NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

Preparation Of Early Lambing Ewes For Mating

The costs involved in early lamb production are high, and it is clearly advantageous to achieve high lambing percentages. However, many early lambing flocks struggle to achieve lambing percentages above 120% and barren rates often exceed 10%. In many cases these problems are associated with the fact that the ewes are seasonally anoestrous (unable to breed), or only just beginning to show oestrous behaviour at the time of ram introduction.

ADVANCING THE BREEDING SEASON

British ewes are all seasonal breeders. While the length of the breeding season varies with different breeds of sheep, in most cases peak fertility does not occur until late autumn. January lambing ewes require to be mated in August. However, while some Suffolk ewes may exhibit oestrus behaviour as early as July, the highest lambing percentages are generally achieved when they are mated in late October or November.

Sexual activity in ewes begins with declining day length, which results in secretion of the hormone melatonin. The onset of the breeding season is also influenced by -

ü ewe breed (being earliest in the Dorset horn and latest in hill breeds)

ü latitude and altitude (being earliest in lowground areas in the south of Britain)

ü age of the ewe (being earlier in adult ewes than in hoggs or gimmers)

ü stress (stressful husbandry such as shearing may delay the onset of oestrus activity)

ü the ram effect (vasectomised rams)

ü PMSG injections following a period of progesterone priming

ü melatonin implants

Cyclical follicular activity occurs in ewes throughout the year, but during the non-breeding season, follicles do not mature and ovulate, and the ewes do not show signs of oestrus behaviour. Ovulation only occurs when adequate amounts of luteinising hormone are released under the influence of melatonin, which is secreted in response to decreasing day length. At the beginning of the breeding season (the transitional period), in the absence of adequate progesterone priming, ovulation is not accompanied by behavioural oestrus.

The length of the transitional period can be reduced, thus advancing the breeding season by one to two weeks by –

ü the presence of rams (the ram effect)

ü the use of PMSG injections following a period of progesterone priming using intravaginal progestogen sponges

The ram effect

Sexually active rams secrete pheromones from sebaceous and scent glands in their skin. These pheromones cause a rapid increase in the frequency of luteinising hormone pulses in ewes which are approaching their natural breeding season. Provided that they have been kept away from the sight or smell of rams for at least 3 weeks, most ewes have a silent ovulation within 3-4 days of ram introduction. In some, the subsequent corpus luteum (tissue in the ovary which produces progesterone) is maintained for a normal period, followed after 18 – 20 days by normal oestrus activity. In the others, the corpus luteum regresses after 6 – 8 days followed by silent ovulation and normal oestrus activity after another 18 days. Thus, normal oestrus activity commences with two peaks of activity in the flock between 18 – 26 days after ram introduction.

Vasectomised teaser rams can be used in this way to advance peak oestrus activity by one to two weeks and tighten up the lambing period. Two vasectomised rams are usually required per 100 ewes. Although vasectomised rams are often left in for a few weeks, only 48 hours exposure is sufficient to achieve results. Furthermore, it is essential that vasectomised rams are removed and replaced with fertile rams no later than 14 days after their introduction. Fertile rams can also be used as teasers, but a small proportion of ewes may become pregnant earlier than required. Because oestrous activity is synchronised following the use of teaser rams, a ratio of one sound fertile ram per 25 – 30 ewes is required. Your vet can advise you about the use of vasectomised rams in your flock.

Progestogen sponges must be inserted carefully and hygienically to avoid trauma and infection of the ewe’s reproductive tract. The dose of PMSG must be selected carefully to avoid the risk of excessively large litter sizes.

The timing of ram introduction following oestrus synchronisation is crucial. Rams should be introduced 36-40 hours after sponge withdrawal and left in for at least 48 hours. If rams are introduced too soon, they will repeatedly serve the first ewes to show behavioural oestrus, depleting their semen reserves for the later ewes. Rams should be reintroduced 16 days after sponge withdrawal.

