The effect of phosphorus fertilization on a Brazilian subtropical grassland intercropped with winter grass

Brazilian subtropical grasslands present generally low yield and quality due to low soil fertility and, as their forage production is concentrated on the spring and summer period, present a winter feed gap problem for producers. To overcome this feed gap, producers can intercrop their subtropical native pastures with winter active grasses. In order to establish the optimal P application rate for subtropical grassland intercropped with winter active grasses (oats - Avena sativa L.) and Italian ryegrass (Lolium multiflorum Lam.), five rates of P were applied (0, 45, 90, 180 and 360 kg ha-1 P2O5), in autumn 2010. The yield and quality (nutritional values of forage and mycorrhizal colonization of the roots of the winter active grasses) and botanical composition of the forage were monitored for 4.5 years. Grassland and winter forage yields increased average 2.6- and 6.0-fold, respectively, decreasing seasonality of feed supply. Phosphorus application increased concentrations of P, K, Ca, Mg and Cu in the winter active grasses. Conversely, only a small increase in P concentration was observed in the native, summer active grasses. Mycorrhizal colonization of the winter active grasses was sensitive to P application, reducing from 90 and 50% at control, to 50 and 15% at highest phosphorus rate, for oats and ryegrass, respectively. The phosphate fertilization affected species composition of the pasture, decreasing the number of species, but, did not compromised the ecological diversity index (Shannon-Wiener (H’), Simpson diversity and the Pielou equitability (J') indices). The capacity of Mehlich-1 extractor to predict soil P availability was discussed.