RESEARCH ON THE CORRELATION BETWEEN THE AGE AT THE FIRST CALVING AND MILK PRODUCTION CHARACTERS

The paper aimed to present the genetic determinism of milk yield traits and age at first calving using a number of 2,237 half sibs, offspring of 989 Friesian bulls and raised in various farms in Romania. Taking into account the peculiarities of half sibs families, it was used a linear mathematical model for unbiased cross classification. Average 305-days milk yield at the first lactation was 3,034.88 ± 14.67 kg, while the average milk fat percentage registered 3.79 ±0.004 %. Average 305 days fat yield accounted for 115.12±0.59 kg, average age at first calving regietered 839.25±1.43 days ( 27.5 months). Between milk yield and fat percentage, it was found a correlation of rG - 0.245, reflecting a weak but positive relation between the two traits. The phenotypic correlation was low and a negative one (rF = -0.181), showing a weak and reverse link between the two traits. Between milk yield and fat yield, both the genotypic and phenotypic correlations were very high and had positive values, rG = 0.971 and rF = 0.964. Between fat percentage and fat yield, it was found a genotypic and phenotypic positive correlation, rG = 0.465, and, respectively, rF = 0.240. The correlation between age at first calving and milk yield was extremely weak and negative, rG = - 0.257 şi rF = - 0.090. Between age at first calving and fat percentage, the correlation was very small and had a negative value (rG = - 0.187) and (rF = - 0.032). Between age at first calving and fat yield, both the genetic and phenotypic correlations were weak and negative (rG = -0.288 and rF = - 0.093). The values of the heritability and genotypic correlations pointed out the effect of the aditive action of the genes, but the phenotypic correlations indicated that some traits were also determined by the environment factors. Cow selection based on milk yield has a positive impact on the future milk performance. In the breeding programmes, age at first calving should not be neglected as it could lead to a decline of production if it does not fit to the optimal thresholds of the the Friesian breed.