What causes forage brassicas and turnips to turn yeallow?

I'm not sure, but here's my best guess. Either they are shaded and not getting enough sunlight, could be stressed due to drought, or they may not be getting enough nitrogen. These are the three most common reasons why most any plants leaveswill turn yellow. Sometimes magnesium deficiencies will cause them to turn yellow. Could you give more details?

could be low pH also. i limed MOST of a field a few years ago, and have planted small grains in it regularly. this spring, i cleared up another portion of the same field that hadn't been limed, then fertilized and planted the entire field. the new portion that didn't get lime had sunflowers that were quite yellow and much shorter than their counterparts just 10 yards away that were growing on limed soil.

Your ph is too high, it could be a magnesium defeciency try using Manganese or try 10 lbs of Epsom salt to 20 gals of water to the acre , spray it on and dont use a sprayer that has ever had 2-4-D in it , in it's lifetime....

Or they could just be maturing , That's what brassicas do when they reach maturity .