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In recent decades, the decreasing fertility of dairy cattle parallel to the increase in milk production has been widely reported - presumably as a result of intensive selection. Based on these publications a general belief has evolved that there might be an inevitable negative correlation between milk yield and reproductive performance. However, the exact physiological mechanisms connecting these two parameters are still unclear, the results of the experiments are ambiguous; in fact, some studies are definitively in contrast to the causality. The aim of our present study was to investigate the relationship between milk production and fertility in some Holstein-Friesian farms. The experiments were conducted in three large dairy farms in southern Hungary: the data were collected from the period between 01.01.2010. and 18.08.2022. In order to avoid the distorsion of the parameters caused by unspecific health problems like lameness and other diseases only the cows with the first three completed lactations were included in the survey. Milk yield was projected for 305 days and expressed in kg of milk, and fertility was defined by the number of inseminations required for reconception as well as by the length of the service period (in days). Animals completed more than one lactation were regarded as separate individuals per lactation, so a total of 13 012 lactations were studied.

Data analysis with Poisson-regression proved that the number of inseminations is significantly correlated with the milk yield and with the studied farm (p<0.0001), but its correlation with the number of lactations was not significant (p=0.9477). Evaluating the length of the service period with a linear model (ANCOVA) similar relationship was found. Again, we used a multiplicative model with the logarithm of the length of the service period. The results show that a 2000 kg increase in milk production prolonged the length of the service period by 9% and increased the number of inseminations by 13%. In a second run, by allotting the animals into four different groups based on their milk yield (always using quartiles of the experimental cows), the average service period of the highest producing group rose by 41.5 days, and the insemination index by almost 1, compared to the lowest quartile. Although the data obtained in our experiment indicate a definitive decline in reproductive indicators parallel to the increase in milk production, further investigations are needed to dispel the doubts raised against this dogma.