The Back Forty October, 2020

Fall Spraying for Weed Control in Grass Seed Fields

Fall applied herbicides are a practical and effective means of managing weeds in grass seed crops. A number of the more difficult to control winter annual, biennial and perennial weeds such as dandelion, narrow-leaved hawk’s-beard, volunteer clover, rough cinquefoil, stork’s-bill, scentless chamomile and flixweed tend to be more susceptible to herbicides in the fall than at other times of the year.

Grass seed stands that are going into their first year of seed production are generally ideal candidates for a September herbicide application. These fields tend to have more weeds present compared to stands where a seed crop has already been harvested. Generally the weeds in newly established stands are also in better condition to spray.

The herbicide selected will depend on what weeds are present and the tolerance of the grass seed crops to the herbicide. Products such as Curtail M, Prestige, Ally and Spectrum (MCPA ester+clopyralid+florasulam) are used. Spectrum is no longer available but can be made by mixing Curtail M and the active ingredient florasulam. Cirpreme+MCPA ester is a relatively new herbicide mix from Corteva and has shown good potential for use on creeping red fescue and timothy.

Fall spraying can also provide more crop safety than spring or summer applications of herbicides such as Ally. Spring or summer spraying of Ally on a number of grass seed crops typically stunts the growth of the crop and may cause reduced seed head size, resulting in reduced seed production. Application of Ally in the fall usually does not result in long-term injury or reduced seed yields to grass seed crops such as timothy. Note that fall applied Ally has caused damage to crops such as perrenial ryegrass and tall fescue.

Many trials comparing products for fall spraying in grass seed crops have been done in the Peace Region. The table below shows a summary of what products work best on certain weeds when applied in the fall.

A recent trial comparing fall applied herbicides shows similar results to previous studies .

Canada thistle in grass seed stands is difficult to control. Emergence of Canada thistle is later in the spring than many other weeds. Spring herbicide applications on grass seed crops are generally made in late May to early June. At that time there may be a few thistles present but there will be more coming. Unfortunately, controlling thistle in grass seed crops with fall herbicide applications is very inconsistent.

Fall spraying grass seed stands is an effective practice to control a number of winter annual and perennial weeds that are often more difficult to control in the spring. Mid- September applications are ideal as temperatures are generally still good and weeds are actively growing. The herbicide used will depend on tolerance of the grass seed crop to the specific herbicide and the weeds present. If spraying timothy stands where hay or straw is being baled and sold into the export hay market check with the buyers to see which herbicides are okay to use as some herbicides will leave residue in the product.

Calvin Yoder Forage Seed Specialist SARDA Ag Research/Peace Region Forage Seed Association

Tall Fescue