Stardew Valley is all about managing your farm, and if you have animals, you need to feed them hay. Hay is very easy to find, but if you haven’t built a building to store it, you can’t actually get it other than buying it. Here’s how to collect hay and what you need to do beforehand.
How to get hay

Hay can be obtained by cutting grass with a scythe, but only if you have a silo with space to store hay. A carpenter can build a silo for a low cost of 100 stone, 10 clay, 5 copper ingots, and 100G. A silo can hold up to 240 bales of hay. Cutting grass with a regular scythe has a 50% chance of getting hay, while using a golden scythe has a 75% chance of getting hay. Any hay obtained from cutting grass on the farm with a scythe is automatically sent to the silo as long as there is space. If the silo is full, you will not be able to mow hay.
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During most seasons, your farm will occasionally start randomly growing grass, although this is much less likely during the winter. You can start a patch of grass yourself using a Grass Starter, which can be purchased from Pierre’s for 100G, or made using 10 fibers after purchasing the 1,000G recipe from Pierre’s.