Canadian Journal of Plant Science (2003) 83, 199-204

W.E. May, C.D. Myhre, H.A. Loeppky, D.C. Murrell and J.J. Soroka (2003)
Preharvest glyphosate in alfalfa for seed production: Control of Canada thistle
Canadian Journal of Plant Science 83 (1), 199-204
Abstract: Canada thistle [Cirsium arvense (L.) Scop.] is increasing in both frequency and density in Saskatchewan alfalfa (Medicago sativa L.) seed fields. Application of preharvest glyphosate is an effective means of controlling Canada thistle in annual crops. This study was conducted to determine the effects of preharvest glyphosate on Canada thistle control and crop tolerance in alfalfa seed fields. Five field experiments were conducted in northeast Saskatchewan between 1995 and 1997 in which preharvest glyphosate was applied at 0, 220, 440, 660, 880, and 1760 g a.i. ha^-1 when 60-70% of alfalfa seed pods were brown. Glyphosate did not affect seed yield in the year of application at any application rate, nor germination and subsequent emergence of the harvested seed. Alfalfa regrowth in the year following application declined linearly as the rate of preharvest glyphosate increased. Seed yield was also reduced in the year following glyphosate application. Glyphosate application at 220 g a.i. ha^-1 significantly reduced Canada thistle regrowth in the year following application. Canada thistle density decreased in 2 of 4 site years with increasing rates of preharvest glyphosate. Using preharvest glyphosate at rates higher than 220 g a.i. ha^-1 in seed alfalfa is not recommended when future seed harvest is planned. However, higher rates of preharvest glyphosate could be used in the final year of seed production to facilitate alfalfa removal.
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Research topic(s) for pests/diseases/weeds:
control - general

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Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
	Cirsium arvense (weed)	Alfalfa/lucerne (Medicago sativa)	Canada (west)