Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2016
Publication Date: 1/10/2017
Citation: Kesoju, S., Boydston, R.A., Greene, S.L. 2017. Effect of synthetic auxin herbicides on seed development and viability in genetically-engineered glyphosate-resistant alfalfa. Weed Technology. 30:860-868.
Interpretive Summary: Feral alfalfa contributes to pollen contamination and lowers genetic purity in alfalfa seed production fields when it is located in the vicinity of foraging pollinators working commercial alfalfa seed fields. Glyphosate resistant (GR) feral alfalfa is particularly troublesome in conventional and organic alfalfa seed production regions as feral plants can act as sources of pollen contamination. Pollinators visiting a GR feral plant and moving on to a conventional plant can result in seed carrying the GR trait resulting in reduced marketability of the seed. Therefore it is important to control feral plants found along roadsides, waste areas, and field borders. Feral alfalfa plants are often treated with herbicides in later developmental stages when plants are easier to detect. This study was conducted to determine the effect of four auxin inhibitor herbicides; dicamba, 2,4-D, triclopyr and aminopyralid on seed development in glyphosate resistant alfalfa when applied to plants in the early seed development stage (green seed pods). The four auxin inhibitor herbicides tested did not lower germination of alfalfa seed harvested from treated plants. However, all four herbicides decreased alfalfa seed yield per plant. Three of the four herbicides (dicamba, 2,4-D, and triclopyr) also caused abnormal seedlings to develop, that when planted in soil, often failed to emerge. The combined effects of lower seed yield and reduction in seedling emergence from seed from dicamba, 2,4-D, and triclopyr treated plants could greatly reduce the ability of feral alfalfa plants to reproduce. As a result, these three herbicides could be useful components of an integrated management program for feral alfalfa in seed producing areas.
Technical Abstract: Feral populations of cultivated crops have the potential to function as bridges and reservoirs that contribute to the unwanted movement of novel genetically engineered (GE) traits. Recognizing that feral alfalfa has the potential to lower genetic purity in alfalfa seed production fields when it is growing in the vicinity of foraging pollinators working alfalfa seed fields, industry has established production standards to control feral plants. But with the commercialization of genetically engineered (GE) glyphosate resistant alfalfa and the need to support the coexistence of both GE and non GE production, effective methods to control transgenic feral alfalfa need to be developed. Therefore we conducted this study to determine the effect of several auxin inhibitor herbicides on seed development in glyphosate resistant alfalfa. These studies were conducted in 2012, 2013, and 2014. GR alfalfa, var. Genuity (R44BD16), was treated with dicamba, 2,4-D, triclopyr, and aminopyralid when alfalfa plants contained green seed pods. Two weeks after herbicide application, plants were harvested, air dried, and seed yield, seed germination, and seedling emergence from the soil determined. In 2013, dicamba, triclopyr, and 2,4-D - decreased alfalfa seed yield per plant compared to untreated plants while in 2014, all four herbicides decreased alfalfa seed yield per plant 24 to 49% (by weight) compared to untreated plants. The same trend was evident in 2012, but seed yield was variable and was not significantly different among treatments. Seed germination averaged 43, 50, and 72% in 2012, 2013, and 2014, respectively, and was not significantly affected by treating with the four herbicides during early pod fill stage. However, seeds harvested from plants treated with dicamba, 2,4-D, and triclopyr often produced deformed and abnormal seedlings and when planted in soil frequently failed to emerge. The combined effects of dicamba, 2,4-D, and triclopyr in reducing seed yield, seedling emergence, and seedling growth could contribute to managing feral alfalfa populations.