Leiv Sigbjørn Eikje defended his PhD thesis on June 17th 2009

Ane Gro Siri Skjelfjord

Sheep breeding schemes for Norway based on artificial insemination

The main aim of this thesis is to contribute knowledge as how to implement artificial insemination (AI) in the Norwegian breeding scheme with about 120 000 adult ewes of Norwegian White Sheep (NWS). Intitially, the aim was to summarise and evaluate the breeding scheme for NWS prior to increased use of AI. Thus, Paper I focuses primarily on the period from the early 1990s to the start of the present decade, but references are also made to the start of systematic breeding in the 1960s. A description of the organisation of the breeding population in ram circles is given, before genetic and phenotypic change is estimated and discussed. Genetic gain was found for all traits in the aggregate genotype, except for fleece weight and fat grade for which the trend was close to zero and negative, respectively. Weaning and carcass weight had higher genetic than phenotypic gain, whereas the opposite was observed for carcass and fat grade and number of lambs born.

In Paper II, a procedure is developed for sampling of estimated breeding values in stochastic simulation. Information about true breeding values and approximated prediction error (co )variances were made use of, eliminating need for phenotypes. The method developed is advantageous for simulation of large populations with many traits in the breeding goal, such as NWS.

In Paper III, the method from Paper II is implemented in a simulation mimicking three breeding schemes with various use of AI, and a reference scheme with only natural mating. Comparison was based on change in aggregate genotype, with 9 traits, when restricting rate of inbreeding. A scheme in which both test rams and the youngest selected proven rams (elite rams) were used in natural mating in ram circles and older elite rams were used in AI gave 25% larger genetic gain than the reference scheme.

The preferred use of rams in the scheme was: 30% of the ewes mated/inseminated to elite rams, with 900 ewes per elite ram in AI and 50 ewes per test ram. Two other schemes had 40% larger genetic gain than the reference scheme; either test and elite rams used in AI or test rams used in natural mating in ram circles and all elite rams used in AI.

In the first of these two schemes, an elite mating percentage of 10% with 300 ewes per elite ram and 200 ewes per test ram gave the best result. Corresponding numbers for the latter scheme, which is the recommended one: 30% elite mating, and 700 and 50 ewes per elite and test ram, respectively.

Keywords: breeding scheme, genetic change, phenotypic change, sheep, genetic gain, rate of inbreeding, stochastic simulation

Updated: 01.07.09
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