Effects of different rates and timing of application of nitrogen as slurry and mineral fertilizer on yield of herbage and nitrate‐leaching potential of a maize/Italian ryegrass cropping system in north‐west Portugal

Efficient use of cattle‐slurry to avoid nitrogen (N) leaching and other losses is important in designing intensive dairy systems to minimize pollution of air and water. The response in dry‐matter (DM) yield of herbage and nitrate‐leaching potential to different rates and timing of application of N as cattle slurry and/or mineral fertilizer in a double‐cropping system producing maize (Zea mays L.) silage and Italian ryegrass (Lolium multiflorum Lam.) was investigated in north‐west Portugal. Nine treatments with different rates and combinations of cattle slurry, and with or without mineral‐N fertilizer, applied at sowing and as a top‐dressing to both crops, were tested and measurements were made of DM yield of herbage, N concentration of herbage, uptake of N by herbage and amounts of residual soil nitrate‐N to a depth of 1 m, in a 3‐year experiment. Regression analysis showed that the application of 150 and 100 kg of available N ha−1 to maize and Italian ryegrass, respectively, resulted in 0·95 of maximum DM yields of herbage and 0·90 of maximum N uptake by herbage. Residual amounts of nitrate‐N in soil after maize ranged from 48 to 278 kg N ha−1 with an exponential increase in response to the amount of N applied; there were higher values of nitrate‐leaching potential when mineral‐N fertilizer was applied. The results suggest that it is possible in highly productive maize/Italian ryegrass systems to obtain high DM yields of herbage for maize silage and Italian ryegrass herbage with minimal leaching losses by using slurry exclusively at annual rates of up to 250 kg available N ha−1 (equivalent to 480 kg total N ha−1) in three applications.