Abstract:
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 num-
ber 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 respect-
ively. 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 com-
mercial sector (ONyre). Utilizing young rams in a closed nucleus system
for the dissemination of superior genes resulted in higher annual mon-
etary genetic gain than utilization of old rams. Profit per ewe was signi-
ficantly higher for FLOCK and IR in ONyre. In a closed system that
allowed for downward movement of dams from the nucleus to the com-
mercial 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.