Weed Control in Grass Forages: When to Spray?

By Dwight Lingenfelter

As many farmers are cutting forages during this time of year, they are noticing weeds in the field. This prompts many questions to us about how and when to control weeds. The answer really depends on a few important factors, including: kinds of weeds in the field; their lifecycle (i.e., summer annual, biennial, warm-season or cool-season perennial, etc.); time of year; condition of the stand; and environmental conditions (e.g., drought).

Once the forage has been mowed and baled, now is the time to prepare for a timely herbicide application. Do not spray immediately after harvest. Weeds must have time to regrow – enough leaf foliage must exist to get proper spray coverage. Summer annuals such as ragweed, pigweeds, lambsquarters, smartweeds, etc. need several days to recover from cutting and then can be sprayed with a systemic herbicide if necessary. It is best to allow perennial weeds to grow about 12-15 inches or so (usually about a month after cutting) before an herbicide application.

Warm-season perennials such as horsenettle, hemp dogbane, common milkweed, smooth groundcherry, pokeweed, bindweed, poison ivy, and brambles should be sprayed with a systemic herbicide(s) from September 1 through October 15 or before a hard frost. In general, applications by October 1 may be more effective. In northern areas of Pennsylvania, consider making the application before October 1. An additional 2 to 4-week or more application window can exist for Canada thistle and quackgrass, because of their cool-season habit of growth.

The most common herbicides used for control of many broadleaf weeds in the late summer/early fall is 2,4-D and/or dicamba (Clarity, etc.) and other systemic products such as triclopyr (e.g., Crossbow, Candor, Crossroad, Remedy Ultra) or metsulfuron (Cimarron) can be options as well. (However, be cautious of crop rotational restrictions with triclopyr and metsulfuron.) A combination of these products may be the best solution for a mixture of different perennial weeds. Late summer is not the best time to control annual grassy weeds such as yellow and giant foxtail, and large crabgrass with herbicides.

No selective, effective, or economical products exist for this application, so mowing is the best option to keep the weeds from setting seeds. Quackgrass can be difficult to control in grass forages. Either spot spray or use a wiper applicator, if possible, with glyphosate to control this weed. Also, keep in mind that drought can cause weeds (and forages) to have slower regrowth and make it more difficult to get the herbicide into the weeds. Under droughty conditions make sure to use higher spray volumes (15-20 gallons/acre) and include any necessary adjuvants in the spray mixture. And finally, if the stand is severely declining, fall (not spring) is a best time to kill it in order to prepare for rotation to another crop.