Effects of dietary protein and lysine content on growth performances, carcass traits and estimated nitrogen input-output flow of growing pigs

This study was aimed to evaluate the effects of crude protein (CP) and standardized ileal digestible (SID) lysine on the performance of growing pigs of two commercial crossbreds (A and B) from 30 to 145 kg BW. The pigs (30.4 ± 2.0 kg of initial BW) were allotted in 8 pens (6 A and 6 B per pen) and received until around 60kg BW the same diet containing 164g/kg CP and 11.0g/kg SID lysine. Afterwards, pigs of 4 pens were fed diets containing 163 to 146 g/kg CP and 9.4 to 8.5g/kg SID lysine (HP), whereas the others were fed diets containing 158 to 126 g/kg CP and 8.0 to 6.5 g/kg SID Lys (LP). Individual feed intake was recorded daily by automated feeding stations (Compident Pig - MLP, Shauer Agrotronic, Austria) and animals were weighed weekly from the start to the end of the trial (145±6.2kg BW). From 60kg BW onwards, the P2 subcutaneous fat thickness (P2BF) of all pigs was measured by ultrasound scanner equipped with a 5.5–10.5MHz linear probe (Mylab OneVET - ESAOTE S.p.A., Genova, Italy), with a 3-week interval. The nitrogen (N) input-output flow was computed as difference between N intake and N retention, estimated either assuming a reten- tion of 24 g N/kg BW or using the relationships between BW, P2BF and the estimated body protein content. At laughterhouse, carcasses and main cuts were weighed and individual samples of longissimus lumborum were collected for chemical and physical analysis. Data were analysed according to a two way factorial mixed model including the effects of diet, crossbreds, their interaction and the random effect of pen within diet. Average growth rate of pigs in the 30 to 145kg BW interval exceeded 0.95kg/d, with an average feed conversion ratio of 2.57. Pigs fed LP diet showed similar growing performance and carcass and meat quality traits when compared to HP fed pigs, but a significant (p<.001) lower N intake and a significant (p<.001) lower estimated N excretion, irrespective of the method used for N retention prediction. Even if crossbreds differentiated (p<.05) for growth rate, feed intake and feed efficiency, no interaction was detected between CP/SID lysine dietary content and pig genetic line. In conclusion, an average reduction of 10 to 20% of CP and SID lysine dietary content did not impair pig per- formance, but greatly reduced the N output and the inclusion of soybean meal in the diets. This response does not seem affected by the genetic type of pigs. Study supported by University of Padova (DOR1638484/16) and Regione Veneto under grant P.O.R. - F.S.E. n. 2105-41- 2121-2015. The authors acknowledge Veronesi SpA (Quinto di Valpantena, Verona, Italy) for financial and technical support.