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  • Does withholding forage feed affect the faecal bacterial microbiota in healthy horses?

    Changes in faecal bacteria populations are associated with gastrointestinal disease in a range of species. Horses presenting with colic have been shown to have a decreased diversity and species richness of faecal and colonic bacterial populations, compared to healthy controls1. Horses with chronic colic (longer than 60 hours) have a decreased diversity of faecal bacteria compared to horses with acute colic (less than 60 hours)2.

    The abundance, species richness and population diversity of the faecal bacterial microbiota in healthy mares allowed free access to feed, or with feed withheld for 24 hours, was examined in a recent study3. The study aimed was to identify if alterations in equine faecal microbiota were a consequence of withholding feed (as is common in colic management), as the effect of feed restriction alone on microbiota changes had not been previously examined.

    Eight mares of varied breeds and aged between 14-23 years old were involved in a cross-over trial of two study periods, each of three days duration, separated by a two-week rest period, meaning each horse acted as their own control. Horses were randomly allocated to a ‘fed’ (control, n=4) or ‘feed withheld’ (n = 4) group.

    On day 1, horses were collected from pasture at 8 am (time 0) and stabled for the remainder of the study. Rectal faecal samples were collected every 6 hours from time 0, and both groups had free access to water and forage. On day 2, the ‘feed withheld’ group were muzzled, and forage removed for 24 hours (but with free water access). ‘Control’ horses had free access to water and forage. Every two hours, faecal samples were collected from all study horses. During the ‘feed withheld’ period, all study horses were fed 170g of concentrate feed every 6 hours for humane and equivalence reasons . On day 3, forage was reintroduced at 8 am to the ‘withheld’ group and two-hourly faecal collection was continued for 12 hours, after which, faeces were then collected at 6-hour intervals until study conclusion 12 hours later, when horses were returned to pasture and permitted a two-week ‘rest’ period before the study was repeated with crossing over of the study horses. Composition of the faecal bacterial microbiome was determined by extracting and sequencing bacterial genetic material to determine diversity, relative abundance, and species richness.

    Results indicate that withholding feed (especially after 10 hours) significantly impacts on the composition and diversity of equine faecal microbiota. Within 24 hours of feed being reintroduced, the faecal microbiome showed signs of restoration to normal composition. There was no effect of stabling horses on the faecal microbiota (time 0; P = 0.0424). Faecal microbiota was not affected by day/night conditions (P = 0.165). Horses in the ‘feed withheld’ group had a significantly different composition of their faecal bacterial community when compared to horses fed as normal (P < 0.05). Horses with withheld feed had a reduction in bacterial species number and a less diverse population composition. Study data suggests that withholding feed can significantly impact the equine faecal microbiota however the clinical relevance of this is unknown. Feed restriction should be acknowledged in considering studies relating to the microbiome composition.


    1. Stewart HL, Southwood LL, Indugu N, Vecchiarelli B, Engiles JB, Pitta D. Differences in the equine faecal microbiota between horses presenting to a tertiary referral hospital for colic compared to an elective surgical procedure. Equine Vet J. 2019;51(3):336–42.

    2. Stewart HL, Pitta D, Indugu R, Vecchiarelli B, Engiles JB, Southwood LL. Changes in the fecal microbiota during hospitalization of horses with colic and the effect of different causes of colic. Equine Vet J. 2020. Available HERE

    3. Willette, J.A., Pitta, D., Indugu, N. et al. Experimental crossover study on the effects of withholding feed for 24 h on the equine faecal bacterial microbiota in healthy mares. BMC Vet Res 17, 3 (2021). Available HERE

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