Data on 369 060 test-day yields of 22 997 Polish Black-and-White Holstein-Friesian cows were used to analyse the effect of lactation month, daily milk yield, herd size and season of the year on the proportion of milk samples with specific protein and urea content. The cows were milk recorded in the Pomerania and Kujavia regions, first calved in 2000 and 2001 and were used until 2008. Statistical calculations were made using the FREQ procedure of SAS. All of the factors analysed had a significant effect on the proportion of milk samples with specific protein and urea content. Too small a proportion of milk samples indicating the balancing of energy and protein in the rations, especially with regard to cows at 2 months of lactation (6.6%), during winter (12.7%) and in herds with up to 50 cows (about 14%), as well as the fact that almost 85% of the samples from the second month of lactation show different degrees of energy deficit in the ration leads us to recognize the need for more frequent use of milk recording results to adjust the rations.

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