It has taken me several years of experimenting with small grain/brassica mixes to assure neither chokes the other out. For me the brassicas were far more likely to choke out the small grains as they jumped up faster with large leaves. Irrespective of small grain planting rates I go with 3 lbs/daikon radishes and 1 lb/ turnips per acre. Get a good stand of the brassicas quickly but then as the small grains start getting tall the brassicas are well established.