Background Info: Location: Northern Virginia. Brought a home last year and the lawn was infested with 90% weeds. Took the complete renovation project last fall. Killed all the vegetation using roundup around 2nd week of September. Got irrigation installed De thatched, Aerated and planted TTTF with compost and fertilizers. Lawn came up beautifully. This spring lot of chick weed showed up. Didn't use any herbicides till March '12. Sprayed Weed b gone Max twice - Once in April and again during 1st week of May. Weeds are gone. But I see brown patches in my lawn. Don't have any pets. Lawnmower is 9 months old. I've attached the pics. What could be the cause of these patches and how do I narrow the root cause?