How the Horse Digestive System Works:

Hindgut Fermenters

Horses have evolved with the specialised ability to utilise a range of feedstuffs. The hindgut (cecum and colon) constitutes 60% of the horse digestive system and is the primary site for microbial fermentation of fibrous plant material.

Unlike the human stomach, the equine stomach has a limited digestive and absorptive capacity. Therefore, problems arise when there is rapid fermentation of non-structural carbohydrates (NSC’s), in particular, starch and sugars. Cereal grains such as oats, barley and corn contain a high starch content which is able to be rapidly utilised as a source of energy, however, when consumed in large quantities and not adequately digested, there is a possibility of rapid fermentation following dumping into the hindgut.  This subsequently causes proliferation of acid producing bacteria and a drop in hindgut pH (causing the hindgut to become more acidic, which is not favourable for the resident microflora population).

Signs of Hindgut acidosis:

  • Laminitis/founder
  • Colic
  • Behavioural stereotypes e.g. cribbing and weaving
  • Increased incidence of dehydration
  • Reduced appetite and performance
  • Poor feed efficiency
  • Loose, frothy faeces

Causes:

  • Excessive fermentation of high starch grains within the hindgut.  This commonly occurs when grains are provided in excess and do not undergo adequate digestion ie; high volume ration consumed quickly.
  • Insufficient roughage provision, resulting in reduced residence time and limited pre-caecal digestion (digestive process prior to hindgut fermentation).
  • Gastric ulcers
  • Rapid changes in concentrate rations.
  • Access to lush pasture containing high levels of soluble sugars/fructans

Prevention:

  • Implement gradual changes in the ration.
  • If using grains, consider pre-caecal starch digestibility (Table 1).
  • Using alternative sources of energy such as Grand Prix Oil®, rice bran or super fibres e.g. soy hulls, beet pulp or high protein roughage.
  • Feed little and often where possible to maximise the digestive capacity of the stomach. Horses have evolved as grazing animals with the capacity to digest small, frequent meals.
  • Minimum 1% body weight in roughage should be provided daily in order to slow the rate of passage through the gastrointestinal tract and increase pre-caecal digestibility of starch based grains
  • Restrict access to lush, cool season pastures particularly during the afternoon when fructans/soluble sugars are highest

Table 1: Starch composition and digestibility in the horse digestive system

Grain

Digestible Energy (MJ/Kg) Starch (%) Pre-caecal Starch Digestibility (%)
Oats 11.4 44.3

94.9

Barley 12.8 53.9

70.5

Corn 14.1 63.8

66.6

 

Source; AustbØ, D. Rosenfeld, I. Digestion of cereals in the equine gastro-intestinal tract measured by the mobile bag technique on caecally cannulated horses.  Animal Feed Science and Technology 150 (2009) pp.249-258. NRC, 2007. Nutrient Requirements of Horses, 6th ed. The National Academies Press,Washington, DC

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