Difference in short-term responses to a high-fiber diet in pigs divergently selected for residual feed intake

Residual feed intake (RFI), defined as the difference between observed and predicted feed intakes, has been explored as a relevant selection criterion to improve feed efficiency in growing pigs. Previous studies exploring the genetic and physiological bases of RFI have been focused on pigs fed a regular diet. The aim of this study was to evaluate the effects of RFI selection on pigs' ability to digest and metabolize a diet enriched in fiber. After 11 wk of age, pigs of 2 lines divergently selected for RFI (low or high; n = 7 pairs of littermates in each line) were fed either a control diet (133 g NDF and 9.69 MJ NE/kg) or a high-fiber diet (244 g NDF and 8.36 MJ NE/kg) for 3 wk. Irrespective of diet, ADFI and water consumption were lower (P < 0.001) in the low RFI line, whereas G:F over the test period was 16% greater (P < 0.001) in that line compared with the high RFI line. There was no line-associated difference in the digestibility of nutrients and energy, despite a lighter digestive tract (-6.4%, P = 0.04) and a lower colon weight (-8.6%, P = 0.03) in the low RFI pigs than in the high RFI pigs. As compared with the control diet, ADFI was reduced (-7.5%, P < 0.001) in both lines when fed a high-fiber diet, but ADG and G:F were lowered for the high RFI line only (P < 0.05). The high-fiber diet decreased (P < 0.01) the digestibility values of nutrients and energy similarly in both lines. However, it increased the ratio of acetate to propionate concentrations in the distal part of the gastrointestinal tract for the high RFI line only (P < 0.07). The pattern of response of plasma metabolites and hormones to a meal test was altered by RFI selection, but it was not affected by the diet. The increase in glucose concentration after meal ingestion was more pronounced (P < 0.05) in the low RFI line, and long-lasting plasma concentrations of lactate tended to be lower (P < 0.10) in the low RFI line than in high RFI line. Plasma NEFA concentrations were greater in that line compared with the high RFI line. The activity levels of catabolic enzymes in muscle and the liver did not differ between lines and diets. In conclusion, the low RFI pigs seem to better utilize a high-fiber diet than high RFI pigs. This was not associated with marked changes in digestion, and the underlying metabolic mechanisms remain to be clarified.