Planted alfalfa early April, did not come up until about May 1 since no rain until end of April. Stand is fair and has lots of weeds in it. Pigweed are about 8" tall, alfalfa is 1" to 6" I am thinking that we would mow it with a bushhog. If we let pigweed get 2 to 3 feet tall before mowing would the pigweed shade out and kill the alfalfa?