Ram effect in ewes

Ewes ovulate naturally in response to the shortening day length that occurs during autumn and early winter. During the hours of darkness, ewes produce melatonin, and when this is increased it stimulates a complicated chain of hormonal activities that ultimately leads to ovulation.

However, ewes can be stimulated to ovulate at other times of the year using the ram effect.

How it works

Rams produce a chemical substance called a pheromone, which stimulates the onset of oestrus.

If ewes have had no contact with rams, are totally isolated from each other and are not cycling, they can be stimulated to start ovulation by the sudden introduction of rams.

After the introduction of the rams, ovulation will occur at 3–4 days but will be an undetectable 'silent heat'.

A silent heat is characteristic of the first ovulation:

  • of the breeding season, usually March to the end of May
  • induced by the ram effect
  • of puberty.

Following this 'silent heat', there will be 2 normal oestrus 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. You can expect that 60–70% of ewes will conceive at their first normal oestrus. Of the ewes that don't, 60–70% will do so at the next cycle and so on until the end of joining.

If ewes and rams are in constant contact, through sight or smell, pheromones are much less effective at inducing ovulation. Generally the ram effect only works if the rams and ewes have had no contact (sight, sound, smell) for at least 6 weeks. Keeping them 1 to 2km apart is usually a sufficient distance.


The great value of the ram effect is in the synchronisation of oestrus activity. Large numbers of ewes ovulate, conceive and lamb over a short period of time and in concentrated bursts.

This also:

  • maximises conception rates in a controlled joining
  • increases the effectiveness of predator control and other lambing management procedures
  • ensures lambs are a similar size at marking, weaning and first shearing
  • improves the likelihood of a majority of maiden ewes reaching the required weight at their first joining.


Ewes that are stimulated to ovulate by the ram effect during poor seasonal conditions, and don't conceive at the first normal oestrous, may fail to exhibit normal oestrous at their next cycle. Instead, they revert to anoestrous and exhibit an undetectable 'silent heat' at the subsequent cycle. For this reason, it's important to have the correct number of young, healthy rams with your ewes right from the start of joining.

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 (minimum 40 to 45kg) and condition at joining, which in turn determines the timing of the onset of puberty.

Teaser wethers

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 don't interfere with the mating activity of the rams and don't have to be removed from the ewes before the rams are introduced.

Consult your veterinarian regarding preparation of teasers.