Doctoral Degrees (Animal, Wildlife and Grassland Sciences)

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Browsing Doctoral Degrees (Animal, Wildlife and Grassland Sciences) by Author "Du Toit, Jacobus"

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    A genetic evaluation of productive herd life in dairy cattle
    (University of the Free State, 2011-11) Du Toit, Jacobus; Van Wyk, J. B.; Maiwashe, A.
    English: The length of productive life is of major economic importance in dairy cattle production. Simple breeding objectives such as selection for increased production in dairy cattle have led to a significant decline in fitness traits. A multi-purpose breeding objective that includes other traits such as length of productive life should be considered. Herd life reflects the ability of a cow to avoid being culled for low production, low fertility, or illness. Herd life can be used in breeding programs if genetic parameters are known. The objectives of the study were to: (1) estimate genetic parameters for functional herd life for the South African Jersey breed using a multiple trait linear model, (2) develop a prototype breeding value for functional herd life for the South African Jersey breed, (3) estimate genetic relationships between functional herd life and conformation traits in the South African Jersey breed and (4) assess inbreeding depression for functional herd life in the South African Jersey breed based on level and rate of inbreeding. A measure of herd life called functional herd life was considered in the current study. Functional herd life refers to herd life adjusted for milk production in the first lactation. In this study functional herd life was defined as survival in each of the first three lactations. Functional herd life was denoted by a 1 if a cow survived and 0 otherwise. Analyses to estimate genetic parameters for functional herd life were carried out as follows. Data and pedigree records on purebred Jersey cows that participated in National Milk Recording and Improvement Scheme were analyzed. Data before editing comprised test-day and lactation yields on milk, fat and protein yields from 252 629 Jersey cows born between 1968 and 2005. After editing, 181 269 cow records from 636 herds recorded over 16 years were available for analysis. Estimates of genetic parameters for herd life were obtained using REML procedures fitting a multiple-trait linear animal and sire models. Heritability estimates (0.02 to 0.03) from the two models were somewhat similar for all lactations. However, heritability estimates for lactations 2 and 3 were slightly higher with the sire model compared to the animal model. The genetic correlation between lactations 1 and 2 from both the sire and animal models was higher than that between lactations 2 and 3. Genetic correlations from the sire model ranged from 0.68 to 0.99 and corresponding estimates from the animal model ranged from 0.76 to 0.99. Genetic parameters obtained in the current study suggest that sufficient genetic variation exist for herd life to allow for genetic improvement and that early selection for functional herd life is feasible. The development of a prototype breeding value for functional herd life for the South African Jersey breed was carried out as follows. Test-day and lactation data on cows that participated in the National Dairy Cattle Improvement Scheme were considered. A multiple-trait linear animal model was used to estimate breeding values using Parameter ESTimation (PEST) software package. A complete (co)variance structure for the additive genetic and residual effects for the three traits were used. These (co)variances were estimated in the first objective. Reliabilities were approximated using the effective number of daughters. Estimated breeding values were scaled so that the average breeding value was a 100. Estimated breeding values for sires ranged from 79 to 114. The rate of genetic progress per year for the period 1985 to 2002 was statistically non-significant (b = 0.02±0.05 per year). The mean reliability was 33.43% and reflective of the low heritability of functional herd life. However, it should be noted that while direct selection for functional herd life could lead to genetic progress, this genetic response could be relatively slow due to the low heritability. The genetic relationship between conformation traits and functional herd life of the South African Jersey population was investigated. Data on conformation traits (n = 46 238) and functional herd life (n = 90 530) on registered South African Jersey cows calving between 1989 and 2008 were obtained from the Integrated Registration and Genetic Information System. Conformation traits were scored using a subjective linear scoring system ranging from 1 to 9, except for foot angle with a maximum score of 8. Conformation traits included stature, chest width, body depth, dairy strength, rump angle, thurl width, rear leg side view, foot angle, fore udder attachment, rear udder height, rear udder width, udder support, udder depth, front teat placement, rear teat placement and front teat length. Genetic correlations between conformation traits and functional herd life were estimated using a series of bivariate analyses. The highest correlations were estimated for udder traits. Significant moderate to high positive genetic correlations between most udder traits and functional herd life (0.23 to 0.63) were estimated. The most important udder traits related to functional herd life were fore udder attachment, rear udder height, and udder depth. Correlations between the majority of body structure and functional herd life were variable. Most of the body structure traits had a low to moderate negative correlation with functional herd life (-0.04 to -0.27). The structural body traits of rump angle and foot angle were estimated to have a moderate positive genetic correlation with functional herd life. The genetic relationships between functional herd life and conformation traits in the South African Jersey breed indicate that conformation traits could be used to enhance the accuracy of genetic evaluation for functional herd life. The effect of inbreeding depression on functional herd life in the South African Jersey population based on individual level and rate of inbreeding was investigated. 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 which 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 single trait 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 2009 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 (P<0.05) unfavourable relationship between inbreeding and functional herd life in the first and second lactation. 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 to be 0.68%. The corresponding estimate for the second lactation is 1.70%. These results indicate that the current level or rate of inbreeding has reached levels that are detrimental to functional herd life. Therefore, individual inbreeding coefficient should be considered when breeding decisions are made by the Jersey breeders in addition to genetic merit.