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Calving interval is a fertility trait that can be used in selection programs to minimize the negative association of selection for production on fertility. Genetic parameters were therefore calculated for South African dairy breeds to estimate breeding values that can be used in the National Dairy Genetic Evaluations of South Africa. The research by B.E. Mostert and co-workers was published in the South African Journal of Animal Science, Volume 40 of 2010, pages 156 to 162, under the title: Calving interval genetic parameters and trends for dairy breeds in South Africa.
The genetic relationship between milk production and fertility traits is antagonistic, which means that continuous selection for increased milk production will affect fertility negatively. However, the effect can be minimized by including fertility traits in selection indices. Calving interval is a convenient fertility trait since it is readily available from milk recording data. In the present study genetic parameters for calving interval were calculated for the Ayrshire, Guernsey, Holstein and Jersey breeds, using the software package VCE4 and from these breeding values, using the genetic software package PEST. The estimated breeding values were combined into an index value for use in the selection programs.
The data showed that calving interval increased genetically in all breeds, which is expected because of the continuous selection for milk production over years. The Holsteins recorded the highest increase of about 1.25 days per year, followed by the Ayrshires with 0.71 days per year, the Guernsey with 0.57 days per year, whereas the Jerseys recorded the lowest increase of 0.50 days per year since 1980. Part of the explanation for the more favourable result for Jerseys is probably because Inter-calving Period and Age at Calving standards in the selection of bull dams were implemented by the Jersey Breed Society since the early nineties. For practical implementation in the South African Dairy Industry, the estimated calving interval breeding values will forthwith be released to the industry as a valuable selection tool to monitor and control fertility of all breeds.