I wonder if nettle would survive mixed in with a field of hay? One doesn't see nettle scattered about in nature in hay fields. It grows in large clumps/groupings of stems partly because of its rhizomatous nature. It thrives in a particular niche - a nitrogen rich loose soil and moist conditions. As such, I would grow it in a separate patch with high organic nitrogen and maybe some extra irrigation water if it isn't in a lower wet spot. Being planted separately, it could then also be harvested at the optimum timing.
Besides, there probably won't be enough nettle growth in the first year, if grown from seed, to harvest without hurting the plant; perennial plants I've worked with like MIscanthus giganteus or Silphium perfoliatum are slow to establish and need at least a year without harvesting to develop strong enough rhizomes to survive the winter. But I'm just starting to work with nettle, so I can't say anything for sure yet.
Whatever, I won't be seeding nettle. Agriculture Manitoba ( http://www.gov.mb.ca/cgi-bin/print_hit_bold.pl/agriculture/crops/medicinal/bkq00s07.html?print
) contends that seed germination is very poor and recommends using rhizomes. For this winter, I've planted my selections in waste coco fibre in the greenhouse. This coming spring I'll plant them out in the wettest richest soil we have, and hope they stay ahead of the weeds!