Longevity of broom snakeweed (Gutierrezia sarothrae) control
Conoly, James Martin
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Two questions commonly asked concerning broom snakeweed (Gutierrezia sarothrae) control are: (1) How long does the control last? and (2) What is the rate of infestation? Ranches with excellent control (> 90% reduction in snakeweed) were selected to evaluate the longevity of commercial:y sprayed (0.28 and 0.56 kg a.e./ha with picloram) broom snakeweed infested rangelands. Sprayed and nonsprayed pastures (with similar degrees of infestation) were selected in which 10 cages were established in each pasture for measuring grass and snakeweed production. Pastures were selected that were sprayed in 1982, 1984, 1985, and 1986. Grass and snakeweed production were measured in 1987, 1988, and 1989. Also, broom snakeweed seedlings were counted (numbers of seedlings/m2} in 1987, 1988, and 1989. Beginning in 1988, broom snakeweed began to "die-off" presumably because of the patterns and amount of precipitation. However, in 1987 broom snakeweed yields were significantly lower in sprayed than in unsprayed pastures, regardless of 2- 5 years post-herbicide application. Although, broom snakeweed yields were substantially reduced in both sprayed and unsprayed pastures in 1988 and 1989, the yields in the sprayed pastures were often less than in the unsprayed pastures. One of the more interesting results of the st~d~· was that regardless of the year sprayed or the year evaluated, grass yields were significantly greater in the sprayej than in the unsprayed pastures. Perhaps we should emphasize grass response following control of broom snakeweed rather than measure how rapidly snakeweed infestations reach pre-spraying conditions. Longevity of control or the decision to "respray'' should be based on the point at which grass production is adversely affected by broom snakeweed, not when the broom snakeweed yields reach a certain level of productivity.