Crop response to carfentrazone–ethyl can be affected by environmental conditions. Field research was initiated to determine the effect of irrigation and light intensity prior to herbicide treatment on crop response to carfentrazone–ethyl. Wheat, corn, and soybean response was evaluated in 1996 and 1997, 2 yr that differed significantly in rainfall. It was difficult to distinguish differences in visible crop injury between irrigated and nonirrigated crops within the same year; however, injury was much higher in 1996 than in 1997. In 1996, the study area received timely rainfall prior to treatment of each crop, but in 1997, no precipitation was recorded during the treatment period. Overall, irrigated plants appear to be slightly more sensitive than nonirrigated plants. In contrast, crop injury was significantly higher in response to low light intensity prior to herbicide treatment. Soybean plants covered with 80% shade cloth for 5 d prior to carfentrazone–ethyl application were injured 24 to 41% more than nonshaded plants. Corn was relatively insensitive to either condition. Soybean plants were very sensitive to carfentrazone–ethyl and were highly influenced by both light intensity and irrigation. Wheat response to carfentrazone–ethyl was not influenced within 1 yr by irrigation, but injury in 1996 was four times higher than in 1997. Light intensity prior to treatment influenced wheat response to carfentrazone–ethyl, where shading before treatment increased visible injury in wheat, but by less than 10%. The risk of crop injury increases when carfentrazone–ethyl is applied to irrigated plants or to crops following several cloudy days.

Nomenclature: Carfentrazone–ethyl; corn, Zea mays L. ‘Pioneer 3655’; soybean, Glycine max (L.) Merr. ‘Conrad'; spring wheat, Triticum aestivum L. ’Blanca'.

Additional index words: Herbicide selectivity, membrane disruptors, protoporphyrin IX; protoporphyrinogen oxidase, shading; triazolinone.

Abbreviations: DAT, days after treatment; PPFD, photosynthetic photon flux density; PPIX, protoporphyrin IX; Protogen, protoporphyrinogen IX; Protox, protoporphyrinogen oxidase; RH, relative humidity.