PREHARVEST APPLICATIONS OF GLYPHOSATE FOR CANADA THISTLE (CIRSIUM-ARVENSE) CONTROL

The effectiveness of single and repeated annual preharvest glyphosate applications (1 to 2 wk before crop harvest) to reduce Canada thistle populations was evaluated in field experiments near Bezanson, AB, Scot t, SK, and St. Albert, AB. Single applications of glyphosate at 0.45 t o 1.8 kg/ha at Bezanson and St. Albert reduced Canada thistle shoot de nsity by more than 75% but in the three experiments at Scott 1.8 kg/ha was required to consistently obtain a similar reduction. Four consecu tive, annual applications of glyphosate at 0.45 kg/ha or more did not eliminate Canada thistle but reduced shoot densities by more than 98% at Bezanson. At Scott, similar reductions occurred in two of three sit es following two consecutive, annual preharvest applications of glypho sate at the same rates. In the third Scott site the 1.8 kg/ha rate of glyphosate provided a similar reduction but lower rates did not. At Be zanson, barley yields following the first, second, and third applicati ons of glyphosate at 0.45 kg/ha or more were 14% greater than check pl ots where MCPA was applied in the spring. Yields of wheat seeded 1 yr after a single application of glyphosate at St. Albert were 26% greate r than from unsprayed checks. Barley yields seeded 1 yr after each gly phosate application at Scott were not affected because Canada thistle populations were sparse. Nomenclature: Glyphosate, N-(phosphonomethyl) glycine; MCPA, [(4-chloro-2-methylphenoxy)acetic acid]; Canada thistle , Cirsium arvense (L.) Scop. #3 CIRAR; barley, Hordeum vulgare L.; whe at, Triticum aestivum L.