I have two Rock Roosters. One is 18 months old and the other is 11 months. These strapping young lads are HUGE they strutt around like they own the place (which they do) but they DO NOT DO THERE JOB. They younger rooster has just started crowing the last few mornings...
Can you give Viagra to chooks ?

No, I don't think you can give viagra to chickens (at least it's never been done), but if they've just started crowing, they should start "doing their job". My barred rock bantams are very fast developers. They start crowing at about 2 months of age and my 4 month old pullet just started laying eggs. My roosters typically start courting their hens at about 4 months, but they don't actually mount them until about 6 months or so. Be patient, if they're like typical rocks than you should have no problems.