Natural Glyphosate Tolerance in Sainfoin (Onybrychis viciifolia).

Sainfoin (Onobrychis viciifolia Scop.), a non-bloating forage legume, is purported to have tolerance to glyphosate [N-(phosphonomethyl) glycine]. The effect on biomass yield (BMY) and survival of eight rates of glyphosate on seedlings and mature plants were determined. Treatment rates were 0, 210, 420, 841, 1,681, 3,363, 6,725, and 13,450 g ae/ha. Alfalfa (Medicago sativa L.) was included as susceptible check. The dose required to produce a 50% reduction (GR50) in sainfoin seedling BMY was 868 and 2,984 g ae ha-1 at the first and second harvests, respectively. Alfalfa seedling BMY GR50 values were 331 and 463 g ae/ha at the first and second harvests, respectively. The dose estimated to cause 50% mortality (LD50) in sainfoin seedlings was beyond the highest treatment rate at 14,016 g ae ha-1, compared to alfalfa at 2,068 g ae/ha. Four weeks after a spring treatment, sainfoin and alfalfa mature plant BMY GR50 values were 2,622 and 208 g ae/ha, respectively. Biomass yield GR15 values from the same spring treatment were 1,030 g ae/ha for sainfoin and 66 g ae ha-1 for alfalfa.  Thirteen months following the spring treatments sainfoin and alfalfa GR50 values averaged 11,007 and 1,093 g ae/ha, respectively.  Fall applied glyphosate BMY GR50 values from harvests the following spring were 12,949 g ae/ha for sainfoin and 755 g ae/ha for alfalfa. The LD50 for sainfoin spring and fall treatments on mature plants was estimated to be well beyond the highest treatment rate 13,450 g ae/ha. The average LD50 for mature alfalfa plants was 583 g ae/ha. Based on the LD50 values, sainfoin seedlings are over six times more tolerant, and mature plants over 20 times more tolerant to glyphosate than alfalfa.  Based on the GR50 values, glyphosate has a 10-fold greater negative impact on alfalfa BMY than it does on sainfoin; however, the GR15 values indicate a likely reduction in sainfoin BMY at glyphosate rates in the range of 841 g ae/ha.