Well, you have a few choices IMOHO:
I would simply tell her that the feed is not appropriate and I would say specifically why. Then I would accept the two bags of feed since you agreed to get two bags to try. And then I'd ask her to return the other eight bags. Then I would make it clear to her that there would be no more bartering in the future. It just complicates things. If you're gong to barter, you need more information about the items being bartered and they should have equal value.
OR, You can also ask her for the receipt and return all of them, that way, it's a wash. Then I would make it clear to her that there would be no more bartering in the future.
OR, you can just suck it up and accept the ten bags and make sure it is crystal-clear to her that there would be no more bartering in the future.
If you think for a second she will ask you to reimburse you for the ten bags, I would load them back onto her truck and have her return them. I would not pay for an acceptable product.
I'm sure her intentions were honorable (or maybe not, after all, I don't know her), but she probably thought she could bring down your expenses. Sometimes cheaper products work and sometimes they don't. But both parties have to agree on the value of the bartered items. It's not something one person can just take upon themselves to decide.
And in the future, I would not have anyone get anything for you "to try". If you were to exchange goods for feed, I'd give the potential barter a copy of acceptable feed with item numbers and no negotiations. I would never try anything new in trade, you just never know what you'll get. And frankly, it could be gold, but if you don't find it acceptable, it's not worth the paper it was bagged in.
We, as our animals' caregivers, are very particular about what we want for our livestock/pets. Whether it's feed, medications, supplies, whatever, we should get what we use, not what's cheaper for someone else. I know someone who used to borrow stuff from me and then replace it with a cheaper product. After third time, when the person asked if she could borrow something and replace it, I specifically told her that I wanted THAT specific item to replace the ones she borrowed. She doesn't ask anymore. I also know someone who swears by a particular feed for her animals, and she and her animals love it. But when I tried a bag, I had to feed it to the chickens because no one else would eat it. It works for her, didn't work for me. That just happens.
Let us know what you decide, and good luck