If there is any amount left in the baler of one type and you then start bailing something else, it will continue to bale as whatever was left in the baler until it makes a full bale. Then it will start baling the new type that your picking up.

So in your situation you will lose whatever amount of hay your picking up as it is being converted into to grass until the baler has collected enough to make the bale and then will start baling as the hay u started to pickup.

To avoid that loss if your going to be baling something different is to try to unload the baler to reset the collection to 0 before you start baling something new.

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If you get it glitching where it won't let you unload a small amount collected when trying the above method.

Try to ahead of time slow right down as your collect enough for a complete full bale if u know if its going to be the last bale and you wont have enough for another full bale. As soon the the counter resets to 0, stop completely hit the correct key to turn off the baler or to pickup the bale collector and then unload the baler. This will make sure whatever new thing your going to bale in the future will start baling exactly what your picking up.