Forages: Weed Control

Excerpt from Agronomy Guide for Field Crops (Chapter 5)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Critical Period for Weed Control
2. Crop Restrictions
3. Crop Losses Due to Weeds
4. Integrated Weed Management
5. Updates on Forages: Weed Control
6. Related links...

Critical Period for Weed Control

There are no critical periods established for forage crops. When establishing forages, it is very important to control weeds early in the growth of the forage crop. The product label for postemergence herbicides indicates the growth stage of the weeds when optimum control can be obtained. Young forage legumes are very sensitive to herbicides, and the correct rate and crop growth stage are essential. The timing of weed control should optimize the critical period for the crop and the optimum growth stage of the weed.

In established forages and pastures, most herbicides that control biennial weeds are best applied in early fall to first-year growth or in late spring to second-year growth unless otherwise noted on the label. Mechanical control of biennial broadleaf weeds is most successful when done at the time of flowering. Perennial weeds are usually best controlled when treated the end of May to mid-June when the weeds are actively growing (unless otherwise noted on the label). If you are wick weeding pastures for Canada thistle or milkweed, wait until they are in the bud-to-flowering stage.

Some herbicides prohibit feeding the year of treatment. Consult the herbicide label for more information on the period of time that livestock should not be grazed in the treated area or for the period between treatment and harvest of hay or hay silage.

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Crop Restrictions

Note that some herbicides used in corn and soybeans can leave soil residues that may injure forage seedlings. Some of these may be dependent on soil pH or organic matter. Consult the label of last year's herbicides to note any restrictions before establishing new seedlings. See the section on "Herbicide Crop Rotation Restrictions" in OMAFRA Publication 75, Guide to Weed Control. (Order Publication 75)

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Crop Losses Due to Weeds

Yield losses will vary with the species and the density of the weed as well as when the weeds emerge. Later-emerging plants tend to be less competitive and produce fewer seeds per plant. Established forages are more competitive with weeds than newly established forages.

Crop quality should also be a consideration when making weed control decisions. The presence of weeds can reduce the quality and the value of the hay. Some weeds can also reduce the palatability of the hay or forage.

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Integrated Weed Management

Crop rotations can be effective in reducing certain perennial weeds that may grow best in a particular crop. Forages are known to reduce the populations of annuals in the first year of the next crop, but annual weeds can be a problem when establishing forages. Use vigorous, high-quality seed to achieve uniform stands at the recommended plant populations to compete with the weeds. Early planting gives the crop a head start on the weeds to assist with crop competition. Planting deep will delay emergence and favour the weeds.

Equipment can tend to move weed seeds from field to field. Harvesting equipment, tillage equipment and soil erosion can all transport weeds. Weeds can also infest a field from manure or other soil amendments. Proper sanitation and cleaning of equipment along with maintenance of field border areas all affect the long-term weed management in the field.

Refer to OMAFRA Publication 75, Guide to Weed Control, for more information, including herbicide products and rates. (Order Publication 75)

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Updates on Forages: Weed Control

Refer to OMAFRA Publication 75, Guide to Weed Control. (Order Publication 75)

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Related links...

• Weeds
• Forages
• OMAFRA Publication 19, Pasture Production
• Publication 75, Guide to Weed Control
• Field Crop Protection Guide - Publication 812

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