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The influence of the development of body condition throughout lactation concerning milk yield and fertility has been discussed in the first two parts of this essay. In the third part reference values for body condition and change in body condition have been computed. These reference values were based on ultrasound measurenments of backfat thickness at 46111 cows of 78 different farms. In the presented fourth part the usefulness of this method as a practical tool in dairy herd management will be discussed. For this purpose, two dairy herds with an average milk yield of about 9500 kg FCM per year and cow were visited periodically for one year and compared. It has been shown that high milk yield does not have to be at expense of good reproductive performance. An efficient herd management can offset depression in fertility, which commmonly is combined with increasing milk yield.