I've found manure is quite a good way of bringing in weeds, particularly fat hen and dock, but don't let that put you off. I'm a big believer in digging a full spit deep as it gets the air and weather into the soil, and I'd probably just spread the dung on top and let the worms take it down as it's easier like that, and then dig it in in the autumn. I'd be tempted to plant a whole plot full of trailing marrows to keep the weeds down.