- Email: [email protected]
Lamb Production Systems for Increasing Productivity
Lamb Production Systems for Increasina Productivity .
-#
~
by Hudson A. Glimp1 and Frank H. Baker2, 3 Winrock International, Rt. 3, Morrilton, AR 72110
Summary
Introduction
This paper discusses modifying the biological cycles of sheep to increase the productivity of sheep in intensive systems of management. The 1 47 ·day gestation period of sheep theoretically permits a ewe to produce a iamb every six months . However, the normal photoperiodic mechanisms regulating mating habits and reproduction capabilities must be adjusted by genetic selection or environmental manipulation to actually permit lambing more often than once a year. Adjusting a ewe flock to an accelerated lambing system usually requires higher resource inputs to achieve the optimum outputs of lamb and wool per unit of resources used . An increase in pounds of lamb marketed per ewe can be achieved by three methods: -increasing the number of lambs per lambing - increasing the slaughter weight of lambs -increasing the lambing frequency to more than one lamb crop per year Essential traits for breeds and breed crosses for ac· celerated lambing systems are: -ewe breeds that approach or exceed two lambs per lambing -ewe breeds that mate and reproduce any time during the year -sire breeds with hardiness, ease of lambing, ability to mate throughout the year, rate of gain, and efficiency of gain to heavier slaughter weights Systems of accelerated lambing under experimentation by research stations and under test by producers are described. These include: -three lamb crops in 2 years -the Cornell University Star System which is designed to produce five lamb crops in three years -two lamb crops per year
Accelerated lambing is defined as the attempt to mate and lamb ewes more often than once per year. The term "intensive lambing systems" is often used interchangeably with "accelerated lambing systems." Intensification generally implies the increased use of appropriate available resources-management, capital, land, genetics, nutrition-to economically increase productivity. Productivity is measured by pounds of lamb and wool marketed (output) per unit of labor, capital, and other resources (inputs) . Those involved in accelerated lambing systems, either for research or commercial purposes, would agree that increased resource inputs are necessary for optimum productivity. The basis of assumption that accelerated lambing is possible is based on 1) the relatively short gestation period (14 7 ± 7 days) and 2) the postpartum interval to completion of involution of the uterus (30 to 40 days) of the ewe. These factors suggest that lambing intervals of less than 180 days are possible. Several exogenous and endogenous factors, however, influence the interval from parturition to mating and conception . These have been extensively studied, and include factors such as photoperiod, endocrinology, ambient temperature, lactation, breed, nutrition, and age of ewe. Extensive reviews have been published by Shelton (10), Hulet (4), Ortavant (6), Thimonier and Cognie (12), and many others. Extensive research investigations are continuing in many of these areas, which have been summarized for the United States by the SID Research Digest (11). Three basic methods exist for increasing the pounds of lamb marketed per ewe: 1) increasing the number of lambs reared per lambing; 2) increasing the slaughter weight of lambs; and 3) increasing the lambing frequency to more than one lamb crop per year (accelerated lambing). These methods are listed in the order of our understanding of how best to use them in intensive production systems. Increasing the number of lambs marketed per ewe depends on both the genetic capability of the ewe to produce more lambs and the management capability to save the lambs born and efficiently grow them to market
1 Consultant, present address, Agricultural Research Service, USDA Southwestern Livestock and Forage Research Station, EI Reno, 73306. 2Senior program officer. 3Reviewed by U. S. Garrigus and John R. Romans.
