Fall-mated ewes that lamb in the spring is the goal of the most common breeding program. However, to produce lambs for more lucrative markets, more producers are lambing in the winter (eg, January for the Easter lamb market in March or April) or fall (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, vaginal pessaries, or MGA; photoperiod manipulation). However, producers that lamb out of season rarely stick 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 fall; instead, these producers adopt an accelerated program to take advantage of the ewe's ability to lamb more than once/yr.
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 that the producer manage 2 flocks that lamb every 8 mo (January, May, and September), alternating first Flock A and then Flock B. For example, in the northern hemisphere, Flock A lambs in January and the lambs are weaned in early April, at which time estrus would be induced in the ewes and they would be exposed to the ram, usually for only 1 cycle at that time of year. The ewes would be evaluated for pregnancy in June. Pregnant ewes from Flock A would lamb in September, and their lambs would be weaned in November. The ewes would then be exposed to the ram during their normal ovulatory period in December for <30 days, to lamb again in May. Then Flock A, after lambing in May, would have the lambs weaned in early August and be rebred during what is likely to be the transitional season (ram effect, melatonin, or exogenous progestagens) to lamb again in January. Thus, Flock A has lambed every 8 mo: January, September, May, and then again in January, or 3 times in 2 years. Flock B would be off-set from Flock A, thus lambing (in this example) in May, January, and then September.
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 5 times in 3 years. Length of exposure to the ram, time for ewes to wean lambs and be rebred, lambing, and lactation are all very short.
While 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 fall 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, labor, and housing (for winter lambing).
Last full review/revision July 2011 by Paula I. Menzies, DVM, MPVM, DECS-RHM