Nutritional value of crude and partially defatted olive cake in finishing pigs and effects on nitrogen balance and gaseous emissions

By-products from the food industry can be valuable ingredients in animal feeds. One example is olive cake (OC), generated in large amounts by the olive oil industry, which contains oil with a high proportion of oleic acid and polyphenols. An experiment was performed using pigs to determine the nutritional value of crude (COC) and partially defatted (PDOC) olive cake, and to evaluate their effect on nutrient balance, slurry properties and potential ammonia (NH3) and methane (CH4) emissions. Five experimental feeds were designed; a basal diet and another four diets produced by substituting 100 or 200 g/kg of the basal diet with either COC or PDOC. Thirty finishing male pigs (76.1 ± 4.2 kg initial BW) were used in the experiment (6 animals/treatment). After a 14-day adaptation period, faeces and urine were collected separately for 7 days to measure nutrient digestibility and the excretory patterns of nitrogen. Potential NH3 and CH4 emissions were measured in reconstituted slurry samples over 11 and 100 days, respectively. The dry matter (DM), crude protein (CP), cellulose, starch and energy coefficients of total tract apparent digestibility (CTTAD) were negative and linearly (P < 0.05) affected by OC inclusion level. However, the type of OC did not influence any of the digestion efficiencies studied. The energy digestibility of the ingredients tested, estimated by substitution, were 0.479 (±0.040, SEM) and 0.327 (±0.049) for COC and PDOC, respectively. Overall, the results indicate a digestible energy (DE) value from COC and PDOC that account respectively for around 80 or 60% of the DE provided by barley grain in pigs. Faecal content of cellulose, polyphenols and gross energy (GE) increased linearly with OC inclusion, whereas ash content decreased. The total N content of urine decreased linearly with OC inclusion, but benzoic and hippuric acid contents increased, which resulted in lower pH values for the OC diets. The ratio between faecal and urine N excretion decreased from 2.48 in the basal diet to 1.01 on average in the 200 g/kg OC diets. As a result, increasing both COC and PDOC levels in diets resulted in lower NH3 emissions per volume of slurry and in a lower biochemical CH4 potential. Although slurry excretion increased with OC inclusion, daily NH3 emissions still decreased with increasing OC inclusion. However, potential CH4 emissions per animal increased. A global perspective throughout the production chain is needed to assess the impact of including OC in pig diets on gaseous emissions.