Field Surfing an Organic Alternative?

This is Derek this morning topping out the yellow flowered Charlock in the field growing stubble turnips and forage rape for the lambs to feed on over the winter. Charlock is a real pain as it belongs to the same family 'brassica' as the actual crop so we can not spray it out. We weed surfer is like a flymo that is carried along behind the tractor trimming of the flowering heads and some of the pods that have already been set. The trick is to get the surfer low enough to cut the charlock but not to low as to damage the actual crop. Some of the outer leaves will be chopped a little but they will recover after the rain we have just had and the warm soil temperatures. The lambs will be running out of grass during October and so they will need to be grazing the turnips in November and hopefully they should last the lambs until March. Anybody know how many grazing days an acre of turnips should last?