Haylage is cut grass that is baled before completely drying, usually within a few days, and then wrapped in plastic to eliminate oxygen. Natural fermentation occurs, turning the grass to haylage and retaining up to 90% of the feed value of the grass.
The wrap must not be pierced to allow oxygen in, which would then cause mould to form.
Haylage has a much higher feed value than hay, with a considerably higher level of protein and calories, and is therefore not suitable for fat ponies.
Because of the higher feed value, you do not need to feed as much haylage as hay, but because of the higher moisture content, it is much heavier than hay. Therefore, if you are going by weight, a 10kg hay net of hay will be much larger than a 10kg net of haylage, but will contain less feed value.
Haylage should not smell yeasty or burnt, and it should contain no heat as this could easily cause colic.
As with hay, it is essential to look through your haylage to see what grasses are in there. Good grasses are found in the hay section. It is very important that you also check for ragwort or other poisonous plants, as they have been cut and therefore they will be more palatable for your horse, and could cause colic and even death.
Once opened, you should use your haylage in about 3-5 days in the summer, a little longer in the winter, this is because it will start to ferment again. If it does, it will smell yeasty and feel warm, in which case you should not feed it.
Final Thoughts
As with anything you feed your horse, it is essential you understand its value and its pros and cons.
You are responsible for making sure what you feed is good for your horse, and not going to harm him in any way.