24
ACCELERATED LAMBING weight. It has been estimated that over 15% of the lambs born in the United States do not survive to weaning-management problems such as starvation, predators, and respiratory diseases are the primary causes of lamb mortality. These levels of lamb losses due to management problems are not acceptable in intensive production systems. Breeds and breed crosses are available in the United States that have the genetic potential to produce more than two lambs per ewe per lambing. Several commercial producers are marketing over two lambs per ewe per year from once-a-year lambing systems. Increasing the market weight of lambs is feasible if the breeds used have the genetic potential to grow to heavier weights without becoming too fat. The average slaughter weight of lambs in the United States has increased by over 30 lb. (13.6 kg) in the last 40 years, and this trend is expected to continue. There are genetic lines within the Suffolk, Hampshire, and Columbia breeds that will approach 150 lb. (68.2 kg) in 150 days without excessive fat. Rams from these lines are excellent candidates for terminal sires in intensive production systems. Two points of caution: 1) as litter size increases, optimum slaughter weight may decrease due to the smaller mature size of the more productive ewe breeds and the smaller size of lambs from multiple births; and 2) the rate of increase in slaughter weight acceptable to the consumer is largely dependent on our ability to produce lambs with less fat content than those currently produced. Research results from accelerated lambing programs in the United States have been reported by Carter (1), Doane (2), Outhouse (7), Schwulst and Banbury (9), Shelton (1 O), and Whiteman et al. (14). Several research centers and sheep producers are continuing to develop improved genetic resources and management programs for accelerated lambing systems. These studies, depending on the breeds, accelerated system, and management practices used, generally reported an increased lambing frequency to 1.2 to 1.4 lambings per ewe per year, and an increase of 0.3 to 0.7 lambs weaned per ewe per year over conventional lambing systems. Accelerated lambing systems are used more routinely in several European countries, particularly the Mediterranean countries with important sheep-dairy industries. Several examples of accelerated lambing may be observed in higher potential equatorial zones of Africa and other continents. Breeds and Crosses for Accelerated Lambing The breeds to use in accelerated lambing must, to the extent possible, have the following characteristics. -The ewe breeds must approach or exceed two lambs per lambing. The added investments cannot be justified at lower lambing rates. -The ewes must breed practically any time of the year. Sheep are "photoperiodic" in their mating habits
25
and reproductive capabilities. Breeds developed closer to the equator, where day-length variation is minimized, tend to have longer mating seasons. There also appears to be some positive relationship between high fecundity and the ability of the ewe to mate out of season. It is not possible to develop an accelerated lambing system without at least one-third of the mating periods being in the months when fertility and ovulation rates are lower. -Commercial programs will use terminal sires on a large portion of the ewe flock. Important terminal-sirebreed traits to consider include hardiness, ease of lambing, ability of the ram to mate at any season, rate of gain, efficiency of gain to heavier slaughter weights, and desirable lean to fat ratios in the carcass. The Finnsheep, Rambouillet, and Dorset breeds as purebreds, as specific crosses, or as synthetic breeds combining various levels of these three breeds are being evaluated in accelerated lambing systems at research centers or are being used by commercial producers in the United States. The use of these breeds as purebreds in accelerated lambing programs may have limited commercial application, but is highly desirable from the standpoint of long-term progress through selection. The Finnsheep is the most prolific breed available in the United States. Research and practical experience indicate that, with good management, the lambing rate will be increased at least 1 % for every 1 % of Finnsheep breeding added to the genetic base of the ewe flock. The ewe flock should include at least 25% Finnsheep breeding, and up to 50% if management capabilities, nutrition, and other resources are adequate. Fall-born Rambouillet and Merino ewes from the southern United States are more likely than other available breeds to breed any time (Hulet, 4). Fall-born purebred Dorsets are also likely to breed out of season. Dorset ewes are more prolific (150% to 180% lamb crop) than Rambouillet and Merino ewes (130% to 150%). The Polypay, a new breed developed at the U.S. Sheep Experiment Station, Dubois, ID, that is composed of % Finnsheep X % Dorset X 114 Rambouillet X % Targhee, appears to have good potential for success in accelerated lambing systems. Progressive Polypay breeders are averaging lambing rates of 180% to 200%, and several are on accelerated lambing or fall-lambing schedules. Wool production does not appear to be adversely affected with good management in accelerated lambing systems. Although income from lamb is far more important to the commercial producer than is the income from wool, decisions on breed selections could be influenced by marginal differences in wool income. Current government programs have unduly increased the importance of wool income, as illustrated by the following example: With average lamb prices at $.70 per lb., the Dorset ewe would have to produce approximately 9 more pounds (4.1 kg) of lamb to equal the economic productivity of the Rambouillet ewe without the government pro-
26
GLiMP AND BAKER
EXAMPLE Ave . Lb. wooP market Total per ewe price/lb . per ewe
Breed
Rambouillet (fine wool) Dorset (medium wool)
$1.00
$11.00
$16.50
$27 .50
8
.60
4.80
7.20
12.00
6.20
=
Total wool income /ewe
11
Difference per ewe
1.2.2 lb.
