Genetic analysis of age at first calving and calving interval in South African Holstein cattle

This study was carried out to estimate genetic parameters for Age at First Calving (AFC) and first, second, third and fourth and later calving intervals (CI₁, CI₂, CI₃ and CI₄, respectively) in South African Holstein cattle. These parameters were subsequently used to Estimate Breeding Values (EBVs) for each trait. Mean phenotypic values and EBVs for AFC and CI were plotted against year of birth, to determine phenotypic and genetic trends for each trait. Data consisted of performance records of 20420 South African Holstein cows in 502 milk-recorded herds. Genetic parameters and EBVs were estimated by REML and BLUP procedures, respectively, using a multi-trait animal model. Heritability estimates were low for calving interval (0.03±0.01, 0.04±0.01, 0.04±0.01 and 0.03±0.01, respectively for CI₁, CI₂, CI ₃ and CI₄) and moderate for AFC (0.26±0.02). Genetic conelations among calving intervals in different lactations were high (average of 0.78) indicating that these may be treated as the same trait. Phenotypically, AFC decreased at an average rate of 0.2 months per year, while CI increased by 1.9 days per year, for the period 1982-1998. On the other hand, average EBVs for AFC decreased by 0.06 months per year and increased by an average of 0.27 days per year for CI, in the same period. Thus, there was an increased genetic merit for heifers to attain early sexual maturity (reduced AFC) but post partum reproductive performance (indicated by calving interval) deteriorated. The consistently declining trend in genetic merit for CI underscores the need to include post partum female fertility in the breeding objective for South African Holstein cattle.