Chemical Winter Weed Control in Crop Land

Charles Stichler
Extension Agronomist
Uvalde, Texas

Introduction:

In many areas of south and central Texas, winter weeds are often abundant in crop land if fall and winter rains moisten the soil sufficiently for germination of weed seed. Winter weeds provide a habitat for insects (cutworms), cause planting problems in conservation tillage systems and may dry out soils sufficiently to prevent planting or germination if moisture is deficient. Henbit is particularly difficult to control and often very abundant. Thistles such as sow thistle, prickly lettuce and others are also troublesome weeds.

With rotations and residue concerns, the selection of herbicides in conservation tillage systems are particularly important. The test was initiated to evaluate several commonly used and recently labeled products.

Study Details

An area between two fields at the Luling Foundation Farm was selected for its variety of weeds and heavy infestation of henbit. Common mallow, Rescue grass, and thistles were also abundant in the area with a limited number of sunflowers.

Treatments were applied with a C02 backpack sprayer equipped with 11015 Turbo TeeJet spray tips. Treatment rate was 12 gallons of water under 30 psi spray pressure. R-11 surfactant was added to all treatments at 0.5% except those where Roundup Ultra was included.

The plots were sprayed on February 17, 2001, air temperature was approximately 58 ^o F. Night time temperatures were approximately 38 ^o F at the time of application and were generally in the 40's for the next three weeks. Several light showers and misty weather conditions were common during the period.

The henbit had stems measuring 12 inches or longer, Rescue grass was 8-10 inches tall and thistles ranged from the rosette stage to starting to develop seed stems. Other broadleaf weeds included some early germinating sunflowers which were 2-4 true leaf stage and common mallow which was generally 12-15 inches tall.

Plots were rated on Feb. 26 and March 7 (17 DAT). The Feb. 26 rating date was too early, the weeds had not yet begun to die sufficiently to determine control. The March 7 rating was intermediate, so another rating was done on March 19.

The results of the two ratings is found below.

Results:

Table 1. Percent control of selected weeds

* Some henbit was still slightly green - although it appeared to be dying

⁽¹⁾ It was very evident there was antagonism to the Roundup due to product inactivation. The lack of control of Rescue grass when mixed with the other noted products indicates the chemical incompatibility.

Table 2. March 19 Rating - Percent Control

Other Broadleaf Weeds includes mustard - but not sufficient to rate percent control.

Boa in the earlier rating, resulted in desiccated leaves, however the growing point was not killed and weeds began to regrow, hence, lower ratings. Only small thistles were killed, larger thistles began to regrow.

Table 2. Per Acre Cost

These costs are estimations based on a random sampling of prices.

Discussion:

Roundup Ultra mixed with 2,4-D LV ester or Harmony Extra will provide good broad spectrum weed control. Roundup alone may not control some large weeds such as common mallow or large thistles. (No ammonium sulfate was mixed with Roundup in this test. Results should be better if mixed with ammonium sulfate.)

The control of Rescue grass when Roundup Ultra alone at a quart/acre or Roundup plus Harmony Extra compared to Roundup Ultra mixed with Direx, Linex, Aatrex and Salvo, very strikingly shows an interaction of the Roundup when mixed with these products. Caution should be used to make sure the products are compatible in combinations with any glyphosate product.

If henbit or other broadleaf weeds are a problem in fields without grasses, then diuron (Direx,), linuron (Linex), or atrazine (Aatrex) can be used to control many broadleaf weeds. If applied earlier when weeds are smaller, increased control can be expected. Under most circumstances, the larger the weed, the harder it is to control. In addition, when temperatures are cool, the activity of any herbicide is reduced, and control of target weeds will take longer. If this test had been established sooner when weeds were smaller, weed control would have occurred sooner and the chemical rate could possibly be reduced.

Crop rotation, planting interval and labeled crop usage are important issues and should be closely observed to prevent crop injury.

Acknowledgments:

Appreciation is given to the following companies for supplying products for evaluation.

Roundup Ultra is a product of the Monsanto Company

Direx is a product of the Griffin Corporation.

Linex is a product of the Griffin Corporation.

Boa is a product of the Griffin Corporation.

R-11 surfactant is the product of Wilbur-Ellis.

Aatrex is a product of Syngenta.

Harmony Extra is a product of DuPont Company.

Salvo is a 2,4-D LV ester product of United Ag. Products.

The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Texas Agricultural Extension Service is implied.