The "ram effect" is when the sudden introduction of a ram or "trailer" stimulates ovulation in a non-cycling ewe. Rams produce chemicals called pheromones, the scent of which stimulates the onset of estrus. Pheromones are much less effective in inducing estrus when the ewe and ram are in constant contact (visual or olfactory). Ewes that are not cycling when a ram (or trailer) is introduced will ovulate within 3 to 4 days. This first ovulation will be a "silent" heat, which is undetectable by the ram. After this silent heat, there will be two normal estrus peaks, with some ewes cycling around day 18 and the rest around day 25. Ewes that do not become pregnant during these times may return to heat for another 17 days. It is expected that 60 to 70 percent of ewes will become pregnant during the first normal estrus. Of the remaining ewes, 60 to 70 percent should conceive during the second estrus.

Although recommendations vary, ewes should be isolated from rams for at least 6 weeks to allow the ram effect to work. The ewes must have no contact with the rams by sight or smell, which means they must be separated by distance. The ram lambing effect is not as effective. The ram effect is most effective during the transition period when the ewes have not yet started cycling but are almost ready. The breed of ram can influence how the ewe responds to the ram effect, with breeds that have fewer seasons being more effective in bringing non-cycling ewes into estrus.

The great value of the ram effect is the synchronization of estrous activity, which will result in a large number of ewes ovulating, conceiving and lambing in a relatively short period of time. To be effective, it is important to have a sufficient number of young, healthy rams. Prepped (vasectomized) rams or testosterone-treated teats can also stimulate the ram effect.

Selection: Selection is another way to obtain an early lambing flock, but it requires a long-term commitment. The heritability is generally thought to be less than 10%. By selecting for spring fertility, Virginia Tech researchers were able to reduce seasonal brooding to 11 days with a crossbred flock of 1/2 Dorset x 1/4 Rambouillet x 1/4 Finnsheep.