Weeds are the No. 1 pest of both corn and soybeans. The 2013 Ohio Weed Control Guide for Ohio and Indiana contains lots of information including a couple of excellent sections on corn herbicide and soybean herbicide management strategies.
Hopefully, the following will entice you to purchase a copy of the publication from your local Extension office or check it out free online at www.agcrops.osu. edu/publications
For corn, preemergence herbicide programs have long been the mainstay of weed management in corn, due in large part to the low cost of atrazine and its broad spectrum of control.
The commonly used premix of atrazine plus acetamide herbicide (Bicep II Magnum, Degree Xtra, Keystone, etc) can be supplemented as necessary with Balance, Python, Callisto (Lexar, Lumax), Hornet, or simazine to improve control of weeds such as fall panicum, triazine-resistant lambs quarters, giant ragweed, and velvetleaf.
In moderate to high weed populations, a pre-emergence plus post-emergence approach will prove more consistent control with less risk of corn injury. Numerous options are available for this type of program at a reasonable cost. The Weed Control Guide states that a PRE plus a POST approach is especially effective in fields with giant ragweed, burcucumber, moderate to high annual infestations of annual grasses and triazine resistant lambsquarters, and perennial weeds.
Many effective total post emergence herbicide programs are also available.
However, university research indicates that total POST programs lacking residual activity should be used only in fields with low weed populations.
A total POST herbicide program should be applied before most weeds in a field exceed 2 to 4 inches in height, and reinfestation with later emerging weeds is likely if a herbicide with residual activity is not included.
In soybeans, OSU research has shown that almost any type of approach to herbicide management can be used in fields with low weed pressure with little risk of crop yield loss. These approaches include: total PRE, PRE followed by POST, and total POST.
However, if you couple the biology of some weed species with the slow early development of no-till soybeans, effective weed control can be difficult with a single application of PRE or POST herbicides. For this reason, a PRE followed by a POST program or a two pass POST program usually provides the most consistent control.
The planned PRE plus POST approach provides more consistent weed control than any one pass attack in most fields. This strategy also helps solve some of the problems in management of glyphosate and other POST herbicides.
The most complete PRE plus POST program includes use of a PRE herbicide, with activity on key broadleaf species and at least some early season control of grasses, followed by glyphosate in Roundup Ready soybeans; or another POST treatment with activity on grass and broadleaf weeds in non RR soybeans.
The PRE herbicide can control or reduce the population of some problem broadleaf weeds, such lambsquarters, waterhemp and giant ragweed, making it relatively easy to control later emerging weeds with the POST treatment.
A major advantage of the PRE plus POST approach, compared to total POST, is that the PRE herbicide often provides enough weed control to prevent major problems if weather delays the POST application.
The PRE plus POST approach can allow for a slightly delayed POST application, resulting in more consistent control of late emerging weeds such as foxtails, giant ragweed, black nightshade, water hemp, and perennials.
For specific recommendations, check out the 2013 Weed Control Guide, money and time well invested.