Perhaps the most often asked question i get asked to me at seminars is also the most common problem i read on sites is.."The deer wont eat brassicas i planted" and "how do I grow brassicas"?
Here is my reasons why this is.
1) your soil pH is too low. Low soil pH effects everything from plant crude protein content to sugar %. pH is a factor in palatability of all forages.
2) Improper fertilization. Most in the USA use recommendations by corn-soybean agronomists and they push more nitrogen. The brassica genetics comes from New Zealand and i have posted on here before what the New Zealanders do for fertilizing brassicas and thats what i recommend.
3) Plant diseases. This is why genetics comes into play when asking how to grow your brassicas. Many companies use a higher amount of rape and hunter brassica. There are many newer genetics that have a much better disease resistance. I did a brassica trial a couple years back where plant disease issues are very evident with some brassicas and others there was no diease problems. Any brassica that is stressed from plant diseases will be less palatable and won't grow correctly.
4) Insect pressures. Insect pressures are something i rarely hear anyone mention to me or online but they are very common for brassicas. I see pictures of this and witnessed it first hand in a small trial i was doing this past year. Again, insect damage is a plant stress and will effect palatability. It only costs $2 an acre to use silencer, warrior or other inecticides.
5) Plants are too mature when they stop growing or during the time when they should be consummed. Every forage has an average maturity. Some brassicas mature in 6-8 weeks, some in 8-10 weeks and others take as long as 18-22 weeks to mature. Any forage that has went past it's maturity date will be low in the major nutrients, sugar and digestibility. Old forage isn't consummed by ruminants unless there is nothing else for them to eat.
6) Genetics. Anything of the kale family tends to be more palatable. This is why i use mainly kale and winfred brassica in my fall mixes. There is one new discovery i made this past year and i'll be adding one more preferred brassica to some of my mixes.
7) SULFUR. Many people are sulfur deficient around the US as we have cleaned up the power plants. Everyone talks Nitrogen but if your low in sulfur you won't have good nitrogen conversion, resulting in lower plant protein. Sulfur also tends to "sweeten" most forages. But again, take a soil test to make sure your sulfur is not in excess.
8) Biodiversity. If your deer haven't been exposed to brassicas before, it might take a bit to get them use to them. This is why i push a mix called grains and greens. It has triticale, oats and peas in it along with radish, winfred brassica and turnip. The grains and peas are preferrable to deer and radish is one of the most palatable brassicas to deer. This mix everyone raves about it is about as fool proof as can be, and anyone can start growing brassicas that get eaten right away.