Context . Fertility control of females with levonorgestrel or deslorelin implants shows promise for managing populations of overabundant eastern grey kangaroos (Macropus giganteus). Although these implants have been tested separately in captive and free-ranging kangaroos, there has been no direct comparison under equivalent field conditions.

Aims . We investigated the long-term efficacy of levonorgestrel and deslorelin implants, together with the cost of treatment, ease of administration, and the welfare of the animals, in a side-by-side trial under realistic management conditions.

Methods . We captured 65 adult female kangaroos over 11 days at a golf course in Anglesea, Victoria, Australia. We assigned each female to one of the following three experimental groups: levonorgestrel (210 mg, n = 18), deslorelin (9.4 mg, n = 24) or procedural control (n = 23). We monitored reproductive success for 8 years, by observing young in the pouch in winter and spring.

Key results . Natural fertility was high; in most years, less than 20% of control females failed to reproduce. For deslorelin-treated females, the odds of failing to reproduce were four times higher than for the control group; for levonorgestrel-treated females, these odds were 74 times higher. Deslorelin was ineffective after 3 years, whereas levonorgestrel was effective for at least 5 years.

Conclusions . Levonorgestrel was markedly superior in efficacy, as shown by a stronger contraceptive effect persisting for longer. In other respects, the two implants were comparable, being similar in cost and ease of delivery, and equally safe.

Implications . Only levonorgestrel implants fulfill their promise for non-lethal, long-term control of kangaroo populations. Deslorelin implants cannot be recommended for this purpose.