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does alfalfa have weed seeds

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Weeds can have their greatest impact during the seedling year of alfalfa. Failure of crop establishment can occur if competition from weeds is high. The type of weed and pressure can vary with the timing of alfalfa seeding. In northern areas, spring seeding is generally preferred due to a greater chance of successful stand establishment. In southern areas, late-summer or early fall seeding is generally preferred because of the opportunity to establish alfalfa after growing another crop. Summer annual weeds can cause problems in spring seedings, and winter annual weeds in late-summer or fall seedings.

Ashigh, J., Craig, M., and Lauriault, L. 2014. Managing weeds in alfalfa. New Mexico State University Cooperative Extension Guide A-325. http://www.aces.nmsu.edu

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Once a healthy alfalfa stand is established, problems associated with weeds can become less because the alfalfa becomes more competitive. However, weeds can become a problem in established stands because of poor soil fertility, improper irrigation or harvest management, disease and insect pressure, or winter kill of alfalfa. There are many herbicides that can be used in established alfalfa. Selective soil-residual herbicides can be applied in the late fall or early spring to dormant alfalfa. Non-selective herbicides such as glyphosate can be used in Genuity ® Roundup Ready ® Alfalfa, and paraquat can be used in dormant alfalfa.

The objective of weed management in established stands should be to preserve or improve the level of control achieved prior to planting and in the seedling year. Weeds can reduce the quality of forage in established stands of alfalfa and are generally the main reason that alfalfa fields are taken out of production. Therefore, continuing weed management practices in established alfalfa helps to improve forage quality and extend the life of the stand.

Autotoxicity? In young stands, we don’t worry about autotoxicity much (chemical residues from existing plants that prevent germination of seedlings)– the more important issues are competition from existing plants (where they exist) and keeping moisture in the top inch to encourage seeds to germinate. Thus, sprinklers may be needed, when rains are unreliable. Flood irrigating seedlings up is a technique that requires skill and luck but works on level, well drained checks. The key is to not allow standing water beyond 6-8 hrs. Seedling diseases from saturated conditions are more problematic when flooding fields in warmer temperatures and poor drainage soils.

A tough decision in producing alfalfa hay is what to do with a seedling stand that’s marginal, at best. Do you keep it, overseed with alfalfa or another forage, or do you replant or alternatively rotate to another crop? If so, when and how?

Overseeding Alfalfa to bolster the stand. If a new stand falls below about 10-20 plants/ft2, it’s a good candidate for overseeding with additional alfalfa. January to March is the ideal window for seeding alfalfa into alfalfa to bolster the stand, which unfortunately now is rapidly closing. These longer days promote seedling growth, there’s less competition from summer weeds, and winter weeds are phasing out. For seeding, probably using a shallow-seeded (controlled depth) grain drill would be a first choice (no till). Broadcasting seed and rolling it would be a second choice, since seeding depth isn’t as consistent and more seed may be needed. Very light tillage (e.g. 1″ scratch) may help in developing a somewhat acceptable seedbed.

What about Weeds? Weed germination will be important to deal with for late winter plantings. If you have a Roundup Ready stand that needs help, that’s a good thing because this allows more flexibility in weed control. When you have a mixed stand of established plants and seedlings, restrictions with conventional herbicides are a factor, depending on the age of the stand (new versus older). In addition, some conventional herbicides have plant back restrictions that might prohibit seeding alfalfa or other forages in the stand (important to check the label). For costs, keep in mind that typically the Roundup Ready technology fee (about $3/lb seed) is waived on a re-plant situation for Roundup Ready fields, so that makes it cheaper to overseed. You can overseed Roundup Ready alfalfa seed into a conventional variety, but of course the conventional variety will be sprayed out and those plants lost.

In other situations, poor stands result from poor soil preparation, improper seeding depth, or bad timing of planting (nearly ALWAYS critical to plant in early fall periods in CA – and this doesn’t mean November).

All of these will be important in evaluating whether it’s a better idea to overseed alfalfa to bolster the stand, to overseed with another forage, to till it up and start over, or to rotate to anther crop and then come back later.

Mother Nature Strikes! Sometimes factors beyond our control result in alfalfa stand losses. For example, this past winter, the long dry spell and lack of access to water, stressed seedling development. This, along with freezing temperatures, resulted in seedlings dying. Last year we had too much water, leading to saturated soils, disease problems and stand loss.