Roundup-Ready soybean and perennial weed control

Perennial weed problems in corn and soybean have generally increased across Iowa with the adoption of reduced tillage systems. While most perennials are not as competitive as annual weeds and they usually spread at a lower pace within fields, over time they can reach levels that interfere with crop production. Most products used for selective weed control in corn and soybean have limited activity on perennials, thus control options are limited as long as the field is maintained in row crop production.

A new option for controlling perennials is the use of Roundup Ultra in combination with Roundup-Ready soybeans. This technology will allow Roundup to be applied later in the season than was feasible in the past. The availability of Roundup-Ready soybeans has some weed managers thinking that problems with perennial weeds are solved.

Jerry Doll at the University of Wisconsin evaluated hemp dogbane control in this system during the 1995 and 1996 growing seasons. Hemp dogbane control increased as Roundup applications were delayed (see table). The effect of hemp dogbane maturity on control could be partially overcome by increasing the rate of Roundup. For example, 1.5 pint of Roundup Ultra per acre at full flower provided 88 percent control, compared to 73 percent control at the bud stage. Increasing the rate of Roundup at the bud stage to 3 pints per acre resulted in dogbane control that was equivalent to the 1.5 pint per acre rate at full flower.

The researchers also evaluated the impact of the Roundup Ultra treatments on hemp dogbane the year following application. While all treatments reduced hemp dogbane populations the year following application, applications made at flowering provided more effective control than Roundup applications made earlier in the season. For example, the 2 pint per acre rate at early or full flowering reduced hemp dogbane populations by 95 percent the year following application, whereas, the bud stage treatment reduced populations by 80 percent.

Can we expect similar results with other perennials as was observed on hemp dogbane? Keep in mind that hemp dogbane is one of the more susceptible perennial broadleaf weeds to Roundup (this is one reason the rope-wick applicator works well for hemp dogbane but not for many other perennials). Other perennial weeds may require higher rates than is necessary for hemp dogbane. Also, the optimum time for controlling perennials usually will not coincide with the best time for controlling annual weeds. Since annuals have a greater potential to impact yields than perennials, these weeds should take priority when determining the optimum time for application. While Roundup-Ready beans provide an excellent opportunity to manage perennials, it will take efficient management of the product to achieve maximum performance.

Influence of Roundup Ultra rate and application timing on hemp dogbane control in Roundup-Ready soybeans.

Percent hemp dogbane control1,2

Hemp dogbane and (soybean) growth stage at application timing
Roundup Ultra (pints per acre) Vegetative


(1 trifoliolate)
Early flower

(2-3 trifoliolate)
Full flower

(5-9 trifoliolate)
1.5 22 73 64 88
2.0 58 72 93 99
3.0 43 91 92 100
4.0 57 78 100 100

Source: Jerry Doll, 1996, University of Wisconsin.

1 Control ratings made on August 1.

2 LSD0.05 = 11%.

This article originally appeared on page 83 of the IC-478(11) -- June 2, 1997 issue.

Updated 06/01/1997 - 1:00pm