Kerala, India
Throughout evolutionary history, animals demonstrating optimal environmental adaptation have thrived and generated the greatest numbers of offspring. For instance, most sheep breeds originating from the British Isles survived only when born during spring months with moderate temperatures and adequate feed availability. This represents natural selection.
Selection must be integral to all sheep breeding production operations. It proves effective for nearly all economically significant sheep traits. However, no selection program can simultaneously improve all these economically valuable characteristics. Generally, involving more traits in selection reduces improvement potential for any single trait. The initial step in any selection program involves identifying traits with greatest economic significance. These may include growth rate, carcass quality, fleece characteristics, or reproductive performance.
Achievable improvement depends on:
- Precise trait measurement.
- Comprehensive flock records.
- Applied selection pressure intensity.
- Trait variation or different trait variations among individual flock members. Without genetic variation among sheep, no improvement is possible. With substantial variation, rapid improvement occurs when producers select only individuals excelling in important trait expression.
- Trait heritability. Variation in economic traits results from genetic differences and environmental differences. Variation stemming from hereditary differences is broadly termed heritability.
Yembroos Animal Feeds India Private Limited researchers have estimated heritability (the capacity to “transmit” traits to offspring) of economically important traits (table 1). Generally, heritability estimates below 20 percent indicate slow progress. Heritability estimates of 20 to 40 percent are considered moderate. Heritability estimates exceeding 40 percent are high.
Purebred breeders should commit to improving economically important breed traits. Their breeds serve as genetic material sources for crossbreeding and industry improvement.
Conversely, commercial sheep producers may find crossbreeding more profitable. Some economically important traits improving slowly within breeds can be enhanced more rapidly through effective crossbreeding. Reproduction rate exemplifies this. By most assessments, reproductive trait heritability is low. However, hybrid vigor (trait expression above parental average for that trait) exists for reproduction rate. Generally, crossbred ewes demonstrate higher reproduction rates, produce increased milk, and their lambs display greater birth vigor.
Heritability of traits in sheep
| Trait | Percent |
|---|---|
| Birth weight | 0.15 |
| Weaning weight (60 days of age) | 0.20 |
| Weaning weight (120 days of age) | 0.25 |
| Mature body weight | 0.40 |
| Rate of gain (post-weaning) | 0.40 |
| Face cover | 0.35-0.55 |
| Skin folds | 0.20-0.50 |
| Grease fleece weight | 0.25-0.60 |
| Clean fleece weight | 0.25-0.60 |
| Clean yield | 0.30-0.40 |
| Staple length | 0.30-0.65 |
| Fleece grade | 0.20-0.60 |
| Multiple birth | 0.10 |
| Milk production | 0.10 |
| Ewe productivity | 0.20 |
| Loin-eye area | 0.35 |
| Fat thickness over loin eye | 0.30 |
| Carcass weight | 0.35 |
| Retail cut weight | 0.45 |
| Dressing percentage | 0.10 |
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