Genetic relationship between the lifetime milk production, longevity and first lactation milk yield in Slovenian Brown cattle breed

To study the genetic relationship between the lifetime milk production (LMP), first lactation milk yield (MY305), and length of productive life (PL) in the Slovenian Brown cattle population, first lactation records from 14,389 cows and lifetime performance records (LMP and PL) from 12,416 cows were used. Records of LMP and PL were normalised using the square root transformation of the raw values. The fixed effect of the first calving year-season oscillated between different seasons within a year for all the traits. These changes followed a seasonal pattern in MY305, where cows with first calving in colder seasons of a year had the highest MY305 and cows with first calving in the warmer seasons of a year had the lowest. The estimated heritability was 0.12 +/- 0.017 for LMP, 0.09 +/- 0.015 for PL, and 0.15 +/- 0.001 for MY305. Genetic correlation between LMP and PL was very strong (0.96 +/- 0.008), while it was moderate (0.48 +/- 0.067) between LMP and MY305 and weak between PL and MY305 (0.23 +/- 0.085). The common herd environment correlation between PL and MY305 was moderate and negative (-0.41 +/- 0.052), whereas it was weak and positive between the LMP and MY305 (0.26 +/- 0.057) and strong and positive between LMP and PL (0.74 +/- 0.025). For the last 40 years, a positive genetic trend was observed for LMP and MY305, while the genetic trend for PL remained stable.