An analysis of the effect of milk urea and protein levels in four consecutive lactations on fertility indices of Blackand-White Polish Holstein-Friesian cows, milk recorded in the Kujawsko-Pomorskie province has been made. Poorerfertility indices were found in first-calf heifers and second lactation cows receiving energy-deficient diets and inolder (third and fourth lactation) cows receiving excess dietary protein and energy. Best fertility was found in youngcows fed excess protein (>3.60%) regardless of milk urea levels, and in older cows having lower and optimum levelsregardless of protein levels. Cow fertility is differentiated more by milk protein levels than by urea content. Fertilityparameters were poorer in first and second lactation cows than in older cows. The coefficients of correlation betweenmilk urea and protein levels and fertility indices were very low, with the only significant differences between proteincontent vs. calving interval and reproductive rest period.
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