Key points
- Ewes cycle naturally in response to shortening day length, which stimulates melatonin production and ultimately leads to ovulation (peak March to end May).
- Merino ewes are not strongly seasonal breeders so they can be stimulated to ovulate at other times of the year using the ram effect. In most British breeds the ram effect will only work about six weeks before the natural breeding season begins.
- The ram effect occurs when non-cycling ewes are stimulated to ovulate by the smell of pheromones produced by rams (or teasers).
- The ram effect relies on ewes and rams being totally isolated from each other by about two kilometres for six weeks.
- The initial ovulation will be a non-detectible “silent heat” at three to four days after the introduction of rams (or teasers). Two peaks of oestrous activity around days 18 and 25 will follow this. Ewes that do not conceive may cycle again in about 17 days time. Poor seasonal conditions can result in ewes reverting to anoestrous at subsequent cycles.
- Sixty to seventy per cent of ewes usually conceive at the first normal oestrous, 60-70 per cent of the remainder will conceive at the next cycle, and so on until the end of joining.
- The ram effect may not be as effective with maidens as body weight and condition at joining influence ovulation more.
Ewe breeding season
Ewes ovulate naturally in response to the shortening day length that occurs during autumn and early winter. As daylight hours decrease (or more importantly, night time hours increase) the pineal gland in the upper part of the brain is stimulated to produce increasing amounts of a substance called melatonin. Melatonin is only produced during the hours of darkness. This increase in melatonin production stimulates a complicated chain of hormonal activities, which ultimately leads to ovulation.
The natural breeding season for ewes usually lasts from about February to June, with the peak from about March through to the end of May (stimulated by decreasing day length). British breeds enter the non-breeding season (ovulation will not occur) abruptly in June-July while Merinos gradually drift towards the non-breeding season in October-December (some Merino strains continue to cycle all year round).
Since merino ewes are not strongly seasonal breeders they can be stimulated to ovulate in a regular cycle at other times of the year by using the ram effect. Where as rams can only stimulate ovulation in most of the British breeds about six weeks before the normal time of onset of the breeding season (i.e. from early to mid January). A few British breeds, notably the Dorset, have a less rigidly defined breeding season.
How does the ram effect work?
Ewes that are not cycling and have had no contact with rams can be stimulated to commence ovulation by the sudden introduction of rams (or teasers). Rams produce a chemical substance called a pheromone, the smell of which stimulates the onset of oestrous. Pheromones are located in the suint of ram’s wool (and teasers for a short time). When ewes and rams are in constant contact (sight or smell) pheromones are much less effective at inducing ovulation.
Ewes that are not cycling when the rams (or teasers) are introduced will ovulate within three to four days. This initial ovulation will be a “silent heat”, which cannot be detected by the rams. A silent heat is characteristic of:
- the first ovulation of the breeding season (i.e. March to end of May)
- the first ovulation induced by the ram effect
- probably the first ovulation of puberty.
Following this “silent heat”, there will be two normal oestrous peaks, with some ewes cycling around day 18 and the remainder around day 25. Ewes that do not conceive at either of these times may cycle again after approximately a further 17 days. It can be expected that 60 to70 per cent of ewes will conceive at their first normal oestrous. Of the ewes that don’t, 60 to 70 per cent will do so at the next cycle and so on until the end of joining.
It is important to remember that under poor seasonal conditions, ewes that are stimulated to ovulate by the ram effect and do not conceive at the first normal oestrous may fail to exhibit normal oestrous at their next cycle. Instead, they revert to anoestrous and exhibit a non-detectible ‘silent heat’ at the subsequent cycle. For this reason, it is important to have the correct number of young, healthy rams with the ewes right from the start of joining. When teasers are used to stimulate the onset of oestrous, rams must be with the ewes before the first cycle. This normally means putting the rams out on about day 14 after the introduction of teasers.
The great value of the ram effect is in the synchronisation of oestrous activity. Large numbers of ewes ovulate, conceive and subsequently lamb over a short period of time and in concentrated bursts. This maximises conception rates in a controlled joining, increases the effectiveness of predator control and other lambing management procedures, ensures lambs are of more even size at marking, weaning and first shearing and improves the likelihood of a majority of maiden ewes reaching the required weight at their first joining.
How long do ewes need to be isolated?
Although current research has shown conflicting results, it is generally best to keep rams isolated from ewes for a minimum period of six weeks. Ewes must have no contact by either sight or smell, which means they need to be separated from the rams by a distance of approximately two kilometres. This also includes neighbours rams.
Teasers
Teaser wethers can also stimulate the ram effect. They produce pheromones and exhibit mating behaviour similar to that of rams for a short period of time. This is sufficient to induce ovulation and the teasers do not necessarily have to mount the ewe to be effective. Teaser wethers do not interfere with the mating activity of the rams and do not have to be removed from the ewes before the rams are introduced. Consult your veterinarian regarding preparation of teasers.
Maiden ewes
The ram effect is not as consistent or effective with maiden ewes. The reasons for this are not clear. Ovulation in maiden ewes is influenced more by body weight (about 35 kg) and condition at joining which in turn determines the timing of the onset of puberty.
Author: Ken Wilson, formerly of DPI&F
Revised by: Deborah Maxwell, DPI&F
