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History of herbicide

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1. 2,4-D debuts in 1946. This broadleaf killer kicks off the row-crop herbicide era. It amazes wide-eyed farmers with its activity on broadleaves, but it has a nasty drawback – drift. Several generations of farmers and their spouses get used to covering tomato plants when it's applied to nearby fields.

2. Resistance tip: Be competitive. “Maintaining crop competition and avoiding skips will prevent weeds from getting a foothold,” says Matt Liebman, Iowa State University (ISU) agronomist. This step is still as applicable today as it was back in the 1940s to farmers who just had 2,4-D to use.

3. Atrazine debuts in 1958. This popular corn herbicide controls a number of broadleaves and grasses. The photosynthesis-inhibitor mode of action that includes atrazine will be joined in the 1960s by microtuble inhibitors (trifluralin) and shoot inhibitors (Lasso).

4. Resistance tip: Rotate crops. Like back in the 1950s and 1960s, crop rotation remains a weed-control tool. A multiyear ISU study examined a corn-soybean rotation and showed inclusion of these crops with small grain and legumes in three- and four-year rotations effectively suppressed weeds. “This was despite backing off herbicides in the three- and four-year systems,” says Liebman.

5. Weed scientists confirm a common groundsel biotype resistant to atrazine's mode of action in Washington in 1970. Over time, more weeds develop resistance. Fortunately, weed resistance to atrazine doesn't deter its use. Atrazine is contained in over 60% of corn herbicides today.

6. Seven herbicide modes of action for corn and soybeans debut during the Golden Age of Herbicides from 1970 to 1989.

● GS inhibitors: (glufosinate, contained in Liberty)

● EPSP inhibitors: (glyphosate)

● ACCase inhibitors: (fenoxaprop, included in Fusion)

● HPPD inhibitors: (Isoxaflutole, contained in Balance that was later commercialized in 1999)

● PPO inhibitors: (Lactofen, contained in Cobra)

● ALS inhibitors: (Imazethapyr, contained in Pursuit)

● Carotenoid Synthesis inhibitors: (Norflurazon, contained in Zorial)

7. Resistance tip: Cultivate rough spots. Think you can dig your cultivator out from the back of your machine shed or tree claim? Well, it can still play a role in ridding your fields of herbicide-resistant weeds.

“The lion's share of weed control will still come from herbicides,” says Mike Owen, ISU Extension weed scientist. However, cultivating those areas with hard-to-control weeds can help manage them and prevent them from going to seed.

8. Roundup Ready debuts in soybeans in 1996. This nonselective herbicide technology enables glyphosate to blanket the crop while obliterating nearly all weeds. Previously to the development of glyphosate-tolerant systems, glyphosate is used as a burndown at an 8- to 10-ounce-per-acre rate. Supplemented with 2,4-D, its cost covers $90 to $100 per gallon, says Owen. “In 1996, glyphosate was used in an entirely different manner on entirely different suites of weeds,” says Owen. In essence, Roundup Ready dumbs down weed control, breaking it down to one and later two postemergence applications of just glyphosate. It soon catches on with corn, too. With weed-control worries over, Roundup Ready frees up considerable time for farmers to take on more land or to take a summer vacation.

9. In 2000, Mark VanGessel, University of Delaware Extension weed scientist, works with growers stumped by marestail that won't die when sprayed by glyphosate. In 2000, glyphosate-resistant marestail is identified in Delaware.

10. Resistance tip: Ramp up the residual. “A soil-applied residual herbicide is absolutely critical in corn and soybeans,” says Owen. Besides adding another way to kill weeds via a different mode of action, it also gets your crop off to a quick weedfree first few weeks in the field.

11. From 2000, resistance to glyphosate snowballs. By 2004, glyphosate-resistant marestail spreads across 500,000 acres. In western Tennessee, glyphosate-resistant marestail infests over 80% of cotton acres. It puts the skids to no-till cotton and eventually soybeans on infested acres.

“No-till cotton production was clearly a time and money savings, but farmers had to revert back to an aggressive tillage system because of this one weed,” says Owen. “This happened a lot faster than what we had ever experienced.”

12. Resistance tip: Don't cut rates. Lower-use rates may have contributed to glyphosate-resistant weeds. It enabled resistant biotypes to escape glyphosate's scythe-like control and eventually multiply into widespread resistance across fields.

Cutting rates can also reduce residual time with preemergence products. “With postemergence products, you also get into problems with resistance if you don't apply the full rate,” says Rick Cole, Monsanto technology development manager.

13. Glyphosate use races through the Midwest, reaching a point where today it is used on 95% of U.S. soybean acres and 65% of U.S. corn acres. Predictably, resistance races across the Midwest.

