Use pre-emptive approach to avoid weed shifts, resistance in RR alfalfa

In the game of golf, watching someone putt on the same line as your have is called “going to school” on someone else’s putt.

Farmers, Pest Control Advisors and others in the business of growing alfalfa have a chance to “go to school” on those who have grown glyphosate-resistant corn and cotton to prevent major problems in weed shifts and weed resistance in the newly released Roundup Ready alfalfa.

Made available commercially in the fall of 2005, RR alfalfa is the first perennial transgenic, herbicide-resistant crop.

That poses challenges in avoiding weed shifts and resistance and some, according to a trio of University of California weed specialists. However, there is an advantage to having a Roundup-resistant perennial crop.

Farm Advisors Steve Roof of Siskiyou County and Mick Canevari of San Joaquin County and UC Davis weed specialist Tom Lanini say less rotation opportunities with alfalfa and the fact resistance and weed shifts are believed to evolve more rapidly in solid seeded crops like alfalfa with relatively low value and grown on large acreages, favors weed shifts and resistant weeds with Roundup Ready technology.

However, alfalfa is an aggressive competitor with most weeds and many weed species do not tolerate frequent cutting, a plus in managing the weed resistance/shift challenge.

One of the biggest pluses in meeting the shift/resistance challenge is alfalfa growers can learn from the experience gained from other Roundup Ready crops introduced over the past two decades, according to the trio.

They are calling for a pre-emptive strategy of minimizing the problem in alfalfa by integrating tillage between alfalfa plantings, crop rotation, rotation with herbicides with different modes of action (preferably soil applied herbicides) and tank mixes of different herbicides.

And by all means, do not wait until weed shifts and resistance occur before utilizing herbicide rotation strategies, said the three weed management specialists.

Worth effort

The challenges will be worth the effort since the herbicide-resistance technology in alfalfa allows grower to “deal with some of the most difficult-to-control weed species.”

This has proven especially effective with seedling alfalfa where the technology has resulted in less alfalfa injury and superior weed control, particularly in suppressing perennial weeds like dandelion, nutsedge, Bermudagrass and quackgrass.

Fortunately, according to the researchers, resistance to Roundup is not as common as resistance to many other herbicides. This may bode well for the first herbicide-resistant transgenic crop where producer can leave alfalfa stands in from as few as 2 years and as long as 14 years in the West.

Nevertheless, the researchers say weed shifts or resistant weeds are unavoidable. Resistance to Roundup has already been documented in the West in ryegrass and marestail.

“Roundup is the most effective broad-spectrum post-emergence herbicide available, and it would be a shame to lose its effectiveness as a result of mismanagement,” according to Orloff, Canevari and Lanini.

Based on experience with other herbicide-resistant crops, weed shifts or resistance show up after approximately six years of continual use.

This should set up a pre-emptive strategy in RR alfalfa. This strategy includes using Roundup to control weeds in seedling alfalfa for improved weed control, reduced crop injury, cost savings and to eliminate alfalfa seedlings that do not carry the Roundup-resistance gene.

“Ordinarily, the one-pound active ingredient rate (22 ounces of Roundup UltraMax) is sufficient, according to the researchers.

Justified rates

However, higher rates may be necessary if the field contains tolerant weeds like burning nettle and little mallow. Then a tank mix may be wise with Raptor or Pursuit if burning nettle is in the field or tank mixing with Prism if there is ryegrass.

It is not a good idea to use Roundup for weed control throughout the life of the stand because this would encourage weed shifts and resistance and in most cases weed control would diminish over time.

The researchers “strongly recommend” Roundup not be used for winter weed control for the duration of the stand. Another herbicide or tank mix should be used at least once in the middle of the life of a stand and perhaps twice if the stand is to be in more than five years.

Fortunately, there are several herbicides to use during dormant season. They are Velpar, Karmex and Sencor either alone or in a tank mix. The choice will be based on the weeds in the field.

Paraquat (Gramoxone) is another candidate for rotation or tank mixing, however, paraquat, like Roundup, is applied late in the dormant season. This could potentially select out early-emerging weeds that may be too large to control with Roundup or Gramoxone.

Controlling weeds in the final year of a stand can be challenging because the stand is typically less dense and less competitive against weeds. There are also fewer herbicide options due to plant-back restrictions for many of the soil-applied herbicides.

This makes Roundup a good choice for controlling weeds in the final year of an herbicide-resistance alfalfa field. This also underscores the importance of rotating herbicides before this final year to ensure that Roundup will remain effective.

Summer annuals

Summer annual grasses like yellow and green foxtail; barnyard grass, cutgrass and jungle rice and sometimes pigweed can be problematic in established alfalfa, according to the UC experts.

These weeds emerge over an extended period of time, typically from late winter or early spring throughout the summer.

Applying Roundup or any foliar herbicide season long for summer grass control can promote resistance. Rather than applying Roundup two to three times, the researchers suggest using a pre-emergence like Treflan and follow-up with Roundup as needed for escapes.

Herbicide rotation should be done for both dormant control of winter weeds and for spring/summer control of annual weeds.

The researchers have no hard and fast rules about how often herbicides should be rotated. However, they are adamant about being diligent in monitoring for weed escapes.

“If the relative frequency of occurrence of a weed species increases dramatically, chances are that it is tolerant to Roundup and immediately rotating tank mixing is advised,” write the researchers.

If a few weeds survive among a weed species normally controlled by Roundup, it could be an indication of weed resistance, assuming misapplication or other factors can be eliminated as possible causes.

Roundup Ready alfalfa has the potential to simplify weed management while also improving the spectrum of weed control. However, growers need to go to school on the other herbicide-resistant crops and stay alert for weed shifts and resistance and avoid year-after-year, application after application of Roundup.

“A grower should not wait for there to be a problem before he employs herbicide rotation or tank mixes; a pre-emptive approach is strongly encouraged.”

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