Diet selection in pigs: choices made by growing pigs when given foods differing in nutrient density

Two experiments were conducted to corroborate or refute the theory that ani mals will choose a food that will allow, them to use it with maximum effici ency. Pigs have been shown to utilize foods of high nutrient density more e fficiently than those of low density, so the choices made by pigs when offe red such foods could be used to test the above optimization theory. In expe riment 1, 48 Large White x Landrace gilts were used, for an 8-week period s tarting at 22 kg live weight, while in experiment 2, 48 boars of the same c ross but of a genetically improved strain were used from 24 to 60 kg live w eight. In both experiments use was made of high nutrient density summit foo ds which were used alone, or diluted in the ratio 80 summit :20 milled sunf lower husk to provide the low density foods. In experiment 1, the high dens ity diet (H1) contained 7.5 g lysine per kg and 13.20 MJ digestible energy (DE) per kg, whereas in experiment 2 two summit foods were formulated, the first diet (H2) was offered for 3 weeks from 24 kg live weight and the seco nd (H3) followed until 60 kg live weight. Foods H2 and H3 contained 11.0 an d 8.40 g lysine per kg respectively and 15.0 and 14.0 MJ DE per kg, respect ively Both experiments made use of a high (H1 and H2, respectively) and a l ow nutrient density (L1 and L2, respectively) control treatment in which pi gs were given ad libitum access to H1 and H2/H3, and L1 and L2/L3 in experi ments 1 and 2 respectively (no. = 4). In addition, a medium density treatme nt (MI) consisting of a 50 :50 mixture of H1 and L1 (no. = 4) was given in experiment I. Two choice;feeding treatments where used in both experiments, the first in which H1 and H2/H3 were placed in the left bin (CL1 (no. = 18 ) and CL2 (no. = 20), respectively) and the appropriate dilution diet in th e right bin, and the second in which H1 and H2/H3 were placed in the right bin (CR1 (no. = 18) and CR2 (no = 20)). There were no differences in averag e daily growth rates between treatments within experiments but there were s ignificant differences (P < 0.05) in food intakes and efficiency of food ut ilization (FCE) between treatments. The highest intakes and lowest FCE were obtained on the L1 and L2 treatments while the lowest intakes were recorde d on the choice-feeding treatments. There were no significant differences i n FCE neither between HI, CL1 and CX1 nor between H2, CL2 and CX2. Only in experiment 2 were there significant differences (P < 0.05) between choice-f eeding treatments on the basis of the position of the food bill but there w as no preference for a particular position. The results indicated that pigs were able to differentiate successfully between two foods on the basis of their nutrient density, that bin position was not used as a cue in the choi ce made, that a small amount of the 'unwanted' food tons consumed throughou t the experiment and that the diet selected maximized FCE.