AbstractBACKGROUNDIn terms of whole‐body metabolism, visceral tissues have a disproportionate influence with respect to their masses, and under certain circumstances their high metabolic rate may compromise nutrient availability to the tissues of productive/economic interest. The low energy efficiency of Iberian compared to modern breeds could be partly explained by the contribution of portal‐drained viscera to heat production. The objective of this study was to determine the relative contribution of portal‐drained viscera heat production (PDVHP) to total heat production (THP) in Iberian and Landrace gilts fed a diet with low protein content.RESULTSCompared to Landrace, Iberian gilts (29 ± 0.9 kg body weight) had lower pre‐ and postprandial portal vein blood flow (654 vs. 965, and 746 vs. 1133 mL min−1, respectively; P < 0.01), postprandial PDVHP (3.09 vs. 3.88 kJ h−1 kg−0.75; P < 0.01) and PDVHP/THP ratio (0.08 vs. 0.13; P < 0.05), and greater THP (38.5 vs. 31.8 kJ h−1 kg−0.75; P < 0.05), when offered 0.25 of their daily ration in a 6 h sampling period.CONCLUSIONIn spite of the low energetic efficiency and growth rate of Iberian compared to modern breeds, the contribution of PDV energy expenditure relative to THP was lower in Iberian compared to Landrace gilts in the present study. © 2015 Society of Chemical Industry