The cost of glyphosate resistance: is there a fitness penalty associated with glyphosate resistance in annual ryegrass?

Glyphosate is a broad spectrum herbicide that is a critical weed control tool in Australia for agricultural, horticultural, and viticultural farming systems. The long term use of glyphosate worldwide has led to the evolution of resistance in nine different weed species. In Australia at least 54 populations of annual ryegrass (Lolium rigidum Gaudin) have been confirmed resistant to glyphosate. Many of these resistant populations appear to have altered translocation of glyphosate within the plants. We have investigated whether this mechanism of resistance causes a reduction in fitness in resistant individuals in a competitive field crop situation. F2 seedlings of four different glyphosate resistant annual ryegrass populations from NSW and SA were planted in a wheat crop in 2003 and allowed to cross only within each F2 family by weeding all wild ryegrass from the trial. Seed was collected at the end of the 2003 season, and seedlings from this seed planted back in the same plots the following season. The frequency of glyphosate resistance was determined in the original F2 seed, and the seed collected in 2003 and 2004 to determine the change in frequency of glyphosate resistance over time in the absence of selection pressure by glyphosate. In all four populations the frequency of glyphosate resistant individuals decreased by 40% in the first year and 20% in the second year.