The demand for lamb is consistent but the supply is not. Because sheep are seasonal breeders, it’s difficult for farmers to supply lambs of consistent weight and quality throughout the year.
Dr. Daniel Poole, Department of Animal Science, North Carolina State University, discussed several factors that influence reproductive success in sheep, including breed, ewe age, time of year ewes are bred and nutrition.
In general, yearling ewes often have a single, triplets can be frequent and twins are most common. “We know some breeds are more prolific than others,” said Poole. “The number of offspring is determined by the ewe and the sex of offspring is determined by the ram.”
While seasonality is an obstacle to expanding the sheep and goat industry, out of season breeding can potentially increase profits by improving market efficiency. Poole said it’s important to understand the estrous cycle and hormonal influences of natural estrus because the same hormones can be manipulated to produce out of season lambs.
“The estrous cycle is 16 or 17 days,” said Poole. “The window during which the ewe is receptive to the ram and can be successfully become pregnant is small – about 24 to 36 hours. She needs estrogen to ovulate, and progesterone controls the set length of her cycle.”
Most sheep breeds are naturally seasonally polyestrous, which means they are sexually receptive during a given season. “Small ruminants are short-day breeders,” said Poole. “When daylength is short, they come into estrus. About 80% of females will breed naturally between early September and December 31.”
As daylength starts to increase after the peak of winter, sexual receptivity decreases. The lack of estrus during summer is due to the amount of daylength perceived by the eye and, ultimately, the pineal gland in the brain. The pineal gland is responsible for producing the hormone melatonin, and with decreased light and increased darkness, the pineal gland is simulated to produce melatonin.
“The increased melatonin goes to other parts of the brain that are specifically responsible for regulating reproduction,” said Poole. “The hypothalamus produces a gonadotropin-releasing hormone (GnRH). There are several commercially available products that do the same thing. This stimulates the gonadotropin follicle stimulating hormone and luteinizing hormone, which is important for ovulation. During fall, this activation of the pathway within the brain allows ewes to display sexual receptivity and, more importantly, ovarian stimulation.”
Ewes now have follicles growing and will ovulate when the proper signals are present. They will now have the opportunity to become pregnant. “This is the system in place in fall, and it’s what shuts down as we transition from winter to spring,” said Poole. “In spring we have increase in daylight, which inhibits the pineal gland. The result is decrease in melatonin, and a decrease in critical hormones that are responsible for stimulating the ovaries and sexual receptivity.”
Without the influence of essential hormones, ewes enter an anestrus (no estrus) period and ovaries are not stimulated during periods of extended daylength.
This hormonal challenge is what must be overcome in order to control the reproductive cycle and breed ewes when daylight is increasing. The understanding and technology necessary to synchronize estrus is available to bring females into heat to obtain lambs out of season or have lambs born to suit markets. With a shorter breeding season, farmers can create more concentrated lambing periods. Flock improvement through AI or ET is also more efficient when ewes are synchronized.
Synchronization can be useful within the normal breeding season to tighten up the lambing window for more uniform lambs and concentrated use of labor – or used outside the normal breeding season for out of season lambs.
Shepherds who choose to synchronize ewes should have good recordkeeping and management skills, appropriate facilities and sufficient labor for several short periods of intense work. For optimum response, facilities should be designed to minimize stress on sheep, and shepherds should understand when and how to properly administer hormone devices and/or injections.
One of the most common methods to synchronize estrus is through the use of CIDRs (controlled internal drug release).
During the normal estrous cycle, ovulation occurs and progesterone levels increase as the corpus luteus (CL) forms and releases progesterone. Progesterone remains high until the ewe doesn’t become pregnant, then the CL dies. The decrease in progesterone allows another opportunity for the female to display estrus. A group of ewes will typically display estrus at various times throughout a 16- to 17-day cycle, but synchronization with CIDRs condenses the process.
Commercially available CIDRs approved for sheep and goats contains 0.3 mg of progesterone, which is enough to suppress fertility for approximately 21 days. CIDRs can be used alone for simple synchronization or in combination with other hormone products such as prostaglandins or gonadotropins for timed AI or ET.
The CIDR device is inserted into the vagina with a “tail” sticking out. The tail is later used to remove the device. While in the ewe, the CIDR secretes progesterone, which is absorbed by the bloodstream. Blood progesterone levels rise steadily, suppressing other hormones.
“Animals show estrus two to three days after the CIDR is removed,” said Poole. “It’s simple to administer and provides strong synchronization because we know each ewe is getting a specific amount of progesterone.”
After the device is removed, usually after five days for simple synchronization, progesterone levels decline rapidly, and ewes will begin to show signs of standing heat and readiness for breeding.
Although CIDRs can be purchased without a prescription, Poole suggested sheep farmers maintain a good client-patient relationship with veterinarian for both planning and drug purchasing. They should work with their vet to monitor flock health and suitability for synchronization, and to ensure the devices and any additional hormones are being used properly.
Poole also cautioned anyone working with hormone therapies in sheep to wear gloves while handling materials and watch withdrawal times on all drugs.
by Sally Colby