By mid-September, options for seeding cover crops in the Northern Plains become more limited and in some cases seeding rates need to be bumped up to get a decent stand. My typical cut-off date for most mixes that include broadleaf cover crops like radish, turnip or peas is Aug. 15. By September, I take broadleaf plants out of the mix because I can't justify the cost of the seed for the anticipated limited amount of growth.
By September, I am only seeding the grass component of mixes, whether it be cereal rye, oats or barley. I keep the mix cheap and get it seeded in the ground for the best possible stand. Most of the time, cereal rye is the best bet since it overwinters, but bump up the rates each week we get later into the season. For example, farmers I work with on high clay soils have been known to seed cereal rye at the end of November if conditions are right. However, their seeding rates will be up around 100 pounds per acre this late in the season, nearly double the seeding rate of what they would put down in August. Barley and oats can still be used, but set your expectations that there won't be a lot of growth and they will winter kill.
If you don't have time to cover entire fields with cover crops like you may have originally planned, for example with cereal rye, focus on seeding only the areas that have excess water, headlands or saline spots. I really like seeing this "spot seeding" in fields, where cereal rye is seeded in drainage ditches or at high rates on headlands to get some roots in there to repair compaction. Saline spots need the extra attention, any time of year, but be reasonable with your cover crops in these areas and don't overspend. Focus on making them less of a headache for you to deal with in the spring.
Be careful of which fields you put cereal rye into and make sure you take good notes of the fields where you seeded cereal rye in the fall. DO NOT put cereal rye in fields where you will be growing wheat the following year because you will contaminate your wheat crop. If you plan to plant corn onto a field where you have cereal rye, it will need to be terminated 10-14 days in advance of planting corn. Unless you have cereal rye in limited problem areas (not the entire field) and you feel that it will help you get the crop planted next spring. I'm talking about only 10 acres of a field where you can justify taking a potential yield hit to keep from mudding up the planter in those problem spots. Terminate the cereal rye in those small areas immediately after planting the corn in this case.
You can look back at Soil Health Minute articles for tips on late season cover crop, they are all posted on the NDSU Soil Health webpage (ndsu.edu/soilhealth). There are also about 80 YouTube videos (all about 2-3 minutes) posted on that same webpage.