What is in it? There are no GM feeds grown in the UK at present, although I think the EU has allowed some GM maize to be grown this year in European countries.
The main GM crops are soya/cotton/maize and many countries, from the USA to Asia have been growing and eating them for years. (not cotton obviously!)
Horse feeds do sometimes contain soya, but if so it usually says if it is GM free, not that I would personally be worrried about it anyway. The USA population has been eating GM and hormone fed animals and it hasn't shown to do any harm.
PS - in the EU beef and sheep don't have artificfial hormones, not sure about pigs (being phased out) and poultry.
they have to state on their labels if the product contains anything that might have come from a GM source. Chances are that soya and soya oils are from GM or potentially GM sources, but not much else in this country.