The first glyphosate-resistant waterhemp biotype in the Midwest is found in Missouri in 2005. In 2008, a survey of 200 Iowa common waterhemp populations shows one third resisted glyphosate. That number has increased since then, says Owen.

14. Resistance tip: Use different systems. Arkansas farmers are starting to rotate between Roundup Ready soybeans, LibertyLink soybeans, and conventional soybeans, says Bob Scott, University of Arkansas Extension weed specialist. “This breaks the cycle of continuous Roundup use,” he says.

15. Resistance woes compound when a weed biotype resists multiple herbicide modes of action. In 2005, weed scientists identify triple-stack resistance of glyphosate, PPO inhibitor, and ALS inhibitor resistance in waterhemp. By 2009, quad-stack resistance to glyphosate, ALS inhibitors, triazines, and PPO inhibitors to an Illinois waterhemp biotype is identified. By 2009, HPPD-inhibitor resistance surfaces in both Iowa and Illinois in triple stacks with other modes of action.

16. Resistance tip: Jump off the transgenic treadmill. Promising new technology – featuring soybean tolerance to dicamba, 2,4-D, and HPPD inhibitors – is slated for later this decade and will join Roundup Ready and LibertyLink systems. Just don't get stuck on one type as you did with Roundup Ready.

“For example, if you get great control with a herbicide (in a herbicide-tolerant system), change it the next year to keep it working,” says Stephen Powles, a director of the Australian Herbicide Resistance Initiative at the University of Western Australia. “You don't want to get on the transgenic treadmill. Changing (to another system) is a way to keep glyphosate and other herbicides working as long as possible.”

17. This troubling member of the pigweed family races across the South since migrating in the 1950s from western states. For a time, glyphosate keeps it in check.

In 2006, though, Malcolm Haigwood, Newport, Arkansas, notices glyphosate isn't killing Palmer amaranth the way it used to. “I was in denial for two years,” he says. “I thought maybe it was an application error, maybe there were escapes from preplant herbicide.”

18. Resistance tip: Usemultiple modes of action. This kills any weed biotype beginning to resist a herbicide mode of action.

“I start clean with a timely burndown before planting,” says Haigwood. A residual herbicide permits him to plant soybeans prior to postemergence applications. On fields with glyphosate-resistant weeds, the postemergence application is a PPO-inhibitor herbicide.

Be cautious, though. Multiple modes may not work for preexisting resistance. For example, ALS herbicides will not control waterhemp in Iowa, given most populations already resist ALS herbicides, says Owen.

19. “In the South, we essentially have driven the (Roundup Ready) technology off the cliff,” says Ford Baldwin, co-owner of Practical Weed Consultants and retired University of Arkansas Extension weed scientist. “We did the same thing over and over until it didn't work anymore. It failed first on marestail and now it's failed on Palmer amaranth.

“It's to the point where farmers have gotten land taken away from them for weed issues,” he says. “Or landlords have trouble renting land to good farmers who want to take over clean fields and don't want to take over a mess.”

20. Resistance Tip: Manage weed-seed production. If all else fails, rouging mature plants that sneak by can keep fields productive. Removing a rouge Palmer amaranth plant that sneaks through your canopy can prevent this plant from releasing up to 1 million seeds.

“In the South, a zero-tolerance policy aims at preventing plants from ever reaching maturity,” says Aaron Hager, University of Illinois Extension weed scientist.

21. In 2009, weed scientists find a 2,4-D-resistant waterhemp biotype in a Nebraska grass field where multiple 2,4-D applications occurred over multiple years.

New technologies like those that resist 2,4-D, dicamba (Banvel), and HPPD inhibitors will come in soybeans later this decade. They will be useful if used with other tools. Used alone, weed resistance will eventually develop.

“Companies will say they have new tools to control it,” says Pat Tranel, University of Illinois weed scientist. “But waterhemp has already evolved resistance to six modes of action. It will evolve resistance to the seventh, eighth, or ninth ones, too.”

22. Resistance tip: Be timely. It's critical to control weeds like waterhemp and Palmer amaranth when they are small in the 2- to 4-inch stage. Waterhemp can grow 1 to 1½ inches per day, while Palmer amaranth can grow 2 to 3 inches per day. Waiting a day to control weeds like these can reduce control, notes Hager.

23. Pry open your pocketbook. Diversifying weed management can cost more, particularly if you already have a herbicide-resistance problem.

“At one time in the 1990s, three to four applications of glyphosate cost as little as $12 per acre,” says Haigwood. “Now, with the different herbicides we're applying, costs can run up to $65 per acre.”

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