Testosterone levels are generally higher in entire male sheep when photoperiod is short as this coincides with the mating period and oestrous cycles of the female. This may increase management problems of entire male sheep through augmented agonistic and sexual behaviour. By delaying puberty in the entire male lamb, management problems may be avoided, while retaining the full benefits of some testosterone secretion to promote growth and leanness. This experiment was initiated to examine the possibility that lamb growth will be enhanced under either constant long or constant short photoperiod without increased behavioural problems.

At birth (mean birth date 19 March 1994 ± 3 days), 24 twin male lambs were assigned as pairs to either long (L) or short (S) photoperiod treatments alternately. One lamb from each pair was left entire (E) and the other lamb was castrated (C) at 12 hours of age using elastrator bands.