The aim of this study was to determine correlations between changes in the levels of body energy reserves assessed based on backfat thickness and the fertility and productivity of dairy cows at the initial stage of lactation. The experiment was conducted under production conditions, in two herds of 108 Polish Holstein-Friesian Black-and-White cows. The body condition of cows was determined and backfat thickness was measured by ultrasonography at thirty-day intervals. The average differences in body fat reserves between the value obtained after calving and that corresponding to the lowest point of the lactation curve were 0.5 BCS unit, 36 kg body weight and 4.4 mm backfat thickness. The cows in which differences in backfat thickness measured postpartum and in week 9–12 of lactation remained within the 1–5 mm range were characterized by the highest fertility. Gradual mobilization of body fat reserves (1–10 mm) resulted in the highest milk yield. This suggests that intense mobilization of body energy reserves does not ensure the highest production levels.

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