Growth rate. Consumer preference for heavier lambs with less body fat has created considerable change within the sheep industry to increase size and weight. Lambs that grow rapidly reach market weights at younger ages, which generally means they require a shorter feeding period and have less risk of death loss with improved feed efficiency.
Growth rate is easy to measure. Lambs can be weighed at weaning time or at a later age. Most 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. Therefore, placing lambs in a controlled feeding program after weaning is useful in growth rate selection. Producers who use such performance testing programs select ram lambs after they have been weaned and place them on a uniform feeding test for approximately 90 days.
Weights at birth, at weaning, and at 12 to 16 months of age are related, but it is important to maintain a relatively low birth weight to minimize dystocia (birthing problems) and lamb mortality. Therefore, select primarily for the growth traits of weaning weight or post-weaning weight, but try to maintain low birth weights.
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.
Factors for adjusting lamb weights for age. Multiply 90–, 120–, or 140–day weight by the appropriate factor.
| Age of dam | |||
| 3 to 6 years | 2 years, or 6+ years | 1 year | |
| Ewe lamb | |||
| Single | 1.00 | 1.08 | 1.13 |
| Twin, raised as twin | 1.19 | 1.29 | 1.38 |
| Twin, raised as single | 1.10 | 1.19 | 1.29 |
| Triplet, raised as triplet | 1.38 | 1.54 | 1.80 |
| Triplet, raised as twin | 1.27 | 1.38 | 1.51 |
| Triplet, raised as single | 1.18 | 1.28 | 1.40 |
| Wether lamb | |||
| Single | .98 | 1.05 | 1.10 |
| Twin, raised as twin | 1.16 | 1.26 | 1.33 |
| Twin, raised as single | 1.08 | 1.16 | 1.25 |
| Triplet, raised as triplet | 1.33 | 1.50 | 1.72 |
| Triplet, raised as twin | 1.24 | 1.35 | 1.45 |
| Triplet, raised as single | 1.15 | 1.25 | 1.36 |
| Ram lamb | |||
| Single | .98 | 1.05 | 1.10 |
| Twin, raised as twin | 1.16 | 1.26 | 1.33 |
| Twin, raised as single | 1.08 | 1.16 | 1.25 |
| Triplet, raised as triplet | 1.33 | 1.50 | 1.72 |
| Triplet, raised as twin | 1.24 | 1.35 | 1.45 |
| Triplet, raised as single | 1.15 | 1.25 | 1.25 |
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 have the potential for multiple births, especially in farm flocks. Therefore, select twins for replacements when possible. With good management, mortality of twins should not be much higher than that of singles.
Measures of reproductive efficiency include age at puberty, fertility, lambing rate, and length of breeding season. Reproduction in sheep is strongly influenced by environment. By most estimates, the heritability of reproductive rate is low, but breed differences exist. Fine-wool breeds are highly fertile and have been used successfully in crossbreeding programs to improve reproductive rate. Breeds that have been used under intensive management systems to increase lambing rate include the Finnish Landrace, Border Leicester, and Suffolk.
Another aspect of reproductive efficiency is frequency of lambing. Fine-wool breeds, Dorset, and fine-wool crossbred ewes have been used successfully in accelerated lambing programs. To increase reproduction rate, select for number of lambs born within a given year or frequency of multiple births. Older ewes twin more frequently than younger ewes. This is environmental rather than genetic. The heritability of barrenness in sheep is low. However, 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, however, than selecting for traits that can be measured accurately on the live animal.
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. Sire progeny group summaries can be compiled from progeny data. Ultrasound technology can be used to estimate fat thickness and loin eye area. This technology will allow selection for carcass merit in potential sires.
Wool traits. Wool can account for as much as a 20 percent of the total gross income. Of all the economically important traits in sheep, those related to 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. Beware, however, of selecting entirely on pounds of wool produced because ewes that are dry or have singles rather than twin lambs may have an advantage in wool production but not in economic return. Milk production is negatively correlated with wool growth, particularly when feed is limited. Keep records on lamb production and wool production. Furthermore, if selection is placed entirely on pounds of wool, it is conceivable that the coarser fleeces may be selected.
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.
Staple length has an important effect on the monetary value of a fleece. Ordinarily, this trait is 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. When selecting replacements, also emphasize uniformity of grade throughout the fleece. Fleeces with a high degree of variation in grade are undesirable and have a lower monetary value. To detect such variation, examine fleeces of ewes and rams before shearing. Cull ewes that have belly-type wool extending up the sides.
Other fleece traits that should 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.