In some areas, the climate during typical establishment periods is favorable and typically soil moisture is adequate to prevent reductions in establishment.

While light can be limiting, mowing/harvesting the first cutting at the appropriate timing can limit this effect. So why do we still manage weeds in establishing forages?

These weeds can result in reductions during establishment in abnormally dry years and lower forage quality in the first and sometimes second cutting.

This same principle holds when establishing grass-legume mixtures. Although much interest exists in combining legumes and grasses, integrating these two forages limits weed control options.

The sections below highlight what the limitations and options are for weed management in establishing and established stands, as management decisions will need to be made at each of these timings of the crop.

Establishing mixture
Currently, most producers rely on herbicides to assist in the establishment of perennial grasses or legumes. These herbicides typically target annual and biennial weeds, as they are the most common in newly seeded fields.

If perennial weeds are present, consider managing prior to planting as they are difficult to control while establishing forages.

Of the herbicides registered for use in establishing legumes, none of the herbicide labels clearly address the use of herbicides in mixed alfalfa/grass stands.

Butyrac (2,4-DB) can be used to aid in establishment of alfalfa/grass mixtures in CRP programs, but it is illegal to feed the treated forage to animals.

Buctril (bromoxynil) can be used in combination with small grains (e.g. wheat, barley, oats, or rye), but is not registered for use with perennial grasses.

While herbicides could be utilized, companies have not obtained the appropriate approvals to allow them to be used. Thus the only management method available is mowing or harvesting.

Mowing, if timed correctly, can eliminate 80 to 90 percent of weeds in the following cutting. If weed density is high, consider harvesting the field early before weeds have flowered, as this will provide a higher quality of forage and reduce the change of stand failure.

Established mixture
If establishment of grass-legume mixtures is successful, likely little management of weeds will be necessary until stands thin due to age, disease or weather.

Weeds in established forages are typically perennials and are difficult to remove. Pursuit (imazethapyr) and metribuzin are the only two herbicides clearly labeled for use in established grass/alfalfa mixtures.

Both are safe to alfalfa, but depending on the environmental conditions can reduce grass stands by more than 20 percent after application, although rarely does this injury persist past one cutting.

Consider using metribuzin in fields that have perennial weeds and Pursuit in fields with annual weeds as this will provide the best level of weed control.

Typically, as these mixtures age, the legume component begins to decline and the grass forages dominate.

As legumes decline, if forage species cannot fill the void left, perennial weeds will appear and require management with a herbicide. While these can be controlled, typically this results in extensive injury to the remaining legumes.

If growers wish to keep the forage grass-only field in production, they can use a range of herbicides registered for use in pastures to suppress weeds in question.

Although this treatment will be effective, it will also likely remove the remaining legumes of the field. If a significant legume component is desired, field renovation can be conducted to replace this feature, but planting of the legume should wait until after the plant-back interval listing in the herbicide has passed.  FG

References omitted due to space but are available upon request. Click here to email an editor.

—From Wisconsin Crop Management Conference Proceedings, Vol. 50