Dose-response experiments showed that three samples of green foxtail collected from fields in southern Manitoba where trifluralin had not controlled the weed in 1987 were resistant to the herbicide. GR50 values indicated that the resistant populations were about 5 times more resistant to trifluralin than susceptible populations, about twice as resistant as wild oat, and about equally resistant as wheat. An additional 33 samples collected in 1988 from southwestern Manitoba where trifluralin has been used for nearly 20 yr in both cereal and oilseed crops were resistant to trifluralin.

A curved-linear Fresnel lens was investigated to test the effect of concentrated solar radiation on surface and buried weed seed. The lens produced a line focus 1 by 150 cm with a mean temperature of 309 C. A 20-s exposure to seed on the soil surface was 100% lethal to green foxtail, kochia, common lambsquarters, common purslane, and wild buckwheat. In a separate study, emergence from kochia and yellow foxtail seed was reduced 100% at 10 mm soil depth after 15 min in soil of 35 and 93 g/kg moisture and 20 to 40% at 178 g/kg moisture content. Concentrated solar radiation from a Fresnel lens has the greatest potential for affecting weed seed on the soil surface. A series of Fresnel lenses and/or larger lenses may be required for many practical field applications.

In a petri dish assay, radicle growth of trifluralin-resistant (R) green foxtail exposed to trifluralin concentrations of up to 0.4 ppm (w/v) was not inhibited. Radicle growth of trifluralinsusceptible (S) biotypes was completely inhibited. Shoots were more sensitive to trifluralin than roots, with shoot growth being inhibited for both R and S biotypes at trifluralin concentrations of 0.2 ppm or more. Best discrimination between R and S green foxtail biotypes was achieved by measuring radicle length after incubation of pregerminated caryopses at 0.3 ppm trifluralin in the dark for 5 d at 22 C.