PS4-7. Hindgut Fermentative Responses of Horses Fed Increasing Levels of Dried Corn Distillers Grains Plus Solubles.

Abstract Agro-industrial by-products such as dried corn distillers grains plus solubles (DDGS), have gained interest in equine nutrition. However, their inclusion requires careful evaluation, as hindgut fermentation is sensitive to soluble carbohydrates. This study aimed to evaluate the effects of increasing levels of DDGS in equine diets on cecal fermentative activity, estimated by fecal short-chain fatty acid (SCFA) production, lactic acid concentration, and pH. We hypothesized that DDGS inclusion in the concentrate would not compromise hindgut fermentative stability. The experiment was conducted at the Research Laboratory of Digestive Health and Equine Performance (LabEqui), School of Veterinary Medicine and Animal Science, University of São Paulo, using eight Arabian geldings in a replicated 4 × 4 Latin square design. Experimental periods consisted of a 15-day dietary adaptation, 5 days of sample collection and 15-day washout between periods. Treatments consisted of a control diet without DDGS (TC) and diets containing 15% (T15), 30% (T30), or 45% (T45) DDGS in the concentrate. Total daily intake was set at 1.75% of body weight (dry matter basis), with a forage-to-concentrate ratio of 60:40. Fecal samples from the first daily defecation were collected to determine SCFA, lactic acid, and pH as indicators of fermentative activity. Data were analyzed using PROC MIXED (SAS 9.4), considering repeated measures over time and a significance level of 5%. Mixed-effects modeling included animal as a random effect and treatment, period, and square as fixed effects, along with Pearson and Spearman correlation analyses, ordinary least squares linear regression when model singularity occurred, polynomial contrasts, and Tukey's test for multiple comparisons. No significant differences were observed among treatments for SCFA concentrations (P > 0.05). The means and standard deviations of SCFA concentrations for acetic acid, propionic acid, and butyric acid were 37.65 ± 4.73 m, 15.05 ± 1.44 mM, and 2.52 ± 0.27 mM, respectively. A strong positive Pearson correlation was observed between fecal lactic acid and fecal pH (r = 0.75; P < 0.001), indicating a stable fermentative environment. Correlations were moderate to strong in TC (r = 0.73; P = 0.040) and very strong in T15 (r = 0.88; P = 0.0038) and T30 (r = 0.96; P = 0,0002), suggesting microbial adaptability to DDGS inclusion. In contrast, T45 showed no linear relationship between lactate and pH (r = 0.004; p = 0.992). Overall, DDGS inclusion up to 30% maintained hindgut fermentative stability, whereas 45% inclusion altered the metabolic relationship between lactate and pH, indicating functional adjustments in the fermentative microbiota.