The relationships between the somatic cell counts in the milk and the fertility of Polish Holstein-Friesian cows

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 > 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 > 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.