the mares reproductive system shuts down during the winter and starts again for the spring and summer.

mares normally show there seasons by becoming fizzy or temperamental. Normally between March and October/November.

Try a supplement such as oestuss by Naf, start around Feb, try it until at least december. 

Yes - I wonder if I have hit on a new phenomena! Mare's late season, seasons having an affect of their behaviour. She was fine in the summer (well from April, May onwards when I took her on). Yes, snatching at the bit, jogging, sweating up, rushing, not standing still too. Drove me mad and really scared me on occasions! About November she reverted back to normal, phew. I might have to consider some kind of herbal/hormonal treatement next year?