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The following is an excerpt from the Sheepman's Production Handbook.
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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
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
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
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
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
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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