The data consisted of 37118 TD milk record of 5851 primiparous Holstein cows of Tunisia. Herd-years of calving means for milk production traits were clustered in three levels using the CLUSTER procedure in SAS software. Production levels included low, medium and high levels. Genotype by environment interaction were investigated by applying a sire model. Generally, the highest heritability estimates of 305-d milk production traits were found in high level rather than low level. Genetic correlation between expression in low and high level was 0,14 for milk yield, 0,22 for fat yield and 0,18 for protein yield. Low spearman correlations (ranged from 0,08 to 0,50) between estimated of the 74 common sires showed re-ranking of sires for these levels. Results from this research indicated that milk production of daughters of the same sires depends greatly on the production environment.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume Genetic Improvement Programs: Selection for harsh environments and management of animal genetic resources (Posters), , 421, 2014
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