The effect of milk urea level on fertility parameters in Holstein-Friesian dairy cows

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).