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 H. Trindade*, J. Coutinho†, S. Jarvis‡ and N. Moreira* *CITAB – Centre for the Research and Technology of Agro-Environment and Biological Sciences, Department of Plant Science and Agricultural Engineering, Universidade de Tra ´s-os-Montes e Alto Douro, Vila Real, Portugal, †Centro de Quı ´mica, Department of Soil Science, Universidade de Tra ´s-os-Montes e Alto Douro, Vila Real, Portugal, and ‡Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon, UK Abstract 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 multiflo- rum 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 0Æ95 of maximum DM yields of herbage and 0Æ90 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 min- eral-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 leach- ing 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. Keywords: slurry, nitrate leaching, nitrogen fertilizer, intensive dairy farming, maize silage Introduction Dairy farming is a major agricultural enterprise in north-west Portugal where an intensive double- cropping forage system that uses maize (Zea mays L.) as a summer crop for silage and Italian (annual) ryegrass (Lolium multiflorum Lam.) as a cover crop in winter. These are used for silage or for zero-grazing plus a final cut for silage or hay, a practice that has developed over the last few decades (Moreira, 1994). Such double-cropping systems provide important man- agement options for increasing forage production (Helsel and Wedin, 1981; Hughes, 1985), particularly where the area of the system is limited (Lloveras, 1987). Substantial quantities of slurry [liquid manure, typi- cally with a dry-matter (DM) content of 60–80 g kg )1 ] are produced in this intensive zero-grazing system, and the slurry is used with complementary dressings of mineral-N fertilizer to achieve high annual DM yields of herbage (c. 30 Mg ha )1 ) with the result that there are large N losses, particularly by nitrate-leaching from the system (Trindade et al., 1997). The efficiency of use of N in slurry needs to increase in order to reduce losses of N to the environment while maintaining high yields and nutritive value of forage (Neeteson, 2000; Aarts, 2003). To meet these dual requirements, different strategies have been evaluated including Correspondence to: H. Trindade, CITAB, Department of Plant Science and Agricultural Engineering, Universidade de Tra ´s-os-Montes e Alto Douro, PO Box 1013, 5001-801 Vila Real, Portugal. E-mail: htrindad@utad.pt Received 28 April 2008; revised 20 June 2008 Ó 2008 The Authors Journal Compilation Ó 2008 Blackwell Publishing Ltd. Grass and Forage Science, 64, 2–11 doi: 10.1111/j.1365-2494.2008.00664.x 2