No abstract

Variance components and breeding values of production traits and somatic cell score of South African Guernsey, Ayrshire, Holstein and Jersey breeds have been estimated using a multi-lactation repeatability test-day model, including tests of the first three lactations as repeated measures and fitting the permanent environmental effect across lactations. Multitrait evaluations were done for the production traits (milk, butterfat and protein) and single trait evaluations for somatic cell score. Heritability estimates were comparable with yield and somatic cell score estimates obtained by test-day models from other countries (17-24% for milk yield; 10-13% for butterfat yield; 14-19% for protein yield and 6-8% for somatic cell score). Proofs of qualifying sires were sent to the International Bull Evaluation Service (INTERBULL) for participation in the March 2005 test runs. Genetic correlations between South Africa and other participating countries, estimated by INTERBULL, compared well with those amongst the other participating countries. Trend validation tests were successful using this methodology for all traits and breeds except for somatic cell score of the Guernsey breed, due to insufficient data for this trait. South Africa can now participate in routine INTERBULL evaluations to obtain Multiple Across Country Evaluation (MACE) breeding values, using this methodology.

Lactation records of South African Holstein and Jersey cows were classified into six calving seasons and 18 age x lactation classes using several trials involving analysis of variance and estimation of fixed effects of calving age on test-day as well as 305-day yields. Best Linear Unbiased Estimates for the effect of season and age x lactation were derived using an animal model following the prediction of daily milk, butterfat and protein yields on fixed days-in-milk and of cumulative 305-day yields by linear interpolation. For both breeds, test-day and cumulative 305-day yields were higher for cows calving in mid-winter (June/July) compared to cows calving in mid-summer (December/January). Daily yields early in the lactation were more influenced by calving age than daily yields towards the end of the lactation. Cows that calved at younger ages in lactations 1 and 2 produced less milk compared to cows calving at older ages over the entire lactation. Second and later parity cows had higher test-day yields than heifers for most stages of the lactation. These estimates can be used to adjust test-day and cumulative yields for the effect of season and age class, thereby enabling the determination of herd levels and the derivation of standard lactation curves and projection factors for South African Holstein and Jersey cows.

In an animal model evaluation of breeding values it is assumed that the base animals are all at the same genetic level. However, in the South African Holstein population, animals of different genetic levels were imported from foreign countries, thus causing a deviation from this assumption. The effect of this deviation is considered using first lactation records from 393 458 Holstein cows. Genetic trend estimation is studied through a time trend analysis of within-Bull-yearly-Daughter Yield Deviations, or DYD's. Bias in the estimation of trend was reduced when phantom parent groups were taken into account. The 109 385 base animals were replaced by 64 phantom parent groups. Phantom parent groups were constructed by combining year of birth, country of birth and selection intensity of the phantom parents. In recent years, foreign sires have been more affected by the exclusion of phantom parents groups in the model, than local sires, although ranking coefficients for the 15 771 sires in the analysis were in excess of 90%. Ranking coefficients for cows were also high.

________________________________________________________________________________ AbstractCalving interval (CI) is a fertility trait that can be used in selection programmes to minimize the negative effects that selection for production have on fertility. CI can be derived from milk recording data, therefore this fertility trait can easily be implemented in the National Dairy Genetic Evaluations of South Africa. The aim of this study was to estimate genetic parameters for CI for estimation of breeding values and genetic trends to enable South African dairy breeders to assess and select for improved fertility. Breeds included in the study were Ayrshire, Guernsey, Holstein and Jersey. Genetic parameters and trends were based on the first three CIs for all breeds. The genetic software package, VCE4, was used to estimate genetic parameters for CI. Heritabilities ranged from 0.011 for CI 1 to 0.069 for CI 2 , both for the Guernsey breed. Genetic correlations ranged from 0.606 between CI 2 and CI 3 for the Guernsey breed, to 0.810 between CI 1 and CI 2 for the Ayrshire breed. Breeding value estimation was done using the genetic software package, PEST. The estimated breeding values were combined into an index value, using weighting factors based on the amount of information available for each trait. Genetic trends were calculated by averaging the CI indices of measured cows per year of birth. These genetic trends indicated that CI increased genetically for all breeds. The genetic parameters will now be implemented for the estimation of CI breeding values, which can be included in selection programmes for improvement of fertility of dairy breeds in South Africa. ________________________________________________________________________________