Alfalfa weed control depends on timing, herbicide decisions

Weed control in alfalfa is mainly a timing thing.

Mick Canevari, San Joaquin County farm advisor, gave an account proving that point during the 31st California Alfalfa and Forage Symposium in Modesto.

While there are all sorts of reasons to do a good job of weed control, in alfalfa the benefits range from simply directing moisture and nutrients to the crop for higher yield and quality to avoiding poisonous weeds that can make a stand unmarketable.

In any event, Canevari pointed out, “the economic return from producing weed-free alfalfa can offset the extra management practices of the use of herbicide.”

Outlining the options for cultural practices, he reminded that a single approach is usually not sufficient. More successful are combinations of cultural and chemical means. Land leveling and proper nutrition also have a place in promoting a healthy population of the crop to better compete with weeds.

In some cases, weeds are best managed in the crop preceding alfalfa. “For example,” he said, “many winter weeds can be controlled in a wheat or oat crop with a phenoxy herbicide or summer weeds can be controlled by growing corn and using selective herbicides and cultivations.”

Even preirrigation can play a role in the effort. It gets the crop under way with more moisture to set deeper roots.

Fallowing may help in reducing soil pathogens, such as pythium, phytopthora, and rhizoctonia, that depend on a high moisture environment.

Planting temperature

Getting back to the timing issue, Canevari said the optimum soil temperature range for alfalfa planting, so the crop gets the jump on weeds, is from 69 to 76 degrees. The object is to avoid planting when soil temperatures are less than 50 degrees or more than 90 degrees, which is the range that favors weed seed germination.

“Herbicides,” Canevari said, “also perform better when applied to vigorously growing weeds. Identifying site-specific weed areas at least one year in advance will help with time of planting decisions and the herbicides that should be used.”

The long-time standard period for planting alfalfa in the San Joaquin Valley was November and December and relying on rainfall for germination of the crop.

The problem with this method is the crop grows slowly in these temperatures and the stand can become irregular with different sizes of plants. What's more, the cooler temperatures during that time favor mustards, chickweed, certain grasses, and other weeds.

The irregular plants can interfere with timely herbicide application, and the delayed treatment will have to deal with larger weeds. Most postemergence herbicides are limited to a certain growth stage or size of alfalfa large enough to avoid crop injury.

“An alternative planting time for the San Joaquin Valley is between September and October,” he said, explaining that by then summer weeds have completed their growth cycle and are less a problem. The fall temperatures enable the alfalfa to get a head start on winter weeds, which generally reach peak germination in December, when the alfalfa is well established.

Avoids competition

Planting during February and March can head-off competition from summer weeds such as nutsedge, bermudagrass, foxtail, and barnyardgrass. For best results, this practice calls for preparing the seedbed and forming irrigation borders the preceding fall.

“This is an advantage to working wet soil and to avoid compaction in the spring. Weeds that grow during the winter can be controlled easily with glyphosate or paraquat so only minimal tillage of the seedbed is needed before seeding,” he said.

Generally, treating smaller weeds is more effective because they require less herbicide, lower spray volumes need fewer tank loads to cover the acreage, and competition from smaller weeds is minimal without effect on long-term yield, he said.

“Identifying the weed and understanding its biology are important in selecting the correct control measure. For example, curly dock is a biennial or can act as a perennial, depending on the environment.

“The most effective control is using a systemic herbicide, which is able to move into the root system. To accomplish this in the fall, when carbohydrates are moving downward, is more effective than in the springtime when the flow is upward.”

The main summer weed in alfalfa, he continued, is yellow or green foxtail. “It has adapted to the multiple harvest intervals of alfalfa and produces viable seeds within a cutting cycle.”

One way to deal with the weed is extending the harvest interval to promote a stronger, more competitive alfalfa plant, although this practice can compromise hay quality. Delaying irrigation until alfalfa regrowth is six to 10 inches tall and has shaded the soil surface will also inhibit weed germination.

Treatment studies

Timing of application on foxtail also has bearing on herbicide effectiveness. Canevari said studies in 2001 measured effects of treatments made at different soil moisture levels.

The first timing of treatments with several systemic herbicides was in June after the third-cutting bales were removed and the field was quite dry.

The second timing was about a week later, three days after an irrigation when the soil was moist and both weeds and crop were growing.

“Evaluations for weed control were made several times the next two months,” Canevari said. “The results were amazing and showed as much as 50 percent decrease in foxtail control from the herbicides sprayed under dry conditions.”

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