Might have something to do with letting the new herd of Blackbuck settle in within the reserve with out cars driving through it.
I believe its just new animals added to the existing group.The old herd was quite large, have they moved on to another collection?
Oh dear:
Longleat Safari Park near Warminster has sealed off its drive-through monkey enclosure after one of the females tested positive for the Simian Herpes virus.
Safari park seals off monkey enclosure after virus discovery (From Wiltshire Times)
A couple of years ago, both Longleat and Woburn destroyed hundreds of macaques for this exact reason. I think the entire troop was shot dead.
Yeah, it seems as if in actual fact Longleat never culled their macaques, . An IPPL article implies all of the safari park stock originated in the US.
I'm pretty certain there has been no large-scale cull of any baboon species at the safari parks.