Extension Tips To Control Winter Annual Weeds In Alfalfa

Extension Tips To Control Winter Annual Weeds In Alfalfa

ISU Extension agronomists offer timely tips on early spring management of alfalfaweed control, evaluating stands, planting a new crop

Iowa State University Extension agronomists point out that now is the time to begin evaluating how well your alfalfa stand survived last year's drought and the unusual winter of the past several months. Early spring is also the time to control winter annual weeds in alfalfa fields.

EVALUATE FORAGE STANDS: Do you have a winter-injured alfalfa stand? Is it worth keeping? What about grass pastures? "We had a drought last year and an unusual winter in recent months," says Brian Lang, ISU Extension field agronomist. "Some varieties of alfalfa and grasses are more winter-hardy than others. Now's the time to evaluate your stand and see how healthy and viable it is. Some stands may not be worth saving to try to get another year of production from them."

* Evaluating overwintering alfalfa stands: "So far this spring I don't see any significant problems with the overwintering alfalfa other than from the 'frozen pond areas' in some fields," says Lang. "Alfalfa can only survive for about three weeks under thick ice sheets. However, also consider that older stands eventually decline due to disease, wheel traffic damage and inherent aging issues. So alfalfa stands, particularly older stands, should still be evaluated for stand density and root health."

Steve Barnhart, ISU Extension forage specialist, recently posted information on stand evaluation of alfalfa and other forages in the ISU ICM Newsletter here. An alternative evaluation to using plant counts is to use the stem count method to evaluate a stand. "Once alfalfa plants reach 6 to 8 inches tall, we could count stems per square foot for a better estimate of stand potential," says Barnhart. A University of Wisconsin publication explains the stem count method at this link. Alfalfa stands averaging 55 or more stems per square foot are excellent stands, says Barnhart.

* Control winter annual weeds in alfalfa now: "Every May I get phone calls on outbreaks of weeds in alfalfa fields like Shepherds purse, Field pennycress, and/or Pepperweed," says ISU's Lang. "However, by that time there isn't much of anything that we can do about it. These are among the most common winter annual weeds that show up in alfalfa fields. They are not very palatability or high in quality."~~~PAGE_BREAK_HERE~~~

Winter annuals germinate in the fall, develop a rosette-type growth like this Shepherds purse plant. The plants overwinter as the rosette, and then bolt (rapid growth of upright stems) in early spring producing seed for next fall. Once these plants bolt, herbicides are not very effective. The time to control these weeds is while alfalfa is still dormant, using herbicides such as Pursuit or Velpar. Velpar is the better choice for dandelion control. The only way to know if these weeds are present is to scout the fields. However, realize that most winter annual weed situations are not heavy enough to significantly interfere with yield and quality of the overall forage, so the herbicides are not usually recommended. If the alfalfa field is a marginal stand and with weed problems, rotating to corn may be the best option.

* Planting small grains and alfalfa: You should plant small grains as soon as spring soil conditions permit. "For alfalfa we prefer to wait until soil temperatures are in the 40 degree F range, usually April," says Lang. Small grains germinate at soil temperatures in the mid- to high 30s, whereas alfalfa germinates at soil temperatures in the high 40's. On average, planting after mid-April, oats and wheat lose an average of 10% yield per week, and planting after May 1 they lose about 15% yield per week compared to a mid-April planting. 

Seeding rates for a pure small grain stands should be about 30 seeds per square foot, which is about 3 bushels per acre for oats, 2 bushels per acre for barley, and 1.7 bushels per acre for wheat and triticale. As a companion crop with alfalfa, you should reduce small grain seeding rates by one-third to one-half to reduce the competition on the alfalfa seedlings. Oats are seeded at about 1-inch deep, and alfalfa and other forage legumes and grasses at about ¼-inch to ½-inch deep followed by press wheels or a cultipacker. More details on alfalfa establishment can be found starting on page 10 in the Alfalfa Management Guide here.

* Soybean cyst nematode: "I've also recently received questions about sampling soil and having it tested for soybean cyst nematode," says Lang. "Yes, there is still time to check your field for SCN before planting beans. All it takes is a soil sample." At this link is the sample submission form with instructions on how to collect a sample. For everything else about SCN click here.

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