I notice that after the first mowing of my lawn, the tips of the blades of grass yellowed. Same thing with several other lawns in the neighborhood. I applied fertilizer and crabgrass preventer on March 13th - because the wetaher was nice here in Northern Pennsylvania - and I mowed for the first time on March 29th. The yellowing occurred mostly in the front unshaded lawn. Not so much on the back shaded lawn. Grasses are a mix. Any idea what is going on?