Energy, amino acid, and phosphorus digestibility of phytase transgenic corn for growing pigs

Three experiments were conducted to evaluate energy, AA, and P digestibility in a phytase transgenic corn (PTC) containing a phytase gene (phyA2) isolated from Aspergillus niger compared with a nontransgenic near-isoline conventional corn (CC) grown in the same environmental conditions for growing pigs. Experiment 1 was an energy balance experiment conducted to measure DE and ME in PTC and CC. Eighteen growing barrows (initial BW 25.8±0.3 kg) from 9 litters were allotted by BW and litter to 1 of 2 dietary treatments with 9 pigs per treatment in a randomized complete block design. Pigs were individually placed in metabolism cages and fed diets based on the 2 corns. The DE and ME in PTC (3,967 and 3,941 kcal/kg of DM, respectively) were greater (P<0.05) than those in CC (3,917 and 3,848 kcal/kg of DM, respectively). Experiment 2 was conducted to measure apparent ileal digestibility (AID) and standardized ileal digestibility (SID) values of CP and AA in the 2 corns. Eighteen growing barrows (initial BW 41.8±0.7 kg) were equipped with a T-cannula in the distal ileum. Pigs were placed in metabolism cages in a completely randomized design with 3 dietary treatments of 6 pigs each. An N-free diet was used to estimate basal endogenous losses of CP and AA. The AID and SID values for CP and all AA did not differ between the 2 corns. Experiment 3 was conducted to measure apparent total tract digestibility (ATTD) and standardized total tract digestibility (STTD) values of P in the 2 corns. Eighteen growing pigs (initial BW 30.5±0.5 kg) from 6 litters were placed in metabolism cages in a randomized complete block design with 3 dietary treatments of 6 pigs each based on BW and litter. Two diets were based on the 2 corns, and a P-free diet was used to measure endogenous P losses. The ATTD and STTD values of P were greater (P<0.05) in the PTC diet (71.4% and 76.9%, respectively) than those in the CC diet (27.6% and 33.4%, respectively). Pigs fed the PTC diet had a greater (P<0.05) P retention (70.7%) than those fed the CC diet (27.1%). It was concluded that PTC had a greater digestibility of energy and P than CC for growing pigs. As a consequence, if PTC replaces CC in a pig diet, the DE and ME in the diet will increase, and less inorganic P will need to be supplemented to the diet, and thus P excretion in manure will be decreased.