Reduction in dietary crude
protein and addition of fiber could mitigate the incidence and severity of post-weaning
diarrhea, a common gastrointestinal condition in newly weaned pigs. Therefore, 360 weanling pigs, initially 5.0 ± 0.10 kg, were used to evaluate the effects of crude
protein (CP) level and fiber source on growth performance and fecal microbial communities. At weaning, pigs were randomly assigned to pens and allotted to 1 of 8 dietary treatments in a 2 × 4 factorial with main effects of CP (21 or 18%) and fiber source (none, coarse
wheat bran, oat hulls, or
cellulose). There were 5 pigs per pen and 9 pens per treatment. Experimental diets were formulated in two dietary phases from d 0 to 10 and 10 to 24, with a common post-treatment diet fed from 24 to 45. The 21% CP diets contained 1.40% standardized ileal digestible (
SID) Lys in phase 1 and 1.35%
SID Lys in phase 2. By using a maximum
SID Lys:digestible CP ratio of 6.35%, the 18% CP diets contained 1.25%
SID Lys in both phases. Diets containing a fiber source were formulated to the level of insoluble fiber provided by 4% coarse
wheat bran, resulting in the addition of 1.85% oat hulls and 1.55%
cellulose. No fiber source × CP level interactions (P > 0.05) were observed. Decreasing CP (and subsequently
SID lysine) decreased (P = 0.05) ADG and G:F during the experimental period. From d 0 to 45, ADG decreased (P = 0.05) for pigs fed 18% CP diets compared to pigs fed 21% CP. No effect of fiber source was observed for growth performance. Fecal DM on d 17 increased (P < 0.001) for pigs fed 18% CP diets compared to pigs fed 21% CP diets. Pigs fed diets with added
cellulose had increased (P < 0.05) fecal dry matter during the experimental period compared to pigs fed no fiber source or
wheat bran. Bacterial community structure was investigated by sequencing the V4 region of the
16S rRNA gene. Analysis indicated a significant difference between CP content at d 24 (P = 0.023) using a Weighted UniFrac distance matrix. Further investigation identified five differential Amplicon Sequence Variants associated with CP content at d 24. In conclusion, reducing crude
protein (and subsequently
SID Lys) decreased growth performance but increased fecal dry matter content. The source of
dietary fiber in nursery diets had no impact on growth performance; but pigs fed added
cellulose had increased fecal DM compared with other treatments. Microbial analysis identified differential taxa associated with CP content.