I'm with tomkat. Sevin is a known insecticide and will wipe out your apiary. Round Up is also a poison designed to kill pests, in this case "broad leafed" weeds. It can also severely hurt an apiary.
Spraying a fence line and a little grass around the hives is NOT the same thing as a sprayer rig driving up and down the rows spraying "Round Up Ready" corn or soybeans. Move the hives for a week. Remember, a farmer spraying is aiming for a time window so that he can spray, then have at least 2-3 clear days without rain to let the poison work. Any nectar or pollen collected in that time frame will have Round Up on it, which will be taken back to the hive. Also, bees collecting around that area will physically come into contact with the RU on their bodies, wings, and feet, bringing it back to the hive in that manner.
Yes, I am very well aware that RU is marketed as "safe for bees". It will not kill them outright, immediately, like Sevin. But it is not in any way good for them, and wild bee populations are very low in areas where it is used wholesale on RU Ready crops. It does cause harm in my experience, and I avoid it. No, thank you, I choose not to debate about it again, as the last "gentleman" who wanted to debate about it, lowered himself to vicious name calling, gender bashing and libel. I won't play. :no: :shhhh: :gh:
Good luck, and I hope your bees thrive this year!
Summer
At least the farmer told you what he was going to do, instead of just letting you find your hives dead. That was decent of him.