Autumn Hormones and Horse Behaviour: What the Science Says

Why your horse may seem different this autumn

Quick answer: As days shorten, horses go through a predictable hormonal shift. Rising melatonin, a natural surge in the stress-axis hormone ACTH that peaks in late September and early October, changes in insulin sensitivity and, in mares, the end of the breeding season all alter how a horse feels. Behaviour is often the first place those internal changes show.

If your horse has seemed quieter, grumpier, more food-focused or less keen to move in recent weeks, it may not be a training issue at all. Autumn is one of the most physiologically active times of the equine year. The research explains a great deal of what owners notice, and it points to where extra attention matters most.

This article draws on peer-reviewed equine endocrinology and ethology to explain what is happening under the coat, and how to read it through behaviour.

How do horses know the season is changing?

Horses read day length, and they read it through hormones. The pineal gland releases melatonin during darkness, so longer nights mean a longer nightly melatonin signal. That signal acts on the hypothalamus and pituitary and sets the timing for reproduction, coat growth and metabolism across the year (Guerin et al., 1995).

Temperature, forage quality and weather then layer on top of this light-driven clock. The result is a horse whose internal state in late September differs meaningfully from the same horse in June, even when routine, feed and handling have not changed.

The autumn ACTH rise: why late September matters

Every horse and pony experiences a seasonal rise in adrenocorticotropic hormone (ACTH), the pituitary hormone that signals the adrenal glands to release cortisol. It was first documented in healthy ponies and horses in Pennsylvania (Donaldson et al., 2005). A year-long study later found ACTH was the only hormone measured with a clear seasonal pattern, often sitting above the standard reference range from August to October in healthy horses (Place et al., 2010).

UK data are especially precise. Monthly reference intervals from the Liphook Equine Hospital showed August to October samples differ significantly from the rest of the year (Copas and Durham, 2012). Follow-up analysis of more than 35,000 horses and ponies suggested the rise begins shortly after the summer solstice and peaks in the last week of September or the first week of October (Durham, 2014). In other words, many horses are at or near their annual ACTH peak right now.

For most healthy horses, this is a normal, adaptive change. For horses in the early stages of pituitary pars intermedia dysfunction (PPID), the rise can be exaggerated well beyond the usual range. Research comparing PPID horses with healthy controls also found lower dopamine output in the PPID group in summer and autumn (Haritou et al., 2008). Dopamine is central to motivation and reward, so this is worth bearing in mind when an older horse becomes noticeably flatter or less engaged at this time of year.

The direct effect of the normal ACTH rise on behaviour in healthy horses has not been well studied, so it is best treated as context rather than an explanation in itself. Its knock-on effects on metabolism, however, are well established.

Autumn metabolism, insulin and laminitis risk

Autumn is a second high-risk window for laminitis, not only spring. Cortisol, driven by ACTH, reduces insulin sensitivity, and horses with insulin dysregulation or early PPID are least able to absorb that extra load. Oral sugar testing across the year has shown seasonal shifts in insulin responses in both healthy and insulin-dysregulated horses (Macon et al., 2022).

Grass adds another layer. A 2026 study of Hucul mares found pasture sugars (water-soluble carbohydrates) fell to their lowest in October, but starch was far higher in October than in May. Mares with a history of laminitis had significantly higher glucose and fructosamine than controls in October (Mlyneková et al., 2026). Lower sugar does not automatically mean lower risk.

Why this matters for behaviour: early laminitic pain rarely looks dramatic. It more often shows as subtle changes before any obvious lameness:

• shifting weight, or standing with a foot pointed or rested more than usual
• reluctance to walk on hard or stony ground, or to turn tightly
• lying down more, or being slow to get up
• irritability when feet are picked up, or tension around the farrier
• a shorter fuse with herd mates, or withdrawal from the group

These are easy to misread as attitude or a training problem. Reading them as possible pain signals first is both kinder and safer.

Mares in autumn: the end of the breeding season

Mares are long-day breeders. As nights lengthen, the longer melatonin signal suppresses the reproductive hormones GnRH and LH, and ovarian activity winds down. Around the autumnal equinox, roughly 85% of horse mares and over 90% of pony mares stop cycling, and most have stopped entirely by the winter solstice (Williams et al., 2012).

