Some alfalfa stands may be damaged or depleted this summer, and no longer as productive as desirable. This could be due to a combination of factors: freeze injury, winterkill, early-season alfalfa weevil, diseases, flooding, or pea aphids.
If the stand appears to be recovering slowly or too thin to be worth saving, producers may want to kill the stand this summer, plant the field to wheat or oats this fall, and replant to alfalfa in the fall of 2016. It would be best to rotate the field out of alfalfa for a year before replanting due to allelopathy and disease concerns.
Evaluating the stand this summer is better than waiting until fall because it gives producers more time to plan out what they want to do with the stand.
Producers should count the number of stems per square foot at several locations in the field. Only stems over two inches tall should be counted. Research at the University of Wisconsin has found that a stem count is a much more accurate method of estimating the yield potential of an alfalfa field than plant count. Plant density is a poor estimator of yield potential because an individual plant may have few shoots and contribute little to yield. A stand of alfalfa should have at least 20-25 healthy stems per square foot to justify keeping it for another year. If a stand has 20 stems per square foot or less, producers should consider destroying it.
When evaluating the stand, producers should also dig up some plants and examine the crowns for size, symmetry, and the number of shoots present. Roots should be cut lengthwise to check for rot or discoloration in the crown and root. Healthy stands have fewer than 30 percent of the plants with significant discoloration or rot.
To kill an alfalfa stand, producers should spray when the plants have about 8-12 inches of regrowth. At that point, the flow of carbohydrates is moving down again, into the root systems, so herbicide uptake will be increased. If the herbicide is applied before that time, carbohydrates are still moving upward from the root reserves into topgrowth.
Producers could also wait until fall to kill the stand. A light frost in the fall won’t affect alfalfa growth or herbicide uptake significantly, but a hard freeze will. So herbicides should be applied before a hard freeze, when temperatures get down into the mid-20’s.
To kill an alfalfa stand, it’s best to use 1 quart of 2,4-D ester and a half-pint of dicamba. Producers could also use two quarts of 2,4-D alone, but that’s generally not as effective as the 2,4-D/dicamba combination. Mixing glyphosate with the 2,4-D or dicamba can also provide good alfalfa control, as well as control any grasses that may also be present in the field.
Jim Shroyer, Crop Production Specialist Emeritus
Dallas Peterson, Weed Management Specialist