Corn Gluten meal is a byproduct of a milling process. It is a light source of nitrogen and that was the property that originally brought it into the horticultural sphere.
A researcher, Dr. Nick Christians, from Iowa State accidentally found that it had herbicidal products and began investigating its use for the suppression of turf weeds.
Corn Gluten Meal inhibits the root formation during germination. It is currently said to control crabgrass, barnyard grass, foxtails, lambsquarters, pigweed and purslane.
Corn Gluten Meal needs to be applied before the germination of the weed seeds. It is useful for 3 - 5 weeks, so your timing must be good. Labels on products suggest that it be applied in early to mid-spring and again in August. The rate will vary with the product.
President's Choice Green products has a lawn product that is mostly corn gluten meal with a bit of bone meal and some source of potash. They claim that all the ingredients are organic and give the mixture a rating of 8-2-4. It is recommended for early spring, late summer and fall applications. But remember that you can't overseed your lawn if you use it. Corn gluten meal does not play favourites.
I use it in the spring to get the turf areas off to a good start. In the fall, I topdress with a mixture of compost, manure and grass seed.