Sheep & Goat Research Journal. Volume 17, No. 3: 2002 -- Special Issue: Breeding for Meat Production
Preface and Overview
On a world basis, sheep are kept for a variety of reasons, but in this country the primary uses have been meat and fiber production. At present there is also a limited and growing interest in milk production from sheep. One of the more recent and growing roles for sheep is that of vegetation management, including optimum grazing and range management practices (Havstad, 1994), control or assisting in the control of noxious vegetation (Olson and Lacey, 1994), reduction of fuel loads for fire control or retardation (Taylor, 1994), and reducing vegetative competition in reforestation efforts (Sharrow, 1994). Even flocks used for vegetative management must produce a marketable commodity to justify their costs or to provide an outlet for surplus animals. Because a majority of the world?s sheep are wool producers (at some level) it seems likely that in earlier periods fiber production was viewed as their more important contribution. Historically wool was an important item in world trade, but this special place is declining.
Selection for Reproductive Efficiency
Number of lambs weaned per breeding ewe has a greater influence on productivity of the sheep enterprise than any other trait. Net reproductive rate is determined by several components, with fertility, prolificacy (litter size) and lamb livability having the greatest influence (Wang and Dickerson, 1991). Age at puberty, prenatal viability and, in some enterprises, out-of-season fertility, can also contribute.
Genetic and Environmental Impacts on Prenatal Lamb Loss
Four 'facts' apply to most commercial sheep flocks:
- Number of lambs sold has a greater influence on flock income than any other factor under the producer's control.
- Ewes producing single lambs are moneylosers!
- Producers put in great effort at lambing to maximize lamb survival and then fight predators to keep lambs alive until marketing.
- MANY SHEEP FLOCKS LOSE MORE LAMBS BEFORE LAMBING THAN AFTER.
The average flock loses about 15% of lambs from birth to weaning. Numerous studies have shown that the embryonic loss rate in the first 30 days after mating often exceeds 20%. These are potential lambs which the producer never knew existed; however, they are just as surely lambs not marketed as are lambs eaten by coyotes, although lambs that die after birth will usually represent greater investment of feed and labor than those lost prenatally.
To the producer, the most important prenatal losses occur in ewes that ovulate two eggs but give birth to only one lamb. The ewe's costs for maintenance, labor, and depreciation are unchanged and usually more than the sale value of her resulting single lamb. Research results indicate that the percentage of twin-ovulators that lose one embryo ranges from about 10% to over 40%.
This paper will look at loss of potential lambs before lambing and some of the factors affecting loss rates.
It has been established that net reproductive rate (lamb crops weaned) is the largest contributing factor to efficiency of lamb meat production (Large, 1970). With the present low rate of return from wool production, it is imperative that producers who will survive must produce meat more efficiently. There is also a need to increase overall numbers of lambs produced in order to justify the maintenance of the necessary infrastructure to sustain the industry. On a flock basis there are a number of components of net reproductive efficiency including age at sexual maturity, length of productive life, seasonality of reproduction, frequency of lambing, ewe fertility, ovula-tion rate, embryo mortality and lamb survival. Among these, it has been suggested that under some conditions, reducing lamb mortality offers the greatest opportunity to improve the efficiency of the flock (Wang and Dickerson, 1991).
Opportunities to Reduce Seasonality of Breeding in Sheep by Selection
Seasonal reproduction is a serious problem for the sheep industry, reducing effectiveness of accelerated lambing programs, restricting flexibility to integrate lambing into other farm activities, and limiting access to favorable seasonal markets. Environmental or hormonal stimulation of reproduction requires increased investment in feed, labor, and (or) facilities, increases cost of production, often requires access to products that are not readily available or not approved for use in sheep, and may not be feasible in extensive or semi-extensive production systems. However, less intensive and less costly management interventions are available to improve reproduction; chief among these is use of the ram effect (Oldham and Fisher, 1992). In addition, substantial evidence exists to document genetic differences in seasonality of breeding, leading to opportunities to reduce seasonality by selection.
This review will address potential for genetic improvement of reproduction in sheep in both annual autumn and accelerated lambing systems. Satisfactory reproductive performance in both systems is mainly limited by the need to lengthen the breeding season to encompass spring and summer matings. In annual lambing, a shift in the annual pattern of reproductive behavior may be sufficient to meet the needs of the program, and ram effect is a useful tool for induction of estrus. In contrast, accelerated lambing systems place a premium on rapid rebreed-ing which is not required in annual lambing. Accelerated systems thus generally require more careful timing of ram effect and greater genetic sensitivity of ewes to ram introduction.
