The horses hindgut
The horses hindgut is a complex section of the digestive system, it is nearly 30ft in length, comprising of the cecum, small and large colon and the rectum (figure 1). It is responsible for many critical processes within the body, most significantly the breakdown of fibre.

Converting the fibre into a viable energy source, which is absorbed through the gut wall and used by the horse. This process is driven by the action of multiple types of bacteria housed within the hindgut, which are collectively known as the gut biome. The ideal number, ratio and balance of these different bacteria types are critical for the health status of a horse. A healthy gut biome supports a horses overall health, assisting the maintenance of an optimal body condition, driving the effectiveness of the immune system, regulating the inflammatory processes, producing certain vitamins, and even helping to regulate a horses behaviour and optimise it’s performance [1,2].

Figure 1: The horses hindgut
(https://gustavomirabalcastro.online/en/health/the-horses-digestive-system/)

One of the golden rules of feeding horses is to make any changes to a diet, including forages, gradually with a transition phase. A transition phase involves reducing the amount of the current diet consumed while increasing the amounts of the new diet in stages. Regarding bucket or hard feeds this transition phase should take place over one week, but regarding forages this should be extended over a period of two weeks. Changes to forage sources often requires a process of gradually reducing the amount of grass consumed, by reducing grazing hours, while introducing or increasing the amount of dry forages (hay or haylage) by increasing the hours a horse is stabled. A transition phase affords the gut biome the essential time required to adapt and continue to thrive. Failure to comply with a sufficient transition phase can result in the accumulation of certain bacteria, the production of lactic acid within the caecum and colon, the destruction of the beneficial bacteria, irritation, inflammation and damage to the gut wall. The consequences of which can have widespread implications, including [1,3];

• Loss of body condition
• Reduction of appetite
• Decreased rates of healing
•.Increased risk of colic
• Increased risk of infections
• Diahroea
• Increased stereotypical behaviours, such as box walking
• Increased flight responses, such as spooking or being reluctant to interact with an unknown person e.g. vet.

Box rest and the hindgut
Unfortunately in certain circumstance other aspects of caring for a horse may need to take immediate priority over a transition phase. One such circumstance is box rest, which is usually the result of an injury or illness. When a clinical condition takes priority, changes to management, and diet may need to occur with the complete absence of any transition phase. In many situations the forage source changes immediately from grass to hay or haylage, and depending on the hard feed portion of the ration this may also require immediate changes to reduce the energy provided. If a hard feed has been appropriate for higher exercise levels, intensities and maintaining body condition, it could result in excitable behaviours or excessive weight gains during a period of box rest. Neither of which are conducive to healing, rehabilitation, or managing an ill or injured horse.

Supporting the gut biome during box rest
Supporting a well-balanced gut biome is of vital importance for horse health, especially during periods of injury, disease, stress, or sudden changes of management [1]. One method of supporting a healthy gut biome , which research has validated, is the use of pre and probiotics [4].

Probiotics
Probiotics are the live micro-organisms that have the capacity to colonize the gut and support the diversity and function of the gut biome, such as yeast. There are several types of yeast, but in relation to the horse, a yeast called Saccharomyces cerevisia is the most widely researched and demonstrated to be effective [4].

Saccharomyces cerevisia
The supplementation of Saccharomyces cerevisia has been shown to restore the gut biome balance and increase a horses ability to manage changes [1;5;6;7]. This yeast is able to scavenge oxygen present within the digestive tract, and in doing so creates an environment more suitable for the beneficial, anaerobic bacteria. Potentially mitigating some of the stress impact from box rest [7;8].

Prebiotics
Prebiotics are not living microorganisms, rather they provide an available food source for the beneficial bacteria. Helping to ensure they are maintained in sufficient numbers and have the stability required to support hindgut health. Two prebiotics recognised as beneficial in the horse are Fructooligosaccharides (FOS) and Mannanoligosaccharides (MOS).

FOS
Supplementation with FOS is known to reduce disruptions to the gut biome during periods of stress and negate some of the negative impacts of changes to feed and forages, even reducing the risk of colic [5;9]. Improving the health and welfare of a horse during challenging periods [9].

MOS
The supplementation of MOS is associated with supporting and improving indicators for health, and recovery. As well as reducing oxidative stress and inflammatory effects, which are common in disease and injury processes [10].

Summary
Even with careful management injury and illness that requires immediate box rest can occur. This can directly impact the health of the bacterial population within the gut, and indirectly impact the healing, recovery and overall health of a horse. The supplementation of suitable pre and probiotics is an evidence based strategy for limiting the impacts of disease, injury and box rest [11].

References

[1] Chaucheyras-Durand, F., Sacy, A., Karges, K., Apper, E. 2022 Gastro-Intestinal Microbiota in Equines and its role in health and disease: the black box opens. Microorganisms (10) 2517
[2] Linderberg,F, Krych,L., Kot,W., Fielden, J., Frokiaer,H., van Galen,G., Nielsen,D.S, Hansen,A.K. 2019 Development of the equine gut microbiota. Sci. Rep. (9) 14427
[3] Mach, N., Ruet, A., Clark, A., Bars-Cortina, D., Ramayo-Caldas, Y., Crisci, E., Pennarun, S., Dhorne-Pollet, S., Foury, A., Moisan, M.P, 2020 Priming for welfare: gut microbiota is associated with equitation conditions and behaviour in horse athletes. Sci. Rep. (10) 8311
[4] Perricone, V., Sandrini, S., Irshad, N., Comi, M., Lecchi, C., Savoini, G., & Agazzi, A., 2022 The role of yeast saccharomyces cerevisiae in supporting gut health in horses: An updated review on its effects on digestibility and intestinal faecal microbiota. Animals 12, 3475
[5] Coverdale, J.A., 2016. Horse species symposium: can the microbiome of the horse be altered to improve digestion? J. Anim. Sci. (94) 2275-2281
[6] Geor, R.J., Harris, P.A., & Coenen, M., 2013 Equine Applied and clinical nutrition. Health welfare and performance. Saunders Elsevier, 575-576
[7] Valigura, H.C., Leatherwood, J.L., Martines, R.E., Norton, S.A., White-Springr, S.H., Dietary supplementation of a Saccharomyces cerevisia fermentation product attenuates exercise-induced stress markers in young horses. J. Anim Sci 99, 199
[8] Newbold, C.J., Wallace, R.J., McIntosh, F.M., 1996 Mode of action of the yeast Saccaromyces cerevisiae as a feed additive for ruminants Br. J. Nutr. 76, 249-261
[9] Respondek,F., Goachet,A.G., Julliand,V. 2008 Effects of dietary short-chain fructooligosaccahrides on the intestinal microflora of horses subjected to a sudden change in diet. Journal of Animal Science. 86 (2) 316-323
[10] Zhao, W., Chen, L., Tan, W., Li, Y., Sun, L., Zhu, X., Wang, S., Gao, P., Zhu, C., Wang, L., Jiang, Q. 2023 Mannan Oligosaccharides promoted skeletal muscle hypertrophy through the gut microbiome and microbial metabolites in mice. Foods, 12, 357
[11] Lyte, M. 2013 Microbial endocrinology in the microbiome-gut-brain axis: How bacterial production and utilization of neurochemicals influence behaviour. PLoS Pathog (9) e1003726