Feed efficiency and fecal microbiome of nursery pigs from parents with divergent breeding value for feed conversion ratio.

Improving feed efficiency (FE) is essential for the swine industry's economic and environmental sustainability. Genetic selection, particularly through estimating breeding values for feed conversion ratio (EBV_FCR), is a common strategy to enhance FE. However, the biological mechanisms underlying phenotypic variations in FE between pigs with different EBV_FCR values are not fully understood. This study investigates these mechanisms by examining growth performance, nutrient and energy digestibility, and fecal microbiota composition and functionality of pigs at the nursery stage. The study involved 128 pigs, weaned at 21 d (±2 d) and with an initial body weight of 6.87 kg (±0.34 kg). These pigs, selected from dam and sire lines with divergent EBV_FCR values, were randomly assigned to 32 pens with four pigs each. Pigs were fed a corn and soybean meal-based diet, divided into two feeding phases of 2 wk each, under similar rearing conditions. Results indicated no significant differences in average daily feed intake (ADFI), average daily body weight gain (ADG), or feed efficiency (FE, gain:feed) between pigs from different EBV_FCR lines (P > 0.05). Similarly, nutrient digestibility showed no significant variation (P > 0.05). While the overall fecal microbiota taxonomic composition was similar between the groups, there was a trend toward higher beta diversity in the microbiota of pigs from parents with lower EBV_FCR (high efficiency pigs, H pigs) (P < 0.083). Carbohydrate and amino acid metabolism were predominant in all pigs, regardless of genetic background, with similar predicted microbiota functionality across groups. The study concluded that genetic differences based on parents divergent EBV_FCR did not affect growth performance, nutrient utilization, or microbiota characteristics at the nursery stage. This suggests that while EBV_FCR based genetic selection does not impact early-stage performance or microbiome responses, its effects may differ in older pigs, warranting further research.