People from all over the
country raise sheep for a variety of reasons.
Here in Virginia we have several different active segments of the sheep
industry which comprise a variety of breeds or crosses in a wide ranging set of
management protocols and systems. Each enterprise
may function in different role or have a different role, however we all strive
to be efficient and economical.
Improving the
flock
Every flock owner strives to make a better or new and
improved lamb. In order to select
replacement animals in an objective manner and make "progress" or
improvement, there must be at least minimal performance records for the flock.
Selection should then based on estimated genetic merit of each individual, performance
records and in many cases phenotypic traits are used to estimate the genetic
merit (Remember that preferred phenotypic traits may vary greatly from one
producer to the other).
Let’s start with the most important issue right off
the top. Record keeping! It can be the most important task on the
farm. It doesn’t have to be extremely
difficult and can help you determine if the flock is actually making you money
or just a expensive, time consuming hobby!
Records can be kept in a little red or black book as many sheepmen do,
or you can go modern and hook computerized systems right to the chute or in the
barn. It doesn’t really matter how you
obtain the data, it only matters that you collect the data.
A performance record is any recorded observation or
measurement of a specific trait for an individual or a group. Average daily
gain, wool weight and mature body size are all examples of traits that can be
measured. If you able to use ultrasound or are retaining ownership or can get
the data collected loin eye area, fat thickness and flank streaking may be
important to you. There are an infinite
number of traits that could be measured - the traits that should be measured
are those that are economically important, directly affect the profitability of
your operation, and are heritable.
The main objective of collecting performance records
is to determine the:
- genetic
merit of each individual animal, and
- performance
of the breeding flock in your farming operation.
This information allows you to determine where, from a
genetic and management standpoint, improvements can be made.
Accuracy
All animals in the flock must be uniquely identified.
Record sex, birth date, breed, contemporary group, sire and dam for each
animal, along with the measurement or observation of the trait to make up the
performance record. Since selection and management
decisions will be made based on the performance records, the authenticity,
accuracy and completeness of all records is essential to achieve improvement in
the flock.
Phenotype
The phenotype or the expression of a trait as observed
in an animal that is measured is made up of two factors -- environment and
genetics. This is often expressed as P = G + E (phenotype = genetics +
environment). The purpose of a performance-testing program is to identify
animals that possess the genetics for a particular trait. To accomplish this,
animals of a similar age are "tested" in the same environment and any
differences in performance.
Your selection program should concentrate on genetics,
which equals phenotype minus environmental effects (G = P - E). Therefore, you
measure the phenotype, but you need to be able to subtract the environmental
effects. Many times this is done by making all the animals the same, like
raising them all in a barn and subjected to similar stresses and creating a
contemporary group.
Contemporary Group
A contemporary group is a group of animals raised in
exactly the same environment, with exactly the same management or as close to
this as is practical. The phenotype or the expression of a trait is made up of
environment and genetics. The environment includes such things as:
- number
of lambs being raised by a ewe
- the
weather
- feed
availability
- ability
of ewe to produce milk
- competition
for creep feed
- incidence
of disease
- season
of the year, i.e. temperature
Stressors like varying temperature and weather
conditions can have a large influence on traits like feed efficiency and growth
rate. The location of the barn an animal is raised in can make a difference.
For example, ventilation, lighting, and animal density are factors that can
affect performance.
As you can tell, It is almost impossible to make the
environment for even just three or four lambs exactly the same. Even with
twins, the ewe may milk better in one half or it may simply favor one lamb over
the other. It is important to make the environment as uniform as possible in
order to evaluate a trait accurately. A uniform environment necessitates a
small age range, as animals of different ages encounter different environments.
The size of the contemporary group also affects
accuracy and the group needs to be as large as possible while maintaining a
uniform environment. A group composed of a large number of lambs from several
sires and within a small age range gives the most accurate estimate of genetic
merit.
Testing the whole Flock Is
Important
It is important when developing a selection program to
test or measure all animals in the flock. This ensures a larger contemporary
group and provides as much information as possible on all animals in the flock,
thereby increasing the accuracy of the estimates of genetic merit in
individuals and to measure flock performance. Complete flock testing is also
the only way to determine what, on average, is happening in your operation.
Performance Traits to
Measure
Performance information is normally collected in three
general areas: ewe productivity, lamb growth and carcass merit.
Performance
Traits
Ewe Productivity
- Prolificacy
- Lambing
interval
- Fertility
- Pregnancy
rate
Growth
- Pre-weaning
gain
- Post-weaning
gain
Carcass
- Backfat
thickness
- Loin
eye area
- Lean
meat yield
Improving Economically Important Traits
Consumer preference
for heavier lambs has created considerable change within the sheep industry to
increase size and weight and lambs that are heavily muscled can retain
acceptable yield grades. Lambs that grow rapidly reach market weights at
younger ages, which require a shorter feeding period and have less risk of
death loss with improved feed efficiency.
Growth rate is easy to
measure. Many producers with commercial flocks weigh lambs at weaning. A ewe's
milk production greatly influences her lamb's weaning weight, but lamb weaning
weight is still a valuable trait to select for because the maternal trait of
producing more milk can be transmitted to replacement ewe lambs.
The heritability of
growth rate is higher for post-weaning weights. Producers who use performance
testing programs select ram lambs after they have been weaned and place them on
a uniform feeding test for approximately 90-120-140 days.
Weights at birth are
important to maintain a relatively low birth weight to minimize dystocia and
lamb mortality.
