Abstract
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. Poorer fertility indices were found in first-calf heifers and second lactation cows receiving energy-deficient diets and in older (third and fourth lactation) cows receiving excess dietary protein and energy. Best fertility was found in young cows fed excess protein (>3.60%) regardless of milk urea levels, and in older cows having lower and optimum levels regardless of protein levels. Cow fertility is differentiated more by milk protein levels than by urea content. Fertility parameters were poorer in first and second lactation cows than in older cows. The coefficients of correlation between milk urea and protein levels and fertility indices were very low, with the only significant differences between protein content vs. calving interval and reproductive rest period.
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