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Reproduction Chapter
The following is an excerpt from the Sheepman's Production Handbook. To learn more about this subject, please order the Sheepman's Production Handbook. Details on ordering this invaluable resource can be obtained at https://sheepusa.org/about/materials.shtml .
Introduction
Reproductive efficiency, or percentage of lamb crop raised and marketed, is the major factor affecting profitability of a sheep flock. It may not always be economically feasible to maximize the lamb crop raised, but increasing the percentage of lamb crop or pounds of lamb marketed should be a primary goal of U.S. producers. Assuming that a sheep enterprise is being conducted at a profit, it will generally be desirable to maximize output from the most limiting resource. In most cases this is feed or land area on which to grow feed. Thus, an increased reproductive rate will often be a more efficient way to increase income rather than an increase in numbers of ewes. The primary factors affecting nutritional efficiency of lamb production are percentage of lamb crop raised and weight of lamb marketed relative to weight of ewe maintained (Figure 1). Approximately 70 percent of the feed required to produce a lamb is that consumed by the ewe. Thus, it is advantageous to spread ewe maintenance cost over more and/or larger lambs. In range production systems, a portion of ewe maintenance cost can be recovered through wool sales. However, the gain in nutritional or economic efficiency associated with increased number or size of lambs still offers the greatest potential for increased profitability of most range sheep enterprises. Figure 1. Influence of Reproductive Rate and Slaughter Weight on Efficiency of Lamb Meat Production.
Improved lamb production may result from (1) more lambs per lambing, (2) more frequent lambing (including fewer dry ewes), (3) increased proportion of total sheep in the breeding flock (i.e., mating ewe lambs and increasing longevity of the ewe flock), and (4) reduced death loss of lambs from birth to market. The logic in most of these factors is self-evident, but perhaps not in all. For instance, more frequent lambing generally implies some type of accelerated lambing, but the presence of dry ewes reduces lambing frequency on a flock basis. Increasing longevity of the ewe flock reduces replacement requirements and increases the proportion of the lamb crop that can be marketed. Reproductive efficiency should be measured as net lamb crop raised to weaning or market. This emphasizes the importance of death loss of lambs as a source of loss in reproductive efficiency.
Ovulation rate (number of ova shed from the ovary at estrus) sets the upper limit for the lamb crop from any given breeding. However, the presence of an ovum does not ensure a live lamb at market, as reproductive wastage is substantial. Wastage may occur due to failure of fertilization or implantation, abortion or death of the fetus, or death of the lamb following birth. Thus, attempts to improve reproductive efficiency must be concerned with increasing ovulation rate and reducing embryo wastage and lamb losses. The first step in improving reproductive efficiency is realizing a flock's genetic potential through effective management. Simultaneously, attempts should be made to increase the genetic potential. Options include selecting for reproductive efficiency within the flock, switching to a more prolific breed or developing a crossbreeding program that increases the genetic potential. These approaches are not mutually exclusive. There should be considerable interaction among environmental conditions, management systems, choice of breeds and selection. Genetic change tends to be permanent and may be a more economical way to improve reproductive efficiency than environmental modifications or management changes, which usually must be repeated regularly. However, improved management may permit utilization of a genotype with a higher lambing potential.
Intensive Systems for Reproductive Management
Overview
Accelerated lambing refers to ewes lambing more frequently than once per year. Intensive reproductive management can reduce maintenance costs of breeding stock per offspring reared, will often increase net return and will provide a more uniform supply of lamb throughout the year. Ewes in annual production systems are pregnant approximately 145 to 150 days per year, lactate 45 to 120 days and are idle 95 to 160 days. Studies have shown that ewes which lamb in the fall may breed back as soon as 40 days afterward. This short postpartum interval in lactating ewes occurs only during the peak of the breeding season. Clearly, an average ewe could lamb every 190 days instead of every 365 days, if nutritional level and seasonal anestrus were not limiting factors. Whether one is interested in accelerated lambing or simply out-of-season lambing, certain techniques and genetic stock are essential to success. Primary advantages of out-of-season lambing are (1) to market lambs when lamb supply is low and prices are high and (2) to produce lambs when labor supply or feeding conditions (rate of growth and cost of feed) may be more favorable. Some potential disadvantages which may be encountered include lower fertility, lower prolificacy and smaller birth weights. One can partially mitigate some of these disadvantages by exposing ewes to rams for a second time during the normal breeding season to settle any ewes which failed to conceive early in the year. Use of prolific, long-season breeds with continued selection for twinning and normal fall birth weights should help to offset the problem of low prolificacy and light birth weight in fall-lambing ewes.
