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Factors Affecting Preemergence Bioactivity of Diclofop: Rainfall, Straw Retention, and Plant Growth Stage

USDA-ARS, P.O. Box 400, Akron, CO 80720

^* Corresponding author.

Diclofop {(±)-2-[4-(2,4-dichlorophenoxy)phenoxy]propanoic acid} controls downy brome (Bromus tectorum L.) when mechanically incorporated after application, but preemergence surface applications result in erratic control of downy brome. To determine the physical factors that enhance diclofop bioactivity, laboratory and greenhouse studies were conducted with two soils: sand (Ustic Torripsamment) and sandy clay loam (Aridic Paleustoll). An oat (Avena saliva) root bioassay was used to measure bioactivity of diclofop. Movement of diclofop downward by simulated rainfall was affected by soil type and time of rainfall event. Oat root growth of seed planted 25 mm deep was reduced by >80% in the sand when 25 mm of rain was applied, but less than 30% in the sandy clay loam. Diclofop movement was reduced if the rainfall event was delayed by 4 d after application to the sand soil, indicating the need for precipitation within a few days after diclofop application. Surface straw interception and retention reduced diclofop activity by 20 to 30%, but simulated rainfall reduced this retention. Diclofop bioactivity was greatest when applied before oat seed germination. Diclofop applied to the soil surface after oat seedlings had emerged reduced plant growth 20 to 30%, while diclofop applied before oat germination reduced plant growth 75 to 85%. Preemergence surface applications of diclofop to the sand soil would be suitable for irrigated wheat (Triticum aestivum L.) production on the sand, provided that diclofop was applied before weed germination and an irrigation of 25 mm occurred within 2 to 4 d after application.

Received for publication November 19, 1987.