A ratio of more than one ram to 10 ewes is required. Alternatively, synchronised ewes can be mated by artificial insemination. Your vet can advise you about the use of progestogen sponges and PMSG injections in your flock.

Melatonin implants.

Melatonin is a natural hormone, which is released from the pineal gland in the brain in response to declining day length. Under natural conditions, melatonin release increases about 10 minutes after the onset of darkness and returns to basal levels about 10 minutes after dawn. Increasing darkness triggers seasonal sexual activity. Under British conditions melatonin implants can override the effect of short day-length to give a modest advance in the breeding season. Ovulation rates can be increased to those of the peak breeding season, although the response depends on the breed of ewe and the depth of anoestrous at the time of implant administration.

Peak mating activity will occur about 25 days after ram introduction. Vasectomised rams and progestogen sponges can be introduced to the system to achieve further control of breeding. Your vet can advise you on the use of melatonin implants in your flock.

INCREASING THE OVULATION RATE

The lambing percentage is influenced by several factors, including ram soundness, ewe losses, abortion and perinatal lamb mortality. However, the maximum possible number of lambs is determined by the ovulation rate. A high ovulation rate is therefore the key to the achievement of a high lambing percentage.

The ovulation rate can be influenced by –

ü stage in the breeding season

ü body condition and plane of nutrition

ü ewe breed and type

ü age of the ewe

ü PMSG injections

Stage in the breeding season

The ovulation rate is generally highest in late autumn during the middle of the breeding season, and lowest at the beginning of the breeding season. Provided that the date of ram introduction is unaltered, any management practice which advances the onset of the breeding season in early lambing ewes will, therefore, result in an increased ovulation rate at the time of mating.

Body condition and plane of nutrition

The benefits of flushing ewes before mating are well recognised. Ewes which are in moderate body condition (score 2.5 – 3.0) generally respond best to a rising plane of nutrition for at least 3 weeks before and during mating. The response is less defined in heavy ewes (score > 4.0), but weight loss should be avoided during this period, because it generally results in a reduced ovulation rate. There is some breed variation in response to flushing with Scottish Blackfaces responding well while some types of Cheviots respond poorly.

Early lambing ewes are often in fat body condition at mating, following a long summer dry period. Excessive body condition can also result in reduced ovulation rates, so if possible ewes should be stocked tightly on medium quality pasture after weaning, to avoid the development of excessive body condition.

Ewe breed

The ovulation rate of breeds such as the Finnish Landrace are higher than those of hill breeds. However, there is also significant within breed variation in ovulation rates, which in some cases such as the Booroola Merino and Inverdale Romney can be selected for.

Age of ewe

The ovulation rates of hoggs and gimmers are generally lower than that of adult ewes.

PMSG injections

PMSG can superovulate ewes, but the response is variable. Some ewes don’t respond, while others over-respond. The main uses of PMSG injections are to induce ovulation outwith the breeding season, or to ensure good oestrus synchronisation as part of synchronisation and fixed time artificial insemination programme.

FERTILISATION

Rams should be checked for breeding soundness well in advance of the mating season. Rams need to be in good body condition for mating (score 3.5 – 4.0). It is also important that they are in good body condition and free of disease for at least two months before mating, during which time sperm production occurs.

Where possible ewes should be mated in small flat fields. Provided that the ewes are showing behavioural oestrus and sufficient sound rams are used, at least 95% of ewes should be pregnant by the end of a two cycle (34 – 40) day mating period.

NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Preparation Of Early Lambing Ewes For Mating

ADVANCING THE BREEDING SEASON

The ram effect

Summary: the use of vasectomised rams

Progestogen sponges and PMSG injections

Implants can be injected under the skin of the ear of early-lambing ewes in June to improve lambing percentages in January. A standard treatment regime would be –

Day 1 (30 weeks before lambing starts) – remove all rams from sight or smell of the ewes

Day 7 – implant ewes

Day 42 (35 days after implantation) – introduce rams