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.

Pedigree information on the registered South African Ayrshire (n = 47 116), Guernsey (n = 18 766), Holstein (n = 892 458) and Jersey (n = 314 403) breeds was analyzed to determine the rate of inbreeding and effective population sizes for the period 1960 to 2003. Inbreeding coefficients were calculated using the Animal Breeder's Tool Kit. The mean inbreeding coefficients for 2003 were 2.02%, 2.04%, 2.30%, and 3.05% for the Ayrshire, Guernsey, Holstein and Jersey, respectively. The corresponding rates of inbreeding per year were 0.05%, 0.05%, 0.06%, and 0.07% indicating that inbreeding is accumulating at a slightly higher rate in Jersey compared to the other three breeds. However, the rates of inbreeding in the current study are still considerably lower than the acceptable rate of less than 0.5% per year. Estimates of effective population sizes were 148, 165, 137, and 108 for the Ayrshire, Guernsey, Holstein and Jersey, respectively. Results indicate that the impact of inbreeding on genetic variability is still minimal. However, the impact of inbreeding on phenotypic performance on traits of economic importance was not investigated in the current study and should therefore receive future consideration. _______________________________________________________________________________________

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.

Two fixed regression testday models were applied for variance component estimation and prediction of breeding values for somatic cell score, using testday records of the first three lactations of South African Holstein and Jersey cows. The first model (ML-model) considered the testdays of the different lactations as different traits in a multiple-trait animal model and the second analysis (RM-model) treated later lactation records as repeated measures of the first lactation. Heritabilities from the RM-model were more in the range of literature estimates compared to that of the ML-model, i.e. 0.19 ± 0.003 for the Holstein breed and 0.18 ± 0.003 for the Jersey breed. Rank correlations indicated that minor changes occur in the ranking of proven sires between breeding values obtained from the ML-and RM-models. Although genetic correlations between parities are not unity, the RM-model estimates more competitive variances and requires extensively less computer time to predict breeding values compared to the ML-model and are therefore recommended for breeding value estimation on a national basis. _______________________________________________________________________________________

Body measurements as selection criteria for growth in South African hereford cattle Summary Birth weight (BW), weaning weight (WW), yearling weight (YW) and seven body measurements (cannon bone length at birth (CB); hip height at weaning (HHW); hip height at yearling (HHY); body length at weaning (BLW); body length at yearling (BLY); scrotum circumference at weaning (SCW) and scrotum circumference at yearling (SCY), measured in a Hereford stud (1497 animals) over a period of 19 years, were used in estimating genetic (co)variances, heritabilities and correlations. Nine different multitrait animal model evaluations were carried out whereby (co)variance components were estimated using the REML VCE 3.0 package. Estimates from different evaluations were pooled, weighing each estimate by the inverse of the sampling variance to calculate weighted mean variance ratios among the different traits. Generally, structural traits tended to have lower heritability estimates (CB = 0.24; HHW = 0.28; HHY = 0.33; BLW = 0.22; BLY= 0.14 ) when compared with literature values , while estimates for production traits compared well with estimates reported for the South African National Evaluation. Weights and linear body measurements were positively correlated (0.4 -0.9). Negative correlations were found for scrotum, circumference at weaning and yearling with BW and CB respectively. SCW and BW: -0.54; SCY and BW: -0.65; SCW and CB: -0.66; SCY and CB: -.58. It is concluded that CB could serve as an early indicator of mature size and weights at different ages.