Rancher, I have been breeding them since 1970, when I started my fullblood herd in the then Rhodesia.
I was in the hot, paracite infested Bushveld in Matabeleland where the breed was origionally developed from selected native Sanga cattle.
Under these conditions, they are heat resistant, fertile and resistant to tick bourne diseases such as Anaplasmosis, babesiaisis and heartwater,and withstand the tick and fly infestations. Due to their unique genetics they cross with all the popular breeds in the 'States, expressing heterosis even with other Bos Taurus breeds. The F1 female makes an exelent mother in a rotational or terminal crossbreeding program.
I have started with Tuli again here in America, and local commercial breeders are showing interest in crossing with them for heat resistance and fertility.
I am on the company farm at present (break time!), but will post some links when I get home later, and will answer any futher questions as best I can.