Breeding manipultion is useful for two reasons; firstly to make the lambing perid more compact, and secondly to force ewes to lamb earlier than ususal to catch the higher prices of the new season lamb markets.
There are three main ways to manipulate a sheep’s breeding cycle - melatonin implants, teaser rams, and progesterone sponges - but firstly, it is helpful to have a brief understanding of the ewe’s reproductive cycle, says vet Anna Whitehead, of Ribble Vets, Preston.
She explains: “Generally most breeds of sheep in the UK are seasonal breeders - with exception of the Poll Dorset and Dorset Horn, which are capable of breeding throughout the year.
“The onset of the breeding season occurs as the days shorten and nights lengthen from late summer onwards. The longer nights and shorter days cause an increase in the hormone melatonin to be released from the pineal gland in the brain.
“This causes a chain of events to occur and leads to a maturation of follicles within the ovary and the ewe to come into oestrus. The ewe will then come into oestrus approximately every 15 to 17 days during the breeding season unless she conceives.”
Melatonin implants are injected under the skin of the ear and release the hormone melatonin, explains Mrs Whitehead.
She says: “The hormone overrides the natural melatonin production of the sheep and advances the onset of the breeding season.
Teaser rams, or vasectomised rams, can be used to both advance the onset of oestrus activity by about a week, and tighten up the lambing period. This is because of the ram effect, says Mrs Whitehead.
She explains: “As well as being affected by day length, a sheep’s breeding behaviour can also be influenced by the presence of sexually mature rams.
“Sexually mature rams secrete pheromones which cause an immediate and rapid increase in the hormone required in the ewe for ovulating and coming into oestrus at the start of the breeding season.
“Providing that the ewes have been kept away from the sight and smell of rams for at least three weeks prior to the ram being introduced, most ewes will have a silent ovulation without showing any outward signs of coming into oestrus within three to four days of ram introduction.
“Approximately half of these sheep will then have a normal inter-oestrus period and will exhibit normal oestrus behaviour 18 to 20 days later. The rest of the sheep will have another silent oestrus six to eight days later, followed by a normal inter-oestrus period.
“Normal oestrus activity will therefore commence with two peaks in activity in the flock between 18 and 26 days after ram introduction and the majority of the ewes will be mated at these times.
“Once the ewes have started showing signs of normal oestrus activity their oestrus cycle is unaffected by the presence of rams.
The vasectomy of a ram is a simple procedure which can be undertaken by the vet either on farm or at the local veterinary practice, says Mrs Whitehead.
She explains: “It is generally carried out under sedation and local anaesthetic. When choosing suitable rams as teasers it is important to pick rams which are well grown, sexually mature and are showing good libido. Tail end rams are inappropriate.
“Some people also use Dorset breeds as vasectomised rams, because they are thought to produce higher levels of pheromones out of the normal breeding season.”
Keep an eye on the following when using teaser rams:
Progestogen sponges contain a synthetic progesterone and can be used either within the breeding season, or out of the breeding season in combination with a gonadotrophin injection to synchronise a group of ewes coming into oestrus, says Mrs Whitehead.
She explains: “A progestogen sponge is inserted for 12 to 14 days and the progesterone mimics the activity of a corpus luteum in the ovary.
“When the sponge is removed there is a sudden drop in progesterone levels, which during the normal breeding season, leads to ovulation and oestrus activity within 36 to 48 hours. Ewes which fail to hold in lamb at first service will cycle normally for the rest of the breeding season.”
Out of the breeding season, there is not enough pituitary activity and gonadotrophin release for oestrus to occur, so a gonadotrophin hormone injection should be given at the time of sponge removal. Generally pregnant mares serum gonadotrophin (PMSG) is used, says Mrs Whitehead.
Ewes which fail to hold in lamb to first service with this regime are unlikely to cycle normally until the start of the ewe’s normal breeding season.
Mrs Whitehead says: “When progestogen sponges are used to synchronise a group of sheep, the ewes should be introduced to the ram about 36 hours after sponge removal and the ram should stay with the ewes for 48 hours, by which time the majority of the ewes will have been mated.
“If the rams are introduced sooner than this, a small percentage can come into season earlier and the rams will become tired from repeatedly serving these sheep before the rest come into oestrus – potentially leading to poorer conception rates in the rest of the group.
“Where a large number of ewes are synchronised a ratio of one ewe to 10 rams should be used. Any more than this and the rams will not cope with the number of services required within this period, again leading to poorer conception rates.”