Selection to reduce seasonality of breeding involves application of the principles well-established. Selection among existing breeds is used to establish a flock with desirable initial characteristics. A breeding program is then designed to appropriately utilize complementary breed effects and hybrid vigor. And finally, selection within the flock is implemented to generate genetic improvement in economically important traits.
Strategies for Genetic Improvement of Carcass Value in Lambs
Improving carcass composition is one factor that can have an impact on lamb consumption and demand (Ward, 1995; Purcell, 1998). Increased size of cuts and decreased fatness are two factors that affect consumer acceptability of lamb (Jeremiah et al., 1993). The lamb producer that markets superior carcasses, with greater consumer appeal, expects to realize financial rewards from doing so. The expectation of greater income received from marketing superior lambs is motivation for producers to develop a strategy to improve carcass value through selection of genetically superior breeding stock. Using genetic selection to change traits measured on carcasses is different from many traits that can be measured on live animals because direct measurements are not available on the animals to be used for breeding stock. However, progress from selection on correlated traits can yield substantial changes over time.
The importance of increasing our knowledge of lamb carcass composition has been recognized for years. There were several publications from US scientists in the 1960?s that addressed prediction of lamb carcass composition, (Field et al., 1963; Judge et al., 1966; Spurlock & Bradford, 1965) lamb carcass value, (Carpenter et al., 1964; Carpenter et al., 1969; Cunningham et al., 1967) and genetic selection for improvements in carcass traits (Botkin, et al., 1969; Bradford, 1967). These US publications were preceded by earlier work of scientists in New Zealand (Barton and Kirton, 1958; Kirton and Barton, 1962; Kirton et al., 1962) and the UK (Bichard and Yalcin, 1964; Bowman et al., 1968). The 57th Annual Meeting of the American Society of Animal Science, held in 1965, included an invited presentation by Dr. G. E. Bradford (1967) titled: "Genetic and economic aspects of selecting for lamb carcass quality". The working definition of quality in this paper was "percent of lean meat, especially in the preferred cuts, and having desirable eating quality." One of Dr. Bradford's conclusions was "... significant genetic improvement in lamb carcass quality will depend upon the development of reasonably accurate live animal measures of carcass quality."
Considerable developments have occurred in the technology available to measure body composition in live animals. However, the change in carcass composition of US lambs has been limited. Although the technology to measure body composition is available, the financial incentive to make genetic improvement in body composition has not been large enough to encourage breeders to place much emphasis on carcass traits. Therefore, the issue of genetic improvement of carcass composition involves not only genetics and measurement of body composition, but also economics. Nsoso et al. (1999) reviewed several aspects of selection for growth and carcass composition. The purpose of this paper is to review issues relevant to developing a strategy for US lamb producers to select for improved carcass value in lambs.
Relationships Among Traits: Growth Rate, Mature Size, Carcass Composition and Reproduction
Genetic variation in any one trait is often associated with variation in other traits. Thus successful selection for one performance trait may impact other traits affecting efficiency of production. The correlated changes may be favorable or unfavorable, depending on the nature of the genetic relationships among the traits and the contribution of each to production efficiency.
Composite Trait Selection for Improving Lamb Production
When the value of wool is low, as at present, there is a greater potential for increasing both biological and economic efficiency of sheep production through improvement in meat production. It has been suggested that biological and economic efficiency can be increased more through genetic selection for improved reproductive rate than in growth rate or body composition (Fogarty et al., 1982). Reproductive rate is the most important component of total litter weight, which is clearly the single most important economic trait in American commercial sheep production. Loss of the Wool Incentive Program and lower wool prices in recent years have increased the economic importance of the total litter weight weaned per ewe. Current farm prices for wool and lamb indicate gross income from lamb exceeds that from wool by up to sixteen fold for most commercial producers of western white faced sheep. Hence, genetically increasing marketable litter weight per ewe is one of the most important contributions genetics can make to the economy of the sheep industry.
Increases in litter weight weaned can be made quickly through crossbreeding especially with prolific breeds. However, introduction of new breeds, often exotic, can result in unadapted genotypes with or without other desirable characteristics. Also, after crossbreeding has been thoroughly exploited, the only recourse for continued genetic progress is via selection for genetically superior individuals within breeds or crosses. It is important, therefore, to determine the relative effectiveness of alternative selection procedures for improving litter weight weaned.
The trait, litter weight weaned, is a composite trait affected by the expression of several genetically influenced traits. Variation in these component traits contributes to the phenotypic variation in the composite trait. Litter weight weaned is a combination of several different aspects of ewe reproduction (fertility, and litter size), ewe viability and offspring growth rate (mothering ability, milking performance, lamb survival, lamb growth rate). Thus, it is a convenient biological and economic measure of ewe productivity (Martin and Smith, 1980; Ercanbrack and Knight, 1985).