1985 Incentive payment @ 150%
15.50
1 kilogram (kg).
gram, and 22 more pounds (10 kg) of lamb with the 1985 government-program payments. This economic difference is an important consideration, yet it may be risky to rely on it too heavily in the long term due to the transitory nature of government programs and potential shifts in demand for the various grades of wool. Control of Photoperiod
Day length may be artificially controlled to override the effects of photoperiod on mating and ovulation (6). The major disadvantage is that total confinement and light control are required, which restricts managements' options to use crop residues and forages harvested or grazed by sheep during the lower-input postweaning and mating periods. Research results in the United States and other countries indicate that implants of melatonin may reduce the influence of day length on the mating
Producers have developed a variation of this system that provides for a more continuous lambing schedule. The year is divided into 2-month intervals, and the flock is divided into 4 groups on staggered 8-month lambing intervai scheduies. if a ewe faiis to conceive witrl her group, she has a second chance to mate 2 months later, or on a 1 O-month lambing interval. A ewe that missed only one mating period in three cycles (2 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% increase in percentage lamb crop marketed per ewe with Hampshire X Rambouillet and Suffolk X Rambouillet ewes in Virginia (1 ). to a 43.5% increase in lamb production with Rambouillet ewes in Texas (10). Whiteman and Thomas (13) 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% increase over conventional once-a-year lambings. Outhouse (7) reported that Rambouillet ewes performed better than Columbia ewes on the accelerated schedule. Producers using the staggered 2-month-interval schedule have reported up to 40% increases 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 and that increased management attention can be given to critical lambing and early lactation periods since all ewes are not lambing at the same time. Cornell University STAR System
The STAR system has evolved from 1 5 years of research at Cornell by Hogue and Magee (3) on ac-
_ __ ~ s= ea=s~0~n~(~8~).~N=a=~_r~a~l~m~e~l=a~~~n=in~l~e~ ve~l~s~ in~c~ffiTIa~s~e~a~S~d~aY~_ _ ~c~ellile~r~ffi~arohWogffimh~from~e length decreases, and decrease as day length increases. beginning used purebred Dorset sheep, with % Finnsheep X % Dorset ewes added in 1984. In this system, A private company is conSidering seeking Food and Drug the calendar year is divided into five segments (the points Administration approval of melatonin implants, but is awaiting further research documentation and analyses of of the star) that each represent 1/5 of a ye a,r0r 73 days . market potential. Two-fifths of a year is 146 days, which is approximately the gestation length of a ewe. The flock can be divided Three Lamb Crops in 2 Years into three groups in this system. When the system is in operation, during the first 30 days of each segment one Several variations of this system have been tried to group lambs at the same time that another group is being achieve a lambing interval of 8 months, or a lambing frebred. The next 35 days in each segment would reprequency of 1 .5 lambings per ewe per year. Research sent lactation for one group, late gestation for the second results have been reported by Carter (1). Doane (2). group, and early gestation for the third group. The lacSchwulst and Banbury (9). Shelton (10). and Whiteman tating ewes would be weaned from their lambs 7 to 8 and Thomas (1 3) . These systems have generally been' days prior to the beginning of the next breeding/lambing characterized by a fixed mating and lambing schedule period . Ewes bred at the first point of the star would lamb similar to the following : December mating-May lambing; 146 days later at the third point and cou ld mate 73 days August mating-January lambing; April later at the fourth point to lamb 1 46 days later at the first mating- September lambing, etc. Others have modified point in the next year. these dates slightly to 7 -7 -10 month or 7 -8-9 month inThe STAR system/actually produces five lamb crops tervals to better fit their climatic, management, and/or in 3 years, at a 7 .2-month lambing interval. A ewe that did feed resources. If a ewe misses once in 2 years, her not miss a mating period in 3 years would lamb at each potential is one lambing per year.
