Horses and ponies showed similar abilities to digest nutrients in an Australian study, suggesting that the trend for “easy keeper” breeds to put on fat reflects metabolic differences.
Researchers in a University of Melbourne study, writing in the journal Veterinary Science, conducted a comparison of feed digestibility between ponies, Standardbreds and Andalusian horses fed three different diets.
Samantha Potter and her fellow researchers said ponies and some horse breeds, such as Andalusians, tend to be “easy keepers” and tend to become obese more readily than other breeds such as Standardbreds.
Various reasons have been proposed, including differences in appetite or metabolic efficiency.
A likely explanation for the tendency toward increased fat deposits in these breeds is the observation of increased insulin responses to oral non-structural carbohydrates, even when in moderate body condition, as insulin is known to promote fat deposition.
However, it is also possible that rates of weight gain and changes in body condition may be attributed to differences in metabolic efficiency or nutrient digestibility between breeds.
The mixed-breed ponies, Standardbreds and Andalusian horses used in the study numbered 33 in all, comprising 11 of each.
The animals were divided into three groups, with nine adapted to consuming a control fibre-based diet, 12 put on a high-calorie cereal-rich diet, and 12 given a high-calorie fat-rich diet over 20 weeks.
Total faecal collection was performed over 24 hours to determine apparent total tract digestibility of gross energy, dry matter, neutral detergent fibre, starch, crude protein and crude fat.
The study team found that breed had no effect on apparent digestibility for any of the nutrients studied.
However, there was a significant effect of diet, with the horses consuming the cereal-rich or fat-rich diets demonstrating higher digestibility of gross energy, dry matter, neutral detergent fibre and crude protein compared with those on the control diet.
The horses adapted to the cereal-rich diet demonstrated higher digestibility of starch, and animals adapted to the fat-rich diet demonstrated higher digestibility of fat.
In conclusion, the study found that horses and ponies had similar nutrient digestibility when adapted to the same diets and management conditions.
“The tendency towards increased adiposity in ponies and Andalusian-type horse breeds is more likely to reflect differences in metabolism, rather than differences in feed digestibility,” they said.
Discussing their findings, the authors said the findings failed to support their hypothesis that ponies and Andalusian horses would have higher digestibility of nutrients compared to Standardbreds when adapted to the same diets.
They acknowledged that limitations of their study included the relatively small number of animals from each breed per diet group, and the short period of total faecal collection. “The possibility of differences in nutrient digestibility between equine breeds, in general, cannot be discounted on the basis of our findings,” the researchers said.
“However, the animals studied were well characterised regarding their metabolic traits, which did show clear differences between breeds, despite the absence of detectable differences in apparent digestibility.”
The authors said the equine digestive tract, remarkably, seems to adapt very well to diets that contain large amounts of fat/oil.
Previous research has shown that in most equine diets with added fat/oil, the apparent digestibility for the added fat may be in the order of 95 to 100%.
Potter and her fellow researchers said their findings show that even the significant quantity of dietary fat given to the horses in the fat-rich group had little adverse effect on the apparent digestibility of the other nutrients evaluated.
They cautioned that high-fat diets have the potential to reduce mineral absorption due to the formation of mineral soaps in the small intestine, but few studies have examined this in equids.
Cereal grains are common feedstuffs used in equine rations to cost-effectively increase the energy density of the diet, with the most common grains fed being oats, maize, and barley.
The presence of undigested starch in the caecum, where it can undergo microbial fermentation, can result in hindgut acidosis, hindgut dysfunction, colic, and laminitis.
They said starch digestion was close to 100% in the cereal-rich diet group, but about 80% in the control and fat-rich diet groups. This, they said, was somewhat surprising since it would be expected that any starch escaping digestion in the small intestine would be subjected to microbial fermentation in the large intestine.
They noted that the starch levels were very low in the fat-rich complimentary meals, and also relatively low in the control complimentary meals, all provided by soya bean hulls, chaff and hay. Some of this starch may have been either non-fermentable or unavailable to the hindgut microbes, which conceivably accounts for the 80% figure.
The study team comprised Potter, Nicholas Bamford, Courtnay Baskerville and Simon Bailey, all with the University of Melbourne; and Patricia Harris, with the Waltham Petcare Science Institute in England.
Potter, S.J.; Bamford, N.J.; Baskerville, C.L.; Harris, P.A.; Bailey, S.R. Comparison of Feed Digestibility between Ponies, Standardbreds and Andalusian Horses Fed Three Different Diets. Vet. Sci. 2022, 9, 15. https://doi.org/10.3390/vetsci9010015