Q.Can you use two year old bales of straw or hay to grow a garden? it’s been in a barn for 2 years.
The bales are two years old. Will we still have nutrients once I break down with nitrogen?

Certified GKH Gardening Expert
It will, still, have nutrients in it, though adding nitrogen is not going to break it down, rather it will contribute to the nutrient content. You will still have to add phosphorus and potassium, along with all of the micro and trace nutrients. Straw alone won't contain a whole lot to begin with.
These articles will help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/fertilizer-numbers-npk.htm
https://www.gardeningknowhow.com/composting/ingredients/how-to-compost-hay-bales.htm
https://www.gardeningknowhow.com/composting/basics/ultimate-beginners-guide-composting.htm