ACCELERATED LAMBING point of the star, and average 1.67 lambings per year. Missing one 73-day cycle in 3 years would result in an average of 1.56 lambings per year, while missing two cycles would result in 1 .4 7 lambings per year, and missing three cycles would reduce it to 1.33 lambings per year. The Cornell data on the STAR system provided by Hogue and Magee (3) indicate that 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 Y2 Finnsheep X Y2 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 % Finnsheep ewes. For example: -Dorset ewes @ 1.7 lambs/lambing X 1 .5 lambings/year = 2.55 lambs/year % Finnsheep X % Dorset ewes @ 2.2 Iambs/lambing X 1.33 lambings/year = 2.93 lambs/year Two Lamb Crops per Year
Whiteman et al. (14), in a study involving Dorset, Rambouillet, and Dorset X Rambouillet ewes, reported that percentage lamb crop born was increased by 25% to 30% by lambing twice a year. The crossbred ewes performed better than either of the parent breeds. Lindahl and Terrill (5) reported on studies involving 15 years of selection for ability to lamb anytime of the year, which indicated that effects of season on mating ability can largely be removed by selection . Current studies on developing new breeds or lines of sheep that will lamb at 6-month intervals are in progress at the U.S. Sheep Experiment Station at Dubois, Idaho, and 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. Even though this system is not recommended for commercial use at this time, these important studies are likely to result in breeds with the highest genetic potential for success in accelerated lambing systems. Other Factors to Consider
This report has considered the types of accelerated lambing systems and the appropriate genetic resources for these systems. These considerations are primary determinants of success, since the ewes must have the genetic potential for high prolificacy and frequent lambing and the system must permit optimum expression of thes~
27
traits. Of equal importance, but not considered in detail in this paper, are such factors as management capabilities, feed resources, facilities required for total or partial confinement systems, capital investment requirements, and other factors that are essential to optimum productivity. All of these must be carefully considered before accelerated lambing can be recommended to the individual producer. As an example, the more uniform distribution of labor requirements throughout the year may be considered an advantage to the person interested in specializing in sheep production, yet it may be a disadvantage for the person having large investments in other enterprises that require high labor inputs at certain times of the year. Literature Cited
1. Carter, R. C. 1968. The Shepherd 13: 14. 2. Doane, B. B. 1967. Illinois Sheep Day Report. p. 13. 3. Hogue, D. E. and B. Magee. 1985 . Proc. Northeast Sheep Management Workshop, July 1985. 4. Hulet, C. V. 1977. Proc. Management of Reproduction in Sheep and Goats Symposium. Sheep Industry Development Program, Denver, CO. p. 119. 5. Lindahl, I. L. and C. E. Terrill. 1974. J. Anim. Sci. 38:328. 6. Ortavant, R. 1977 . Proc. Management of Reproduction in Sheep and Goats Symposium. Sheep Industry Development Program, Denver, CO. p. 58. 7. Outhouse, J . B. 1974. Purdue Univ. Agric. Expt. Sta. Bul. No. 49, p. 1. 8. Parker, C. F. 1986. ARS-USDA, Dubois, Idaho, Personal communication. 9. Schwulst, F. J. and E. E. Banbury. 1976. Progress Report 262, Colby Sheep Day, Cont. No. 62-5, p. 1.
10. Shelton, M. 1968. Proc. Symposium on Physiology and Reproduction in Sheep. Okla. State Univ., Stillwater. 11 . SID Research Digest. 1985. Vol. 1, NO.3. 12. Thimonier, J. and Y. Cognie. 1977. Proc. Management of Reproduction in Sheep and Goats Symposium. Sheep Industry Development Program, Denver, CO. p. 109. 13. Whiteman, J. V. and D. L. Thomas. 1977. Okla. Station Report. Proc. NC-111 Tech. Comm. Meeting. 14. Whiteman, J. V., W. A. Zollinger, F. A. Thrift, and M. B. Gould. 1972. J. Anim. Sci. 35 :836.
