Age-related effects on fecal particle size and composition

There is limited information about the fiber digesting capacity in the gastrointestinal tract of young foals. A greater understanding of how the foal's fiber digesting capacity develops over time may lead to better feeding practices. Fecal particle size (FPS) is considered a proxy of digesta particle size in horses and previous work suggested FPS of yearlings is similar to adults. This 2-phase study aimed to extend prior work by examining FPS and fecal composition of foals at 1 and 4 mo of age. We first hypothesized that FPS of foals would differ from their dams at 1 mo but not at 4 mo. A single free-catch sample was collected from 6 Thoroughbred mares and their nursing foals when foals were 1 and 4 mo of age. Mare and foal pairs had shared access to pasture, alfalfa hay, and a commercial concentrate feed. The FPS distribution (FPSD) was determined by wet sieving. Mean FPS was calculated from the weighted average of dry matter (DM) retained on the sieves with manual measurement of maximal particle size. The FPSD was calculated as DM retained on sieves as a percentage of total DM retained. For FPSD comparisons, sieves were grouped into large (4–2 mm), medium (1–0.5 mm), and small (0.25–0.063 mm) particle sizes. Data were analyzed using PROC MIXED in SAS with repeated measures and Tukey comparisons, with significance set at P < 0.05. At 1 mo of age, foals had smaller mean FPS, a smaller percentage of large particles and a greater percentage of small particles than mares (P < 0.05). At 4 mo, foals had smaller mean FPS than mares (P < 0.05) but the percentage of large and small particles were similar to mares, suggesting progressive changes in ingestive or digestive function from 1 to 4 mo of age. Phase 2 investigated the chemical composition of different fecal particle fractions. Particles were grouped into large-medium (LMP) and small (SP) based on their physical appearance and limited DM retained on some sieves. We hypothesized that acid detergent fiber (ADF), neutral detergent fiber (NDF) and the carbon-to-nitrogen ratio (C:N) of LMP and SP would differ between mares and foals. Compared with SP, LMP had greater NDF, ADF, and C:N ratio for mares and foals (P < 0.05). Fecal NDF content increased with foal age in LMP and SP (P < 0.05) but there were no differences between foals and mares. At 1 mo, foals had a lower C:N ratio in LMP compared with mares (P < 0.05) and by 4 mo of age, foals had similar C:N ratio in LMP and SP as mares. Differences in composition may be due to changes in diet, fiber digestibility, or microbial biomass. Further studies on changes in FPS with age may contribute to our understanding of fiber digesting capacity in growing horses.