Sowing date and nitrogen supply determine the outcome of competition between dallisgrass (Paspalum dilatatum Poir.) and tall fescue (Festuca arundinacea Schreb.) in the Pampas region of Argentina

The effects of sowing date and nitrogen (N) fertilizer on the inter‐specific competition between dallisgrass (Paspalum dilatatum Poir.) and tall fescue (Festuca arundinacea Schreb.) in the humid Pampas of Argentina were investigated in two pot experiments where a constant soil moisture content was maintained. Tall fescue and dallisgrass seeds were sown either in the spring (October 2000) or in the autumn (March 2001) in mixed and mono‐specific stands with 0 or 100 kg N ha^?1. In the spring, competition from tall fescue depressed dry‐matter (DM) yield of dallisgrass from 1·53 to 0·36 g DM per plant and tiller number from 9·4 to 3·7 tillers per plant in mixed and in mono‐specific stands, respectively, while tall fescue had 3–4 times higher DM yields in mixed stands. Leaf extension rate (LER) of tall fescue was higher (1·3 mm d^?1) than that of dallisgrass (0·53 mm d^?1). In the autumn, inter‐specific competition did not affect DM yield of dallisgrass and N fertilizer increased DM yield from 0·53 to 2·07 g DM per plant, tiller number from 6·8 to 14·2 tillers per plant and LER at the beginning of autumn from 1·2 to 2·12 mm d^?1 in both species. As temperature decreased, LER was reduced in both species to 0·31 mm d^?1 by late autumn. The number of leaves per tiller was not affected by treatment. Nitrogen fertilizer increased N concentration of above‐ground tissues of both species (18 g kg^?1 DM in autumn and 20 g kg^?1 DM in spring). It was concluded that a productive mixed pasture of dallisgrass and tall fescue can be obtained by sowing early in the autumn. The application of N fertilizer in this season is essential to ensure a high herbage yield and quality.