The weeks either side of that shift are a transition, not a switch. Follicle activity becomes irregular, and oestrous behaviour can still appear even when a mare is no longer ovulating (Coelho et al., 2023), so some mares seem erratic for a few weeks before settling into winter anoestrus. Owners may notice:

• temporary changes in tolerance of other horses, particularly geldings
• sensitivity around the flank or girth that comes and goes
• restlessness or attention to neighbouring fields
• a noticeably calmer, more settled mare once anoestrus is established

Where a mare's behaviour changes sharply or does not follow this pattern, a veterinary reproductive check is worthwhile. Ovarian and uterine conditions, as well as pain elsewhere, can look very similar to seasonal behaviour.

Why horses slow down and eat more as winter approaches

Horses carry an ancient seasonal energy strategy. In free-ranging Takhi horses, heart rate (a measure of metabolic rate) roughly halved from a spring peak of 89 beats per minute to 44 in December and January (Arnold et al., 2006). The same research group found food intake peaked in autumn, building reserves before winter (Kuntz et al., 2006).

This is not only true of wild equids. Shetland ponies kept outdoors showed clear seasonal changes in behaviour and physiology, including signs of winter hypometabolism, a lowered heart rate and body-surface temperature that conserve energy (Brinkmann et al., 2012). Free-ranging Takhi (Przewalski's) horses also showed lower heart rate variability in winter, consistent with a physiological shift when resources are scarce (Pohlin et al., 2017).

What owners often notice:

• more time spent eating, and stronger food motivation
• less spontaneous play and movement, more standing
• more time dozing in sheltered spots
• slower responses to requests that took little effort in summer

Closer to home, a study of Icelandic horses living out on Shetland found they foraged for about 12.7 hours a day in winter against 17 in spring, stood far more, and lost body condition over winter (Taylor et al., 2025). These are healthy adaptations, not laziness. They matter for welfare because a horse conserving energy has less to spare, and asking for the same output as in July may cost more than it appears.

What to do this autumn: a practical checklist

The aim is not to override the seasonal shift but to work with it, and to catch the changes that are not seasonal at all.

1. Know your horse's baseline. Seasonal change is only visible against a record of what normal looks like. Note appetite, water intake, movement, lying behaviour, social interactions and responses to handling, week by week.
2. Treat new grumpiness as a question, not a verdict. Ask first whether pain, especially foot pain, could be involved.
3. Talk to your vet about autumn testing. UK data suggest late summer and autumn are useful months to test ACTH for PPID when interpreted against seasonal reference ranges (Copas and Durham, 2012). Older horses, and any with a history of laminitis, are the priority.
4. Watch grazing, not just grass sugar. Autumn flushes, frosty mornings and seed heads can all shift what a horse takes in. Horses with insulin dysregulation need the most careful management.
5. Adjust expectations of work. Energy conservation is adaptive. Shorter, lower-demand sessions often suit this time of year better.
6. Protect choice and social contact. Shelter, forage and companions that the horse can choose between help them manage their own changing needs, which supports positive welfare as well as physical health.
7. Keep an eye on mares in transition. Note any cyclical pattern, and seek a veterinary opinion if behaviour is intense, persistent or out of character.

Seeing the season through your horse's eyes

Every point in this article comes back to one skill: noticing change against a known baseline. Seasonal hormones shift gradually, and the behavioural signs are often small enough to miss day to day.

The Equine Voice App was built for exactly this. It brings welfare observations together into a single wellbeing picture over time, so a gradual autumn slide in comfort, engagement or social behaviour becomes visible as a trend rather than a vague feeling. That makes it easier to tell a normal seasonal slowdown from something that needs a closer look, and easier to share clear information with your vet or behaviourist sign up today at Equine Voice.

If this season has raised questions about your own horse, the free Beyond Training live Q&A is a relaxed place to bring them. Sessions run every fortnight online, with time to talk through real horses and real behaviour, and a look at how Equine Voice and our Learning Hub can support what you are seeing. Save your place at the next live Q&A.

Frequently asked questions

Why is my horse grumpy in autumn?
Shortening days trigger hormonal changes, including a natural rise in ACTH that peaks in late September and early October. These can affect metabolism and comfort. Autumn is also a laminitis risk period, so new irritability should prompt a check for pain first.

