Brassicas are a great piece of the food plot puzzle, but not the entire puzzle. Some deer need to learn to eat brassicas, where others will start hitting them the first year. When people think of brassicas they think rape and turnips. By all means this is a great mix. However, I have learned by adding radishes into this mix will improve how deer use a brassica patch. In my experiences and from talking to others, deer will radishes first, then rape then turnips.
Think about it, if you add radishes to the mix and what I said above is true, then you are making the deer search in the plot to eat radishes. They get accustomed to it and they they start sampling the rape and turnip varieties.
I have also heard people say they planted brassicas several years in a row and not a one was touched. Try planting a small area or strips within a main food plot before you go and plant an entire food plot.
Brassicas in the spring can be a pain which is why I try and plant small strips of them instead of entire plots. Brassicas give off chemicals that can prevent small seeded legumes and weeds from growing. I like to frost seed my plots in March with clover. If you have a brassica plot that was uneaten and you have rotting leaves and roots scattered everywhere, these chemicals can prevent clovers from growing. This is why I plant some strips within one food plot. If they don't eat it, I don't have an entire plot sitting idle. Brassicas are also nototrious for regrowing in the spring. This can also be a mess. Spray with 24db or round up to kill them.
For a maintance free easy to care for food plot it's hard to beat rye, oats, peas and white clover. But brassicas do provide deer with valuable late season forage that can help to keep deer in your hunting area. Think about it, once the snow it a couple inches deep, are they going to eat clover? Probably not. Are they going to hit winter rye? Probably not? Will they hit brassica greens that are sticking up above the snow or just under it? Yes. Brassicas, corn and soybeans are great late season attacrtion. Brassicas, if planted and fertilized correctly, can produce literally tons of forage per acre.