In subtropical Mexico, the main critical factor for success of the male effect is the use of sex-ually active males. Bucks, but not females, are thus treated with long days followed by natural days or melatonin to improve the efficiency of the male effect and can stimulate ovulation and oestrous behaviour in female goats better than untreated males (Flores et al., 2000). Availability of sexually active males allows producers to manipulate the annual breeding season according to consumer demands, without the restraint of also keeping females in confine¬ment for teasing.