What is in this article?:
- Avoidance is the best, least costly, approach to herbicide resistant weed management.
- At least 76 weed species have known resistance to some herbicide.
- Specialists offer several recommendations, depending on location, crop choices and management options.
Palmer amaranth is one of the most worrisome herbicide resistant weeds.
Not a new problem
Resistant weeds are not a new problem for Texas farmers. “I’ve been warning farmers to beware of resistant weeds since 2001,” Baumann said. “Using herbicides with a single mode of action multiple times in a season and applying the same herbicide to sequential crops and over several consecutive seasons and without other weed control options,” sets a farm up for herbicide resistant weed populations.
Growers should look for warning signs such as plants that escape herbicide applications and that are surrounded by controlled plants. Also, patches of weeds that persist after herbicide applications may be resistant weeds that grew from plants that went to seed the year before.
Baumann said the problem can spread rapidly. Glyphosate resistant common waterhemp was identified in Wharton County, Texas, in 2005. “We saw more resistance in 2006 and also noticed resistance to other herbicides.” Resistance was also spreading to other counties. In recent years, glyphosate resistant waterhemp has spread from South Texas up into the Blacklands.
Common waterhemp is more difficult to control than the typical pigweed species. “It’s not the same as Palmer amaranth. Some herbicides don’t work as well on waterhemp as they do on Palmer amaranth.” Consequently, proper identification is essential for effective control.
Observers had “a lot of hits” on resistant waterhemp in 2012. “By the time farmers notice they have resistant or uncontrolled weeds, the plants are 12 inches to 16 inches tall,” and harder to manage, Baumann said. “Control could be a timing issue, so it’s important to make sure if it’s resistant.”
That means collecting seed and having them tested. A recent survey collected seed from 20 plants from 17 locations where weeds were not adequately controlled. The seed were grown in a greenhouse and exposed to various rates of glyphosate, from a half-rate to four times the normal rate. Of the 20 plants, 19 had more than 50 percent survival with a half-rate; 17 had more than a 50 percent survival at a full rate; 15 had more than 50 percent survival at two times the normal rate and 8 had more than 50 percent survival at four times the normal application rate. “At that level, we know resistance is an issue,” Baumann said.
The problem is not confined to South and Central Texas. In 2011 Texas AgriLife personnel confirmed Palmer amaranth resistance in Terry County, in the High Plains. “Palmer amaranth is the most common weed in that area,” Baumann said.
In 2012, resistant Palmer amaranth was identified in several more High Plains counties. Of 10 suspected sites, resistant Palmer amaranth was confirmed in eight.