Effects of Nitrogen Fertilization on Phosphorus Uptake in Bermudagrass Forage Grown on High Soil-Test Phosphorus Sites

Common bermudagrass [Cynodon dactylon (L.) Pers.] was harvested from two producer sites (Latta and Stephens) with high soil-test P (286 and 153 ppm, respectively) to assess the effects of N fertilization on P uptake and potential removal in hay or silage. Ammonium nitrate was applied in split applications each year at rates totaling 0, 56, 112, 168, 224, 280, or 336 kg/ha of actual N. At the Stephens site in Yr 1, cumulative DM yield increased linearly (P<0.0001) with N fertilization rate. Mean concentrations of P in these forages declined linearly (P<0.0001) with N fertilization rate, but uptake of P still increased linearly (P=0.0004) with N fertilization, reaching a maximum of 30.0kg of P/ha. At the Latta site, mean yields of DM and uptake of P for Yr 1 and 2 increased linearly (P<0.0001) with N fertilization; the maximum uptake of P was 50.8kg of P/ha. At the Latta site, there was an interaction between concentrations of P in the forage and year (P=0.002). During Yr 1, there was no relationship (P>0.10) between concentration of P and N fertilization rate (overall mean=0.40%), but concentrations of P declined in linear (P<0.0001) and quadratic (P=0.044) patterns from 0.59 to 0.44% during Yr 2. Uptake of P was generally improved with N fertilization, but potential P removal may be limited by reductions in the concentration of P in the forage at greater N fertilization rates.