When does the seasonal ACTH rise happen in the UK?
UK data suggest ACTH starts rising soon after the summer solstice, with the highest levels in the last week of September and first week of October, then declining.

Is autumn laminitis linked to hormones or grass?
Both. Cortisol reduces insulin sensitivity, and autumn pasture can still carry meaningful starch even when sugars are lower. Horses with insulin dysregulation or PPID are at greatest risk.

Do mares behave differently in autumn?
Often, yes. Most mares stop cycling between the autumn equinox and the winter solstice. The transition can bring irregular oestrous behaviour before they settle.

Is it normal for my horse to be less energetic as winter approaches?
Yes. Horses naturally lower their metabolic rate and activity to conserve energy in winter. Sudden or marked lethargy, especially in an older horse, is still worth discussing with your vet.

References

• Arnold, W., Ruf, T. and Kuntz, R. (2006) Seasonal adjustment of energy budget in a large wild mammal, the Przewalski horse II. Energy expenditure. Journal of Experimental Biology, 209, 4566–4573. doi:10.1242/jeb.02536
• Brinkmann, L., Gerken, M. and Riek, A. (2012) Adaptation strategies to seasonal changes in environmental conditions of a domesticated horse breed, the Shetland pony. Journal of Experimental Biology, 215, 1061–1068. doi:10.1242/jeb.064832
• Coelho, L.A. et al. (2023) Seasonal variation of melatonin concentration and mRNA expression of melatonin-related genes in developing ovarian follicles of mares. Animals, 13(6), 1063. doi:10.3390/ani13061063
• Copas, V.E.N. and Durham, A.E. (2012) Circannual variation in plasma adrenocorticotropic hormone concentrations in the UK in normal horses and ponies, and those with pituitary pars intermedia dysfunction. Equine Veterinary Journal, 44, 440–443.
• Donaldson, M.T. et al. (2005) Variation in plasma adrenocorticotropic hormone concentration and dexamethasone suppression test results with season, age, and sex in healthy ponies and horses. Journal of Veterinary Internal Medicine, 19, 217–222.
• Durham, A.E. (2014), weekly ACTH data from over 35,000 equids, summarised in Liphook Equine Hospital: Seasonal Changes in ACTH Secretion
• Guerin, M.V. et al. (1995) Plasma melatonin in the horse: measurements in natural photoperiod and in acutely extended darkness throughout the year. Journal of Pineal Research, 19, 7–15. doi:10.1111/j.1600-079X.1995.tb00165.x
• Haritou, S.J.A. et al. (2008) Seasonal changes in circadian peripheral plasma concentrations of melatonin, serotonin, dopamine and cortisol in aged horses with Cushing's disease under natural photoperiod. Journal of Neuroendocrinology, 20, 988–996. PMC2658710
• Kuntz, R. et al. (2006) Seasonal adjustment of energy budget in a large wild mammal, the Przewalski horse I. Energy intake. Journal of Experimental Biology, 209, 4557–4565. doi:10.1242/jeb.02535
• Macon, E.L., Harris, P., Barker, V.D. and Adams, A.A. (2022) Seasonal insulin responses to the oral sugar test in healthy and insulin dysregulated horses. Journal of Equine Veterinary Science, 113, 103945. doi:10.1016/j.jevs.2022.103945
• Mlyneková, E. et al. (2026) The effect of seasonal changes in non-structural carbohydrates in pasture on the metabolic profile of horses with laminitis. Animals, 16(2), 267. doi:10.3390/ani16020267
• Place, N.J. et al. (2010) Seasonal variation in serum concentrations of selected metabolic hormones in horses. Journal of Veterinary Internal Medicine, 24, 650–654. doi:10.1111/j.1939-1676.2010.0500.x
• Pohlin, F. et al. (2017) Seasonal variations in heart rate variability as an indicator of stress in free-ranging pregnant Przewalski's horses. Frontiers in Physiology, 8, 664. doi:10.3389/fphys.2017.00664
• Taylor, D.E.F., Lancaster, B.E. and Ellis, A.D. (2025) Time budgets in domesticated male Icelandic horses on pasture turnout in winter and spring. Animals, 15(21), 3206. doi:10.3390/ani15213206
• Williams, G.L. et al. (2012) Reproductive seasonality in the mare: neuroendocrine basis and pharmacologic control. Domestic Animal Endocrinology, 43(2), 103–115.