Too late to catch up: a high nutrient specification diet in the grower phase does not improve the performance of low birth weight pigs.

Low birth weight pigs are on a different growth trajectory from their normal birth weight littermates and as a consequence they may need to be fed differently, if they are to show compensatory gain. The objective of the experiment was to determine if low birth weight pigs will respond to a higher AA: energy diet, in a manner similar to the response of normal birth weight pigs whose BW gain has been previously reduced as a result of feed restriction. The experiment was a 3 × 2 factorial with 180 pigs and 6 replicates. Treatments were comprised of 3 BW categories (NU = normal birth weight fed unrestrictedly [1.7 to 2.0 kg], NR = normal birth weight but fed restrictedly from d 49 to 63 and L = low birth weight [ ≤ 1.2 kg]) and 2 diet specifications given from d 63 to 91 (high or standard AA: energy ratio). In period 1 (d 0 to d 49), pigs were selected within 24 h of birth and cross fostered into litters by birth weight (11/12 piglets). Pigs were weaned at d 28 and kept in their litters until d 49. In period 2 (d 49 to 63), litters were split to form 2 treatment groups consisting of 5 pigs each: NR pigs received restricted amounts of feed (600 g/d) with the remaining NU and L pigs fed ad libitum on a conventional diet. The aim was for NR and L pigs to have the same BW by d 63. In period 3 (d 63 to 91), groups within litter were randomly allocated to a high or standard AA:energy ratio diet. Body weight on d 63 was 25.8, 21.2, 21.8 kg (0.381 SED) for NU, NR and L pigs respectively; by d 91 there was no difference in the BW of NU and NR pigs (P > 0.05), but L pigs still weighed less (P < 0.001). There was a significant effect of BW category and diet specification during period 3 on the ADG and G:F of pigs (P < 0.05), with NR pigs exhibiting higher ADG and G:F than L and NU pigs. For G:F only, there was a tendency towards significance for an interaction (P = 0.057) with NU and NR pigs having an improved G:F on the high specification diet, but L pigs did not. Average daily feed intake was similar for all BW categories and diets from d 63 to 91. However, both L and NR pigs consumed more feed than NU pigs relative to their BW, but only NR exhibited higher ADG than NU pigs. This suggests that a higher specification diet post weaning may not improve the performance of low birth weight pigs. However, previously restricted normal birth weight pigs were able to exhibit compensatory growth and were more efficient when fed a higher specification diet.