It likely won't work, as the elevator was selling the other seed as feed, not for planting. The farmer was banking on getting RR seed, because he knew the elevator didn't care what kind of seed went into the stuff for "feed/milling/etc".
He was trying to argue that first sale doctrine means the patent can't tell him he can't use the cheap seed for planting. Which is true. But the patent still applies because he can use the seed to grow more seed, and he knew it.
This isn't a case of a farmer's crop being cross-contaminated. This guy was deliberately trying to get around having to honour the plant patent by obtaining the seeds through other means.
Monsanto may have a case against the elevator for not heat-treating the seeds sold as feed to ensure they could not be used for planting. But the farmer does not have a case, as he was banking on the elevator not treating the seed.