Herbage dry-matter yield and nitrogen concentration of grass, legume and herb species grown at different nitrogen-fertiliser rates under irrigation

An important goal in dairy systems is to increase production while achieving environmental targets associated with lower nitrate leaching from soils. One approach is to identify forages that grow more at a given level of nitrogen (N) input and result in a lower N intake per kilogram dry matter (DM) consumed. However, while N responses have been well described for perennial ryegrasses, less information is available for alternative grasses, legumes and herbs. In the present study, conducted on the Canterbury Plains, New Zealand, six species (perennial ryegrass, Italian ryegrass, white clover, lucerne, chicory and plantain) were grown at six N-fertiliser rates ranging from 0 to 450 kg N/ha.year and managed under irrigation and cutting management. Herbage DM yield and N concentration were measured over 12 months. As N-fertiliser rate increased from 0 to 450 kg N/ha, annual herbage yield increased linearly (from 4794 to 14329 kg DM/ha.year in grasses and from 7146 to 13177 kg DM/ha.year in herbs). In contrast, annual herbage yield for legumes was unaffected by N-fertiliser rate and ranged from 11874 to 13112 kg DM/ha. Additionally, there were contrasting responses in herbage N concentration between species. At all N-fertiliser rates, herbage N concentration was highest in legumes (43.4 g N/kg DM), then herbs (30.7 g N/kg DM), and lowest in grasses (25.8 g N/kg DM). The N concentration of legume DM was unaffected by increasing N-fertiliser rate, whereas in grasses and herbs it increased. Results suggested that there were no benefits in using herbs instead of grasses for reducing N intake in livestock in an irrigated Canterbury environment.