Contracting the Lambing Period
'The tup went in for the ewes to start lambing on the 1st March this year. However very few sheep have lambed in the first couple of weeks before they've finally got going around the 20th. Lambing time is going to drag on far too long.'
Does this sound familiar? Having a protracted lambing time can be a common problem. This is because as well as the breeding season of sheep being affected by day length, it is also influenced by the presence of sexually mature rams, known as the ram effect. Sexually mature rams secrete pheromones, causing an immediate and rapid increase in the hormone required in ewes 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 3 weeks, most ewes will have a silent ovulation without showing any outward signs within 3-4 days of ram introduction. In 40 to 60% of these ewes they will have a normal inter-oestrus period and will come into oestrus and be mated 18-20 days later. The remainder of the sheep will usually have a second oestrus 6-8 days later, followed by a normal inter-oestrus period. Therefore normal oestrus activity commences with 2 peaks of activity in the flock between 18 and 26 days after ram introduction, and the majority of ewes will be tupped at these times.
Once the ewes have started showing normal oestrus activity, their oestrus cycle is unaffected by the presence of rams, and this ram effect only occurs at the start of the breeding season. The onset of the breeding season can be manipulated however to advance it by using 1 of 3 methods.
Use of the ram effect
Teasers or vasectomised rams can be used to both advance oestrus activity by about a week, and tighten up the lambing period. When using teaser rams, a number of principles should be followed:
- The ewes should be kept away from the sight or smell of rams for at least 3 weeks before the teasers are introduced.
- They should be in with the ewes for up to 14 days, and no longer, before the rams are introduced.
- About 2 teaser rams should be put with 100 ewes.
- When the teasers are taken out, the stock rams should be put in, but a higher number should be used with approximately 1 ram per 25 to 30 ewes.
Vasectomy of rams is a procedure which can be very easily carried out by the practice. It is usually done on farm under local anaesthetic, with the ram being lightly sedated. 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 ram lambs are inappropriate.
Use of Progestogen Sponges and PMSG injections
During the breeding season intravaginal progestogen sponges can be inserted into the sheep. A progestogen sponge is inserted for 12 to 14 days and mimics the activity of a corpus luteum. When the sponge is removed there is a sudden drop in progesterone levels which in turn 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 however there is too little residual hormone activity for oestrus to occur, so a hormone injection of PMSG should be given at the time of sponge removal. When sponges are used in this way, most sheep which do not hold in lamb at this service will not cycle again until the start of the ewes' normal breeding season. Following removal of sponges the rams should be introduced about 36 hours later, and a ratio of no more than 10 ewes to 1 ram should be used.
Melatonin is released from the pineal gland in the sheep's brain during periods of darkness, and has to reach certain trigger levels for the onset of breeding. Melatonin implants artificially increase the levels of this hormone in the sheep, and when used correctly will advance the breeding season by approximately 1 month. A standard regime for using these implants would be:
Day 1 - remove all rams from sight or smell of the ewes
Day 7 – implant ewes and maintain isolation from sight or smell of rams
Day 42 – introduce rams
If you require any further help or advice with any of these regimes, please do not hesitate to contact the surgery for further information.