For winter-killed cover crops we like a 60%/30% (by weight) blend of oats and Austrian winter peas. In our location (Nova Scotia, Canada) they need to be planted by the first week in September to make plenty of top growth before repeated hard frosts shut them down. We typically get 12" or more of top growth, which by spring is completely dead and lying flat on the soil surface as a nice, protective mulch. Another possibility is tillage radish, which breaks down really fast and will have essentially disappeared by spring planting time. On sloping ground this is bad because you've lost your erosion protection, but on the flat it can make bed prep dead easy.
And here's a link to an article on no-till spinach following winter-killed cover crops. It has a photo of what radish looks like in the spring. https://www.researchgate.net/profile/Raymond_Weil/publication/265757461_No-till_seeded_spinach_after_winterkilled_cover_crops_in_an_organic_production_system/links/541ac53b0cf25ebee988c052.pdf
And finally, here's a graph showing the awesome rooting power of radish. The radishes were planted on August 8 in a sandy loam soil in Denmark, and by mid-October their roots had reached a depth of over 2 meters! Annual ryegrass and winter rye are also shown for comparison. You can read more about it here: http://orgprints.org/32024/1/SSSAJ%202004.pdf [Radish root depth graph](//muut.com/u/notillgrowers/s1/:notillgrowers:ZOws:radishrootdepthgraph.jpg.jpg)
@David: Thanks for the info! What is the last 10% of your blend?
Sorry for the omission! The last 10% is the tillage radish. It's much less by weight than the oats and peas because the seeds are so much smaller. If the cover crop follows a brassica cash crop we'll eliminate the radish for rotational reasons and go 65/35% oats/peas.
I’m in Western South Dakota edge of zone 4/5. Last year I mixed barley with various clovers (some biennial). The barley was great and I chose it because we have a slightly basic soil. If you have time to establish it crimson clover is great and winter kills. I also did a quick round of buckwheat that died with the first hard frost in late September then planted garlic right into the mulch (no straw added, just planted garlic at 5” and 100% is up and at 15” since the soil warmed so quickly with out extra cover). I’m excited to try Austrian peas this year.