Shepherds can achieve their goal of having uniform lambs for certain markets by using controlled internal drug release devices, also known as CIDRs.
Photos by Sally Colby.
At this time of year, many shepherds are thinking about the right time to put the flock ram in with the ewes. The flock has been flushed on new-growth pasture or grain, and ewes are in good condition at the end of summer. With everything in place, there's no reason not to expect that close to 100 percent of the flock will conceive on the first or second heat and deliver healthy lambs. What if most of the ewes were bred to lamb within several days, or even at a different time of year?
It isn't news that most sheep breeds are wired to come into heat when daylight decreases. For many years, this meant that lambs were born in spring, usually at the time pasture started to grow. When shepherds learned to manipulate the initiation of breeding season, ewes would cycle earlier, even in August, so lambs would be ready for the Easter market. However, the increasing demand for lamb at different times of the year and shepherds' desire to maximize profits have led to more interest in altering breeding times.
Dr. Keith Inskeep, professor of reproductive physiology at West Virginia University, has done extensive research with estrus synchronization and out-of-season breeding. He cites improved prices to meet niche markets for specific holidays as one reason for using these techniques. "There are also benefits through lower losses to predators," he added. "In early studies where we looked at losses, feed costs and other factors, we found that there was about a 6 percent loss to predators in spring lambs, but only 3 percent loss for fall lambs. Coyotes are taking young lambs in spring to feed their young."
Early work with synchronization in the 1940s involved progesterone injections every one to three days. In the 1970s, prostaglandin was used to stimulate estrus. In the late 1980s, New Zealander Bob Welch developed the controlled internal drug release device, or CIDR. "It's made of plastic spline with silicone rubber containing the progesterone," explained Inskeep. "The progesterone is delivered from there."
The CIDR is placed into the ewe's vagina with an applicator, and "wings" on the device spread and hold it behind the vulvovaginal sphincter so that it stays in place until it's pulled out. Inskeep said loss rate is low - never more than 2 percent in any of the studies he's done.
In 1998, Inskeep was asked to work toward FDA approval for the use of CIDRs for out-of-season breeding of sheep. After three years of research, Inskeep and his team submitted data to the FDA, and in October 2009 the device was approved. "It's a slow process," said Inskeep. "That explains why many of the products on the market for other species aren't available for sheep and goats; they're considered minor species, and the market isn't big enough for companies to fund research." Inskeep noted that a federal program for minor species and the West Virginia legislature supported CIDR research.
The CIDR works by delivering progesterone that is embedded in the device directly to the reproductive system of the anestrous (not in heat) ewe. The progesterone prepares the animal to show estrus, followed by ovulation in response to the introduction of the ram. "The most powerful tool we're working with is the introduction of the ram," said Inskeep. "Ewes that have been isolated from rams for a month are very responsive to ram introduction." According to Inskeep, what normally happens with ram introduction is that within 10 to 20 minutes, ewes begin to release more luteinizing hormone (LH), which is responsible for triggering ovulation. "The frequency of LH release from the pituitary gland in anestrous ewes is about once every four hours. If we put a new ram in that the ewe hasn't seen before, that changes to once every hour," noted Inskeep. Once estrogen production is stimulated, a surge in LH occurs in about 39 hours. About 24 to 27 hours later, ovulation occurs.
Breeds vary in their response to estrus manipulation. Ewes such as this North Country Cheviot tend to have a short breeding season and don't respond as well to out-of-season breeding in comparison to long-season breeds such as Dorset, Rambouillet, merino and Finnsheep.
However, the initial ovulation results in a corpus luteum (a temporary endocrine structure that produces high levels of progesterone) that lasts only a few days before regressing. The ewe does not show signs of estrus in that first ovulation, and often doesn't show signs in the second ovulation. "Then there's a normal luteal phase, and she finally shows signs of heat about 18 to 23 days after the ram has been introduced," said Inskeep. "The ram is a powerful stimulus for the ewe to ovulate, but it takes a while to get estrus with ovulation. When we pretreat with progesterone, we get estrus and ovulation together, about 45 to 60 hours after the ram is introduced." Inskeep noted that this scenario applies to nonlactating ewes in good body condition.
Breeds vary in response to estrus manipulation. "We know that breeding seasons differ in length among different breeds," noted Inskeep. "We have short breeding seasons in Southdowns, Cheviots, North Country Cheviots and Border Leicesters, and long breeding seasons in Dorsets, Rambouillets, merinos and Finnsheep. Hampshires and Suffolks are intermediate, and St. Croix, Katahdin and other hair sheep breed year-round."
Inskeep reminds producers that breeding season length for sheep is a highly heritable trait - probably the most heritable of all reproductive traits. "We can select for the ability to breed out of season," said Inskeep. "That means if you keep fall-born replacements, you'll select for responsiveness to the CIDR and ram introduction. But if you let that dollar convince you to sell those ewe lambs instead and save ewe lambs from spring breeding, you won't make any progress in increasing responsiveness in your flock."
Variables in feeding methods, feed costs, lamb prices and other factors throughout the year determine whether or not out-of-season breeding will be profitable. "In general, it's more profitable now than it was in the past," said Inskeep. "But if you're paying too much for feed, it won't be as profitable. In a pasture situation, where no grain is being purchased for ewes, you can make good money with this system today."
The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania. Comment or question? Visit http://www.farmingforumsite.com and join in the discussions.