If you have an island in a winter wheat field then yes I would take it down to a very minimal height. Animals like geese tend to avoid areas where there natural predators could hide and ambush them without warning. Hence your tall, thick grass area in the extremely short winter wheat field. I would cut your island down as short as you can to help with that. Also when you place decoys make sure you put some in amongst the eye of grasses so the geese think it is a normal situation and doesn't stick out more.