Wheat competes well with weeds especially when good production techniques result in an initial uniform stand establishment and when loss of stand due to winter injury is minimal. Effective weed control and prevention of weed seed production in prior crops will reduce the risk of weed problems in wheat. Some wheat fields can benefit greatly from herbicide application, and failure to scout fields and take the appropriate measures can result in yield loss and harvesting problems in these fields. The weeds that appear above the wheat canopy late in the season, such as ragweeds and Canada thistle, can often be easily controlled with a spring herbicide treatment. The most common weed problems in wheat include:
- Winter annual weeds, such as common chickweed, purple deadnettle, shepherdspurse, and field pennycress. These weeds become established in the fall along with the wheat and can interfere with early development of wheat in the spring. Dense populations of winter annual weeds should be controlled in late fall or early spring to minimize interference with wheat growth.
- Wild garlic, which contaminates harvested grain with its bulblets. Several herbicides are effective if applied in the spring after garlic has several inches of new growth.
- Canada thistle, which can greatly suppress wheat growth due to its tendency to occur in dense patches. Most wheat herbicides have some activity on thistle and can suppress it adequately, if not applied too early in spring.
- Summer annual broadleaf weeds, such as common and giant ragweed, which can begin to emerge in late March. A healthy wheat crop can adequately suppress these weeds, but herbicide application is occasionally warranted.
It is very important to apply herbicides at the correct stage of growth of the wheat plants in order to avoid herbicide injury to the wheat. When wheat has not yet reached the jointing stage, any herbicide labeled can be safely applied. As the wheat growth stage advances past jointing and then past boot stage, herbicide choices become much more limited. Most herbicides can be applied in nitrogen fertilizer solution when the wheat is top-dressed. This may increase injury somewhat, and some labels recommend adjusting surfactant rates to minimize injury.
Complete information on herbicides available for use on wheat, effectiveness of herbicides on individual weeds, and proper timing of herbicide applications can be found in the small grain section of OSU Extension Bulletin 789, Weed Control Guide for Ohio Field Crops, available from OSU Extension county offices or on Ohioline at http://www.ag.ohio-state.edu/~ohioline/b789/index.html. Consult current product labels for crop rotation restrictions and tank mix recommendations.