Preharvest glyphosate in alfalfa for seed production: Effect on alfalfa seed yield and quality

May, W. E., Loeppky, H. A., Murrell, D. C., Myhre, C. D. and Soroka, J. J. 2003. Preharvest glyphosate in alfalfa for seed production. Effect on alfalfa seed yield and quality. Can J. Plant Sci. 83: 189-197. Preharvest applications of glyphosate have been shown to be effective in controlling Canada thistle in annual crops, but may reduce alfalfa seed yield depending on time and rate of application. The objective of this study was to determine the effect of preharvest applications of glyphosate on subsequent alfalfa seed yield and quality. The effects of timing, 1, 26, 51 and 76% alfalfa seed pod maturity, and rate, 0, 440, 880, 1320, and 1760 g a.i. ha(-1), of a preharvest application of glyphosate on alfalfa seed yield and quality in the year of application, and regrowth and seed yield in the following year were determined. The rate of glyphosate applied did not affect seed yield in the application year; however, in the following year, growth and seed yield were reduced as rates increased. Delaying the application of glyphosate increased seed yields in the application year, but decreased alfalfa regrowth and flowering the following year. Germination and seedling emergence were not affected by the rate or timing of the glyphosate. A preharvest application of glyphosate at 1760 g ad. ha(-1) at 76% pod maturity is a viable option in the last year of seed production.