Carrots used to be a well-accepted horse feed but lately they have been getting a bit of a battering, so it seems like a good time to look at this from a more objective viewpoint.
Carrots are from the group of vegetables known as ‘roots’, which are plants that store nutrients in swollen ‘roots’. Carrot roots, like all root veg, have a high water content, so their nutrient content is ‘diluted’. For every 5 carrots you feed, 4 are actually water, and just one is pure carrot. If you look at the nutrient supply on a dry matter basis – which is the equivalent to about a fifth of a carrot, since about four-fifths is water – then carrots are indeed relatively high in sugar. However, fresh – or on an ‘as fed’ basis, they are low calorie and low in sugar and starch. A 128 g carrot contains just 6 g of sugar and 1.8 g of starch.
To compare, a single 1 kg slice of hay might supply between 100 and 200 g of sugar. And this means that 128 g of that hay would supply between 12.8 and 25.6 g sugar. Even the low sugar hay (10%) contains twice the amount of sugar as the same weight of carrot!
Carrots are a good source of water (at about 80%) so are a useful succulent for horses on a dry diet e.g. stabled horses on hay and compound feed. They are also a rich source of betacarotenes, which are vitamin A precursors, and supply good levels of some minerals, B vitamins, vitamin C and vitamin K. Carrots are low in energy (calories) on a fresh basis, because of their high content of water.
There is no reason why you should not feed carrots to ‘fizzy’ horses or those prone to laminitis, provided the rest of the diet is balanced and appropriate and a limited amount of carrots are fed. For healthy horses i.e. those whose sugar does not need to be severely restricted, you can safely feed 1-2 kg of carrots daily.
Carrots can be fed whole or sliced lengthways. They should never be fed sliced widthways (as circles) because they can cause choke in this shape. You can throw carrots around a (clean) stable bed to encourage foraging, and they make a very healthy treat for reward during training.
The fad for carrot-bashing on forums is quite undeserved and reflects a confusion between the nutrient content of a fresh feed versus that on a dry matter basis (i.e. without the water).