|Yenish, Joe - WASHINGTON STATE UNIV.|
Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 1999
Publication Date: N/A
Interpretive Summary: Specific herbicides may be applied preharvest in cereal grain crops in the Pacific Northwest to control weeds that interfere with harvest and to increase wheat-dry down. Glyphosate, a commonly used preharvest herbicide, cannot be applied to wheat that is being produced for certified seed. However, many growers produce their own wheat for seed, some of which may have been treated with preharvest applications of glyphosate. In a study conducted at Pullman, WA glyphosate was applied to two varieties of soft white spring wheat at five stages of growth ranging from milk stage to 1 day before harvest. Information collected included seed yield and quality of the treated wheat and seedling establishment and seed yield of wheat produced from seed originating from preharvest-treated wheat. Grain yield from wheat treated with preharvest glyphosate applied at the milk stage of growth was reduced up to 77% compared to untreated wheat yield. Seed collected from wheat treated with glyphosate in the milk stage, and plante the next year, had a seedling density reduction ranging from 28 to 99% and seed yield reduction ranging from 12 to 97%. Growers must be aware of the importance of time of application of preharvest glyphosate if they are producing their own seed. Seed quality reduction was probably because of incomplete seed embryo development instead of herbicide residues in the seed.
Technical Abstract: Studies were conducted to determine effects of preharvest applications of glyphosate on the seed and seedling quality of 'Alpowa' and 'Penawawa' soft white spring wheat varieties. Glyphosate was applied at 0.62 or 0.84 kg: ae/ha at the milk, soft dough, or hard dough stage of wheat development, 7 d following the hard dough treatment, and 1 d prior to wheat harvest. In harvest aid applications, wheat yields were only reduced with glyphosate applied at the milk stage of development. Yield reduction ranged from 20 to 77% depending on the year, variety, and glyphosate rate. Likewise, kernel weight and germination were only affected by glyphosate applications at the milk stage with reductions from 19 to 73% and from 2 to 46% for kernel weight and percentage germination, respectively, compared to untreated wheat. Using wheat from harvest-aid glyphosate treatments at the milk stage as seed the following year resulted in reductions ranging from 28 to 99%, 19 to 39%, and 12 to 97% for seedling density, plant height, an seed yield, respectively, compared to seed from untreated wheat. In this study, wheat seed and seedling quality following preharvest glyphosate applications were most greatly influenced by crop maturity stage at time of application than by herbicide rate or variety.