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New weed resistance find a concern

Recent documentation of water hemp resistance to foliar-applied HPPD-inhibitor herbicides in central Illinois should get the attention of Southeastern farmers because of the close biological relationship between water hemp and Palmer amaranth.

Several factors don’t bode well for Southeastern farmers in their quest to avoid the kind of resistance problems documented in central Illinois.

The increase in cotton acreage in the upper Southeast, in particular, could create a return to more frequent use of specific families of herbicides, which could become problematic if the Illinois find in seed corn were to develop in Palmer amaranth in the Southeast.

Corn, prior to its use in ethanol and subsequent price jump, was for many Southeastern growers most valuable as a good rotation crop for more lucrative crops like cotton and peanuts. Corn also played a key role in overall weed resistance management strategies, because herbicide options for corn include choices that aren’t used in cotton, peanuts and other traditional Southeastern crops.

A big concern for Southeastern growers is the number of corn acres abandoned due to severe heat and drought in late June and July. In many cases, fields with low populations of resistant weeds were left to grow to maturity through the drought, leaving behind a good supply of seed.

As of late July several pockets of farmland around the Southeast have suffered from record high temperatures and prolonged drought. Dealing with more weed resistance won’t be good news.

In southeast Virginia, for example, dryland corn has been decimated and peanuts and cotton are only a few hot, dry days from suffering extreme stress-related problems. The Tidewater area has generally been free of glyphosate resistant pigweed, but these climatic and economic conditions are ideal for the spread of resistance.

The latest weed resistance find in central Illinois may or may not have a direct bearing on herbicide weed management strategies in the Southeast — that is yet to be determined. However, past spread of resistant weeds from one crop to another is a good reason for growers in the Southeast to take notice of these latest findings.

Chuck Foresman, weed resistance manager for Syngenta says, “Greenhouse testing has confirmed resistance in this (central Illinois corn field) water hemp population to several post-emergence applied HPPD-inhibitor herbicides including mesotrione, available as Callisto herbicide.

“In addition, Syngenta field studies showed ineffective weed control with post applications of HPPD inhibitors, and ineffective control with post applications of some ALS and triazine herbicides applied alone to the same population,” Foresman says.

“This particular field was used for seed corn production, and for seven consecutive years there was over-reliance on post-emergence HPPD-inhibitor herbicides to control key weeds.

“Hybrid seed corn production systems often preclude the use of important broad-spectrum herbicides like glufosinate or glyphosate. The lack of diversity in both herbicide modes of action and crop rotation in this field led to the development of resistant water hemp,” Foresman says.

These findings significantly ramp up the risk involved in managing weeds in corn and will likely increase the cost of weed management programs in corn, especially in areas in which water hemp is a major problem.

There is a significant biological difference between Palmer amaranth and water hemp. Of greatest concern to Southeast farmers is that Palmer amaranth is at least twice as competitive with crops as is its cousin.

Palmer amaranth is a summer annual (C4 weed) and is problematic in soybeans, where it potentially causes the biggest yield loss, and in cotton. With cotton acreage up, growers need to be particularly aware of Palmer pigweed.

Identification of Palmer pigweed, though virtually every farmer is positive they can identify it in a field, is fairly difficult. Once any member of the pigweed family becomes mature, it is much easier to identify. However, knowing which member of the family you are dealing with at an early level of maturity can be a critical factor for cotton and soybean farmers in the Southeast.

The first true leaves of Palmer amaranth seedlings are ovate in shape, with few or no hairs present. Leaves often have a slightly notched tip and the leaf petioles are usually as long as or longer than the leaf blades.

Mature Palmer amaranth plants are without hairs, with leaves that are diamond or egg-shaped in outline, and petioles that are usually longer than the leaves.

One physical trait that water hemp shares with Palmer amaranth is its ability to adapt to herbicides. That factor ups the ante to growers in the Southeast, who have heretofore not had significant problems with HPPO-inhibiting herbicide resistance in corn.

In Iowa, researchers documented in 2009 and 2010 that water hemp is now resistant to PPO inhibitors and ALS inhibitor herbicides. For all practical purposes, the study indicates all common water hemp in Iowa has evolved resistance to ALS inhibitor herbicides and researchers are thus finding multiple resistances to ALS/glyphosate and ALS/PPO herbicides at an increasing frequency across the state.

Perhaps of greater concern to farmers in the upper Southeast, researchers in Iowa are also finding glyphosate resistance in giant ragweed and marestail populations in increasing numbers.

In general, water hemp is a weed pest north of the Mason-Dixon Line. However, giant ragweed and marestail are problems in many areas of the Southeast.

University of Illinois weed scientist Aaron Hager says researchers across the country are continuing to find weed biotypes that are resistant to various herbicide classes. In Illinois, the recent finding of HPPD-inhibitors is the fifth herbicide family to which water hemp is resistant.

“The number of options growers have for rotation of crops and active ingredients continues to decline. The days of spraying weeds and controlling weeds are rapidly coming to an end. We have to go back and be more integrative in the approaches we use because the options farmers have will continue to decline,” Hager says.

As to whether there is a connection between HPPD-resistant water hemp and HTTP-resistant Palmer amaranth, Hager says, “despite water hemp and Palmer amaranth being similar in many regards, efforts to transfer resistance characteristics that first appeared in water hemp to Palmer amaranth have been unsuccessful.”

This doesn’t mean it can’t happen, but Hager says Palmer amaranth resistance to HTTP-inhibiting herbicides is most likely to evolve through genetic selection within Palmer amaranth, rather than transfer from another amaranth species.

The bottom line is growers in the Southeast don’t want and can’t afford to lose any more herbicides to resistance problems. Doing what it takes to prevent resistance problems is a good alternative to paying the price for overcoming resistance once it occurs.


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