Progress Towards a 2,4-D Tolerant Red Clover (Trifolium pratense).

Incorporation of a legume, such as red clover (Trifolium pratense), into grass pasture systems is advantageous for many reasons. However, susceptibility of red clover to herbicides commonly used in these systems limits its use; the Kentucky pasture weed management guides state “In grass pastures interseeded with clover or other forage legumes, selective herbicide options are not available”.  2,4-D has long been standard for pasture weed management, so a 2,4-D tolerant red clover would be very advantageous.  Sufficient variability in red clover 2,4-D tolerance  was identified, suggesting a 2,4-D tolerant red clover could be selected.  A Florida red clover line with improved 2,4-D tolerance was crossed to 2,4-D susceptible Kenland, and the resulting population was field selected for 2,4-D tolerance (2006-2012).  To assess progress towards 2,4-D tolerance, plants were grown in the greenhouse from seed collected after the 2010 and 2011 selections and treated with 0.0, 0.5, 1.0, 1.5, or 2.0 kg/ha of 2,4-D. Plant fresh weights and injury at two weeks post-treatment were compared to similarly treated parent lines.  The 2,4-D tolerance of the plants grown from the 2010 and 2011 seed, based on fresh weight reductions, was intermediate between the parent lines.  Based on injury ratings, the 2010 and 2011 lines had 2,4-D tolerance similar to the Florida parent.  Thus, while our cross to the 2,4-D tolerant Florida line increased 2,4-D tolerance compared to Kenland, little additional gain has been made in 2,4-D tolerance beyond that of the Florida line, despite numerous rounds of selection for 2,4-D tolerance.  Future work will include studies to determine the role of metabolism in the increased 2,4-D tolerance as well as additional selection at higher rates of 2,4-D (2.24 kg/ha or higher) to increase selection pressure.