The USTA rules are:-
Beginning with the 2009 breeding season, the total mares bred to a stallion standing in the United States that has never bred a mare or had a list of
mares bred filed previously shall not exceed the following:
Breeding Season: 2009 2010 2011 and thereafter
Trotting Stallions: 140 140 140
Pacing Stallions: 160 150 140
These limits on total mares bred may only be exceeded in the event of death of a mare which has been bred in which case a new mare may be added to the mares bred list and bred to the stallion.
Thomas Johnson the trotters more than the pacers have that slight problem, Super Bowl and Speedy Crown. The Aussies are still 5 years behind
NA, they will catch up and will be using the same sulkies on the same tracks and getting the times(Peacheymagic you are correct). Then the stallions will travel with some chance of getting good books of mares.