Management Strategies for Improved Fall-Lambing

Livestock Update, April 1997

Steve Umberger, Animal and Poultry Sciences, Virginia Tech

Of the three major lambing seasons, fall lambing (September to November) is the most difficult to accomplish because of the inability of most breeds of sheep to cycle and breed in the spring and early summer. There are many benefits to fall lambing, some of which include: 1) the ability to take advantage of fall forages for ewes in late gestation and early lactation; 2) weather conditions are usually ideal for pasture lambing; 3) there is more time to feed lambs to heavier weights for the traditionally high spring lamb market; 4) there are fewer problems with internal parasite control; and 5) there are fewer problems with coyote predation. Although fall lambing is a challenge, there are management strategies that can be used by producers to significantly improve their fall-lambing success rate. Research conducted by Virginia Tech Animal Scientists during the past ten years has demonstrated techniques that consistently put October and November lambing rates at 60 percent and higher. In general, these strategies are not dependent upon specific breeds or breed crosses in the ewe flock, but do require strict attention to management detail.

The primary management tool used to improve fall-lambing rate is the "ram effect." Although the term is not new, many producers are unaware of the physiology of the ram effect. The ram effect is a phenomenon whereby anestrus (non-cycling) ewes are induced to begin cycling through the spontaneous introduction of a ram. Ewes must have been previously isolated from rams (no fenceline contact) for a minimum of three to four weeks. Within minutes after ram introduction, there is a hormonal response in the ewe that leads to ovulation within approximately 50 hours. Typically, this ovulation is not accompanied by heat and is commonly referred to as a "silent" estrus. After ovulation, corpora lutea are formed on the ovary, which are responsible for the production of progesterone, which is the primary hormone responsible for maintenance of early pregnancy. In approximately 50 percent of the ewes responding to the ram effect, a premature regression of the corpora lutea occurs by about day six in the estrous cycle, rather than at 15 days when normal regression should occur. Consequently, a second ovulation takes place, which is also accompanied by silent estrus. All subsequent ovulations are accompanied by behavioral estrus at which time the ewes are serviced by the rams. Thus, in ewes responding to the ram effect, there are two peaks of breeding activity that occur in and around days 18 and 24 after ram introduction. The success rate of the ram effect is highest when breeding occurs from mid-spring through early summer.

Breed of ram can influence both the number of ewes responding to the ram effect and their overall pregnancy rate. Research conducted at the Shenandoah Valley Agricultural Research and Extension Center in Steeles Tavern, Virginia, showed a difference in lambing rate and time of lambing for Suffolk x Rambouillet ewes exposed to Dorset or Suffolk rams for June and July breeding. Two weeks before breeding, ewes were penned next to Dorset, Suffolk, or no rams. Dorset rams stimulated 24 percent of the ewes to lamb during the first 14 days of lambing compared to 9 percent of the ewes exposed to Suffolk rams and 10 percent of the ewes with no ram exposure. The overall lambing rate was 75 percent for ewes exposed to Dorset rams for breeding compared to only 54 percent for ewes exposed to Suffolk rams for breeding. Therefore, results from this two-year study indicate an advantage for Dorset rams both in manifesting the ram effect and settling ewes for out-of-season breeding.

Additional research conducted by Virginia Tech has shown the benefit of using synthetic progestogens, which are commercially available for beef cattle, to improve the response of the ram effect. The two products that have been used in these studies are melengestrol acetate (MGA) and Syncro-Mate-B. Melengestrol acetate is administered orally through the feed for the suppression of estrus and ovulation in feedlot heifers. It is also used for estrous synchronization in beef cattle. Syncro-Mate-B is administered by subcutaneous ear implant for estrous synchronization in beef cattle. The use of progestogens cause the initial ovulation after ram introduction to be accompanied by behavioral estrus and may also result in improved pregnancy rates. In Virginia, on-farm studies conducted with these two products have resulted in fall lambing rates ranging from 65 to 90 percent.

Before using progestogens, ewes must be isolated from rams for a period of at least three to four weeks. With the MGA treatment, ewes are fed .3 mg MGA per day at the same time each day for a period of 10 days. A minimum of 10 inches of trough space per ewe must be available to help insure adequate consumption of product each day. Rams are placed with ewes on day 10 when MGA feeding is discontinued. The low cost of the MGA treatment is its primary advantage. The ten-day treatment is less than $1.25 per ewe. When using Syncro-Mate-B, one-half of the cattle implant is inserted subcutaneously on the back of the ear. After ten days, the implant is removed and the ewes are given a 5 mL injection of PG-600, which is a commercially available product for the swine industry containing 400 IU pregnant mare serum gonadotropin (PMSG) and 200 IU human chorionic gonadotropin (hCG). If PG-600 is not available, the Syncro-Mate-B option should not be considered. None of these products are labeled for sheep. Therefore, their use should only be administered under the supervision of a veterinarian with the appropriate veterinarian-client relationship. While the cost of progestogen treatment is lower for MGA, pregnancy rates and synchronization of lambing have been consistently higher for the Syncro-Mate-B/PG-600 treatment. A minimum ram to ewe ratio of 1:15 should be used following progestogen treatment. Rams should be equipped with marking harnesses so that breeding activity can be monitored. A majority of the ewes should exhibit estrus at 36 to 48 hours after ram introduction.

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