-#
~
by Hudson A. Glimp1 and Frank H. Baker2, 3 Winrock International, Rt. 3, Morrilton, AR 72110
Summary
Introduction
This paper discusses modifying the biological cycles of sheep to increase the productivity of sheep in intensive systems of management. The 1 47 ·day gestation period of sheep theoretically permits a ewe to produce a iamb every six months . However, the normal photoperiodic mechanisms regulating mating habits and reproduction capabilities must be adjusted by genetic selection or environmental manipulation to actually permit lambing more often than once a year. Adjusting a ewe flock to an accelerated lambing system usually requires higher resource inputs to achieve the optimum outputs of lamb and wool per unit of resources used . An increase in pounds of lamb marketed per ewe can be achieved by three methods: -increasing the number of lambs per lambing - increasing the slaughter weight of lambs -increasing the lambing frequency to more than one lamb crop per year Essential traits for breeds and breed crosses for ac· celerated lambing systems are: -ewe breeds that approach or exceed two lambs per lambing -ewe breeds that mate and reproduce any time during the year -sire breeds with hardiness, ease of lambing, ability to mate throughout the year, rate of gain, and efficiency of gain to heavier slaughter weights Systems of accelerated lambing under experimentation by research stations and under test by producers are described. These include: -three lamb crops in 2 years -the Cornell University Star System which is designed to produce five lamb crops in three years -two lamb crops per year
Accelerated lambing is defined as the attempt to mate and lamb ewes more often than once per year. The term "intensive lambing systems" is often used interchangeably with "accelerated lambing systems." Intensification generally implies the increased use of appropriate available resources-management, capital, land, genetics, nutrition-to economically increase productivity. Productivity is measured by pounds of lamb and wool marketed (output) per unit of labor, capital, and other resources (inputs) . Those involved in accelerated lambing systems, either for research or commercial purposes, would agree that increased resource inputs are necessary for optimum productivity. The basis of assumption that accelerated lambing is possible is based on 1) the relatively short gestation period (14 7 ± 7 days) and 2) the postpartum interval to completion of involution of the uterus (30 to 40 days) of the ewe. These factors suggest that lambing intervals of less than 180 days are possible. Several exogenous and endogenous factors, however, influence the interval from parturition to mating and conception . These have been extensively studied, and include factors such as photoperiod, endocrinology, ambient temperature, lactation, breed, nutrition, and age of ewe. Extensive reviews have been published by Shelton (10), Hulet (4), Ortavant (6), Thimonier and Cognie (12), and many others. Extensive research investigations are continuing in many of these areas, which have been summarized for the United States by the SID Research Digest (11). Three basic methods exist for increasing the pounds of lamb marketed per ewe: 1) increasing the number of lambs reared per lambing; 2) increasing the slaughter weight of lambs; and 3) increasing the lambing frequency to more than one lamb crop per year (accelerated lambing). These methods are listed in the order of our understanding of how best to use them in intensive production systems. Increasing the number of lambs marketed per ewe depends on both the genetic capability of the ewe to produce more lambs and the management capability to save the lambs born and efficiently grow them to market
1 Consultant, present address, Agricultural Research Service, USDA Southwestern Livestock and Forage Research Station, EI Reno, 73306. 2Senior program officer. 3Reviewed by U. S. Garrigus and John R. Romans.
