Weed Control and Crop Safety with Premixed <i>S</i>-Metolachlor and Sulfentrazone in Sunflower

A preliminary study conducted in the central USA near Colby and Hays, Kansas (KS) in 2010 indicated a premix of S-metolachlor & sulfentrazone codenamed F7583 (Broadaxe?) had good potential for use in sunflower (Helianthus annuus L.). Additional studies were conducted in 2011 at Colby, Hays, Manhattan, KS to refine rate and application timing of F7583 for weed control and crop safety. Four rates of F7583 (860, 1100, 1350 and 1840 g·ha-1) were compared to single rates of S-metolachlor and pendimethalin, and applied 21 days preplant versus preemergence (PRE). F7583 at ≥1100 g·ha-1 applied preplant or PRE controlled Palmer amaranth (Amaranthus palmeri S. Wats.) and kochia [Kochia scoparia (L.) Schrad.] ≥95% and 100%, respectively in neutral pH soils. In slightly acidic soils, PRE application of F7583 was more effective against Palmer amaranth and grass weeds compared to preplant application. No benefit was gained by increasing the rate of F7583 from 1100 to 1350 g·ha-1 at either application timing. Puncturevine (Tribulus terrestris L.) control was not commercially satisfactory with F7583 at any rate or time of application. Both S-metolachlor at 1070 g·ha-1 and pendimethalin at 1600 g·ha-1 applied either preplant or PRE were considerably less effective on all three broadleaf weeds compared to F7583 treatments. Individually, S-metolachlor and pendimethalin were more effective when applied PRE compared to preplant application. F7583 did not reduce sunflower plant population or visibly injure sunflower anytime during the season.

Broadleaf Weed Control in Sunflower (<i>Helianthus annuus</i>) with Preemergence-Applied Pyroxasulfone with and without Sulfentrazone

A field study was conducted at two locations in Kansas, USA in 2011 and 2012 to test weed control efficacy and crop response to preemergence-applied pyroxasulfone alone and in combination with sulfentrazone in sunflower. Treatments included three rates of pyroxasulfone (100, 200 and 400 g·ha-1) applied alone and tank-mixed with sulfentrazone at 70, 140 and 280 g·ha-1. Commercial standards sulfentrazone at 140 g·ha-1 + pendimethalin at 1390 g·ha-1 and sulfentrazone at 140 g·ha-1 + S-metolachlor at 1280 g·ha-1 were also included. Pyroxasulfone at 100 g·ha-1 controlled Palmer amaranth 87% at 3 weeks after application (WAA), but control decreased to 76% at 6 WAA. Increasing pyroxasulfone rate to ≥200 g·ha-1 or tank mixing with sulfentazone at 140 g·ha-1 provided ≥90% Palmer amaranth control for at least 6 WAA. Sulfentrazone alone at 70 g·ha-1 controlled Palmer amaranth 77% at 3 WAA, but control dropped to 69% at 6 WAA. Increasing sulfentrazone rate from 70 to 140 or 280 g·ha-1 increased control to >90% at 3 WAA, but did not maintain acceptable control at 6 WAA. Tank mixing sulfentrazone at 140 g·ha-1 with pendimethalin at 1390 g·ha-1 or S-metolachlor at 1280 g·ha-1 controlled Palmer amaranth ≥90 and 84% at 3 WAA and 6 WAA, respectively. The lowest rate of pyroxasulfone (100 g·ha-1) controlled kochia 98% and the control was complete with all other treatments. However, no treatment provided as much as 90% puncturevine control at 3 WAA and the control was commercially unacceptable (<75%) at 6 WAA. No treatment visibly injured sunflower anytime during the season or reduced sunflower plant population.