Publications

Summary Economic values for resistance to gastrointestinal (GI) helminths in meat sheep were estimated based on previously published genetic and phenotypic parameters in a selection index objectively optimizing gains on yearling weight (YW). A two-trait index of YW and faecal egg count (FEC) were considered when accounting only for tangible roles of sheep production using individual records for index calculation (scheme 1) or including individual, sire, dam and male paternal half-sib information (scheme 2). Schemes 3 and 4 were similar to schemes 1 and 2, respectively, but accounted for both tangible and intangible roles of sheep production. Economic values within each scheme were estimated for five breeding objectives as follows: (i) index response in YW equals response from single trait selection, (ii) index response in YW is maximum, (iii) the level of FEC was held constant, (iv) A predetermined response of -0.22 (×1000) eggs per gram was assumed and (v) Monetary value of response in FEC is minimum. All estimated economic values were negative, and were Kenya Shillings (KSh) -34.90 (US\$-0.50), 0.00, -92.20 (US\$-1.32), -153.31 (US\$-2.19) and -47.90 (US\$-0.68), respectively, for options 1–5 in scheme 1. In all schemes the breeding options ranked 4, 3, 5 and 1 in descending order; however, options 1 and 5 exchanged positions in schemes 2 and 4. Economic values were responsive to changes in heritability of FEC and genetic correlation between the two traits. The magnitude of the economic values reflects the importance of including resistance to GI helminths in the breeding objectives for meat sheep in different production systems.

Summary A deterministic approach was used to genetically and economically evaluate the efficiency of five two-tier nucleus breeding systems for meat sheep in Kenya. The nucleus breeding systems differed in terms of whether the system was closed or open, in the type of animals that were involved in the movement of genetic superiority and in the number of selection pathways in each system. These systems were compared under four alternative breeding objectives based on monetary genetic gain and profit per ewe. The first objective simulated a situation where the flock size cannot be increased due to non-feed related constraints (FLOCK). The second specifically assumed that the flock size is restricted due to limited amount of feed resources (FEED). The third and fourth objectives assumed that sheep performed only tangible roles (TR) and both tangible and intangible roles (IR) in the production system respectively. Monetary genetic gains were highest for all objectives in an open nucleus system with a certain proportion of commercial-born ewes being introduced in the nucleus while at the same time utilizing young rams from the nucleus to breed sires and dams for the nucleus and commercial sector (ONyre). Utilizing young rams in a closed nucleus system for the dissemination of superior genes resulted in higher annual monetary genetic gain than utilization of old rams. Profit per ewe was significantly higher for FLOCK and IR in ONyre. In a closed system that allowed for downward movement of dams from the nucleus to the commercial sector to breed sires and dams, profit per ewe was highest for FEED and TR. The success of a nucleus breeding system should also focus on the profitability and logistics of establishing it. The implication of these results on the choice of two-tier nucleus breeding systems for the improvement of meat sheep is discussed.

Summary A deterministic approach was used to genetically and economically evaluate the efficiency of five two-tier nucleus breeding systems for meat sheep in Kenya. The nucleus breeding systems differed in terms of whether the system was closed or open, in the type of animals that were involved in the movement of genetic superiority and in the number of selection pathways in each system. These systems were compared under four alternative breeding objectives based on monetary genetic gain and profit per ewe. The first objective simulated a situation where the flock size cannot be increased due to non-feed related constraints (FLOCK). The second specifically assumed that the flock size is restricted due to limited amount of feed resources (FEED). The third and fourth objectives assumed that sheep performed only tangible roles (TR) and both tangible and intangible roles (IR) in the production system respectively. Monetary genetic gains were highest for all objectives in an open nucleus system with a certain proportion of commercial-born ewes being introduced in the nucleus while at the same time utilizing young rams from the nucleus to breed sires and dams for the nucleus and commercial sector (ONyre). Utilizing young rams in a closed nucleus system for the dissemination of superior genes resulted in higher annual monetary genetic gain than utilization of old rams. Profit per ewe was significantly higher for FLOCK and IR in ONyre. In a closed system that allowed for downward movement of dams from the nucleus to the commercial sector to breed sires and dams, profit per ewe was highest for FEED and TR. The success of a nucleus breeding system should also focus on the profitability and logistics of establishing it. The implication of these results on the choice of two-tier nucleus breeding systems for the improvement of meat sheep is discussed.