animal health consulting
The amine hypothesis
high-protein diets and behavioural problems in horses
Christine King BVSc, MANZCVS (equine), MVetClinStud
What do we know?
2. Surplus dietary protein likewise spills over into the hindgut.
Dietary protein in horses has not received a lot of attention when it comes to the gut microbes and gut health, but we do know a couple of useful things, thanks to a couple of recent studies.
A 2023 study showed that the amount and source of crude protein (CP) in the horse's diet affects the composition of the microbial community in the hindgut.[15]
The three test diets were grass hay only, and hay + concentrate at two different CP levels (low and high). All three diets provided well over 100% of the recommended daily intake of CP for light exercise.[13] In fact, they provided between 163% and 230% of the recommended daily intake, the hay-only diet supplying 176% of requirement.
For some context, the diet my two neighbour horses were on in May supplied an estimated 465% of requirement, just from the grass alone. That is not a typo; almost 5x the daily requirement.
The study showed that even moderate excesses of dietary protein altered the microbial community in the hindgut, and not necessarily for the better. For example, abundance of the phylum Proteobacteria increased with crude protein intake. This phylum (huge, umbrella category) includes E. coli, Salmonella, and Pseudomonas. These bacteria may all cause endotoxaemia (systemic inflammatory response syndrome) under the right circumstances.
So, what is the threshold for spillover of dietary protein into the hindgut? Evidently, it is less than 163% of requirement. And evidently, a forage-only diet can do it.
. . .
Here is further evidence that grass alone can result in spillover of dietary protein into the hindgut.
A study published this year (2026) had some interesting details in its supplementary materials.[16] This study involved 22 horses kept on pasture at the University of Queensland's research facility in Gatton QLD. The study ran from December one year to October the next. Faecal (manure) samples were collected from each horse every two months, for a total of 6 samples per horse. Other than pasture, the horses were fed lucerne (alfalfa) hay free-choice during the winter months.
Here is what the data look like for faecal protein content, each coloured dot representing a single horse:
Table of Contents
5. The (failed) amine–laminitis hypothesis
6. The amine–behaviour hypothesis
7. What do we know?
© Christine M. King, 2026. All rights reserved.
First published on 31 August, 2026. Last updated 08 September, 2026.
This section continues on the next page. Click on the 'Read on...' link below the Table of Contents.
. . .
Reference
[15] Graham-Thiers P, Bowen L. Relationships between feed protein fractions and the hindgut microbiome in the exercising horse. Conference abstract, Equine Science Society Proceedings. Journal of Equine Veterinary Science, 2023; 124: 104386.
[16] Wang W, Nitert MD, Gobius S, et al. Horses with pituitary pars intermedia dysfunction have an altered fecal short-chain fatty acid profile. American Journal of Veterinary Research, 2026; 18: 1–7.
[17] AussieGRASS historical data for the Lockyer Valley QLD, August 2022–November 2023. The Long Paddock, a Queensland Government initiative. https://data.longpaddock.qld.gov.au/TimeSeriesGraphs/lga/QLD/LockyerValleyRegional_QLD.txt. Accessed 25 August, 2026.
Figure 7 (b). Faecal protein content by month in 22 horses on pasture full-time. Each horse is represented by a different coloured line. Shaded: the horses were also offered lucerne (alfalfa) hay free-choice during the winter months.[16]
Crazy, no? Reminds me of playing pickup sticks as a kid.
Not only are no two horses alike, but the faecal protein content in most horses bounced around from month to month (well, at 2-monthly intervals). And when some horses had an increase in faecal protein, others had a decrease. If you can see a pattern here, please share it with me!
Although the crude protein of the pasture and hay were not measured, this study inadvertently suggests that dietary protein does indeed spill over into the hindgut in horses on pasture, as quite large amounts of protein were measured in the faeces of some horses in some months.
If we were to use a completely abribrary threshold of 1,000 µg/ml, three horses in February (summer) and nine horses in October (spring) exceeded this figure, which represents 1 gram of protein per litre of manure. Add up how many litres or kilos of manure you lift each time you clean up after your horse, and that gets to be quite a lot of excreted protein. And that's just from the pasture.
. . .
Figure 7 (a). Faecal protein content by month in 22 horses on pasture full-time. Each horse is represented by a different coloured dot. Shaded: the horses were also offered lucerne (alfalfa) hay free-choice during the winter months.[16]
There is tremendous individual variation among horses, even within the same month. These horses were all grazing the same pasture throughout, and they all had access to lucerne hay in the winter. And yet values for faecal protein over the year ranged from a low of 7.6 µg/ml to a high of 2,644 µg/ml — a 348-fold difference.
In February alone, there was a 120-fold difference among horses (16–1,921 µg/ml). Southeast Queensland gets most of its annual rainfall in the summer. In the study year (Dec 2022 to Oct 2023), December and January had a cumulative total of 171 mm (6.7 in) of rain in the Lockyer Valley, where UQ's equine research unit is located. Daytime highs in January and February averaged around 31 C, and overnight lows around 17.5 C. [17]
Owing to a very wet spring in 2022, total standing biomass (amount of pasture) peaked in December, and so there was abundant grass through July of the study year.[17] So, there was plenty for these horses to eat in February of that year.
And here is what those data look like when each horse is represented by a single coloured line:
