Just to preempt, I acknowledge that this language is odd:
"Thus a farmer whose field contains seed or plants originating from seed spilled into them, or blown as seed, in swaths from a neighbour's land or even growing from germination by pollen carried into his field from elsewhere by insects, birds, or by the wind, may own the seed or plants on his land even if he did not set about to plant them. He does not, however, own the right to the use of the patented gene, or of the seed or plant containing the patented gene or cell."
However, I don't think you can conclude that "use" arises when Roundup is applied. "Use" in this context is broader than that.
I can't find the citation right now, but last time I looked into this case I found that the court intended to set an explicit precedent that accidentally planting Roundup-ready plants does not constitute "use" of the patented gene ("use" happens when you use Roundup to select for Roundup-resistant plants). The language is not odd when considered in the context of setting such a precedent.
"Thus a farmer whose field contains seed or plants originating from seed spilled into them, or blown as seed, in swaths from a neighbour's land or even growing from germination by pollen carried into his field from elsewhere by insects, birds, or by the wind, may own the seed or plants on his land even if he did not set about to plant them. He does not, however, own the right to the use of the patented gene, or of the seed or plant containing the patented gene or cell."
However, I don't think you can conclude that "use" arises when Roundup is applied. "Use" in this context is broader than that.