Kevjack is correct.
According to our DDH nutrition experts, the occurrence of toxicity is not common. This normally occurs if animals "slug feed" on brassicas
are introduced to it quickly and brassicas make up nearly the entire part of the diet. As you know, corn being fed this way can also be a problem. Even clover and alfalfa can be a problem nutritional if it is fed in this manner from a rumen digestive standpoint. The key is to have a variety of forages for deer to eat and to slowly introduce brassicas to deer. When you plant food plots for deer, at least wild deer, it is introduced over time and even if there are limited other sources of food plot types, deer will use natural browse as part of their diet. In addition, most people plant smaller plots of brassicas ½ to 3 acres in size not larger fields like is commonly done for grazing cattle.
The problem may be more prevalent or likely in penned deer that have access to only one food source and are turned in to a field where they are introduced quickly. So, it could be a problem though rare and if you plant a variety of winter food plots the likelihood would be reduced greatly.
Daniel E. Schmidt, Editor, Deer & Deer Hunting