Maize (Zea mays L.) is a very important crop in many of the irrigated areas of the Ebro Valley (NE Spain). Intensive pig (Sus scrofa domesticus) production is also an important economic activity in these areas, and the use of pig slurry (PS) as a fertiliser for maize is a common practise. From 2002 to 2005, we conducted a field trial with maize in which we compared the application of 0, 30 and 60 m 3 ha −1 of PS combined with 0, 100 and 200 kg ha −1 of mineral N at sidedress. Yield, biomass and other related yield parameters differed from year to year and all of them were greatly influenced by soil NO3 − -N content before planting and by N (organic and/or mineral) fertilisation. All years average grain yield and biomass at maturity ranged from 9.3 and 18.9 Mg ha −1 (0 PS, 0 mineral N) to 14.4 and 29.6 Mg ha −1 (60 m 3 ha −1 of PS, 200 kg ha −1 of mineral N), respectively. Grain and total N biomass uptake average of the studied period ranged from 101 and 155 kg ha −1 (0 PS, 0 mineral N) to 180 and 308 kg ha −1 (60 m 3 ha −1 of PS, 200 kg ha −1 of mineral N), respectively. All years average soil NO3 − -N content before planting and after harvest were very high, and ranged from 138 and 75 kg ha −1 (0 PS, 0 mineral N) to 367 and 457 kg ha −1 (60 m 3 ha −1 of PS, 200 kg ha −1 of mineral N), respectively. The optimal N (organic and/or mineral) rate varied depending on the year and was influenced by the soil NO3 − -N content before planting. For this reason, soil NO3 − -N content before planting should be taken into account in order to improve N fertilisation recommendations. Moreover, the annual optimal N rates also gave the lowest soil NO3 − -N contents after harvest and the lowest N losses, as a consequence they also could be considered as the most environmentally friendly N rates. © 2008 Elsevier B.V. All rights reserved.