Genetic variation in twin calving incidence in herds with a high phenotypic mean

Twin calving data were analysed from five private New Zealand dairy and beef cattle herds, and from the Ruakura experimental twin-research herd founded from industry cows screened-in on their twin calving history. The objective of the study was to estimate the heritability of twin calving rate in herds with a high phenotypic mean for non-identical twin calving. The annual twin calving rate of the private herds averaged 3.36% overall, based on a total of 17 474 calving records. On average, there were 13% of cows from the selected herds with at least one twin calving in their lifetime. Heritabilities were estimated using restricted maximum likelihood (REML) procedures, with the value for annual twin calving rate for the five herds averaging 0.061 +/- 0.010 (individual estimates ranging from 0.016 to 0.090), while estimates for the lifetime incidence of at least one twin calving averaged 0.16 +/- 0.029 (range 0.14-0.19). The Ruakura herd had a mean annual twin calving rate over all years (1985-2000) of 11.2%, and in recent years (1998-2000) of 13.3%. One of the most elite cow-families in the herd (12 cows spanning five generations since the foundation cow) had an annual twin calving rate averaging 31% (seven of the cows producing at least one twin set in their lifetime), while another family of seven cows included three sets of 3-times twinning cows. Heritabilities of annual and lifetime twin calving in the Ruakura herd (cows born in 1983-1998) were 0.081 +/- 0.031 and 0.25 +/- 0.07, respectively. It is concluded, from the five high twinning-rate private herds selected in this study, that they had a higher heritability (average value 0.061 +/- 0.010) than in published studies from industry data in other countries (average value from 13 studies, 0.027).