Analysis was made of the relationship between conformation traits and lifetime production efficiency of the cows that first calved in 2000 and represented the active population in the Pomorze and Kujawy regions of Poland. The CORR Pearson procedures of SAS package were used in the statistical calculations. It was found that there is a statistically significant relationship, weak or low on the Guilford scale, between conformation traits and lifetime production efficiency of the cows, which is slightly higher for milk yield than for longevity. The type and conformation traits appear to be more suitable than the detailed traits for predicting the lifetime production efficiency of cows. Lifetime performance was most strongly related to the overall score and udder score (r = 0.22), followed by the scores for type and conformation and legs and feet (r = 0.13), and detailed traits such as udder width and dairy character (r = 0.14). The highest positive effect on longevity was exerted by udder score and legs and feet (r = 0.11) and among detailed traits by udder placement (r = 0.14) and fore udder attachment (r = 0.10).

Abstract. Lifetime performance was analysed in 6 981 Black and White cows with different proportion of Holstein Friesian inheritance from the active population in the Kujawsko- Pomorskie province according to first lactation yield. It was shown that from the viewpoint of lifetime efficiency, the optimum milk yield of first calvers depends on the milk yield level of the herd. In herds with ≤5 000 kg milk production, the best efficiency was shown by the cows that produced 6 001–7 000 kg milk as first calvers. A decrease in lifetime milk yield, reduced life span and length of productive life, and lower fertility should be taken into account for more efficient first calvers. In herds with 5 000–6 500 and >6 500 kg milk production, lifetime milk yield and milk yield per day of (productive) life increased with increased first lactation milk yield, while fertility deteriorated.

The effect of some factors on relationships between milk urea levels and fertility of 19 028 cows was analysed. The generalized linear model (GLM) of the SAS package (SAS 2004) was used in the statistical calculations. The increase in milk urea concentration was found to increase calving interval (r=0.05, P≤0.01), rest period (r=0.07, P≤0.01), service period (r=0.01, P≤0.01) and the number of services per conception (r=0.02, P≤0.01). Therefore, dairy breeders may find it advantageous to monitor urea concentrations, which could help to improve cow fertility. Herd milk production level, age of cow and lactation period were the factors that had the greatest effect on the relationship between milk urea levels and cow fertility parameters. The relationship between milk urea level and fertility was stronger in cows from herds with >6 000 kg milk yield, in first-calf heifers and in cows at 3 months of lactation.

The objective of the study was to analyse the effect of age at first calving (AFC) on first lactation milk yield, lifetime milk production and longevity of dairy cows. The study used SYMLEK data on the milk yield and culling of 111 857 Polish Holstein-Friesian cows from the active population in Pomerania and Kujawy, Poland. The cows first calved during 2000–2009 and were used or removed from the herds until the end of 2015. For each cow, calculations were made of first lactation milk yield, lifetime milk production, first lactation milk yield per milking day, lifetime milk production per day, as well as the lifespan (length of productive life, duration of rearing) and number of calvings. Based on the age at first calving (duration of the rearing period), the cows were grouped as follows: ≤22, 22.1–24.0, 24.1–26.0, 26.1–28.0, 28.1–30.0, 30.1–32.0 and >32 months. The GLM, CORR PEARSON and FREQ procedures from the SAS package were used in the statistical calculations. Considering first lactation milk yield, lifetime milk production and longevity, it is recommended that the cows should be first milked between 22.1 and 26.0 months of age. This is supported by the approximately 24% higher lifetime milk production in relation to the latest calving cows (P<0.01). Late commencement of the first milking (especially after 28 months) causes a considerable decrease in the first lactation milk yield and lifetime milk production, shortens the productive period, reduces the number of calvings, and increases culling rate due to low milk yield and udder diseases.