It is not possible at present to increase the frequency of lambing and
avoid the complication of seasonal restrictions. However, a knowledge of
reproductive physiology, selection10:08 AM 7/12/02 and useful management
practices has led to successful intensive management systems under some
conditions. These systems usually require higher resource and management
inputs and should be considered only when a producer can provide adequate
nutrition for the ewe and lamb. These systems are not recommended for extensive
range conditions lacking the necessary feed resources and management options.
The economics of an intensive reproductive management system must be studied
carefully. Increased income from extra lambs must compensate for added costs
and labor input. Desirable traits for breeds and breed crosses for optimizing
intensive systems are:
1. Ewes that can breed successfully at any season of the year.
2. Ewes that can mate while lactating, or have the capacity to manage early
weaning.
3. Ewes that have a good lambing rate, i.e., twinning throughout the year.
4. Sires that produce a desirable market lamb and have both the libido and
fertility to bring about conception at any time of year.
Obviously, these traits have not been fully attained. However, much progress has been made in recent years. We now have breeds that are relatively prolific at times other than the normal breeding season. Active selection for shortening or eliminating seasonal anestrus is proceeding at several experiment stations and encouraging results are being seen. Light treatment of the ram (16 hours dark and 8 hours light for 12 weeks) improves libido and fertility during the normal anestrous season with certain management conditions.
Rambouillet, Dorset, Polypay and Barbados Blackbelly are breeds with long breeding seasons which appear most adapted to accelerated lambing and are used most frequently in the United States. Crosses among these breeds and with Finnsheep also exhibit good potential for intensive reproductive management systems as well as an increased lambing rate. Selection of the best seed stock is important when a producer is developing programs which require frequent lambing. Flocks maintained on a successful fall lambing or accelerated lambing system in a geographic location most similar to the producer's appear to be the best source for breeding stock.
The closer the flock is located to the equator, the longer the breeding season and the less complete and shorter will be the seasonal anestrus. The longer the breeding season, the more successful an accelerated lambing program is likely to be. Therefore, accelerated lambing should be easier to manage with respect to seasonality in Texas and other more southern latitudes than in Canada and the northern United States. However, the nearer one lives to the equator, the less intensive the breeding season appears to be, resulting in lower lambing rates even at the most favorable season.
As stated previously, there is always some complication with seasonal restriction in an intensive reproductive management program. It is quite possible to attempt to breed at less favorable periods and end up with fewer lambs per year than with a single-lambing season at the most favorable period. Thus it is important that some of the lambing periods coincide with the more favorable breeding seasons. For example, in a "three lamb crops in two years" system, one might be able to breed successfully in October, June and February. October would result in both high fertility and high ovulation rates. However, in June both fertility and ovulation rate10:16 AM 7/12/02s will be lower, especially in less prolific breeds. In February, prolificacy will be lower than in October, and lactation of the summer lambing ewe may be impaired. Selection might greatly reduce these problems in time. Normally, April is considered to be well into the nonbreeding season. Yet in 1987 at the U.S. Sheep Experiment Station in Dubois, Idaho, 73 percent of Polypay ewes and 69 percent of Rambouillet ewes conceived in April with overall lambing rates of 120 and 114, respectively; those values were much higher than normal for this area. Scientists at Las Cruces, New Mexico, have started selecting for breeding in May. The first year, over half the Rambouillets and Polypays lambed in the fall with lambing rates of 1.5 and 1.7, respectively. Likewise, in Blacksburg, Virginia, a selected flock bred in May each year, is now lambing over 80 percent of the ewes in October and November. The "ram effect" is probably responsible for a part of the success in each case.
Several research centers and sheep producers are working to develop improved genetic resources and management programs for intensive systems. These studies, depending on breed, system and management practices used, report increases in lambing frequency from about 0.93 per year on once per year lambing to 1.2 to 1.5 lambings per ewe per year. The studies also show increases of 0.3 to 0.9 lambs weaned per ewe per year for accelerated systems over conventional lambing systems.
Control of Photoperiod and Out-of-Season Breeding
As stated previously, day length appears to be the primary factor controlling breeding season in sheep. High levels of nutrition and exposing ewes to sterile rams near the end of anestrus are management tools for enhancing early breeding for fall lambing.
Day length can be artificially controlled to induce estrus and ovulation during anestrus. However, total confinement and light control are required, which greatly increase management costs and restrict options to use crop residues and forages harvested or grazed by sheep during the normally lower-cost postweaning and mating periods.
Accelerated Lambing Systems
Some intensive reproductive management systems are described below.
