Isolation Distances for Transgenic Alfalfa Seed Production in the Pacific Northwest

Alfalfa (Medicago sativa L.) was the first perennial crop genetically engineered (GE) to resist glyphosate herbicide. Since alfalfa is insect pollinated, the potential for transgene flow and occurrence of adventitious presence (AP) in conventional varieties is a concern, especially for organic and export producers. In response, the industry developed seed production isolation distances. However, no landscape-scale studies have been conducted to validate industry standards. Our objective was to quantify the extent of gene movement from GE seed fields to commercial conventional seed fields on a landscape level to provide the industry with information to support coexistence strategies. Alfalfa seed fields were mapped in the Touchet area, Walla Walla Valley, Washington, during spring 2013. Fourteen conventional seed fields located at various distances from GE sources were identified, harvested, and tested for the presence of the GE trait. Although other variables affect gene flow, this article focuses on geographic distance, since this variable can be easily controlled by growers and industry. Using distance as the predictor variable, we modeled the relationship between AP proportion and distance to GE source fields at 0.1, 0.5, and 0.9% AP threshold levels using quadratic regression. As distance increased, transgene flow generally decreased. At a threshold level of 0.9% AP, an isolation distance of 330 m between conventional and GE fields is needed to ensure that at least 95% of the samples fall below this threshold level. Spatial prediction maps were consistent with our statistical analysis. At an average distance of 2.2 km, AP was 0.05%, and this decreased to 0.03% at an average distance of 8.8 km from GE seed fields. S.R. Kesoju, Dep. of Agriculture, Columbia Basin College, 2600 N 20th Ave., Pasco, WA 99301; S.L. Greene, Plant and Animal Genetic Resource Preservation Unit, USDA-ARS, 1111 S. Mason St, Fort Collins, CO 80521; R.C. Martin, Forage Seed and Cereal Research Unit, USDA-ARS, 3450 SW Campus Way, Corvallis, OR 97331; M. Kramer, NEA Statistics Group, USDA-ARS, 10300 Baltimore Ave., Beltsville, MD 20705; D.B. Walsh, Irrigated Agriculture Research and Extension Center, Washington State Univ., 24106 N Bunn Rd., Prosser, WA 99350; R.A. Boydston, Grain Legumes Unit, USDA-ARS, 24106 N Bunn Rd., Prosser, WA 99350. Received 4 July 2018. Accepted 12 Mar. 2019. *Corresponding author (skesoju@columbiabasin.edu). Assigned to Associate Editor Glen Ritchie. Abbreviations: AB, alkali bees; ALCB, alfalfa leafcutter bees; AOSCA, Association of Seed Certifying Agencies; AP, adventitious presence; ASSP, Alfalfa Seed Stewardship Program; EPSPS, 5-enolpyruvylshikimate3-phosphate synthase; GE, genetically engineered; GOZ, Grower Opportunity Zones; HB, honey bees; NAFA, National Alfalfa and Forage Alliance; PCR, polymerase chain reaction. Published in Crop Sci. 59:1701–1708 (2019). doi: 10.2135/cropsci2018.07. 0414 © 2019 The Author(s). This is an open access article distributed under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Published April 25, 2019