Kerala, India
Growth rate. Consumer demand for heavier lambs with reduced body fat has driven substantial changes within the sheep industry to enhance size and weight. Fast-growing lambs achieve market weights at younger ages, typically requiring shorter feeding periods with reduced mortality risk and improved feed conversion efficiency.
Growth rate is easily measurable. Lambs can be weighed at weaning or at subsequent ages. Most commercial flock producers weigh lambs at weaning. A ewe’s milk yield significantly influences her lamb’s weaning weight, but lamb weaning weight remains a valuable selection trait because the maternal characteristic of increased milk production can be passed to replacement ewe lambs.
Growth rate heritability is higher for post-weaning weights. Therefore, implementing controlled feeding programs after weaning proves useful for growth rate selection. Producers utilizing such performance testing programs select ram lambs after weaning and place them on uniform feeding tests for approximately 90 days.
Weights at birth, weaning, and 12 to 16 months of age are correlated, but maintaining relatively low birth weights is important to minimize dystocia (birthing difficulties) and lamb mortality. Therefore, prioritize selection for growth traits of weaning weight or post-weaning weight while attempting to maintain low birth weights.
When selecting for weaning weight, correct the weight for age, sex, birth type, rearing type, and dam age. Apply the adjustment factors in table 2.
Factors for adjusting lamb weights for age. Multiply 90–, 120–, or 140–day weight by the appropriate factor.
| Â | Age of dam | Â | Â |
|---|---|---|---|
| Â | 3 to 6 years | 2 years, or 6+ years | 1 year |
| Ewe lamb | Â | Â | Â |
| 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 | Â | Â | Â |
| 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 | Â | Â | Â |
| 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 categories. By listing twins and singles separately and selecting within contemporary groups, birth type is automatically adjusted. Twins should receive selection preference.
Reproductive efficiency. Sheep possess multiple birth potential, especially in farm flocks. Therefore, select twins for replacements when feasible. With proper management, twin mortality should not significantly exceed that of singles.
Reproductive efficiency measures include puberty age, fertility, lambing rate, and breeding season length. Reproduction in sheep is strongly environmentally influenced. By most assessments, reproductive rate heritability is low, but breed differences exist. Fine-wool breeds are highly fertile and have been successfully utilized in crossbreeding programs to enhance reproductive rate. Breeds used under intensive management systems to increase lambing rate include Finnish Landrace, Border Leicester, and Suffolk.
Another reproductive efficiency aspect is lambing frequency. Fine-wool breeds, Dorset, and fine-wool crossbred ewes have been successfully employed in accelerated lambing programs. To increase reproduction rate, select for lambs born within a given year or multiple birth frequency. Older ewes twin more frequently than younger ewes. This is environmental rather than genetic. Barrenness heritability in sheep is low. However, to maintain high flock productivity, cull ewes failing to lamb.
Carcass merit. Most measurable carcass traits are medium to highly heritable, making carcass trait improvement through selection possible. This is more challenging, however, than selecting for traits measurable accurately on live animals.
Among market lambs of identical size, carcass merit is most influenced by cutability (lean meat to fat ratio). Fat is the primary factor in evaluating carcass yield grade (cutability measure) and ultimate consumer value. Fat amount in the carcass at a given weight closely relates to the lamb’s growth curve. Fast-growing lambs reaching market weight at earlier ages generally have higher cutability (lower yield grade). Therefore, one practical method for selecting increased carcass merit is selecting for gain rate.
More precise carcass merit selection methods can be employed 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 measure carcass merit in progeny groups. Sire progeny group summaries can be compiled from progeny data. Ultrasound technology can estimate fat thickness and loin eye area. This technology enables carcass merit selection in potential sires.
Wool traits. Wool can account for up to 20 percent of total gross income. Of all economically important sheep traits, wool-related traits are easiest to improve. Generally, wool traits are highly heritable and easily measured. Traits most directly influencing fleece value include fleece weight, fiber diameter, and staple length. Fleece weight, particularly clean fleece weight, is usually the most valuable trait. Ordinarily, clean fleece weight associates with grease fleece weight (actual fleece weight when shorn).
To increase flock wool production, select sheep producing the most wool. Beware, however, of selecting entirely on wool pounds produced because dry ewes or those with singles rather than twin lambs may have advantages in wool production but not in economic return. Milk production is negatively correlated with wool growth, particularly when feed is limited. Maintain records on lamb production and wool production. Furthermore, if selection is placed entirely on wool pounds, coarser fleeces may conceivably 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 fiber weight produced and staple length. If possible, rank sheep within a given wool grade, within a given age classification, and within a group where lamb numbers raised are known.
Staple length has an important effect on fleece monetary value. Ordinarily, this trait highly correlates with wool pounds produced, and heavier fleeces typically have longer staple length. Measure staple length and fleece weight at shearing time, and select replacements based on combining these two economically important traits.
Fleece grade is also economically important. Fine-wool fleeces ordinarily bring higher per-pound prices than coarse-wool fleeces. Wool grade, or fiber diameter, primarily depends on sheep breed. When selecting replacements, also emphasize grade uniformity throughout the fleece. Fleeces with high grade variation are undesirable and have lower monetary value. To detect such variation, examine ewe and ram fleeces before shearing. Cull ewes with belly-type wool extending up the sides.
Other fleece traits deserving attention are color, handle softness, length and fiber diameter uniformity, and freedom from other defects. Cull sheep with abundant black fiber, hair, or kemp.
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