animal health consulting
The amine hypothesis
high-protein diets and behavioural problems in horses
Christine King BVSc, MANZCVS (equine), MVetClinStud
The problematic pasture
Like almost everyone, I assumed that the problem was the high 'sugar' (nonstructural carbohydrate) content of the lush grass.
Nonstructural carbohydrates (NSC) include starch, simple sugars, and other rapidly fermentable carbohydrates (oligosaccharides and fructan polysaccharides).
So imagine my surprise when we tested the pasture in May (autumn), after an episode of abnormal behaviour, and found that its 'sugar' content was much lower than when the same fields were cut for hay in December (summer).
The results were largely unchanged when we retested the pasture in August (winter). We decided to retest in August because, despite the changes we'd made to the horses' diet in response to the May results, the sporadic episodes of abnormal behaviour continued, when in a typical year they all-but disappear in the winter. More on the weather conditions in part 3.
The thing that had changed the most between summer and autumn was the crude protein (CP) content. The CP content of the autumn pasture (29.3%) was almost 5x higher than that of the summer hay (6.2%). And the CP content had decreased only slightly from autum to winter (26.8%).
BTW, these figures are all on a 100% dry matter (DM) basis. That is, the sample was dried before analysis to remove all of its water. That's essential when comparing feedstuffs with very different water or moisture contents. The water content of a well-cured hay is 10% or less, whereas lush grass may be more than 80% water.
Figure 1 (below) shows the major changes in the pasture's nutrient profile (on a DM basis) from late December, when the mature grass was cut for hay, to May, and then to August. We will test again in the spring, so stay tuned.
Table of Contents
2. The problematic pasture
5. The (failed) amine–laminitis hypothesis
6. The amine–behaviour hypothesis
© Christine M. King, 2026. All rights reserved.
First published on 31 August, 2026. Last updated 08 September, 2026.
Figure 1. Changes in the principal nutrient classes in pasture grasses between summer and winter on a 16-acre farm in Gembrook, Victoria, 2025–2026. The farm holds a small flock of sheep and two horses. Its fields are cut for hay in late December (summer) and are otherwise grazed for most of the year. Each coloured bar represents a separate nutrient class, the proportions of each adding up to 100% of the dry matter in the sample.
'Sugars' is an overly simple category when it comes to equine nutrition, and particularly to the various medical conditions associated with high-NSC diets, so let's tease it apart.
Figure 2 (below) shows the changes in the various components of the NSC on these pastures over the same three seasons.
Figure 2. Changes in the various 'sugars' (rapidly fermentable carbohydrates) in the pasture grasses on the farm represented in Figure 1. NSC, nonstructural carbohydrates (total); WSC, water-soluble carbohydrates (NSC – starch); ESC, ethanol-soluble carbohydrates (simple sugars); OF, oligosaccharides such as oligofructan.
So, if these horses' erratic behaviour is caused by the high 'sugar' content of their pastures, then we should see these abnormal behaviours concentrated in the summer months, not in the autumn and spring (and this year in the winter as well).
High-starch diets are known to increase in behavioural reactivity in horses.[1,2] However, there is no comparable research on high-protein diets in horses, and the research on dietary protein and behavioural reactivity or aggression in dogs is inconsistent and often confounded by high levels of starch in the test diets.[3]
So, what is causing episodes of abnormal behaviour in these two mature and well-handled horses when the grass is lush?
The largest change in nutrient composition is in crude protein, so could it be the high protein content of the lush grass compared with when the same grass, when mature, is cut for hay?
Before I posit some answers to those questions, I need to examine another: Why is the abnormal behaviour episodic (inconsistent)? It appears to be weather-related, but in what way?
That's the subject of the next section.
. . .
References
[1] Bulmer L, McBride S, Williams K, et al. The effects of a high-starch or high-fibre diet on equine reactivity and handling behaviour. Applied Animal Behaviour Science, 2015; 165: 95–102.
[2] Bulmer LS, Murray JA, Burns NM, et al. High-starch diets alter equine faecal microbiota and increase behavioural reactivity. Scientific Reports, 2019; 9: 18621.
[3] King C. Diet Matters. In The Highly Sensitive Dog, second edition. Anima Books, Emerald VIC, 2025; pp. 119–160.