Do I need to treat all my cattle?
No you do not and, in fact, you should not do so for two reasons.
First, most indigenous breeds of cattle have an innate resistance to tick-borne diseases which they acquire by being exposed to ticks when they are still calves (see review by Torr et al., 2002).
Second, tsetse seem to feed preferentially on the older and larger cattle in a herd (Torr et al., 2001; Torr et al. 2007).
So by treating only the older and larger animals we not only allow cattle to develop resistance to tick-borne diseases but also apply the insecticide to the cattle that are more likely to be bitten by tsetse.
As a general rule, you should just treat half the herd, selecting the bigger animals.
Torr, S., Eisler, M., Coleman, P., Morton, J. & Machila, N. (2002). Integrated control of ticks and tsetse: A report for the DFID Advisory and Support services Commission (Project ZV0151; NRI code V0160). 150 pp.
Torr, S.J., Wilson, P.J., Schofield, S., Mangwiro, T.N.C., Akber, S. & White, B.N. (2001). Application of DNA markers to identify the individual-specific hosts of tsetse feeding on cattle. Medical and Veterinary Entomology 15, 78-86.
Torr, S.J., Prior, A., Wilson, P.J. & Schofield, S. (2007). Is there safety in numbers? The effect of cattle herding on biting risk from tsetse flies. Medical and Veterinary Entomology 15, 78-86.