Effect of low protein diets on growth performance, backfat thickness and N excretion of heavy pigs

Conventional diets used in heavy pig production contain about 14.5% of CP and more than 7 g of lysine/kg feed, which are likely to be in excess considering a protein retention of about 100 g/d. The hypothesis tested in this trial is that a replacement of pro- tein with starch in the diet should not influence the growth per- formance, improve the back-fat thickness and reduce the N excretion of pigs reared for the dry cured ham production of pro- tected origin (DOP). Four iso-energetic diets (ME = 12.9 MJ/kg) were formulated to contain 14.2, 13.2, 11.9, 11.4% of CP and 45.8 g of lysine/kg CP, achieved by a progressive replacement of soya- bean meal with wheat grain. In two repeated cycles, a total of 160 pigs were housed in 8 pens in groups of 10 individuals. The pigs were divided in 4 groups, balanced for age and BW, and fed one of the 4 diets following a restricted feeding regime. Automated feeding stations measured daily the individual feed intake. Pigs were weighed at the start of the trial (99.3\ub110.1 kg), every 2 weeks, and before slaughter (168.9\ub19.0 kg). The two cycles last- ed 102 and 112 d. Back-fat thickness was measured by an ultra- sound device. Nitrogen excretion was computed as consumption minus retention, assuming a N content of 0.024 kg/kg weight gain. Data were analysed by ANOVA. No significant effects due to the treatment were observed on average daily gain (ADG= 0.650 kg/d; P=0.66) and feed conversion ratio (FCR=4.17; P=0.16), but back-fat thickness variation during the trial was increased from +7.2 to +8.4 mm with decreasing CP levels (P=0.007). The esti- mated N excretion strongly decreased from 41.6 to 38.4 g/d (P