Induction of fertile cycles in the Blackhead sheep during the anoestrus period

Abstract. A continuous supply of lambs is expected from the market, but not realizable without additional administration of hormones in seasonal breeds kept under extensive conditions. Our study had the aim to work out a possibility to induce fertile cycles in a flock of the Blackhead sheep already in early summer. Analyses of hormones progesterone and LH in untreated ewes were used, together with other tools, to control the different treatments for success, e.g. induction of ovulation, appearance of oestrus signs, regular luteal phases or pregnancy. Annual treatments were performed on altogether 76 ewes and started in June (1998), May (1999–2001), or April (2002). All tested variants had the main aim to overcome the GnRH/LH deficiency in the summer period. First series of experiments with either a single GnRH injection, a pheromone-containing paste or FSH showed unsatisfying results. An improvement was found after a previous progesterone priming which may have simulated a luteal phase of the oestrous cycle. Further modifications of the second part of treatment revealed a successful stimulation after as well a sequential GnRH agonist administration as a PMSG regime detected by plasma progesterone and ultrasonographic analyses. Finally, births of a single lamb and a pair of twins, respectively, already in October demonstrated the essential potency of both treatment regimes. However, a simplification, i.e. reducing the number of injections, and an optimisation of added hormone amounts are necessary. At the same time, changed conditions for keeping both the pregnant ewes and earlier born lambs have to be considered.