This study evaluated the effects of reducing dietary crude protein (CP) and increasingneutral detergent fiber (aNDFom) on growth performance, nutrient digestibility, manurecomposition and carcass parameters of lean pigs as a means of reducing the environmen-tal load of slurry. Sixty-four intact male Landrace × Large-White pigs (13.8, SD 2.3 kg ofinitial body weight (BW)) were assigned to one of two dietary CP levels (high, HP or low,LP) and one of two aNDFom levels (high, HF or normal, NF) in a 2 × 2 factorial design, andsubjected to a three-phase feeding program from 6 to 21 weeks of age (15<br>110 kg of BW).The diets had similar metabolizable energy (ME), total lysine content and ideal amino acids(AA) ratio. Pigs fed HP diets had the highest average daily gain (ADG) and BW from 12weeks of age (P < 0.05), which was associated with a gain:feed ratio that was higher thanin the LP treatment (P < 0.05). Dietary aNDFom did not affect the ADG or gain:feed of pigs(P > 0.05). The coefficient of total tract apparent digestibility (CTTAD) of CP was higher in HPgroups (0.765 ± 0.0075), than it was in the LP groups (0.732 ± 0.0075, respectively), inde-pendent of the dietary aNDFom level. Low dietary CP reduced aNDFom digestibility in pigsfed diets that had a normal aNDFom level (LP-NF: 0.45), but not in pigs that were fed ahigh aNDFom diet (LP-HF: 0.548), compared to pigs fed HP diets (HP-NF: 0.546, and HP-HF:0.583 ± 0.011). Low dietary CP increased the manure output at 21 week of age (P < 0.001)and high dietary aNDFom increased manure output from 16 weeks of age (P < 0.001). Theslurry pH was higher in the HP groups than it was in the LP groups (7.42 vs. 7.18 ± 0.085,P = 0.05), but the level of dietary aNDFom did not alter the pH of slurry (P = 0.66). Back-fat thickness at slaughter was highest in pigs fed low CP (P < 0.001) or high aNDFom diets(P = 0.03). In summary, the effects of dietary CP and aNDFom on growth performance werelargely independent of each other. From 12 to 21 weeks of age, lowering dietary CP up to50 g/kg reduced growth performance and increased back-fat at slaughter. High aNDFomdiets did not reduce growth performance, but hampered lean tissue gain. An increase indietary aNDFom in low CP feed did not reduce the CTTAD of aNDFom in growing-finishingpigs. However, this dietary manipulation had the lowest economic return.This work was supported by the Ministry of Economy and Competitiveness of Spain and the European Union Regional Development Funds (AGL2010-20820). Henris Morazán was supported by a grant from the Ministry of Foreign Affairs and Cooperation of Spain (MAEC-AECID 2009–2011) and Ahmad Reza Seradj was a recipient of a research training grant from the Government of Catalonia (FIDGR 2011-Generalitat de Catalunya). The authors wish to thank the staff of the Centre d’Estudis Porcins (CEP Diputació de Lleida, Spain), David Contreras Tinoco for their technical assistance and Javier García from UPM for total dietary fiber analysis support.