Three Lamb Crops in Two Years
Several variations of this system have been tried in order to attempt an average lambing interval of eight months, or a lambing frequency of 1.5 lambings per ewe per year. These systems have generally been characterized by a fixed mating and lambing schedule such as May mating/october lambing, January mating/june lambing, September mating/february lambing. Others have modified these dates slightly to 7-7-10 or 7-8-9 month intervals to better fit their climatic, management and feed resources. If a ewe misses once in two years, her potential is one lambing per year.
Producers have developed a variation of this system that provides for a more continuous lambing schedule. The flock is divided into four groups on a staggered eight-month lambing interval schedule. If a ewe fails to conceive with her group, she has a second chance to mate two months later, or on a 10-month lambing interval. A ewe that missed only one mating period in three cycles (two years) would average 1.39 lambings per year, and 1.29 lambings per year if she failed to conceive during two mating periods. Research results have varied from a 10 to 15 percent increase in percentage lamb crop marketed per ewe with Hampshire x Rambouillet and Suffolk x Rambouillet ewes in Virginia, to a 43.5 percent increase in lamb production with Rambouillet ewes in Texas. Researchers at Oklahoma State University reported results from studies involving various combinations of the Dorset, Finnsheep, and Rambouillet breeds in which all breed groups averaged over two lambs born per ewe per year. These results represented a 30 to 35 percent increase over conventional annual lambings. Scientists at Purdue University reported that Rambouillet ewes performed better than Columbia ewes on the accelerated schedule.
Producers using the staggered two-month interval schedule have reported up to 40 percent increase in lamb production over previous conventional systems. They also suggested that, by dividing the flock into four groups, substantial savings in facilities' costs are possible. Increased management attention can be given to critical lambing and early lactation periods since all ewes are not lambing at the same time.
Five Lamb Crops in Three Years
In this system, developed by Cornell University and often called the Star system, the calendar year is divided into five segments (the points of the star) that each represent one-fifth of a year, or 73 days (see Figure 23). The star can be rotated to give the most suitable dates. Two-fifths of a year is 146 days, which is approximately the gestation length of a ewe. The flock can be divided into three groups in this system. When the system is in operation, during the first 30 days of each segment one group is lambing at the same time another group is being bred. The next 35 days in each segment would represent lactation for one group, late gestation for the second group, and early gestation for the third group. The second and third groups can be managed together, thus reducing the system to two groups. Lambs from the lactating ewes would be weaned seven to eight days before beginning the next breeding/lambing period. Ewes bred at the first period or point of the star would lamb 146 days later at the third point and could mate 73 days later at the fourth point to lamb 146 days later at the first point in the next year. This system actually produces five lamb crops in three years, at a 7.2-month lambing interval. A ewe that did not miss a mating period in three years would lamb at each point of the star and average 1.67 lambings per year. Missing one 73-day cycle in three years would result in an average of 1.56 lambings per year, while missing two cycles would result in 1.47 lambings per year. Missing three cycles would reduce it to 1.33 lambings per year. The Cornell Dorset flock, which has been on some form of accelerated lambing for 15 years, averages approximately 1.5 lambings per ewe per year. The 1/2 Finnsheep x 1/2 Dorset ewes have had a longer lambing interval, with approximately 1.33 lambings per ewe per year, but this is more than overcome by the extra 0.5 lambs per lambing in the 1/2 Finnsheep ewes. For example: Dorset ewes @ 1.7 lambs/lambing x 1.5 lambings/year = 2.55 lambs born/year and 1/2 Finnsheep x 1/2 Dorset ewes @ 2.2 lambs/lambing x 1.33 lambings/year = 2.93 lambs born/year. More information is needed regarding expected lambing intervals and rates for sheep managed on this system and costs of production. One or another of the five segments may need to be eliminated on a particular farm or in a particular climate. For example, lambing prolific ewes in August in a hot climate could cause too many problems.
Figure 23. STAR Accelerated Lambing System, Cornell University, November, 1983. Copyright ©1984 Cornell Research Foundation.
Two Lamb Crops per Year
Scientists at Oklahoma State University using Dorset, Rambouillet and Dorset x Rambouillet ewes reported that percentage lamb crop born was increased by 25 to 30 percent by lambing twice a year. The crossbred ewes performed better than either of the parent breeds.
Current studies on developing new breeds or lines of sheep that will lamb at six-month intervals are in progress at the Roman L. Hruska Meat Animal Research Center at Clay Center, Nebraska. In theory, this system would permit the maximum number of lambings per ewe, but it is unlikely this will be realized in practice. Even though this system is not recommended for commercial use at this time, it is hoped these important studies will result in production systems which approach twice per year lambing.
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