Nitrogen leaching implications of poor pasture persistence

Abstract

Farmers have indicated that perennial pastures sown in the Lake Taupo catchment revert to low quality species within 8 to 10 years. These may be renewed with perennial pasture species following an autumn then spring cropping regime, or resown pasture-topasture by direct-drilling into glyphosate-sprayed turf or following full cultivation. Vegetation which is desiccated and/or ploughed-under before sowing will decay and release mineral nitrogen (N). The mineral N from these sources is available for newly sown plants but can also be leached. In a large, replicated, rotationally cattle-grazed trial near Lake Taupo, new pasture was established with the high sugar ryegrass (HSG) Aberdart in one treatment only by direct-drilling, following glyphosate application in late summer. Existing pasture remained in Control plots. Renovated pasture leached 63 kg nitrate-N ha-1 in the 8 months following establishment compared 8 kg nitrate-N ha-1 in Control (P<0.05). There were no differences in the amount of leached N amongst treatments in the second year (P=0.1). Nitrogen leaching losses resulting from pasture renovation and forage cropping practices are discussed in relation to the impacts these have on future pasture management and renewal options and on the potential value of new germplasm in the Lake Taupo catchment, whose management is constrained by a N cap. Keywords: nitrogen leaching, pasture establishment, Taupo