24
ACCELERATED LAMBING weight. It has been estimated that over 15% of the lambs born in the United States do not survive to weaning-management problems such as starvation, predators, and respiratory diseases are the primary causes of lamb mortality. These levels of lamb losses due to management problems are not acceptable in intensive production systems. Breeds and breed crosses are available in the United States that have the genetic potential to produce more than two lambs per ewe per lambing. Several commercial producers are marketing over two lambs per ewe per year from once-a-year lambing systems. Increasing the market weight of lambs is feasible if the breeds used have the genetic potential to grow to heavier weights without becoming too fat. The average slaughter weight of lambs in the United States has increased by over 30 lb. (13.6 kg) in the last 40 years, and this trend is expected to continue. There are genetic lines within the Suffolk, Hampshire, and Columbia breeds that will approach 150 lb. (68.2 kg) in 150 days without excessive fat. Rams from these lines are excellent candidates for terminal sires in intensive production systems. Two points of caution: 1) as litter size increases, optimum slaughter weight may decrease due to the smaller mature size of the more productive ewe breeds and the smaller size of lambs from multiple births; and 2) the rate of increase in slaughter weight acceptable to the consumer is largely dependent on our ability to produce lambs with less fat content than those currently produced. Research results from accelerated lambing programs in the United States have been reported by Carter (1), Doane (2), Outhouse (7), Schwulst and Banbury (9), Shelton (1 O), and Whiteman et al. (14). Several research centers and sheep producers are continuing to develop improved genetic resources and management programs for accelerated lambing systems. These studies, depending on the breeds, accelerated system, and management practices used, generally reported an increased lambing frequency to 1.2 to 1.4 lambings per ewe per year, and an increase of 0.3 to 0.7 lambs weaned per ewe per year over conventional lambing systems. Accelerated lambing systems are used more routinely in several European countries, particularly the Mediterranean countries with important sheep-dairy industries. Several examples of accelerated lambing may be observed in higher potential equatorial zones of Africa and other continents. Breeds and Crosses for Accelerated Lambing The breeds to use in accelerated lambing must, to the extent possible, have the following characteristics. -The ewe breeds must approach or exceed two lambs per lambing. The added investments cannot be justified at lower lambing rates. -The ewes must breed practically any time of the year. Sheep are "photoperiodic" in their mating habits
25
and reproductive capabilities. Breeds developed closer to the equator, where day-length variation is minimized, tend to have longer mating seasons. There also appears to be some positive relationship between high fecundity and the ability of the ewe to mate out of season. It is not possible to develop an accelerated lambing system without at least one-third of the mating periods being in the months when fertility and ovulation rates are lower. -Commercial programs will use terminal sires on a large portion of the ewe flock. Important terminal-sirebreed traits to consider include hardiness, ease of lambing, ability of the ram to mate at any season, rate of gain, efficiency of gain to heavier slaughter weights, and desirable lean to fat ratios in the carcass. The Finnsheep, Rambouillet, and Dorset breeds as purebreds, as specific crosses, or as synthetic breeds combining various levels of these three breeds are being evaluated in accelerated lambing systems at research centers or are being used by commercial producers in the United States. The use of these breeds as purebreds in accelerated lambing programs may have limited commercial application, but is highly desirable from the standpoint of long-term progress through selection. The Finnsheep is the most prolific breed available in the United States. Research and practical experience indicate that, with good management, the lambing rate will be increased at least 1 % for every 1 % of Finnsheep breeding added to the genetic base of the ewe flock. The ewe flock should include at least 25% Finnsheep breeding, and up to 50% if management capabilities, nutrition, and other resources are adequate. Fall-born Rambouillet and Merino ewes from the southern United States are more likely than other available breeds to breed any time (Hulet, 4). Fall-born purebred Dorsets are also likely to breed out of season. Dorset ewes are more prolific (150% to 180% lamb crop) than Rambouillet and Merino ewes (130% to 150%). The Polypay, a new breed developed at the U.S. Sheep Experiment Station, Dubois, ID, that is composed of % Finnsheep X % Dorset X 114 Rambouillet X % Targhee, appears to have good potential for success in accelerated lambing systems. Progressive Polypay breeders are averaging lambing rates of 180% to 200%, and several are on accelerated lambing or fall-lambing schedules. Wool production does not appear to be adversely affected with good management in accelerated lambing systems. Although income from lamb is far more important to the commercial producer than is the income from wool, decisions on breed selections could be influenced by marginal differences in wool income. Current government programs have unduly increased the importance of wool income, as illustrated by the following example: With average lamb prices at $.70 per lb., the Dorset ewe would have to produce approximately 9 more pounds (4.1 kg) of lamb to equal the economic productivity of the Rambouillet ewe without the government pro-
26
GLiMP AND BAKER
EXAMPLE Ave . Lb. wooP market Total per ewe price/lb . per ewe
Breed
Rambouillet (fine wool) Dorset (medium wool)
$1.00
$11.00
$16.50
$27 .50
8
.60
4.80
7.20
12.00
6.20
=
Total wool income /ewe
11
Difference per ewe
1.2.2 lb.
