Yield and Environmental Effects of Summer Pig Slurry Applications to Irrigated Alfalfa under Mediterranean Conditions

In the Ebro Valley (Spain), the intensive pig (Sus scrofa) production combined with a limited nearby cereal growing area for spreading pig slurry residue is leading more farmers to apply slurry to alfalfa (Medicago saliva L.). The effects of summer pig slurry applications on irrigated alfalfa yield and the environment have not been adequately established. An experiment was conducted in 12 drainage lysimeters in 2007 and 2008, where two rates of pig slurry (low (LD), 140 kg N ha(-1) yr(-1); high (HD) 340 kg N ha(-1) yr(-1)) were compared to a P-K fertilized control. Application of pig slurry did not affect accumulated forage yield after 2 yr (37.3 Mg ha(-1)). Forage N concentration and total N extractions (1214 kg ha(-1)) were similar for all treatments, revealing a high flexibility of the crop to adjust symbiotic N fixation depending on mineral N availability. The NO3--N and P concentrations and loads in drainage were very low (< 2 kg N ha(-1) yr(-1) and 0.035 kg P ha(-1) yr(-1)) and not affected by the pig slurry applications. Soil P in the surface layer (0-0.3 m) increased by a 19% as a result of both pig slurry and P fertilization. Application of pig slurry did not significantly increase the Zn and Cu content of the soil. These results indicate the feasibility of moderate pig slurry applications to growing alfalfa in the Ebro Valley, which will improve the management of this residue.