I think a suitable bull would be Tonka from Knoxville. They said they wanted to move him so he could have calves, but still nothing. Ali from Jacksonville would also be good. He only lives there with two older cows.
I wish Tonka would go. Founder bull with no offspring, and the zoo seems to have no interest in doing AI from him. Ali would be a good option too, but he at least has 2 offspring through AI, and I'm sure the zoo would be willing to collect semen again. Other good options might be S'dudla from Tampa or Tamani from Kansas?
The issue with the US African elephant population, so many bulls are behavioural non breeders. Think...
Mabu, Jackson, Maclean, and S'dudla are the only bulls who are breeding naturally right now. Msholo, Tuffy, Artie, Czar, Osh, Vusmusi and Louie are all behavioural non breeders, and the only ones who stand a realistic chance of becoming natural breeders are Louie and Vusmusi as they gain experience.
Others, Ali, Callie, Ajani, Tamani, Kedar, Tendaji, Samson, Tonka, and more, have either never had a good chance to breed naturally, or are just newly moved into breeding situations.
US African elephant population is a disgusting mess. Asians at least, most of the bulls will breed naturally, the population is just managed poorly. Africans though, along with bad management, only a fraction of the male population has proven to be able to breed naturally so far. Proven bulls should be moving every 2-3 years to different facilities to breed with all the aging cows and AI born females who are beyond due for first calves, non breeders should be pulled from natural breeding situations and moved to facilities that will collect for AI, and young bulls need MUCH better social rearing to support them becoming good natural sires.
People say the US population of African elephants is sustaining much better than the European one, and I couldn't agree less. US was doing well for a brief period of time from 2008-2012 when there were lots of calves born, but that was it. Europe has much better founder represention, way more natural breeding bulls, and much more consistent calf production (not to mention multiple third and even a fourth generation captive bred animal, while the US only has 5 second gen animals total, and none of them have bred yet. (Technically Osh is second gen too, but he came from Europe, so doesn't count).