Weed Management in Newly Seeded Forage Legumes

Jerry Doll

I. Impact of Weeds in New Legume Seedings

A.  Reduced forage quality in first and possibly second cutting

B.  Risk of stand failure, especially in dry years

C.  If not failure, thinner stands likely in fields with moderate to heavy weed pressure.

D.  Not all weeds have the same impact on forage quality. See Table 4-1 on page 116 of Pest Management bulletin for relative importance of weeds in forages.

II. Weed Management Options in Companion Seedings

A.  Herbicides seldom needed because the small grain acts as "biological control" method

1.  Weeds are displaced by the small grain

B.  If herbicides are needed, there are two postemergence choices

1.  Buctril (contact photosynthesis inhibitor)

a.  very safe to small grain; relatively safe to alfalfa

2.  MCPA (growth regulator)

a.  relatively safe to small grain

b.  serious risk of injury to the legume unless protected by crop canopy

C.  A combination strategy would be to have a temporary companion crop

1.  This would displace most weeds and prevent erosion

2.  Then kill the companion crop when it is 6 to 8 inches tall with Poast Plus or Select

III. Solo (Direct, Clear) Seeding

A.  More each year; never expect it to be the only or preferred method in all cases

B.  Preventive herbicide options (preplant incorporated, PPI)

1.  Eptam (shoot inhibitor)

a.  Strong on annual grasses and several broadleaves

b.  Weak on mustards and black nightshade

2.  Balan and Treflan (root inhibitors)

a.  Strong on annual grasses and some broadleaves

b.  Weak on velvetleaf, nightshade and ragweeds

C.  "Wait and see" options (postemergence)

1.  Butyrac (2,4-DB)

a.  Registered for most forage legumes.

b.  Mode of action: growth regulator that makes its own poison
2,4-DB --------> 2,4-D

c.  Treat when weeds are 2 to 4 inches tall

d.  Safe to legumes as seedlings

e.  Not labeled for use after first cutting

f.  Harvest interval 60 days: a real bottleneck

g.  Strong on pigweed, lambsquarters, velvetleaf, ragweeds

h.  Weak on mustards and nightshade

2.  Poast Plus (sethoxydim) and Select (clethodim) for alfalfa and trefoil; Poast for any clover

a.  Mode of action: lipid synthesis inhibitors (ACCase inhibitors)

b.  Kill only grasses

c.  Treat when grasses are 3 to 8 inches tall (varies by species)

d.  Harvest interval 15 days or less (varies by product and forage use)

e.  Other niches:

• oat regrowth after silage harvest
• volunteer grains in summer seedings
• temporary cover crop
• annual grasses or quackgrass in older stands

3.  Buctril (bromoxynil)

a.  Mode of action: contact photosynthesis inhibitor

b.  Treat when:

• alfalfa has 4 or more trifoliate leaves
• weeds are small
• temperature on the day of application and the next 3 will not exceed 70 F
• this leaves us with a very narrow window for application

c.  Strong on mustards, nightshade and lambsquarters

d.  Weak on pigweed and ragweeds

e.  Harvest interval 30 days

4.  Pursuit

a.  Mode of action: an ALS inhibitor

b.  General information:

• low risk of crop injury; cool weather (< 40 F) biggest risk for injury
• broader spectrum of control than other single active ingredients
• has foliage and soil activity

c.  Apply:

• only in postemergence when alfalfa has 2 or more trifoliate leaves
• when weeds are 1 to 3 inches tall
• with NIS or crop oil concentrate and a fertilizer additive

d.  Strong on mustards, nightshade, smartweeds and pigweeds

e.  Weak on lambsquarters

f.  Harvest interval is 30 days

g.  Should you use Pursuit in alfalfa if you already use it in soybeans?

• little if any risk of resistant weed shifts in alfalfa because multiple harvests each season should prevent weeds from going to seed

IV. Summer seedings

A.  Logical and easy to do after wheat or early season vegetable harvest

B.  No PPI herbicides needed: go with treat as needed only

C.  Use glyphosate as a burndown if no-till seeding

D.  Keep an eye on

1.  volunteer winter wheat: must be treated in the fall

2.  winter annuals like shepherds purse and pennycress

Prices of herbicides for newly seeded forages (1999)

V. For herbicide use to be justified in newly seeded forages, consider the impact of weeds on yield, quality, palatability, stand establishment and other factors.