1985 Incentive payment @ 150%
15.50
1 kilogram (kg).
gram, and 22 more pounds (10 kg) of lamb with the 1985 government-program payments. This economic difference is an important consideration, yet it may be risky to rely on it too heavily in the long term due to the transitory nature of government programs and potential shifts in demand for the various grades of wool. Control of Photoperiod
Day length may be artificially controlled to override the effects of photoperiod on mating and ovulation (6). The major disadvantage is that total confinement and light control are required, which restricts managements' options to use crop residues and forages harvested or grazed by sheep during the lower-input postweaning and mating periods. Research results in the United States and other countries indicate that implants of melatonin may reduce the influence of day length on the mating
Producers have developed a variation of this system that provides for a more continuous lambing schedule. The year is divided into 2-month intervals, and the flock is divided into 4 groups on staggered 8-month lambing intervai scheduies. if a ewe faiis to conceive witrl her group, she has a second chance to mate 2 months later, or on a 1 O-month lambing interval. A ewe that missed only one mating period in three cycles (2 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% increase in percentage lamb crop marketed per ewe with Hampshire X Rambouillet and Suffolk X Rambouillet ewes in Virginia (1 ). to a 43.5% increase in lamb production with Rambouillet ewes in Texas (10). Whiteman and Thomas (13) 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% increase over conventional once-a-year lambings. Outhouse (7) reported that Rambouillet ewes performed better than Columbia ewes on the accelerated schedule. Producers using the staggered 2-month-interval schedule have reported up to 40% increases 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 and that increased management attention can be given to critical lambing and early lactation periods since all ewes are not lambing at the same time. Cornell University STAR System
The STAR system has evolved from 1 5 years of research at Cornell by Hogue and Magee (3) on ac-
_ __ ~ s= ea=s~0~n~(~8~).~N=a=~_r~a~l~m~e~l=a~~~n=in~l~e~ ve~l~s~ in~c~ffiTIa~s~e~a~S~d~aY~_ _ ~c~ellile~r~ffi~arohWogffimh~from~e length decreases, and decrease as day length increases. beginning used purebred Dorset sheep, with % Finnsheep X % Dorset ewes added in 1984. In this system, A private company is conSidering seeking Food and Drug the calendar year is divided into five segments (the points Administration approval of melatonin implants, but is awaiting further research documentation and analyses of of the star) that each represent 1/5 of a ye a,r0r 73 days . market potential. Two-fifths of a year is 146 days, which is approximately the gestation length of a ewe. The flock can be divided Three Lamb Crops in 2 Years into three groups in this system. When the system is in operation, during the first 30 days of each segment one Several variations of this system have been tried to group lambs at the same time that another group is being achieve a lambing interval of 8 months, or a lambing frebred. The next 35 days in each segment would reprequency of 1 .5 lambings per ewe per year. Research sent lactation for one group, late gestation for the second results have been reported by Carter (1). Doane (2). group, and early gestation for the third group. The lacSchwulst and Banbury (9). Shelton (10). and Whiteman tating ewes would be weaned from their lambs 7 to 8 and Thomas (1 3) . These systems have generally been' days prior to the beginning of the next breeding/lambing characterized by a fixed mating and lambing schedule period . Ewes bred at the first point of the star would lamb similar to the following : December mating-May lambing; 146 days later at the third point and cou ld mate 73 days August mating-January lambing; April later at the fourth point to lamb 1 46 days later at the first mating- September lambing, etc. Others have modified point in the next year. these dates slightly to 7 -7 -10 month or 7 -8-9 month inThe STAR system/actually produces five lamb crops tervals to better fit their climatic, management, and/or in 3 years, at a 7 .2-month lambing interval. A ewe that did feed resources. If a ewe misses once in 2 years, her not miss a mating period in 3 years would lamb at each potential is one lambing per year.
