Several studies have shown that the diacylglycerol O-acyltransferase 1 (DGAT1) K232A polymorphism has a major effect on milk production traits. It is less clear how effects of DGAT1 on milk production traits change throughout lactation, if dominance effects of DGAT1 are relevant, and whether DGAT1 also affects lactose content, lactose yield, and total energy output in milk. Results from this study, using test-day records of 3 subsequent parities of around 1,800 cows, confirm previously reported effects of the DGAT1 polymorphism on milk, fat, and protein yield, as well as fat and protein content. In addition, we found significant effects of the DGAT1 polymorphism on lactose content and lactose yield. No significant effects on somatic cell score were detected. The effect of DGAT1 on total energy excreted in milk was only significant in parity 1 and is mainly due to a higher energy output in milk of heterozygous cows. Significant but relatively small dominance effects of DGAT1 on fat content and yield were detected, which are of little practical relevance. Significant DGAT1 by lactation stage interaction was detected for milk yield, lactose yield, fat content, and protein content, indicating that the effect of the DGAT1 polymorphism changes during lactation. In general, the DGAT1 effect shows a large increase during early lactation (from the start of lactation to d 50 to 150) and tends to decrease later in lactation. No DGAT1 by lactation stage interaction for fat yield was observed. Similar to DGAT1, effects of other genes also might vary throughout lactation and, therefore, using longitudinal models is recommended.