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The average ewe’s ovarian cycle takes 17 days. Table 1 shows the average length of the sections in the ewe reproductive cycle and the potential range seen between breeds.
In the breeding season, ewes will naturally ovulate every 17 days, but ovulation is further stimulated by feed changes, especially to green feed or a high-quality supplement, or by rams (see ‘Teasing’ in ‘Best Practice Joining’). One or more eggs are released from the ovary and make their way to the fallopian tube. Some breeds, particularly Merinos, are more likely to release a single egg but there is variation within the breed. The number of eggs released is also affected by season, nutrition and body condition score.
Oestrus and mating
Behavioural oestrus (heat) refers to the time that a ewe will show interest in the ram and stand to be mated. The length of time the ewe spends in oestrus varies from 30 minutes up to 36 hours, mainly affected by age, nutrition and season. Ewes do not show oestrus in the absence of rams. Rams show interest in the ewes in oestrus, as they are attracted by both the ewe’s behavioural cues and the smell of the pheromones released. Rams will sniff the ewes’ vulva region and perform ‘flehmen’, which involves extending the neck, turning the upper lip upwards and sniffing. This stimulates the ewe to stand to be joined. After the oestrus period is finished, the ewe will not stand to be mated.
Figure 1: Anatomy of ewe reproductive organs
Diagram courtesy of Queensland Dept. of Agriculture and Forestry
When the ram serves the ewe, they deliver a dose of semen about 1mL in volume, containing 3 billion sperm. The sperm travel through the vagina, past the cervix and reach the uterus. The strongest sperm then travel to the fallopian tube where they penetrate the waiting egg or eggs, and conception takes place.
The fertilised egg (embryo) sends a signal to the ovary to stop ovulating. The ovary then produces progesterone, the hormone that supports pregnancy. This also stops the ewe showing behavioural oestrus.