New Alfalfa? Weed Control!

Forage seedlings grow slowly and are easily overpowered by rapidly growing weeds. Weeds compete for water, sunlight and nutrients, and reduce yields in newly seeded stands. Some broadleaf weed seedlings can grow up to five times faster than alfalfa seedlings. Costs of establishment are high, and alfalfa stands decline with age. The importance of weed control in new forage stands cannot be overlooked!

Broadleaf weed control is usually required in direct-seeded alfalfa. Alfalfa seeded with a cereal companion crop often does not require a herbicide treatment, especially if the small grain is harvested as silage. The cereal companion crop will act as a biological control, crowding out the weeds.

The following points should be considered for managing weeds in newly seeded alfalfa:

1. Weeds emerging with the crop are the most competitive.
2. Newly seeded alfalfa should be kept relatively weed-free for the first 60 days.
3. Weeds emerging after 60 days will not influence that year's forage yield.
4. In new alfalfa stands, broadleaf weeds are more competitive than grassy weeds.
5. When broadleaf weed pressure is low, clipping or early harvest can be effective in controlling broadleaf weeds in alfalfa stands.

2,4-DB is the broadleaf herbicide of choice. While application rate varies for the various 2,4-DB formulations, the lower rate of the product usually offers a good compromise between weed control and crop safety. Apply the herbicide in at least 60 L/acre (13 gal/acre) of water when the legumes are in the 1^st to 4^th trifoliate stage. The application of 2,4-DB suppresses legume growth for 2-3 weeks. Legume susceptibility to 2,4-DB increases significantly beyond the 4^th trifoliate stage. When crop emergence is uneven, it is less damaging to the crop to apply the herbicide on the early side than on the late side. Under heavy wild mustard infestations, the addition of 28 ml per acre of MCPA to the 2,4-DB spray mix will provide superior mustard control.

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