Dietary fiber in a low-protein diet during gestation affects nitrogen excretion in primiparous gilts, with possible influences from the gut microbiota.

We investigated the effects of dietary fiber (DF) supplementation in normal or low crude protein (CP) diets on reproductive performance and nitrogen (N) utilization in primiparous gilts. In total, 77 Landrace × Yorkshire (LY) pregnant gilts were randomly allocated to four dietary treatments in a 2 × 2 factorial design. The groups comprised 1) equal intake of normal CP (12.82% and 0.61% total lysine), 2) low CP (10.53% and 0.61% total lysine), and 3) with or 4) without DF supplementation (cellulose, inulin, and pectin in a 34:10:1 ratio). A low-protein diet during gestation significantly reduced daily weight gain from days 91-110 of pregnancy (-162.5 g/d, P = 0.004). From N balance trials conducted at days 35-38, 65-68, and 95-98 of pregnancy, DF addition increased fecal N excretion at days 65-68 (+24.1%) and 95-98 (+13.8%) of pregnancy (P < 0.05), but reduced urinary N excretion (P < 0.05), resulting in greater N retention at each gestational stage. DF increased fecal microbial protein levels and excretion during gestation. A low CP diet also reduced urinary N excretion at different gestational stages. An in vitro fermentation trial on culture media with non-protein N urea and ammonium bicarbonate (NH4HCO3) as the only N sources revealed that microbiota derived from feces of gestating gilts fed the high DF diet exhibited a greater capacity to convert non-protein N to microbial protein. Microbial fecal diversity, as measured by 16S rRNA sequencing, revealed significant changes from DF, but not CP diets. Gilts fed a low CP diet had a higher number of stillbirths (+ 0.83 per litter, P = 0.046) and a lower piglet birth weight (1.52 kg vs. 1.37 kg, P = 0.006), regardless of DF levels. Collectively, DF supplementation to gestation diets shifted N excretion from urine to feces in the form of microbial protein, suggesting that the microbiota had a putative role in controlling N utilization from DF. Additionally, a low-protein diet during gestation negatively affected the litter performance of gilts.