Consider weed species, rotational crops and cost per acre in putting together a weed management plan for field corn.
By Dr. Eric P. Prostko
One of the most important aspects of field corn production is weed management. Uncontrolled weeds not only reduce corn yields through their competition for light, nutrients and moisture, but they can also severely reduce harvest efficiency. Before implementing a weed management plan for field corn, consider weed species, rotational crops and cost/A.
The top 10 most troublesome weeds in Georgia field corn are as follows: 1) Texas panicum; 2) crabgrass; 3) morningglory species; 4) pigweed species; 5) sicklepod; 6) nutsedges; 7) johnsongrass; 8) annual ryegrass; 9) Pennsylvania smartweed and 10) Benghal dayflower.
Research has shown that weeds that emerge just prior to or at the same time as corn cause greater yield losses than later-emerging weeds. Consequently, the use of effective weed control programs from 20 to 45 days after planting usually prevents yield losses due to weed competition. Weeds that emerge 45 days after planting will likely not cause competition-related yield losses but can have a negative influence on seed quality and harvest efficiency. Other research has shown that corn can tolerate a certain level of weed pressure and that control strategies should only be implemented when the potential yield losses caused by the weeds exceeds the cost of control (i.e. economic threshold concept).
Field Corn Weed Management Strategies
The most effective weed management programs in corn use a combination of cultural, mechanical and chemical control strategies. Cultural practices include such factors as planting date, planting rate and row spacing. Cultural practices improve weed control by enhancing the competitive ability of the corn. Mechanical practices, such as cultivation, are a non-chemical method for controlling weeds between rows. A multitude of herbicides labeled for use in field corn can be applied preplant incorporated (PPI), preemergence (PRE), postemergence (POST) and post-directed (PDIR).
The foundation of weed management systems in all field corn production systems is atrazine. Atrazine provides broad-spectrum control of many weeds with excellent crop safety. Atrazine can be applied PPI, PRE or POST (up to 12 inches tall). Numerous premixtures are available that contain atrazine + a grass herbicide (Bicep, Bullet, Guardsman, Lexar, Lumax, etc). Generally, these pre-mixtures will provide broad spectrum weed control when applied PRE. However, they are usually not very effective for the control of Texas panicum.
In order to protect both surface and groundwater, it is important to read and follow the label regarding the use of atrazine. When atrazine is applied PRE + POST, a total of 2.5 lb ai/A can be applied per year (2.5 qt/A of 4L or 44 oz/A of 90DF). When atrazine is applied only POST, a total of 2.0 lb ai/A can be applied per year (2 qt/A of 4L or 36 ozs/A of 90DF).
Herbicide-Resistant Crop Management Systems
In 2012, it was estimated that 73 percent of the field corn acreage in the United States was planted using herbicide-resistant corn hybrid technologies. Yield performance of herbicide-resistant corn hybrids has improved to the point where yield drag is no longer a major concern. Herbicide-resistant technologies that can be used by Georgia corn growers include Roundup Ready and LibertyLink.
Roundup Ready Systems (RR): Numerous hybrids are available that are resistant to over-the-top applications of glyphosate. Research in Georgia has shown that two applications of glyphosate, applied approximately 21 and 35 days after planting, are more effective than single applications. It is also recommended that atrazine be included in the RR corn system. Atrazine can be applied either preemergence or in combination with the first postemergence application of glyphosate in the RR corn system. Glyphosate-resistant Palmer amaranth (pigweed) has been discovered in Georgia. Consecutive plantings of RR crops should be avoided.
LibertyLink Systems (LL): LibertyLink corn hybrids are tolerant of postemergence applications of Liberty (glufosinate). Liberty provides good control of many troublesome weeds, including morningglory, Texas panicum and sicklepod. Atrazine should always be included with Liberty to improve the spectrum of control and to provide residual weed control.
Herbicide-resistant weed species can become a serious problem in fields when a single herbicide or herbicides with similar modes of action are used repeatedly. This phenomenon has been documented in Georgia with Palmer amaranth (pigweed) and other weed species. Populations of Palmer amaranth have been found in the state that are resistant to atrazine, glyphosate and/or ALS-inhibiting herbicides. Check with your county Extension agent for updated information about the distribution of herbicide-resistant weeds in your area.
Herbicide-resistant weeds can be managed by using a combination of strategies including crop rotation, narrow row patterns, mechanical cultivation and utilizing herbicides with different modes of action. Atrazine-resistant (AR) Palmer amaranth was confirmed in Macon County in 2007. In 2008 and 2010, additional seed samples were collected to determine the distribution of this type of resistance. At this point, it appears as though AR-Palmer amaranth may be limited to dairy farms that have a long history of continuous atrazine use. If glyphosate resistance is not an issue, AR-Palmer amaranth should be easily controlled with glyphosate. However, growers who are concerned about ALS, AR and GR-Palmer amaranth and other herbicideresistant weeds should consider the weed control programs provided in the 2013 UGA Corn Production Guide.
Rotational Crop Concerns
Advances in herbicide chemistry have led to the development of some exceptional families including the sulfonlyureas (Accent, Beacon and Sandea), imidazolinones (Lightning, Pursuit), sulfonanilides (Python) and others. Many herbicides in these families are used in field corn. However, some of these herbicides have the potential to injure rotational crops if the appropriate replanting interval is not observed. Atrazine also has the potential to cause carryover problems to sensitive crops particularly when used in late plantings.
Because of the diversity of crops that are grown in Georgia, producers must consider the potential effects that herbicides could have on a rotational crop the next year. This information is readily available on nearly all herbicide labels.
Dr. Eric P. Prostko is an Extension weed specialist with UGA. Contact Prostko at (229) 386-3328 or email@example.com.