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Velvetleaf in Roundup Ready soybeans
by Bob Hartzler and Bruce Battles
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December 22, 1998 - One of the hot topics in weed management is what weeds will be the first to create problems in Roundup Ready crops (see related article).  One species that is often mentioned in these discussions is velvetleaf, primarily due to it having greater tolerance to Roundup than many other weed species.  We conducted a study during the 1998 growing season to evaluate potential problems with velvetleaf in RR soybeans.  Of primary interest was evaluating the vigor of velvetleaf plants that survived Roundup Ultra applications.

The study was conducted in Benton County near Mount Auburn.  Roundup Ready soybeans were planted in 15" rows in a field with a heavy infestation of velvetleaf.  Roundup Ultra was applied at 16 or 32 oz per acre at the V2, V4 or V6 soybean stage.   No AMS was added to the spray solution.  At the time of application, 20 velvetleaf in each plot were tagged so that the fate of individual plants could be monitored throughout the growing season.  The experiment had 3 replications, so 60 plants per treatment were monitored.  Hand weeding and spot treatments of Roundup were used to eliminate weeds other than the tagged velvetleaf.  A control treatment with no Roundup application was also included in the experiment.  Results are presented in Table 1.  Percent survival, velvetleaf height at the end of the season, and seed production were determined for the tagged velvetleaf.

Table 1.  Influence of Roundup Ultra on velvetleaf survival and growth.  Mt. Auburn, IA  1998.

Rate

Applic.
timing
Vlvt. ht
(applic)
Survival
(%)
Vlvt ht
(harvest)
Plants prod. seed (%) Seed per
plant
Control - - 90 A 39" A 87 A 286 A
16 oz V2 4-5" 13 A 11" C 0 C 0 B
16 oz V4 6-7" 70 B 20" B 20 B 24 B
16 oz V6 8-10" 53 BC 12" BC 7 BC 7B
32 oz V2 4-5" 0 D 0 D 0 C 0 B
32 oz V4 6-7" 40 C 8 C 3 C 4 B
32 oz V6 8-10" 0 D 0 D 0 C 0 B

Velvetleaf survival ranged from 13 to 53% at the 16 oz rate of Roundup Ultra, whereas the 32 oz rate provided 100% velvetleaf control at the V2 and V 6 timings.   Velvetleaf survival was greatest when applications were made at the V4 stage (velvetleaf  height of 6-7"), with 70% and 40% survival at the 16 and 32 oz/acre rate, respectively.  Although velvetleaf survival in several treatements was higher than would be considered commercially acceptable, it is important to note the relative vigor of surviving plants.  Plants treated with 32 oz when 6-7" tall only reached a height of 8" at the end of the season.  Only 3% of the plants produced seed, and the low seed production by these plants (4 seeds/plant) would have little or no impact on future velvetleaf populations.  While survival of velvetleaf was higher at the 16 oz Roundup Ultra rate, the vigor of these plants was also greatly reduced.   Late-season heights of plants surviving 16 oz Roundup Ultra ranged from 11 to 20".  While 70% of plants treated at the V4 stage survived the Roundup Ultra application, only 20% of the treated velvetleaf produced seed.  Seed production was less than 1/10 of the seed produced by untreated plants.  Previous research has shown that velvetleaf plants stunted by competition or sublethal herbicide treatments frequently fail to provide seeds.

Roundup Ultra was highly effective on velvetleaf in this study.  Although there was significant survival at the 16 oz rate, most plants did not accumulate enough biomass to reduce soybean yields or produce significant quantities of seed.  Although Roundup generally provides acceptable velvetleaf control, many producers have experienced less favorable results than observed in this study.  When evaluating the results of this study, it is important to recognize that the velvetleaf in the untreated control was less vigorous than frequently observed.   Even though these plants emerged at the same time as soybeans, untreated velvetleaf only reached a height of 39", barely breaking through the soybean canopy at the end of the season.  Seed production was also very low, with less than 300 seeds per plant.  Other research has found that velvetleaf in soybean can produce more than 2000 seeds per plant.  Velvetleaf is susceptible to many diseases, and perhaps the above normal early season rainfall resulted in diseases that suppressed the weeds' vigor.   Velvetleaf under stress from other conditions, particularly diseases, would be expected to be more susceptible to herbicides than unstressed plants. This research will be repeated during 1999 to see if similar results occur.

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
http://www.weeds.iastate.edu
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