Weed Management in Field Peas

BOZEMAN - Mar 7/05 - SNS -- Field pea growers in Montana face major problems from weeds, warns Montana State University agronomists.

"Because peas are poor competitors, particularly at the seedling stage, avoiding early season weed interference is critical. Thus, whenever possible, producers should start their pea production by selecting clean fields with a history of low weed pressure," the university said.

Weeds can hinder pea production in many ways. First, weeds can reduce yield through competition for light, moisture, nutrients, and space. Second, weeds may harbor insect pests and pathogens that could threaten crop production. Finally, late season weeds can be a nuisance that reduces harvest efficiency and increases mechanical damage to the pods.

To reduce the risk of developing severe weed infestations, producers should develop an integrated weed management program using mechanical, cultural and chemical control practices, without over-relying on one strategy at any point in time. The goals of such a program should be to limit the introduction and spread of weeds, help the crop compete more efficiently for resources and minimize the chances of any weed species adapting to the specific growing conditions created by the crop.

Pea production represents a relatively new crop in the traditional wheat-fallow system that dominates Montana. Because peas replace summer fallow, they provide new opportunities for weed control. As a broadleaf crop, pea creates environmental conditions that do not favor the growth of the traditional grassy weed communities that dominate wheat fields.

Also, it allows different mechanical and cultural management practices. For example, small weeds can be controlled by harrowing before crop emergence and when peas are 3-to-7 inches tall. Moreover, the inclusion of peas allows the use of herbicides that are very effective in controlling grassy weeds such as quackgrass and wild oat. However, the inclusion of these alternative products could result in crop damage.

To reduce the risk of herbicide injury, it is very important that producers plan rotations considering herbicide carryover restrictions. For some herbicides such as Tordon (plicoram), Glean (chlorsulfuron), and Finesse (chlorsulfuron) these restrictions can last up to 36 months.

Field peas or small grain/pea mixtures produced for forage under high rainfall or irrigated conditions are very effective as "smother" crops. These crops are very competitive for nutrients and moisture, and generally restrict weed growth. Also, the grain stems help support the pea vines, thereby reducing lodging. When harvested for silage or hay, many weeds under the crop canopy are unthrifty and produce fewer seeds. However, planting peas with a small grain crop severely reduces herbicide options.

Producers can modify the pea seeding rate to reduce early weed competition and include a phosphorous and sulfur fertilization to favor crop development. For example, seeding rates with a grain drill could range from 45 to 180 lbs per acre, depending upon the seed size and moisture conditions.

As a general rule, lower seeding rates should be used in drier areas and heavier seeding rates are more suitable for well-fertilized soil under irrigation or in more humid areas.

Several of the weed management practices available were developed in the Pacific Northwest or Western Canada. As such, many herbicides currently used in these regions are not labeled in Montana. Hopefully, as pea acreage expands in Montana, effective herbicides from surrounding states and provinces will be labeled for use in Montana.

Current preplant / preemergence herbicides labeled for peas include Command (clomazone), Dual Magnum (metolachlor), Frontier (dimethenamid), Gramoxone (paraquat), Prowl (pendimethalin), Pursuit (imazethapyr), Roundup (glyphosate), Sencor (metribuzin), Sonalan (ethalfluralin), Spartan (sulfentrazone), and Treflan (trifluralin). Postemergence options include Assure (quizalofop), Basagran (bentazon), Gramoxone, Poast (sethoxydim), Raptor (imazamox), Scythe (pelargonic acid and related fatty acids), and Thistrol (MCPB). Pre-mixed herbicide products labeled for weed control in peas include Pursuit Plus EC (imazethapyr with pendimethalin), and Sequence (metolachlor with glyphosate).

Before using these products, producers should carefully read the label for rates and recommendations. Producers should also refer to the latest version of the Montana, Wyoming and Utah Weed Management Handbook for rates and recommendations. The handbook is in the process of being revised and a new edition is expected by late spring. Contact your local county Extension offices to ask about availability.