Íslenskar landbúnaðarrannsóknir - 01.09.1976, Qupperneq 64

Íslenskar landbúnaðarrannsóknir - 01.09.1976, Qupperneq 64
62 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR Table 2. Average generation interval. Generation interval in years. Sire Sire Dam Dam Farm Son Daugther Son Daughter Hvanneyri .............................. 2.16 2.34 5.18 4.81 Reykholar .............................. 2.07 2.37 4.49 4.51 Skriðuklaustur.......................... 3.18 2.47 4.94 5.15 Hólar .................................. 2.36 2.24 5.08 5.07 The whole material...................... 2.69 2.38 4.94 4.97 In this connection it should be noted that the age distribution of ewes at the farm Reyk- hólar differs from that on the other farms due to the founding of the Reykhólar flock in 1960 and 1961 trough purchase of lambs only. The generation interval is short on the male side but on the female side com- parable to that found by Hallgrímsson (1971). When the phenotypic selection differintial (S), the generation interval (L) and the heri- tability are known, expected annual genetic gain can be calculated as which calculated as percentage becomes A G • 100 = per cent genetic gain per year, b where u is the average corrected autumn weight in the group in question. The expected annual genetic gain due to selection has been calculated within year, farm and sex. The calculations were done in the four following ways. 1. By using the estimated heritability within year, farm and sex. Negative estimates were equalled to zero. 2. By using the estimated heritability within farm and sex. 3. By using the estimated heritability within sex. 4. By using a heritability of 0.25. The heritability estimates are described by Jónmundsson (1976b). The above methods have also been used for estimating genetic gain on the basis of potential selection. For estimating the average genetic gain within sex and farm the selection differential (S.) has been weighted, using the number of lambs kept for breeding (n.) as weights, according to the expression: 2"i sih! 1,2....,6. Here one encounters the problem of over- lappin of generations. As shown by Turner and Young (1969), the additive genetic gain is cumulative. The theoretical assump- tion made here will therefore probably not ho!d, but within the short period of time of just below 2 generations the error introduced can hardly be of great magnitude. The re- sults from these calculations are shown in tables 3a and 3b. It has also been calculated what percentage of the potential selection has been utilized
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