Alfalfa gains important new weed control strategies

Alfalfa is now California's largest acreage crop. Going right along with that ascension to the top of the state's 350-crop list is increasing economic importance of the forage crop, largely related to the burgeoning dairy industry.

This growing importance of alfalfa is running headlong into the latest round of water and air pollution regulations threatening the use of some old standby chemistry.

Fortunately, with the growing importance of alfalfa as a cash crop, several new herbicides have been brought to market to allow California producers to continue to produce high quality hay.

Mick Canevari, San Joaquin County, Calif., University of California Cooperative Extension farm advisor, told the California Weed Science Society 59th annual meeting recently in La Jolla that 75 percent of the state's alfalfa acreage is treated for weeds on an annual basis because the rewards for clean hay are worth the price to pay for it.

“In 2006, the price for No. 1, weed free hay exceeded $160 per ton to the grower compared to weedy hay which sold for $80 per ton,” said the veteran farm advisor.

Managing weeds in a timely manner provides maximum production of high quality alfalfa hay, said Canevari. Conversely, poor weed management can lead to premature stand loss, poor quality hay, unacceptable weed control, alfalfa injury and a loss of money.

Unfortunately, increased air and water quality regulations are impacting many popular alfalfa herbicides which fail to meet new standards under California farming practices. Many herbicides targeted for mitigation are the “mainstays” to annual weed control in alfalfa and other crops.

“Searching for new farming methods that allow critical herbicide usage and development of new herbicides has a high priority to secure crop production and a high quality product,” noted Canevari.

Expands regulations

California recently expanded its groundwater protection regulations. Seven herbicides, two of which are used in alfalfa, Karmex and Zorial (diuron and norflurazon) have been placed on the groundwater protection list and are limited for use in leaching and runoff areas unless certain irrigation management practices are used. However, management alternatives for use of herbicides proposed by regulatory agencies may not be practical.

To use on these herbicides in leaching groundwater protection areas (GWPA) a grower must:

  1. Not apply any irrigation water for six months following application.

  2. Not apply the pesticide to the planted bed or berm above the level of irrigation water for six months following application.

  3. Irrigation shall be managed to apply a net irrigation requirement of 1.33 or less for six months after application.

Management practices required in runoff protection areas are:

Within 7 days before the pesticide is applied, the soil to be treated shall be disturbed by using a disk, harrow, and rotary tiller.

  1. Within 48 hours of application the pesticide shall be incorporated on 90% of the area treated by mechanical tillage or irrigation.

  2. The pesticide shall be applied between April 1 and July 31.

  3. Retain all runoff in the field for 6 months or,

  4. Retain all runoff in a holding area off the field.

Retention and recycling ponds comprise one avenue that provides acceptable alternatives in certain areas where soil types and size of operations make it economically feasible. In the northern San Joaquin Valley, existing drainage ponds and recirculation back onto irrigated crops is being done successfully.

Several alfalfa pesticides are coming under new, more stringent air quality standards in two non-attainment areas of the state, the Central Valley where 60 percent of the state's alfalfa hay crop is produced, and the desert region which produces 20 percent of the state's alfalfa hay crop. The remaining 20 percent of the acreage is in the coastal and mountain areas.

California Department of Pesticide Regulation has identified several of those used on alfalfa as contributing volatile organic compounds (VOC) to air quality problems in California.

Major herbicides

Four of the eight VOC pesticides are herbicides that are important to alfalfa producers: hexazinone, trifluralin, sethoxydim, clethodim.

“If these herbicides were eliminated from use, it would have significant economic impacts to the industry,” said Canevari.

Research is currently under way to study the true economic impacts if these herbicides were removed from the market.

As challenging as these regulations are on a grower's ability to control weeds, there are some new herbicide options that meet air and water quality standards.

The one that has captured the headlines over the past two years is Roundup-tolerant alfalfa. By this spring, an estimated 200,000 acres of the state's 1.1 million acres of alfalfa will be Roundup Ready alfalfa.

Having the flexibility of applying a herbicide without limitations to alfalfa size and having a wide range of rate options allows for a timely and more effective weed control program, especially in the case with larger weeds or perennial weeds that are more difficult to control, said Canevari.

The veteran farm advisor also said it is possible to combine other alfalfa herbicides with Roundup without crop injury problems or compatibility issues to produce weed-free hay.

“Without question, the RR alfalfa system has heightened interest and is gaining popularity. The low cost and flexibility of glyphosate with little threat of crop injury, plant back issues or listed as a problem in groundwater will attribute to a rapid adoption,” he said.

“However, one of the greatest concerns facing rapid adoption of this new technology is the potential overuse leading to weed resistance and weed shifts. In the case where three years of Roundup only was applied, a shift of burning nettle was documented,” he noted.

Across the country in other RR crops, weed shifts and possible herbicide resistance may emerge. The importance of developing new herbicides in alfalfa becomes even more important for rotation and adds balance to an integrated system.

Promising compounds

There are several other herbicides which offer promising weed control scenarios, as well as provide tools for solid resistance management.

One is Sandia, an herbicide well-noted for nutsedge control in field and row crops. Nutsedge has been a serious problem in alfalfa with few management options that are effective. However, it can cause temporary stunting and yellowing when applied during the growing season in the Sacramento and San Joaquin Valley. Less injury and yield loss occurs in the desert region.

Prowl H20 pendimethalin herbicide controls annual grasses and certain broadleaf weeds before they germinate. It will be especially important in alfalfa for summer grass control, dodder control and if used as a tank mix partner for winter dormant weed control. It is safe on established alfalfa once a harvest has been made. Prowl is in the dinitroanline family of herbicides with little known weed resistance issues.

This will be particularly helpful in managing against the development of glyphosate weed resistant in Roundup Ready alfalfa,” said Canevari.

“Initially, it will have a 50-day pre-harvest interval which is restrictive to its most effective use in alfalfa, which is spring and summer applications. That is expected to change to a shorter PHI by the 2008 season,” he explained.

Chateau has been undergoing alfalfa research for several years in California and Arizona for winter weed control in semi dormant alfalfa. Chateau was granted a Section 18 registration in Arizona for groundsel control. It was scheduled for California alfalfa registration this year, but that date has been extended until 2008/09 until ongoing research is completed.

It has contact post emergence action, so it will control small annual weeds. The post emergence action may not be sufficient in most field situations containing emerged weeds, especially large annuals, so a post emergence herbicide should be added. “Tank mixing with paraquat, glyphosate, imazamox and hexazinone have demonstrated excellent results,” said Canevari. Chateau by itself at the low rate of 0.094 pounds active ingredient per acre gives very good control of common chickweed and common groundsel with 80 percent control of annual bluegrass and annual sowthistle, he added. The higher rate of 0.125-pound active ingredient provided excellent control ranging from 92 percent to 100 percent.

Chateau 12 days after treatment and depending upon the rate shows alfalfa burn from 15-27 percent. Alfalfa stunting at 36 days after treatment ranged from 13-28 percent. However, prior to harvest alfalfa had completely recovered with normal yields.

“Chateau would be an excellent addition for alfalfa, especially in winter applications for pre-emergence weed control. It has no restrictions in GWPA or worker safety issues and has been granted fast track registration in California for trees and vine crop use,” Canevari noted.

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