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Effect of emergence date on
woolly cupgrass competitiveness in corn
by Bob Hartzler

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December 29, 1999 -  While the introduction of several new herbicides in recent years has lessened problemsin managing woolly cupgrass during corn production, this weed still remains a formidable foe for many Iowa farmers.  One of the problems in managing woolly cupgrass is maintaining full season control.  Research at ISU has shown that the majority of woolly cupgrass emerges in a relatively short time frame compared to other common species (e.g. giant foxtail, velvetleaf, waterhemp) {related story}.   In a typical year woolly cupgrass begins emergence in central Iowa during the last two weeks of April or first week of May and typically reaches 90% emergence within 3-4 weeks of initial emergence.  However, a small percentage of seeds in the seed bank may continue to emerge into late June, and in fields with a large seed bank there may be enough of these late-emerging plants to be an economic concern.  A recent paper in Weed Science (Mickelson, J. A. and R. G. Harvey. 1999.  Relating Eriochloa villosa emergence to interference in Zea mays.  Weed Sci.   47:571-577) reported on the impact of woolly cupgrass emerging at different dates in corn.  This article will present some of the results reported in this paper.

Experiments were conducted at two sites in Wisconsin in fields with natural infestations of woolly cupgrass.  At the Milton site good woolly cupgrass was obtained in corn the year prior to the experiment, thus few seeds entered the seed bank the year prior to the experiment.  At the Edgerton site woolly cupgrass was not controlled during soybean production the year prior to the experiment, thus there was a large return to the seed bank.  To establish woolly cupgrass populations at different times in relation to the corn, Liberty Link corn was planted at the sites and applications of Liberty were made at different corn stages (e.g. Liberty would be applied at the V4 stage of corn and then cupgrass plants that emerged after the application would be monitored).  Data collected included the number of plants emerging after the herbicide application, biomass and seed production by woolly cupgrass, and corn biomass (Table 1).

Although the woolly cupgrass densities varied at the two sites due to differences in past management, results of the two experiments were similar.   Woolly cupgrass densities declined rapidly with delayed Liberty applications.   Woolly cupgrass densities at Edgerton averaged 1467 plants per m2 following the VE timing, whereas only 203 plants emerged when the Liberty application was delayed until the V1 corn stage (Table 1).  While densities dropped off rapidly with delays in application, significant woolly cupgrass plants continued to emerge as late as the V5 application.  Woolly cupgrass that emerges after corn is less competitive than weeds that emerge at the same time as the crop due to the corn's head start.  This is shown in both the woolly cupgrass and corn biomass results.  At Milton, delaying woolly cupgrass emergence until the V2 stage or later resulted in maximum corn biomass (it is assumed that total corn biomass would be directly correlated with corn grain yield), whereas at Edgerton maximum corn yields were obtained when the Liberty application was made at the V3 stage or later. 


Table 1.  Effect of woolly cupgrass emergence time on woolly cupgrass growth,
seed production and corn growth.

Site Application timing1

W. cupgrass density

(plants / m2)

W. cupgrass biomass

(g / m2)

W. cupgrass Seed Prod.

(seeds / m2)

Corn biomass

(g / m2)

Milton VE

























Edgerton VE


























   1Liberty was applied at the specified corn stage to eliminate all woolly cupgrass plants and then
the plants that emerged after the application were studied.   For the later applications (e.g. V5, V11)
additional applications were made earlier to prevent early emerging cupgrass from competing with

The primary goal of weed management is to protect crops from yield losses due to competition with weeds.  At both locations woolly cupgrass that emerged after the V3 stage of corn did not affect corn yields.  However, significant woolly cupgrass populations did emerge after this application date and these late-emerging weeds did produce seed.  At Edgerton, 6600 seeds per m2 were produced by woolly cupgrass emerging after the V3 stage, whereas approximately 1000 seeds per m2   were produced following the V5 application.  The high seed production at this site may be due to a poor corn canopy brought about by heavy rainfall and saturated soils early in the growing season.  Seed production at Milton was significantly less due to lower weed populations, and Liberty applications at the V5 stage or later reduced seed production to 300 seeds per m2 or less.

This research shows that woolly cupgrass that emerges after typical postemergence applications should not impact corn yields; however, in fields with heavy infestations the late emerging plants may contribute significant quantities of seed to the seed bank.  Since the goal of preventing seed production is unrealistic for most Iowa producers, the question should be how much seed production can be allowed without reducing the effectiveness of control tactics in subsequent years?  Research at ISU found that the addition of 4000 giant foxtail seeds per m2 reduced the effectiveness of Frontier in fields with a high initial weed density, whereas in fields with low initial weed densities the performance of Frontier was not affected by the addition of foxtail seed.  For woolly cupgrass, I might speculate that seed production less than 1000 seeds per m2 should not be a  concern.   While this may seem like a large amount of seed returning to the soil, keep in mind that a large percentage of the seed may be lost due to predation and decay.

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

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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