Phosphorus Use Efficiency and Partitioning Among Forage Legume Genotypes.

Forage legumes require a higher level of soil fertility than many grasses species and the failure to provide these nutrients is one of the common reasons for the poor persistence of these species in forage systems.  In addition, the cost of phosphorus (P) has risen in response to higher global demand and the diminishment of mined reserves which has reduced the amount of this fertilizer applied by producers.  The objectives of this study were to examine the P use efficiency (PUE) of alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), and white clover (T. repens L.) genotypes to evaluate the possibility of selecting for varieties with lower fertility requirements and to elicit possible mechanisms that led to the improved efficiency of this soil nutrient.  Samples were collected from the University of Kentucky (UK) forage variety trial in May 2013 and June 2015 when all species were in the early bud stage of growth.  Despite high soil P concentrations in several locations(> 350 lbs P/A), differences (P < 0.001) in PUE were observed between legume species and may represent differences in P utilization by the plant rather than the difference in P uptake.  Red clover had the highest PUE followed by alfalfa and birdsfoot trefoil.  White clover had the lowest PUE among the species evaluated. Higher PUEs across species were correlated with a greater partitioning of P towards the NDF and ADF fractions of the plant cell wall instead of storage forms (i.e. phytate).  This indicates that there is potential to improve legume tolerance to lower soil fertility by selecting genotypes that place more emphasis on supplying nutrients to active growth.  Alfalfa and white clover also exhibited genotypic differences in PUE (P < 0.01) that was related to stand maturities and levels of glyphosate tolerance.