Hay and silage – what’s the difference?


Large round bales of hay


Hay is the lovely, sweet-smelling end product of drying cut grass in the sun.  The sight and smell of hay to many people is soft, warm and comforting.

But hay is actually something much more practical – winter food, or ‘fodder’, for cows and sheep.  It’s what keeps them healthy through long, dark winters – energy food, grown and preserved by sunshine.

No wonder there is a centuries-old saying ‘make hay while the sun shines’.  Farmers are anxious to dry their grass as quickly as possible after cutting.  A few days in the sun, turned once by a hay-turner machine, is all it usually needs before being baled.  The round or square bales are stored away in a barn or in an outside haystack.  But rain can spell disaster, ruining the haymaking process and leaving farmers without enough winter fodder for animals.


Silage bales near the sea
Wrapped silage bales

Silage is also made from grass and other pasture plants, but it’s completely different from hay.

It’s a fermented, high-moisture fodder which is compacted and stored in airtight conditions.  Because air is excluded, anaerobic micro-organisms ferment the plant material, preserving it so it can be stored for a long time – a bit like pickling vegetables, or making yoghurt or cheese to extend the life of milk.  Because of this process, silage also contains more nutrients than hay – a kind of superfood for cows.

Over the winter, when there are no fresh forage crops, silage can be a vital part of the diet for animals, especially cattle.  The anaerobic fermentation also makes the silage taste slightly sweet or sour, which cows seem to like!

Farmers in the UK typically get two or three ‘cuts’ of grass in summer, to make silage or hay.  Silage can be made from mown grass or other crops such as alfalfa or maize plants.  After mowing, it’s left to’wilt’ in lines for a day or two, but not as long as hay.  The farmer then uses a machine called a ‘forage harvester’ to chop it into small pieces.

Silage is often made in a pit or ‘clamp’, a very large container made with concrete walls, or built into the side of a bank.  The chopped organic matter is built up in layers on top of each other, run over by a tractor to press it down, then covered with plastic sheeting and weights to exclude air.

Nowadays, silage can also be made by tightly wrapping round bales in plastic – watch it here.  These are the familiar black (or white, green or pink!) rolls which can be seen in fields or stacked in farmyards.  This is another way to exclude air, allowing anaerobic fermentation to take place.  In the UK, there are strict rules about disposing of plastic silage film that farmers must follow, using specialist recycling companies.

Farmers need to use the right grass / crop mixture for their to give enough nutrients to their cows.  This is especially important for dairy cows, to make good quality milk.

Fact: apparently, the best time to cut grass for silage is in the afternoon, as that’s when the sugar content of grass is highest.

In case you’re interested, here you can find lots of technical information about making silage!

Tractor with baler


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