The Amounts and Forms of Nitrogen Leached From Autumn and Winter Forage Crops Grazed by Dairy Cows in Southern New Zealand

A 3‐year grazing study was undertaken to quantify nitrogen (N) leaching losses from forage crops that were grazed by dairy cows in late autumn (fodder beet, Beta vulgaris ; FB) or winter (FB and kale, Brassica oleracea ). The practice of mechanical lifting instead of autumn grazing of FB was also evaluated as an option for mitigating N leaching. Mean annual inorganic‐N leaching losses from autumn grazing of FB were large (98 kg N ha −1 ) relative to those measured for winter‐grazed FB. Lifting of FB in autumn had no effect on reducing N loss. Leaching losses from winter‐grazed FB were significantly less than those measured from winter‐grazed kale (52 vs. 82 kg N ha −1 year −1 ), most likely reflecting the effects of the lower protein content of FB and consequent reduced intake and excretion of N by cows. Leaching losses from the forage crop treatments were large relative to those measured from nearby pasture paddocks, confirming the elevated risk associated with forage crops where extended overlapping periods of bare ground coincide with inputs of surplus rainfall. Considerations of grazing and/or harvest timing and crop type are therefore important factors that determine the risk of N leaching losses from grazed forage cropping systems.