The study was conducted during the years 2005-2007 on a private farm located in Księży Dwór near Działdowo, Province of Warmia and Mazury (NE Poland). The experimental materials comprised 368 Holstein-Friesian cows. The aim of the study was to determine the effect of inter-calving interval (ICI) duration in two consecutive lactations on milk yield and composition. Milk yield in the second lactation reached 7769 kg (305-day lactation) and 8317 kg (full lactation), and was higher than in the first lactation by 1218 kg and 959 kg respectively. Value-corrected milk (VCM) yield was substantially higher, i.e. 8744 kg and 9450 kg. The yield of fat, protein, lactose and dry matter was also higher in the second lactation. Cows with the longest ICI were characterized by the highest yield of milk and milk components. Significant differences between mean values obtained for those cows and cows with shorter ICI were recorded at a level of p ≤ 0.01 and p ≤ 0.05. It was found that milk yield in the first lactation and ICI duration may provide a basis for selecting high-performance cows. The high coefficients of simple correlation between ICI duration and milk yield and composition, observed in this study, are indicative of a strong relationship between ICI and cow productivity. The highest values of correlation coefficients (significant at p ≤ 0.01) were noted with respect to the yield of dry matter (r = 0.993), protein (r = 0.973), VCM (r = 0.968) and fat (r = 0.942). The ICI duration x lactation interaction had a significant effect on the yield of milk, VCM, fat, protein, lactose and dry matter (P ≤ 0.01) as well as on the content of fat and protein (p ≤ 0.05).

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