Herbage production and persistence of two tropical perennial grasses and forage sorghum under different nitrogen fertilization and defoliation regimes in a summer-dominant rainfall environment, Australia

An experiment was conducted in inland northern New South Wales (NSW) to assess the response of tropical perennial grasses Chloris gayana (Rhodes grass) cv. Katambora and Digitaria eriantha (digit grass) cv. Premier and annual forage sorghum (Sorghum bicolor ssp. bicolorxS.bicolor ssp.drummondii hybrid) cv. Sweet Jumbo fertilized with five rates of nitrogen (N; 0, 50, 100, 150 and 300kgNha(-1)) and defoliated every 2 or 6weeks over two growing seasons. Tropical perennial grasses were highly responsive to N fertilizer, while there was no significant response by forage sorghum. Herbage production of Rhodes grass increased linearly whereas digit grass had a high response at 50-100kgNha(-1). Nitrogen-use efficiency was highest during the growing season when rainfall was higher. During this season, digit grass had the highest N efficiency (148kgDMkg(-1) N applied) at 50kgNha(-1), and Rhodes grass (66kgDMkg(-1) N applied) at 100kgNha(-1). Plant frequency of both perennial species increased and then stabilized at high levels (>84%, cell size 01 by 01m) during the two growing seasons. Plant frequency of Rhodes grass declined over the winter period, but recovered within 6weeks of commencement of the growing season. Soil nitrate levels indicated that unused nitrate moved down the soil profile during wet winters. Implications of leaching below the rooting zone are discussed.