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Factors affecting alfalfa seeded in late summer

green alfalfa field with tractor
SEEDING SUCCESS: Alfalfa needs at least six weeks of growth after germination to survive through the winter.
Field Fodder: A good rule of thumb is to plant alfalfa six to eight weeks before your typical first killing-frost date.

By Kaitlyn Lance

What do the weather, soil moisture and your forage needs have in common?

All are factors to consider if you plan on seeding alfalfa in August. For most of Wisconsin, the ideal time to seed is Aug. 1-15. The southernmost counties close to the Illinois and Iowa borders can push the date a little later and seed Aug. 15 to Sept. 1.

Since you can’t control the weather, let’s look at the factors you can control to have successful late-summer seeding of alfalfa.

Soil moisture important
For late-summer seeding to be successful, soil moisture is key. If the soil does not have sufficient moisture, the results are most often poor germination and stand loss. Soil moisture can be lost through too much tillage and dry weather conditions. If the weather is not cooperating, soil moisture can be increased through irrigation. It is not recommended to plant companion crops with alfalfa, because they will compete for soil moisture.

Alfalfa needs at least six weeks of growth after germination to survive through the winter. A good rule of thumb is to plant six to eight weeks before your typical first killing-frost date. That may be later or earlier than the recommended dates previously given. However, nobody can predict the weather, and if an early frost happens, there could be damage to the stand.

If you are planning on reseeding a pre-existing stand, then a waiting period is recommended. Alfalfa roots exude a toxin that affects the growth of new seedlings. The length of the waiting period depends on the age of the stand and soil type. Sandier soils leach the toxin faster but may have a higher concentration of the toxin when it is present.

If the stand is older than 2 years, it is recommended that another crop be planted before reseeding the area to alfalfa. If you are unsure of how long you should wait, the University of Wisconsin-Extension Learning Store has a great resource called the Alfalfa Management Guide. In the guide you will find a risk assessment for reseeding.

Better weed control
Seeding in late summer does give you the advantage of better weed control. Typically, fewer weeds germinate in late summer. Weeds can be taken care of with herbicides. This can also help get rid of any lingering vegetation from the previous crop. Tillage from seedbed preparation will also help get rid of existing weeds. If weeds persist through the winter, then cleanup may be necessary in the spring.

Besides weeds, diseases like sclerotinia crown rot can be an issue. When alfalfa is planted in late summer, it sometimes forms a dense canopy; when combined with high humidity, this can create the right conditions for sclerotinia crown rot to cause damage.

This is a brief overview of some of the factors that have an effect on late-summer seeding of alfalfa. The three big things to remember are to make sure there is proper soil moisture, have at least six weeks of growth, and observe a proper wait time if reseeding into alfalfa. More factors that were not mentioned in this article can be found in the Alfalfa Management Guide, or contact your local Extension agriculture agent for more information.

Lance is the Extension agriculture educator in La Crosse County, Wis.

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