Understanding reproduction in sheep helps us be good ranch managers, have highly productive flocks and use resources wisely. One of the most important points to understand is that the number of lambs produced by the ewe flock differs depending on when you breed your sheep. Consider the following information in your flock management practices.
The main determinant of sheep reproduction is the season of the year. Sheep are seasonal breeders, with the majority being short-day breeders that cycle in the fall and lamb in the spring. However, some breeds of sheep will cycle in the spring and have lambs in the fall. Through the eye of the sheep, its brain perceives day length and sends appropriate signals (hormones) to the reproductive system to begin the breeding season. The nonbreeding time for these ewes is called seasonal anestrus.
As ewes come out of seasonal anestrus, they are in a transitional phase. During this phase, some ewes begin to express estrus and ovulate before others in the flock. In mid-breeding season all the ewes should be cycling. As the season comes to an end, a smaller percentage of the flock will be active. A similar situation is true for the number of ovulations (eggs shed) per estrus.
That is, the number of ovulations per cycle starts off low, maximizes during middle of the breeding season and then decreases at the end. In order for the estrus to be fertile, ovulation must accompany estrus. Early in the breeding season, some ewes may express estrus without an ovulation (infertile estrus) and some ovulations occur without estrus (silent estrus). What all this means is that maximum fertility occurs midseason.
Since early, mid, and late seasonal differences occur for number of ewes in estrus, number of ewes ovulating and number of ovulations per estrus, decisions on breeding seasons need to take into account these differences so that ewe pregnancy rates and lambing rates (number of lambs per ewe) are maximized for the system. Usually the more lambs that can be born into a flock, the more profitable the operation is. This is because the overhead of maintaining ewes can be supported by the sale of many lambs. However, some marketing schemes may be profitable with lower reproductive rates because of the advantages of timing sales and specialty markets.
Genetics (across and within breed) also play a part in the seasonality of the estrous cycle. Some breeds are capable of out-of-season breeding (for example, the Dorset breed). And, some animals within a breed are more able to express those characteristics than others. Choose a breed of sheep that matches your season of choice and then monitor your own flock for the animals that best represent that breed. Keep and use ranch records to pick replacement ewes and rams that help meet your objectives.
Interestingly, ranch location also influences the percentage of ewes in estrus and ovulating and the ovulation rate. Since seasonality is driven by day length, longitude or location in relation to the equator influences the strength of this effect on the ewe. Here in the Pacific Northwest we are further north compared to Texas, and so our ewes are more seasonal in their breeding than the same ewes would be there (See Table 1). Use caution when choosing replacements from other locations.
And, I wouldn’t be a good nutritionist if I did not mention that nutritional status affects whether or not a ewe will cycle once the seasonal indicators (day length) are right. Flushing (improving plane of nutrition) and body condition score (fatness or body energy reserves) both influence the reproductive system. Temperature, ewe age and the presence of a ram also influence reproduction.
| Ewes in estrus (%) | Ewes ovulating (%) | Ovulation rate | ||||
|---|---|---|---|---|---|---|
| Idaho | Texas | Idaho | Texas | Idaho | Texas | |
| January | 100 | 100 | 100 | 100 | 1.89 | 1.48 |
| February | 100 | 100 | 100 | 94 | 1.57 | 1.14 |
| March | 89 | 40 | 94 | 52 | 1.50 | 1.12 |
| April | 26 | 38 | 32 | 32 | 1.37 | 1.16 |
| May | 02 | 31 | 02 | 31 | 1.00 | 1.00 |
| June | 07 | 44 | 07 | 75 | 1.00 | 1.24 |
| July | 06 | 94 | 06 | 94 | 1.00 | 1.73 |
| August | 12 | 86 | 41 | 100 | 1.75 | 1.48 |
| September | 88 | 94 | 100 | 94 | 1.72 | 1.66 |
| October | 100 | 94 | 94 | 100 | 1.80 | 1.25 |
| November | 100 | 97 | 94 | 100 | 1.86 | 1.44 |
| December | 100 | 100 | 100 | 100 | 1.88 | 1.44 |
You can find more information on reproduction, nutrition and genetics of sheep on the OSU website. Good luck with your breeding plans this season and the following ones. I would be glad to discuss any of this information with you and help develop flock management plans.
- 1Adapted from Hulet et al. 1074 (from ASIA Sheep Production Handbook. 2002 Edition, Vol. 7)