If weaning weight is
selected for, correct the weight for age, sex, type of birth, type of rearing,
and age of the dam. Use the adjustment factors in table 2. Table 2. Factors for
adjusting lamb weights for age. Multiply 90-, 120-, or 140-day weight by the
appropriate factor.
Example: To find the adjusted 120-day weight of a
ram lamb born and reared as a twin from a 2-year-old ewe that weighed 90 pounds
at 110 days of age, make the following calculations: 90 pounds at 110 days of
age = .82 pounds x 120 = 98 pounds x 1.26 (adjustment factor) = 124 pounds The
adjusted 120-day weight of the lamb would be 124 pounds.
Note: If a lamb is born a single but raised as
a twin, adjust it as a twin-born, twin-raised lamb. Source: Sheep Production
Handbook. 1996. American Sheep Industry Association, Inc., Production,
Education, and Research Council.
When selecting
individual animals within a flock, simply select within sex and within twin and
single groups. By listing twins and singles separately and selecting within
contemporary groups, type of birth is adjusted for automatically. Twins should
be given preference in selection.
Reproductive Efficiency.
Sheep can have for
multiple births, especially in smaller flocks. Therefore, select twins for
replacements when possible. With appropriate shepherding, mortality of twins
should not be much higher than that of singles.
Measures of
reproductive efficiency include age at puberty, fertility, lambing rate, heat
tolerance, and length of breeding season. Reproduction in sheep is strongly
influenced by environment. Although the heritability of reproductive rate is
low, some breed differences exist. Fine-wool breeds or old genetic Dorset’s are
highly fertile and have been used quite successfully in F1 or F2 crossbreeding
programs to improve reproductive rate. If you need a real boost to your reproductive
efficiency you may consider adding some Finn or Barbados Blackbelly to the mix.
Lambing Interval
Increasing the
frequency of lambing or getting a higher number of total lambs per ewe can
impact overall profitability. Fine-wool breeds, Dorset, and fine-wool crossbred
ewes have been used successfully in accelerated lambing programs. Many
producers try to maintain a high productive level within a flock, cull ewes
that fail to lamb.
Carcass Merit.
Most of the measurable
carcass traits are medium to highly heritable, thus it is possible to improve
carcass traits through selection. It is more difficult than selecting traits
that can be measured accurately on the live animal. That is where the slaughter and direct marketing
a few or ultra-sounding a group can tell you your progress.
Among market lambs of
the same size, carcass merit is most influenced by cutability (the ratio of
lean meat to fat). Fat is the primary factor in evaluating the carcass yield
grade (measure of cutability) and eventual value to the consumer. The amount of
fat in the carcass at a given weight is closely related to the growth curve of
the lamb. Lambs that grow rapidly and reach market weight at an earlier age
generally have a higher cutability (lower yield grade). Therefore, one
practical method of selecting for increased carcass merit is to select for rate
of gain.
More exact methods of
selecting for carcass merit can be used if carcass traits of related
individuals or groups can be measured. Carcass weight per day of age, loin-eye
area, fat thickness at the 12th rib, percentage of closely trimmed retail cuts,
and leg-loin index all are used in measuring carcass merit in progeny groups.
Wool Traits.
Here in Virginia, the
value of the wool clip may be diminishing if you are not involved in the direct
market business or have no value if you are raising hair based breeds. However, producers should try to maximize the
value of the wool they produce.
The traits for wool
are the easiest to improve. Generally, wool traits are highly heritable and
easy to measure. Traits that most directly influence the value of a fleece
include fleece weight, fiber diameter, and length of staple. Weight of the
fleece, particularly clean fleece weight, is usually the most valuable trait.
Ordinarily, clean fleece weight is associated with grease fleece weight (actual
weight of the fleece when shorn).
To increase flock wool
production, select sheep that produce the most wool. Remember, however, of
selecting entirely on pounds of wool produced because ewes that do not milk
well or have singles rather than twin lambs and may not have a positive economic
return. Milk production is negatively correlated with wool growth, particularly
when feed is limited. If selection is placed entirely on pounds of wool,
coarser fleeces may be selected inadvertantly.
Staple length has an
important effect on the monetary value of a fleece. This trait is usually highly
correlated with pounds of wool produced, and heavier fleeces typically have a
longer staple length. Measure length of staple and fleece weight at shearing
time, and select replacements based on a combination of these two economically
important traits.
The grade of a fleece
is also economically important. Fine-wool fleeces ordinarily bring higher
prices per pound than do coarse-wool fleeces. The grade, or fiber diameter, of
wool primarily depends on the breed of sheep and nutrition. When selecting
replacements breeding stock remember that fleeces with a high degree of variation
in grade are undesirable and have a lower monetary value.
Other fleece traits
that can be given attention are color, softness of handle, uniformity of length
and of fiber diameter, and freedom from other defects. Cull sheep with a lot of
black fiber, hair, or kemp.
How to Select for Desirable Wool Traits
- Record grease fleece weight and
staple length of each fleece at shearing.
- If possible, obtain clean
fleece weight.
- Rank fleeces according to
weight of fiber produced and staple length.
- If possible, rank sheep within
a given grade of wool, within a given age classification, and within a
group in which the number of lambs raised is known.
In
summary, remember that since selection and management decisions will be made
based on the performance records, the authenticity, accuracy and completeness
of all records is essential to achieve improvement in the flock. For more in information please contact cchilds@vt.edu or your local Virginia Cooperative Extension
agent.
No comments:
Post a Comment