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.

The study involved 55 685 Polish Holstein-Friesian cows whose services per conception (SPC) averaged 2.2. Statistical analysis performed using the GLM procedure of SAS, showed significant differences in SPC value within different factors. The greatest changes were associated with daily milk yield of the cows; as it increased from ≤ 20 kg to >40 kg, the number of services required to conceive increased by 0.6. SPC value deteriorated also with an increasing number of cows in the herd (by 0.37) and with an increase in milk somatic cell count, being indicative of mastitis (by 0.11). The opposite situation existed for an increase in herd production level and lengthening of the interval between calving and first insemination (decrease of SPC by 0.31). It was also found that summer was the least favourable period for insemination and winter the most favourable (SPC=2.28 vs 2.16). Analysis of the results demonstrates that it is necessary and appropriate to account for these factors when aiming to improve cow fertility.

The objective of this study was to reveal the effect of milk urea levels on the fertility of dairy cows, taking into account their age, herd production level, number of cows, season of first insemination, which were analyzed using data from the SYMLEK national system. The study included data on milk urea levels in 88,745 test-day yields of 55,685 Holstein-Friesian cows, which were milked up to 30 days before first insemination, during the period preceding 180 days of first and second lactation. The GLM procedure of the SAS package was used in the statistical calculations. Considering the observed significant relationships and tendencies, a milk urea level higher than 300 mg/L was found to negatively affect reproductive efficiency in dairy cows, although the effects of the urea level could be masked by the more profound impact of environmental factors. Regardless of the factors, an increased milk urea level was paralleled by decreased cow fertility, with statistically significant differences occurring only for the interactions with herd production level and with herd size. The unfavorable effect of the increase in the level of urea in milk (from ≤150 to >300 mg/L) on cow fertility was particularly evident in the highest producing herds (>9000 kg milk).

The aim of the study was to analyse the relationship between the somatic cell counts in the milk (the udder health status indicator) and the cow’s fertility, taking the influence of the selected factors into account. The udder health status was determined based on the somatic cell count of the milk from 88 745 test-day records from 55 685 Polish Holstein-Friesian cows. The test-day measurements were made up to 30 days before the first insemination, during the period up to 180 days of the first and second lactation. Because the somatic cell count showed high variation and is not normally distributed, the data were transformed to the natural logarithm scale. Based on the results of the statistical analysis, it was shown that with the increase in the number of somatic cells in the milk, the calving intervals were extended by 11 days (P ≤ 0.01), the service period by around 4 days (P ≤ 0.05), the increase in the services per conception by 0.11 (P ≤ 0.01). There was a statistically significant correlation, weak on the Guilford scale, between the natural logarithm of the somatic cell count and the fertility of the cows: the calving interval – CI (r = 0.050**, P ≤ 0.01), service period – SP (r = 0.016**) end services per conception – SPC (r = 0.019**). Monitoring the number of somatic cells in the milk could contribute to improving the fertility of the cows in particular: in the second lactation in the double lactation (for the CI (r = 0.059**)); in herds with a production level of 7000–9000 kg of milk (for the CI (r = 0.055**), the SP (r = 0.022**) and the SPC (r = 0.024**)); the daily productivity of &gt; 40 kg (for the CI (r = 0.052**), the SP (r = 0.033**) and the SPC (r = 0.029**)), the number of cows in the herd of &gt; 200 (for the CI (r = 0.061**), the SP (r = 0.034**) and the SPC (r = 0.033**)), in the autumn season of the first insemination (for the CI (r = 0.072**), the SP (r = 0.027**) and the SPC (r = 0.031**)). The magnitude of these correlations varied within the classes of the factors such as the daily production level, the age of cows´ lactation number, the season of the first insemination, the herd production level, and the herd size. It appears that the somatic cell count results obtained from the periodic milk recording, considered as an indirect measure of the udder health and used when deciding on the mastitis treatment, could be a useful tool for controlling the fertility in the cows.