By Jeff Coulter, Extension Corn Agronomist
Planting corn after alfalfa is an effective way to eliminate or greatly reduce the corn nitrogen requirement while increasing corn yield potential due to the rotation effect. However, successful termination of the alfalfa stand is essential in order to fully realize these benefits, as volunteer alfalfa can compete with corn for water, nitrogen, and light. Traditionally, alfalfa stand termination relied on tillage implements such as a moldboard plow or a chisel plow with overlapping sweeps to completely cut off alfalfa roots. From a soil conservation standpoint, however, these tillage implements may not be desirable for all fields. Yet, tillage implements that do not cut the roots from all plants are ineffective for complete termination. As a result, herbicides are an indispensible tool for alfalfa stand termination in many cropping systems.
Since alfalfa is a perennial crop, termination with herbicide should be done in the fall (anytime in mid-September or later), since carbohydrates are being translocated to the roots for overwintering at this time, resulting in more effective termination. However, fall herbicide applications on alfalfa should be delayed until alfalfa regrowth after the last cutting is at least 6 to 8 inches tall. Prior to this height, carbohydrates produced in the alfalfa plant are primarily being used for new growth rather than being translocated to the roots. Thus, herbicide application prior to this regrowth height can have ineffective translocation to roots.
Fall herbicide applications to alfalfa should also occur before the first killing freeze, as herbicides applied after the first killing freeze can have poor translocation. To determine if a killing freeze occurred, use the fingerprint test. This involves going into the low areas of a field on the morning after a suspected killing freeze and pinching individual alfalfa leaves between your fingers. If a fingerprint remains on the leaves after pinching them, that is an indication that the epidermis on the leaves ruptured due to a killing freeze. Since fall herbicide applications to terminate alfalfa should occur after the plants are 6 to 8 inches tall but before the first killing freeze, growers should time their last alfalfa harvest accordingly.
In some fields, fall tillage is desired after a herbicide application for alfalfa termination in order to relieve compaction and produce a darker soil surface that dries out more rapidly in the spring. In general, tillage can occur within 3 to 4 days of the application in the fall, since the majority of the herbicide is translocated within the plant during this time.
Good herbicide options for termination of alfalfa stands include 2,4-D ester or 2,4-D amine. These herbicides are effective and inexpensive. If grasses are present, a good option is to tank-mix glyphosate with 2,4-D. The use of glyphosate alone for alfalfa stand termination often provides only partial control or suppression.
Alfalfa stand termination with herbicide in the spring is risky. This is because corn planting often occurs prior to the time alfalfa regrowth is 6 to 8 inches tall. While 2,4-D can effectively control volunteer alfalfa when applied post-emergence in corn, crop injury can occur if the label is not followed. Most other post-emergence corn herbicides are only labeled for suppression of volunteer alfalfa, and many of these growth-regulator herbicides have high potential for volatilization and injury to off-target crops such as soybean.