Thickening Old Alfalfa Stands — Consider the Risks
March 27, 2009 The other day I received a call: "How can I thicken an old stand of alfalfa with more alfalfa rather than other forages?"
Before trying to add alfalfa to alfalfa, first ask yourself "What do you have to gain, and should you even bother?"
Older, dryland alfalfa stands need only two or three healthy plants with 25 to 35 shoots per square foot to achieve top production. More plants won't help because rainfall won't provide enough moisture for more growth. Irrigated or subirrigated alfalfa, though, can use four to six plants with 40 to 55 shoots. With the extra moisture, more plants can be productive.
Thinner stands can be thickened — sometimes, but problems can and do occur. For example, diseases are common in older fields; these diseases can infect new seedlings, causing the new stand to decline quickly.Also, existing alfalfa plants and weeds will compete with your new seedlings and might prevent them from establishing. This competition must be controlled.
Finally, alfalfa itself produces chemicals that inhibit seedling growth of new alfalfa plants. This is called autotoxicity. We can't predict when this will happen or its severity, but it is less of a problem on irrigated sites, especially on light-textured soils.
If you do try to thicken your alfalfa, make sure you minimize competition. Herbicides like Roundup® and Gramoxone® can help. Also, get your seed in the soil and do it early. Use a drill that will place seed about one-half inch deep.
With some luck you can be successful, but first make sure that rotating to a different crop wouldn't be better.
Extension Forage Specialist