Since 1994, cotton acreage in Grady County, Georgia has increased 144%. Most of these acres (80%) are planted in glyphosate-resistant cotton. Growers have rapidly adopted this technology because of the broad spectrum of weeds controlled by glyphosate, reduction in labor input cost, and ease of the glyphosate-resistant system. With the exception of a dinitroaniline herbicide applied at planting, glyphosate is often the only herbicide used to manage weeds in glyphosate-resistant cotton. Glyphosate products only suppress dayflower growth. With little to no crop rotation and the repeated use of glyphosate, Asiatic dayflower has become a significant pest.

Two experiments were conducted to evaluate early postemergence herbicides applied over-the-top of cotton in the 3- to 4-leaf stage and late post-directed herbicides applied to cotton at least 12 inches tall. Asiatic dayflower ranged from 1 to 5 inches with a plant population of 80 plants per square yard. Visual estimates of weed control were taken 2 and 4 wk after treatment.

In the early postemergence herbicide study, treatments included Roundup Ultra 4L (glyphosate) at 1.5, 2.0, 4.0, and 8.0 pt/A; Roundup Ultra at 1.5 pt/A mixed with MSMA 6.6 L (1 pt/A), Staple (pyrithiobac) (0.6 oz/A), or CGA 362-622 at (0.1 oz/A); Staple at 1.2 oz/A applied alone and mixed with 1 pt/A of MSMA; CGA 362-622 at 0.1 oz/A; and Liberty (glufosinate) at 28 oz/A. A nonionic surfactant was included with Staple and CGA 362-622 and an ammonium sulfate was included with Liberty. All treatments were applied at 14.8 GPA.

At 4 wk after treatment, Roundup Ultra (1.5, 2.0, 4.0, and 8.0 pt/A) controlled Asiatic dayflower 63, 74, 84, and 92%, respectively. Mixing Staple or MSMA with 1.5 pt/A of Roundup Ultra improved dayflower control 23 and 12%, respectively. CGA 362-622 mixed with Roundup Ultra did not increase dayflower control. Applied alone, Staple and Liberty were at least 20% more effective than Roundup Ultra at 2 wk after treatment. MSMA mixed with Staple tended to reduce dayflower control. By 4 wk after treatment, this trend was still apparent. CGA 362-622 applied alone controlled dayflower only 42 and 35% at 2 and 4 wk after treatment, respectively.

In the late postemergence-directed study, treatments included MSMA (2 lb ai/A) applied alone or mixed with Bladex 4L (cyanazine) (1 qt/A), Caparol 4L (prometryn) (1 qt/A), Cobra (lactofen) (12 oz/A), Direx 4L (diuron) (1 qt/A), Harvade (dimethipin) (8 oz/A) + crop oil concentrate (1 pt/A), or Valor (flumioxazin) (2 oz/A). Additional treatments included Direx (1.5 pt/A) + Cobra (8 oz/A) and Roundup Ultra (2 pt/A) alone or Roundup Ultra (1 ¹/2 pt/A) mixed with Aim (carfentrazone) (0.67 oz/A), Caparol (1.5 pt/A), Cobra (8 oz/A), Direx (1.5 pt/A), Harvade (8 oz/A) + crop oil concentrate (1 pt/A), or Valor (2 oz/A). All treatments were applied at 20 GPA. A nonionic surfactant was included with all MSMA treatments and Direx + Cobra.

MSMA applied alone controlled Asiatic dayflower 80%. Control was similar and ranged from 88 to 95% when MSMA was mixed with Bladex, Caparol, Cobra, Direx, or Valor. Leaving out the MSMA and applying Direx plus Cobra reduced dayflower control 30%. Roundup Ultra at 1 qt/A controlled dayflower 74%. When 1¹/2 pints of Roundup Ultra was tank mixed with Aim or Valor control was improved 14 to 15% at two weeks after treatment and the Valor mixture improved control 9% at four weeks after treatments. Cobra mixed with 1 ¹/2 pints of Roundup Ultra increased control 10% at four weeks after treatments. Caparol, Direx, and Harvade mixed with Roundup did not affect control at either rating date.