INFLUENCE OF THE ESTER OF 2,4-D APPLIED AT VARIOUS GROWTH STAGES FOR THE CONTROL OF RUSSIAN THISTLE (SALSOLA PESTIFER NELS.) IN FLAX

Abstract

Linseed flax, growing with naturally occurring infestations of Russian thistle at Lethbridge, Alta., was treated with three rates (2, 3, and 6 oz. acid equivalent per acre) of the butyl ester of 2,4-D at 5-day intervals from the seedling stage to boll formation in each of the 5 years 1952 to 1956, inclusive.Two peaks of weed control were determined, the first being at the early growth stages between 19 and 35 days after crop seeding when some measure of weed stand reduction was obtained. The second peak occurred from 45 to 60 days after seeding when the Russian thistle plants were 6 to 8 in. tall, the control being characterized by a "curl-down" of weed growth which persisted to harvest time. Mature flax height was relatively unaffected except when the crop was treated at or near the first-bud stage of growth. Height of flax above the weed mass was 9.7 and 13.2 in. in 1954 and 1955, respectively, as a result of the best-timed 6-oz. treatment. Such height differences would make it possible to combine-harvest flax seed containing a minimum of weed material. Maturity delays of only 2 or 3 days resulted from treatments made within 1 month after seeding, whereas delays of up to 33 days resulted from treatments made when the flax was in full bloom. On the average for all three treatment rates, flax yields equal to or better than those from untreated weedy plots were obtained in each of the 5 test years when the 2,4-D was applied prior to 36 days after seeding. Thus, for adequate control of a Russian thistle infestation and the undelayed harvest of a good yield of flax seed, it would appear that, 25 to 30 days after seeding, the crop should be treated with a 6-oz. rate of the butyl ester of 2,4-D. Slightly earlier treatment is indicated if cool, wet conditions prevail after seeding.