Seed harvested before it is ripe does not grow. Do not collect only the largest, healthiest plants. Go for diversity of genetic material. Preferably, collect seeds of each species from multiple sites. Do not pull up the entire plant.
Most prairie plants are perennials.
Even though they appear dead, they will grow back from the roots the following Spring. Look for insect damage. Plants in the pea family, especially, will often have small holes in the seeds, evidence of insect damage. These seeds are not viable. Seed content of fruits should be examined during reconnaissance and again immediately before collection to determine whether enough seeds are present to make collection worthwhile.