Heritability

The estimated heritability of different traits varies greatly. Although many production traits, such as carcass, dairy, and wool characteristics, are highly heritable, most reproductive traits are not (Table 85-1).³ Selection pressure for traits with low heritabilities must be intense over several generations for progress to be noted. Genetic progress is generally slower when selecting for multiple traits. It is also important to be aware that selection for some traits may have a negative impact on others. For example, many carcass characteristics and wool characteristics are negatively heritable.

Table 85-1 Heritability Estimates

Scrotal circumference (SC) is probably the most important genetic trait associated with reproduction efficiency. The heritability of SC is estimated at 35%. SC is correlated with sperm output, age at puberty of the female offspring, ovulation rate, and the number of multiple births produced. In addition, some British researchers have reported a positive correlation between SC and the ratio of muscle to fat in the offspring. Failure to properly select for SC has the potential to reduce reproductive performance and production, thereby reducing the potential profitability of subsequent generations.

Cost of Ram Power

The cost of ram power per lamb produced is an important component of the ESE. The cost of ram power is determined by purchase price and by the number of lambs sired by an individual ram. Table 85-2 gives an example of annual costs per ram, exclusive of purchase price. Based on this example, Table 85-3 shows how the ram-to-ewe ratio and lambing percentage affect the annual cost of a ram per lamb produced.

Table 85-2 Annual Ram Costs^*

* These figures are representative of Colorado producers as reported by CSU farm management specialists Rod Sharp and Paul Gutierrez.

Table 85-3 Estimated Cost of Ram Power per Ewe Exposed and per Lamb Produced

Breeding Capacity

The breeding capacity of a ram is determined by sperm production, semen quality, and libido. The breeding capacity of a highly fertile ram is one of the most difficult concepts for many sheep producers to accept. Under most range and semiconfinement conditions, a ram in good physical condition, with high libido, can successfully serve 100 or more ewes during a 17-day breeding cycle. For example, one 5200-ewe western range operation using 1.5 rams per 100 ewes consistently lambs over 90% of the bred ewes during the first 17 days of the lambing period and averages a 140% lamb crop.⁴ In this type of an operation, a higher ram-to-ewe ratio cannot be economically justified.

Libido

Libido refers to the willingness of a ram to breed ewes. A serving capacity test or direct observation of a ram with ewes is an important component of the BSE, but is frequently neglected. It has been estimated that approximately 10% of all rams have no interest in breeding ewes. In addition to not siring lambs, these rams can interfere with other working rams and should be removed from the breeding population. The behavioral evaluation of a ram should be conducted before the evaluation of the reproductive tract and the semen evaluation. Rams with inadequate libido should be culled without incurring further expense.

Physical Examination

Restraint can vary, depending on the number of animals being tested and the number of people available to assist. The most important factor in restraint is that it should allow for a complete examination of both structural and physical characteristics. An adequate examination can be conducted on a standing ram restrained by a halter and pushed against a wall or fence. However, a calf table or sheep cradle makes restraining a ram for proper examination much easier.

Structural soundness refers to the capacity of a ram to remain sound during the breeding season under normal environmental stresses. The feet, legs, and teeth should be examined for structural soundness. Physical soundness refers to the health of the ram. The ram should be examined for conditions that may prevent optimal performance, or that can be transmitted to the ewes, such as foot rot, lip and leg ulcerations (ulcerative dermatitis), and pizzle rot. Body condition should also be noted during the physical examination. A body condition score of 2.5 to 3.5 is recommended for rams entering the breeding season. Structural and physical conditions should be noted at the time blood is collected for the B. ovis ELISA test, in conjunction with libido testing, and before examination of the reproductive tract and semen evaluation. If there are structural or physical conditions that cannot be corrected, the ram should be culled before incurring further expense.