Genetic parameters and trends for calving interval in the first three lactations of Iranian Holsteins |
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Authors: | Navid Ghavi Hossein-Zadeh |
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Institution: | (1) Department of Animal Science, Faculty of Agriculture, University of Guilan, P.O. Box 41635-1314, Rasht, Iran |
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Abstract: | Calving records from the Animal Breeding Center of Iran collected from January 1987 to December 2007 and comprising 292,875
calving events of Holsteins from 1,413 dairy herds were analyzed using univariate and bivariate linear animal models to estimate
heritabilities and genetic correlations for calving intervals in the first three lactations of Holstein cows. Genetic trends
were obtained by regressing yearly mean estimates of breeding values on calving year. Average calving intervals were from
406 to 414 days and decreased over the parities. Heritability estimates for calving intervals varied from 0.03 to 0.04 across
the parities. Also, estimates of genetic correlations between calving intervals in different parities were high and ranged
from 0.67 to 0.89. The average annual phenotypic trends obtained from fitting linear regression of annual mean calving intervals
at parity 1 and 2 were significant (P < 0.01), but the phenotypic trend of calving interval at parity 3 was not significant over the years. On the other hand,
there was an increasing genetic trend for calving interval at parity 1, and there were decreasing genetic trends for calving
intervals at parity 2 and 3 over the years (P < 0.01). The low estimates of heritability obtained in this study imply that much of the improvement in calving interval
traits could be attained by improvement of production environment rather than genetic selection. |
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