Ewes are seasonal short-day length breeders which is controlled by the hormone melatonin. The shorter day lengths triggers an increased amount of melatonin secretion from the pineal gland in the ewe’s brain – bringing the ewe into cycle.
In the UK from mid-December to mid-June, as the days become longer, ewes will be in anoestrus and will not show any signs of heat or ovation. From mid-June to mid-September as the days shorten, ewes will be in a transition period where some will start to show heat behaviour and ovulation.
Then from mid-September to mid-December it is the breeding season. This cycle can vary from breed to breed, with some breeds such as the Dorset Horn able to breed almost all year round. The length of a ewes oestrus cycle is 16-17 days.
Manipulating ewes breeding season
Tightening a lambing period can have many benefits including the ability to manage the ewes nutrition more carefully during the lead up to lambing, and employing the correct workforce for a shorter period of time. Advancement of the ewes breeding season can also ensure an earlier lambing and therefore lambs ready for market before peak supply and better prices/kg.
Before thinking of manipulating a ewe’s breeding season you must ensure:
1. Ewes are in the correct BCS (3-3.5) and provision of good nutrition at breeding.
2. You have the correct facilities and feed supplies for an earlier/tighter lambing period.
3. You have a workforce in place for a more intense lambing.
4. You have the correct number of rams in good fertile health and BCS (4) for tupping.
Different methods of tightening or advancing the breeding season
1. The most basic approach to tightening your lambing period is to ensure that the ram goes in and comes out at the correct time.
2. Introduction of a teaser (vasectomised ram) in the transition period can advance the breeding season by a few weeks or tighten the breeding season by ensuring all ewes are cycling by the time a fertile ram is introduced. Ewes should be kept out of sight or sound of any ram before the introduction of a teaser to be effective. The teaser should be in with the ewes for two weeks, before being removed and fertile rams introduced at a ratio of 1 to 20 or 30. Lambing should occur over a 20-day period.
3. Progestagen Sponges can be used during the breeding season to synchronise a flock. They can also be used alongside an injection of the hormone PMSG outside the breeding season to get the ewes cycling for natural or AI service. They are inserted into the vagina for 12-14 days; removal will trigger oestrus at 36-48 hours. Rams should be introduced at a ratio of 1 to 10 at 36 hours to prevent exhaustion.
The success of the first service depends on many factors and pregnancy rates can vary from 30-80%. Pregnancy rates after a second service should be above 75% giving a tight lambing period of around three and a half weeks.
4. Melatonin implants mimic the effect of naturally produced melatonin during the shorter day lengths. It can slightly advance the breeding season but only in the late transition period and can improve the number of ewes cycling in the early breeding season. Ewes must be removed from all ram contact seven days before implantation and 30 weeks before the intended start of lambing. It is implanted under the skin behind the ear and then rams are introduced 35 days later at a ratio of one to 40 or 50. Expect peak mating activity 25 days after this. Ewes should lamb over a three to four week period.