As you contemplate late-summer, cover-crop establishment, remember that your spring herbicide program may limit your options, say Penn State agronomists.

"Most herbicide labels only include the major cash crops in their rotational information and do not provide specific details about what cover crops can be planted after a herbicide treatment earlier in the season," says Bill Curran, Penn State weed specialist.

Cereal rye, wheat, barley and oats are four common crops that are often listed on a product label, Curran says. Penn State Agronomy Guide Tables 2.2-17 and 2.4-15 or the herbicide label to determine which products have less than or equal to 4-month rotational restriction for covers like rye, field pea, clover, oats and sorghum/sudan.

Agronomist Dwight Lingenfelter says there is no listing for hairy vetch and some of the more obscure clovers or legumes, but by looking at the alfalfa or clover restrictions, you can get a good idea of which herbicides may cause potential injury. Covers like canola/rape, tillage radish and buckwheat can be difficult to determine if herbicide residuals will cause problems.

"In general for corn herbicides, atrazine at less than 1 pound per acre can allow cereal grain establishment and at less than 0.75 pounds per acre may allow for most legume cover crops, mustards and annual ryegrass," Lingenfelter says. "This should be similar for simazine.

"In addition to atrazine and simazine, mesotrione is problematic for legumes and probably mustards like canola. Clopyralid could also affect these small-seeded broadleaves."

For soybean herbicides, chlorimuron, Pursuit or Extreme, and Prefix or Reflex could be a problem for fall-seeded legume or mustard cover crops, Lingenfelter says, but cereal grains should be fine.

Persistence of some of the other corn and soybean herbicides and their impact on legumes or mustard species in particular is less clear, the agronomists add.