Critical periods of competition in corn

Encyclopedia Article

Many growers strive to reduce weed management programs to as few trips across the field as possible.  The one-pass, total post program is the ultimate goal for these growers.  The difficulty in achieving this has been addressed in previous articles posted on this site.  This article will provide a quick summary of some of the research that has investigated the critical period for early-season competition in corn.  The critical period is the length of time following crop planting that weeds can grow with the crop before yields are impacted (i.e. how long can you wait before applying a postemergence herbicide without compromising 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.  Days after planting 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.

Location Year Days after planting to 5% corn yield loss Corn stage at 
5% yield lossa
Kemptville 1988 50 12 lf 3
Elora 1988 22 5 lf 25
Woodstock 1988 24 8 lf 4
Ridgetown 1988 40 10 lf 9
Kemptville 1989 40 10 lf 14
Elora 1989 52 12 lf 14
Woodstock 1989 12 3 lf 56

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 2, 4, 6 or 8 inches in height.  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 and inversely related to the critical period, 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.

Weed height at
application (inches)a
Corn height at
application (inches)
Corn leaf stage (collars) Days after planting Corn yield loss


2 3 2 12 0
4 6 3 18 0
6 12 5 25 10
8 18 6 31 20
Full season competition - - - 68
2 4 2 9 0
4 6 3 15 0
6 12 4 20 0
8 18 4 23 8
Full season competition - - - 49

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.

Prepared by Bob Hartzler, extension weed management specialist

Iowa State Weed Science Online