A seeded mixture of warm-season prairie grasses containing big bluestem (Andropogon gerardi, Vitman), switchgrass (Panicum virgatum L.), Indiangrass (Sorghastrum nutans, L.), and sideoats grama (Bouteloua curtipendula, L.) was fertilized over a four-year period with N, P, K, S and Zn. Yields were increased by N and combinations of N and P, but not by K, S, or Zn. Fertilization did not alter indexes of quality (% protein and % digestible dry matter) in the mature tissue. After four years, the percentage of each species in the mixture had not been altered by fertilizer treatment. Encroachment by cool-season species and weeds was not encouraged by fertilization which was largely attributed to the fact that fertilizers were applied after May 15 of each year.
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Rehm et al. (1972) studied this question.