively.
Norsworthy agreed with Culpepper that hand weeding is increasing in the Mid-South. "Last year in Arkansas, 70 percent of cotton acres had at least one hand weeding," he said.
"We will definitely use more herbicides as a result of glyphosate resistance," Norsworthy said. "As we move back to older herbicides and techniques, some producers will be forced to have fewer acres to achieve the needed level of management. Also, some of the newer technologies such as Liberty Link will aid in the management of glyphosate-resistant weeds in crops like soybean and cotton."
Although herbicide-resistant weeds are more of a challenge in the Mid-South and Southeast than the Corn Belt, both Culpepper and Norsworthy warned that as resistance spreads, those states will likely need to adopt more of these strategies than they ever thought they would have to.
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