Zone 10 lawn improvement

[argh]

I have a cubic feet bag of steer manure that I'd like to apply a one-third inch layer to a section of my lawn (St. Augustine).

1. The product claims to be "Weed-free and fine screened" - why don't other bags of steer manure of similar price offer such? Is it possible there might still be weeds?
2. Temperatures this week range from 84-90 F highs. Will applying now have any detrimental effects?