Longitudinal pattern of growth performance and feeding behaviour in pigs fed varying dietary crude protein levels and categorised by final feed efficiency

Feed efficiency (FE) estimated on pen level does not take individual feed intake or feeding behaviour into account, and thus, varying response among pen mates is impossible to quantify. This study aimed to explore the within-pen variation in FE of pigs fed one of three levels of CP. The study included 60 gilts (30-115 kg) divided into two repetitions (10 pigs/pen with an automatic feeder). Pigs were fed a low (LCP; 120 g SID CP/kg), standard (SCP; 124 g SID CP/kg) or high (HCP; 128 g SID CP/kg) CP diet. Pigs within diet were categorised based on their FCR in the final four growing weeks as the 30% highest (HF), medium 40% (MF), and the 30% lowest (LF). Weekly BW, average daily gain (ADG), feed conversion ratio (FCR), and daily and per-meal feed intake and feeding behaviour were measured per individual. Data were analysed as polynomial mixed models. Growth performance parameters except FCR had significant dietary treatment (DT) × time (T) and feed conversion category × time interactions. During weeks (W) 10-12, BW tended (P ≤ 0.10) to be greater in HCP compared with LCP aligning with the numerically highest intake for HCP in the preceding period. Similarly, HF intermittently had greater BW and preceding higher intake than LF or MF (0.05 ≤ P ≤ 0.10). The ADG of LCP pigs showed the greatest variability over time and intermittently increased faster and slower (0.05 ≤ P ≤ 0.10) than SCP and HCP, respectively. From W10 onwards, the HF pigs had a stable ADG while the ADG of LF and MF increased steeply and consequently was greater (P ≤ 0.05) at W13-14. In line with ADG, FCR varied significantly over time and most strongly for LCP. Overall, LF pigs had the lowest, while HF pigs had the highest FCR (P < 0.001). Meal size (P < 0.001) and duration (P < 0.001) had a significant DT × T interaction, being greater for HCP than LCP and occasionally SCP. In line with differences in growth rate, meal frequency decreased faster (P < 0.05) for HCP than LCP or SCP, resulting in longer intervals and limited differences in daily feeding duration. While meal size and duration tended (P ≤ 0.10) to be greater for HF than MF or LF pigs, differences in daily feeding behaviour were negligible. It was concluded that, independent of CP-level, a distinction in FCR categories could be attributed to differential meal feeding behaviour of HF pigs leading to higher feed intake, whereas differences in ADG changed during the growing-finishing phase.