I found the following interesting.
 " Anecdotally, farmers have observed cereal rye cover crop growth to really take off as temperatures consistently rise. With that in mind, the data from the on-farm trials suggest that those who plant green after 750 heat units accumulate in the spring are likely planting into much more cover crop growth than those who plant green before then. In other words, yield drag from planting green might be potentially avoided if planting corn before temperatures rise and cover crop growth accelerates in spring. Basically, the warmer it gets in the spring, the more likely you’ll suffer yield drag from planting green because of competition from the cover crop."

Corn Following Cereal Rye Cover Crops: Research Sheds Light on Planting Green - Practical Farmers of Iowa