Equine microbiome shifts with dietary mannan-rich fraction inclusion

The equine hindgut microbial community is highly sensitive and disturbances can lead to issues like diarrhea or even colic. Some of these disruptions, such as those caused by transport stress, illness, or necessary medications, are beyond the horse owner's control. Consequently, the inclusion of pre-, pro-, and postbiotics in feeds and supplements to support and maintain gut health has garnered much interest. This study evaluated the microbiome and immune response of healthy horses fed different inclusion rates of a mannan-rich fraction (MRF), derived from yeast cell wall. Eight 2-year-old Quarter horses (3 geldings, 5 fillies) were included in a replicated 4 × 4 Latin square design, so that upon completion of the study each horse had received each treatment (n = 8 per treatment). All experimental and washout periods were 21 d. Treatments included control (0), 1, 2, or 3 g MRF, top-dressed once daily on a 12% CP oat-based concentrate feed (McCauley Bros, Versailles, KY, USA). Horses had ad libitum access to forage (pasture in summer; grass hay of similar composition produced on farm in winter). Horses had been on the same diet for 60 d prior. Fresh fecal samples were collected on d 1 and 21 of each period, from each horse, and subsets of samples stored at −80°C. Samples were analyzed for secretory IgA (sIgA) using an equine ELISA assay (ICL Lab, Portland, OR, USA) while COSMOSID (Germantown, MD, USA) performed microbiome analysis (shallow shotgun sequencing). Data were analyzed as Latin square using ANOVA (SAS 9.4) with significance at P < 0.05. Changes (d 1 vs. d 21) in fecal sIgA concentrations were greater for the control group, consistent with elevated sIgA, compared with the MRF supplemented groups (P < 0.05). Shannon diversity index was lowest for the control group (P < 0.05), with Lachnospiraceae (P < 0.05) and other butyrate-producing taxa positively influenced by MRF supplementation. In addition, butyrate kinase activity was lower in the control group (P < 0.05). Of further interest was a trend for a decline (P = 0.07) in antibiotic-resistant genes (Sulfonimide trimethoprim) when horses received MRF. Although these responses were seen across all 3 MRF treatments, responses were not always linearly correlated with MRF dose, taking on a quadratic type of response. Pooling treatment data, however, maintained similar responses. High levels of sIgA can be associated with an inflammatory response. Butyrate is known to contribute to gastrointestinal cell health. These findings highlight the potential of MRF to support hindgut health in a group of healthy, unchallenged horses.