Nitrogen utilization in tall fescue (Festuca arundinacea Schreb.) pastures fertilized with nitrogen or grown with alfalfa (Medicago sativa L.) or red clover (Trifolium pratense L.)

Use of legumes as an alternative to nitrogen (N) fertilization in pasture management improves forage quality and animal performance and has been suggested to reduce the potential for environmental pollution. "Kentucky 3l" tall fescue fertilized with 160 kg N ha-1 yr-1 (split application) was compared to tall fescue grown with alfalfa or red clover in a 5-yr pasture experiment on a mixed Typic Hapludult. During yr 6, effects of N fertilization or the legume on soil N, forage N concentration, yield, botanical composition, N intake by esophageally fistulated steers grazing the pastures and N utilization by wethers fed the harvested forages were investigated. Soil ammonium was higher (P ≤ .01) in the A and B horizons in the tall fescue-red clover pastures compared to the other treatments and nitrate was lower (P ≤ .05) in the A horizon, but concentrations differed (P ≤ .01) by date. Nitrate in the A horizon averaged 2.65, 1.38 and 2.21 ppm for tall fescue-N, tall fescue-red clover and tall fescue-alfalfa, respectively. In the B horizon, average soil NO3 was .43, .23 and .53 ppm for tall fescueâ N, tall fescue-red clover and tall fescue-alfalfa, respectively. Tall fescue-alfalfa pastures were higher (P ≤ .01) in percentage legume than tall fescue-red clover, overall, but differed by date (P ≤ .01). Alfalfa was generally higher (P ≤ .05) in N concentration than red clover. Total kg N accumulated ha⁻¹ in above-ground herbage was higher (P≤ .05) for the grass-legume mixtures than N-fertilized tall fescue. Esophageally fistulated steers grazing stockpiled tall fescue-alfalfa selected forage higher (P ≤ .05) in N concentration than steers grazing the other pastures. Stockpiled tall fescue-alfalfa fed to wethers in a metabolism trial was higher (P ≤ .01) in N concentration, dry matter digestibility (DMD), apparent N absorption, and N retention than the other treatments. All treatments differed, with wethers fed tall fescue-red clover having the lowest DMD, apparent N absorption and N retention. Wethers fed tall fescue-alfalfa and tall fescue-red clover had higher blood urea N then those fed tall fescue-N. Results of this research demonstrate that soil NO₃ concentrations were low for all three forage treatments and would not contribute to ground water contamination. Legumes supplied adequate N to achieve yields similar to tall fescue fertilized with N and increased N production ha⁻¹ in the above ground biomass. Digestibility and utilization of the N in stockpiled tall fescue were improved by inclusion of alfalfa but not red clover.