Abstract
The objective of this study was to investigate the effect of inbreeding depression on functional herd life in the South African Jersey population based on individual level and rate of inbreeding. A pedigree file of the South African Jersey breed (n = 912 638) was obtained from the Integrated Registration and Genetic Information System (INTERGIS). The data included registered, grade and imported animals. The percentages of animals in the pedigree file with two, one and zero parents unknown were 22%, 18% and 60%, respectively. The inbreeding coefficient for each animal (Fi) and the rate of individual inbreeding (ΔFi) as an alternative measure of inbreeding that is adjusted for the depth of known pedigree were calculated. The effect of inbreeding on functional herd life in each of the first three lactations was estimated, using a singletrait sire model on data collected from 1985 to 2003. Three analyses for survival in each of the first three lactations were conducted. In the first analysis, in addition to fixed and random effects, an individual inbreeding coefficient (Fi) was fitted as a linear covariate. In the second analysis, the inbreeding coefficient was included as a discrete variable with the following classes of inbreeding: 0 < F ≤ 3.125, 3.125 < F ≤ 6.25, 6.25 < F ≤ 12.5 and F > 12.5. In the third analysis, the individual rate of inbreeding (ΔFi) was included in the model as a linear covariate. The level of inbreeding in the SA Jersey population showed a gradual increase for the period 1985 to 1994, while the period 1995 to 2003 showed a rapid increase. The current mean level of inbreeding (for the year 2010) is 4.85% with a minimum and maximum of 0 and 31.34%, respectively. The rate of inbreeding showed a gradual increase from 0.36% to 0.43% between 1985 and 2003. The average rate of inbreeding is currently (for the year 2010) at 0.55%. There was a significant unfavourable relationship between inbreeding and functional herd life in the first and second lactations. The effect of inbreeding was more pronounced in the second lactation for both measures of inbreeding. Based on the current level of inbreeding, the reduction in functional herd life in the first lactation can be estimated as 0.68%. The corresponding estimate for the second lactation is 1.70%. The results from the current study indicate that the current level or rate of inbreeding has reached levels that are detrimental to functional herd life. Therefore, individual inbreeding coefficients should be considered in addition to genetic merit when breeding decisions are made by Jersey breeders.Keywords: Inbreeding, longevity, survival, dairy cattle
Highlights
Inbreeding will occur in any population of finite size unless specific measures are taken to avoid the mating of related individuals
The objective of this study was to investigate inbreeding depression on functional herd life in the South African Jersey population based on individual level and rate of inbreeding
There was a gradual increase in the level of inbreeding for the period 1985 to 1994, while the period 1995 to 2003 showed a rapid increase
Summary
Inbreeding will occur in any population of finite size unless specific measures are taken to avoid the mating of related individuals. Genetic improvement programmes should balance genetic gain with increases in inbreeding levels (Miglior et al, 1992). Increased inbreeding is usually considered undesirable because it leads to a decrease in genetic variation within a population, and to a reduction in performance in traits associated with fitness (e.g. health, fertility, survival). Sci. vol 42 populations is negligible, its effect in a typical livestock population, where selective breeding is practised, cannot be ignored (Maiwashe et al, 2006). It is important to maintain genetic diversity within a breed to ensure that future animals can respond to selection and changes in the environment. The net effect of inbreeding in a selection programme will depend on the magnitude of the selection response relative to the possible depression and the rate of accumulation of inbreeding (Van Wyk et al, 2009)
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