Energy cost of ingesting and excreting indigestible material in growing pigs is minimal

The nutritive value of pig feed ingredients is generally derived from the digestible nutrient supply, while little consideration is given to the impact of indigestible material on nutrient utilization. An experiment was conducted to determine heat production (HP) associated with ingestion and excretion of indigestible material in growing pigs. Five pairs of littermate barrows were assigned to a basal diet (Basal) or to a basal diet mixed with wheat straw in a 4:1 ratio (Straw). The pigs were housed individually in metabolism crates and adjusted to feeding regimes for at least 10 d prior to measuring components of HP using indirect calorimetry. The dynamics of HP was recorded for 5 d during the fed state and during a subsequent 24 h fast. Feeding behavior, physical activity of the animals and N-balance were monitored to associate HP with feeding (HPfeed), activity and energy retention. Mean body weight (BW) of the pigs was similar (62.1 kg) for both diets. Dry matter intake (1783 vs. 2207 gd -1 , SE 9) and fecal DM excretion (200 vs. 588 gd -1 , SE 12) were higher (P < 0.05) in straw fed pigs, while ME intake was similar (2302 vs. 2359 kJkg BW -0.60 d -1 ; SE 28). Based on the difference method, the digestibility of DM and gross energy in straw was 15.0% (SE 3.1) and 13.3% (SE 3.9), respectively. Various aspects of energy utilization (expressed as kJkg BW -0.60 d -1 ; Basal vs. Straw) did not differ (P > 0.10) between treatments: total HP (1374 vs. 1355; SE 14), fasting HP (762 vs. 747; SE 6), activity HP (150 vs. 164; SE 15), HPfeed (452 vs. 444; SE 7), and energy retention (961 vs. 983, SE 10). The overall energy cost of ingesting and excreting indigestible material in growing pigs appears minimal and could not be identified in this experiment.