Long term selection for a composite trait may (but not necessarily) improve each individual component trait. Component traits within a composite trait should not be expected to improve at the same rate because they may differ in the genetic parameters involved. However, selecting for a composite trait should result in a balance among the component traits that produces an adapted animal, while selection for an individual trait can result in a reduction in adaptability. For example, selection response for a non-composite trait such as ovulation rate in sheep may be positive but gains in ovulation rate can be offset by decreased embryo survival (Bradford, 1985). Similarly, selection for increased litter size at birth may not be accompanied by increased milking performance and lamb growth rate. There may be limiting factors associated with favorable major genes such as the Booroola (FecB) allele which increases ovine ovulation rate substantially. While the FecB allele will increase litter size, there are associated decreases in lamb survival and weaning weight (Willingham and Waldron, 2000).
Direct selection for the composite trait of litter weight weaned in mice was three times as effective as selection for litter size for increasing litter weight weaned (Luxford and Beilharz, 1990). Long term selection in Targhee sheep for individual lamb weaning weight, rather than total litter weight weaned, resulted in decreases in lamb survival to weaning and ewe fertility (Bradford et al. 1999). From this last study, it is obvious that single trait selection for growth rate to weaning can improve weaning weight but it does not necessarily increase total lamb production per ewe. Thus, litter weight weaned per ewe exposed is the most appropriate composite trait to be used in selection for increasing total lamb production. The objective of this review is to characterize the composite trait litter weight weaned and its component traits.
Fundamental Aspects of Crossbreeding of Sheep: Use of Breed Diversity to Improve Efficiency of Meat Production
The sheep industry competes against beef, pork, poultry, and fish for food dollars of consumers who have many choices of high-quality meats. To compete effectively, the industry needs to produce uniform, nutritious, lean lamb that satisfies the eating preferences of consumers and to improve reproductive efficiency and reduce labor requirements so that seedstock and commercial flocks are both practical and profitable under a range of production environments. Although this situation indeed represents a difficult challenge, sheep producers have an invaluable resource to make necessary changes - a wealth of biodiversity represented by numerous breeds. Breeds of sheep have evolved over many thousands of years, their utility and function guided by their ability to adapt and survive in specific environments and production systems. Following domestication, further diversification among breeds has stemmed from selection by man for numerous characteristics, for example, appearance, color, size, shape, or wool production. Consequently, breeds of sheep differ markedly in adaptability to different environments and in levels of performance for traits that influence efficiency of production and product quality. Characteristics of each breed have a genetic basis and can therefore be exploited in structured crossbreeding systems designed for specific production-marketing situations. The purpose of this manuscript is to provide guidelines to improve efficiency of meat production through the appropriate use of breeds in crossbreeding systems.
Use of Finnsheep Crosses in a Western Commercial Sheep Operation
The Hamilton family has been engaged in agriculture, including sheep production, in California for more than 130 years. The farming and ranching operation is diversified, and includes row crop farming, dry land grain and safflower farming and livestock which also includes cattle. The sheep flock today consists of about 3800 commercial whiteface ewes that are 3/8 Finn, and about 85 purebred Suffolk ewes.
The climate of the area is Mediterranean, with rain from October or November to March or April, and thus with a dry season of at least six months in most years. Sheep are an integral part of our cropping system, and particularly important to the dryland wheat and barley production, which is our largest cropping enterprise. Following harvest of the grain crop, the sheep graze the crop residues during the summer. The land is left fallow the following season or sometimes two seasons, but with the rains there is a substantial "volunteer" crop of grasses and forbs, which provide good grazing for the sheep. If not grazed, this growth would make crop preparation the following season more difficult, especially in better than average rainfall years. Sheep have an advantage over cattle in this system in that they cause much less compaction of the heavy clay soils. The ranch includes considerable areas of native grass range that are also grazed much of the year to complement the stubble and fallow grazing. The sheep are also used to enhance sensitive native California grasslands for the Nature Conservancy, Solano County Open Space and the California Fish and Game Department.
Ewes are lambed in two seasons, a fall lambing from October 18 to December 15, when 75% or more of the mature ewes lamb, and a winter lambing from January 20 to March 15, when the ewe lambs and remainder of the mature ewes lamb. Ewes in each group are pregancy tested and separated by fetal count and estimated stage of gestation. Mature ewes with singles are generally field lambed, while those carrying multiples and all ewe lambs are barn lambed. A very successful fostering system is used to maximize the number of ewes raising twins and minimize the number of ewes that fail to raise a lamb.