Post Application Timing to Protect Corn Yields
by Bob Hartzler

Revised May 22, 2003 - Postemergence programs have dominated soybean weed management systems for several years and are becoming more popular in corn.  While these systems can provide effective and economical weed control, good management is critical in order to obtain maximum yields and weed control.  Three primary factors need to be considered when determining the optimum time for application:  1) size of weed that can be controlled consistently, 2)  crop tolerance in relation to size, and 3) early season competition between crop and weeds.  Of course, label restrictions regarding application timing need to be considered also.   Timing is especially critical in corn since this crop is more sensitive to early season competition than soybean, and many of the herbicides used in corn do not have the application flexibility of glyphosate.  The following is a summary of several studies investigating how application timing influences corn yields.

Researchers in Canada investigated early-season competition from mixed weed infestations on corn yield in seven experiments (Table 1).  Results were highly variable among locations, with early-season weed competition causing a 5% corn yield loss as soon as two weeks after planting or as long as seven weeks after planting.  Weed density was strongly correlated with the critical period, with sites having high weed populations generally having shorter critical periods than sites with low to moderate infestations.  The shortest critical period occurred at the site having the highest weed population, 56 weeds per sq. ft.

Table 1.  Number of days required for native weed populations to cause 5% yield loss in corn at several Ontario, Canada sites.  
Modified from:  Hall et al.  1992.  The critical period of weed control in corn.  Weed Sci. 40:441-447.

^aNumber of emerged leaves, including those just emerging from the whorl.
^bMixed populations of weeds were present at all locations.  Species present included:  redroot pigweed, common lambsquarters, green foxtail, wild mustard and others.

Researchers in Michigan controlled weeds when they reached heights of 2, 4, 6 or 8 inches.  In 1992, yield losses were first observed when herbicide applications were delayed until weeds reached 6 inches, whereas in 1993 applications losses did not occur until the 8 inch weed height application (Table 2).  The difference in weed densities between the two years was relatively small, thus it appeared that the difference in critical period was due to environmental factors rather than weed populations.   Monsanto sponsored a multi-state study investigating the critical period in Roundup Ready corn (Loux et al. 1998.  Determining the critical period of competition in Roundup Ready corn.  Proc. North Central Weed Sci. Soc.  53:66-67.).   Similar experiments were conducted at 22 sites in the North Central region during 1998.  Roundup was applied at different foxtail heights, ranging from 2" to 15".  Late flushes were controlled with a second application of Roundup.   The critical period ranged from 4" to 12" giant foxtail, with a 6" height being the most common stage where yield losses were first observed.

Table 2.  Effect of weed removal time on yield loss in corn.  Michigan State University. 
Source:  Kells, J.J.   1999.  Weed Competition in corn.  1999 Illinois Crop Protection Conference Proceedings, pp. 63-64.

^aMixed infestation of giant foxtail, common lambsquarter, redroot pigweed, common ragweed and velvetleaf.  Total weed density of approximately 55 - 80 plants/sq. ft.

The results of these studies illustrate the complexity of crop-weed interactions.  The time at which weeds begin to impact yields is influenced by many factors, but weed density probably has the greatest influence, followed by soil moisture availability early in the growing season.  Based on the information provided here, a conservative recommendation would be that initial postemergence applications should be made before weeds reach four to five inches in height.  In fields with low to moderate infestations, there should be little risk of yield loss following this recommendation.  However, in fields with heavier infestations there could occasionally be situations where significant yield losses occur by delaying applications this long.

Related articles:
    Can total post programs provide optimum yields
    Realistic expectations for herbicide performance
    Early season weed competition

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University