Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
There are three issues to consider when thinking of fall herbicide applications for small grains: herbicide effectiveness, weed emergence timing, and weed competition. Herbicides applied in October or November for barley or early planted wheat tend to more effective than spring applications. Fall applications are made when weeds are smaller and more susceptible than spring applications. Winter annual weeds are actively growing throughout November despite heavy frosts. Soil temperatures still maintain active growth of winter annuals and seedlings have not shut down yet. Most of the common weeds have peak emergence in the fall with reduced levels of spring emergence. In addition, wheat is well established by spring — it outcompetes late emerging weeds and they are not competitive. This includes henbit, common chickweed, and annual ryegrass.
Research has shown that weed competition in the fall can reduce yields. Trials examining high weed densities that are removed in the spring resulted in lower yields, even though spring weed control was very good.
Late planted winter wheat can also benefit from fall herbicide applications since the peak emergence of weeds has already passed, however, the crop may be as effective in competing with spring emerging weeds.
Some herbicides need to be applied in the fall due to rotational restrictions for double cropping or crop safety concerns with applications too close to nitrogen applications (i.e. Osprey, Huskie). Fall applications (same as early spring applications) may still require a spring application for garlic.