Breeding Programs of Sheep
The most common breeding program is annual lambing in which autumn-mated ewes lamb in the spring. However, to produce lambs for more lucrative markets, ewes are bred in early autumn or late summer to lamb in winter (eg, January for the Easter lamb market in March or April) or bred in late spring to lamb in autumn (eg, September for the Christmas lamb market). This requires the use of programs that advance the ovulatory season into the late summer (ram effect, melatonin) or induce estrus during the anovulatory period in the spring (progestagens in the form of CIDR devices or MGA; photoperiod manipulation). However, producers whose flocks lamb out of season rarely adhere to an annual breeding program, because overall production is generally too low to account for the increased costs associated with lambs born in the winter or autumn; instead, these producers adopt an accelerated program to take advantage of the ewe’s ability to lamb more than once per year.
The two most popular accelerated lambing programs are the “3 lambings in 2 years” (3 in 2) and the Cornell Star system. The former requires the producer to manage two flocks that lamb every 8 mo (January, May, and September), alternating the first and second flocks. For example, in the northern hemisphere, over 24 mo, a ewe would be bred in the ovulatory season to lamb in the spring (December to lamb in May), then the transition season to lamb in the winter (August to lamb in January, and then the anovulatory season to lamb in the autumn (April to lamb in September). If the ewe was found to be open at pregnancy scanning ~50 days after breeding, it could be “slipped” to the other flock and reexposed to the ram. To be successful, breeding exposures would need to be short to shorten the lambing interval, and lambs would need to be weaned by 2 mo of age to allow breeding back.
The Cornell Star system is similar but more tightly controlled. Ewes lamb every 7.2 mo instead of every 8 mo. This means a ewe can lamb five times in 3 yr. Length of exposure to the ram, time for ewes to wean lambs and be rebred, lambing, and lactation are all very short.
Although the benefits of these programs are considerable in terms of increased productivity and access to lucrative lamb markets, they require a high level of management to work well. Poor flock fertility tends to create a system heavily weighted to spring lambing, because ewes bred in the late spring that do not conceive are rebred in the autumn when more naturally fertile. Disease control programs must be very well managed; there is little “down time” in the barns or pastures for cleanup of contaminated environments. In addition, ewe nutrition must be such that body condition scores do not get too low during lactation, because there is no opportunity for ewes to regain weight after weaning and before breeding. Breeds that are naturally fertile out of season (eg, Dorset, Merino) have an advantage in these systems.
Accelerated lambing systems are commonly used in northeastern USA and Canada and are becoming more common in South Africa and New Zealand. However, the increased revenue from extra market lambs sold into higher value markets must be carefully weighed against the increased input costs such as feed availability, labor, and housing (for winter lambing).