ACCELERATED LAMBING point of the star, and average 1.67 lambings per year. Missing one 73-day cycle in 3 years would result in an average of 1.56 lambings per year, while missing two cycles would result in 1 .4 7 lambings per year, and missing three cycles would reduce it to 1.33 lambings per year. The Cornell data on the STAR system provided by Hogue and Magee (3) indicate that 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 Y2 Finnsheep X Y2 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 % Finnsheep ewes. For example: -Dorset ewes @ 1.7 lambs/lambing X 1 .5 lambings/year = 2.55 lambs/year % Finnsheep X % Dorset ewes @ 2.2 Iambs/lambing X 1.33 lambings/year = 2.93 lambs/year Two Lamb Crops per Year
Whiteman et al. (14), in a study involving Dorset, Rambouillet, and Dorset X Rambouillet ewes, reported that percentage lamb crop born was increased by 25% to 30% by lambing twice a year. The crossbred ewes performed better than either of the parent breeds. Lindahl and Terrill (5) reported on studies involving 15 years of selection for ability to lamb anytime of the year, which indicated that effects of season on mating ability can largely be removed by selection . Current studies on developing new breeds or lines of sheep that will lamb at 6-month intervals are in progress at the U.S. Sheep Experiment Station at Dubois, Idaho, and 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. Even though this system is not recommended for commercial use at this time, these important studies are likely to result in breeds with the highest genetic potential for success in accelerated lambing systems. Other Factors to Consider
This report has considered the types of accelerated lambing systems and the appropriate genetic resources for these systems. These considerations are primary determinants of success, since the ewes must have the genetic potential for high prolificacy and frequent lambing and the system must permit optimum expression of thes~
27
traits. Of equal importance, but not considered in detail in this paper, are such factors as management capabilities, feed resources, facilities required for total or partial confinement systems, capital investment requirements, and other factors that are essential to optimum productivity. All of these must be carefully considered before accelerated lambing can be recommended to the individual producer. As an example, the more uniform distribution of labor requirements throughout the year may be considered an advantage to the person interested in specializing in sheep production, yet it may be a disadvantage for the person having large investments in other enterprises that require high labor inputs at certain times of the year. Literature Cited
1. Carter, R. C. 1968. The Shepherd 13: 14. 2. Doane, B. B. 1967. Illinois Sheep Day Report. p. 13. 3. Hogue, D. E. and B. Magee. 1985 . Proc. Northeast Sheep Management Workshop, July 1985. 4. Hulet, C. V. 1977. Proc. Management of Reproduction in Sheep and Goats Symposium. Sheep Industry Development Program, Denver, CO. p. 119. 5. Lindahl, I. L. and C. E. Terrill. 1974. J. Anim. Sci. 38:328. 6. Ortavant, R. 1977 . Proc. Management of Reproduction in Sheep and Goats Symposium. Sheep Industry Development Program, Denver, CO. p. 58. 7. Outhouse, J . B. 1974. Purdue Univ. Agric. Expt. Sta. Bul. No. 49, p. 1. 8. Parker, C. F. 1986. ARS-USDA, Dubois, Idaho, Personal communication. 9. Schwulst, F. J. and E. E. Banbury. 1976. Progress Report 262, Colby Sheep Day, Cont. No. 62-5, p. 1.
10. Shelton, M. 1968. Proc. Symposium on Physiology and Reproduction in Sheep. Okla. State Univ., Stillwater. 11 . SID Research Digest. 1985. Vol. 1, NO.3. 12. Thimonier, J. and Y. Cognie. 1977. Proc. Management of Reproduction in Sheep and Goats Symposium. Sheep Industry Development Program, Denver, CO. p. 109. 13. Whiteman, J. V. and D. L. Thomas. 1977. Okla. Station Report. Proc. NC-111 Tech. Comm. Meeting. 14. Whiteman, J. V., W. A. Zollinger, F. A. Thrift, and M. B. Gould. 1972. J. Anim. Sci. 35 :836.