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The reason far wrapping the bales I was told was that the grass is fermenting in anaerobic atmosphere, which prevents fungi and rot from developing inside the bales. In Poland these are called "kiszonka" which means "sour grass". The alternative is to dry it out, thus making hay, but it's more difficult as a single rain can ruin the process. On the other hand, cows can digest this "sour grass" while horses for example cannot and they need actual hay.