Extended lactation in high yielding cows : effects on fertility parameters milk yield udder health und body measurements The objective of this thesis was to evaluate the effects of extending the VWP on various reproductive measurements milk production udder health characteristics and development of body condition of high yielding Holstein cows. The experiment was carried out in a large dairy farm in Saxony Germany (Mlichviehanlage Ruppendorf Saxony 1 100 cows; average milk yield 11 450 kg per 305 days). On 40 days in milk (DIM) cows with no history of severe dystocia and postpartum diseases such as third grade metritis and septicemic mastitis were gynecologically examined (transrectal palpation sonography vaginoscopy). Cows without signs of clinical endometritis were blocked by parity and were randomly allocated to 1 of 3 experimental groups with a VWP of 40 120 and 180 days respectively (G40 n = 135; G120 n = 141; G180 n = 139). Cows of G120 and G180 were re examined at the end of the VWP. If natural estrus was detected within 46 days after the end of the VWP an artificial insemination was performed. If no estrus was detected the respective cows were synchronized by applying the classical Ovsynch protocol (i.e. 100 μg of GnRH; 7 days later 0.5 mg of cloprostenol; 48 hours later 100 μg of GnRH; insemination 14 to 20 hours thereafter using frozen thawed semen from multiple sires of proven fertility). Body condition score (BCS) and backfat thickness (BFT) were assessed at parturition and on day 15 30 45 60 90 120 180 postpartum respectively and thereafter every 2 months until the end of the lactation. Cows were milked with an automatic milking system (AMS Lely Astronaut A4 Maassluis Netherlands) and were housed and fed under the same conditions. Considering the effects of the extension of the VWP on the reproductive performance no difference was found in the proportion of cows in which estrus was detected between 40 to 86 DIM or in the days to first estrus between the 3 groups. Estrus detection in this period was lower for cows with BCS<3 on 90 DIM compared to cows with BCS≥3 (61.5 % vs. 76.0 %) and in cows with high energy corrected milk production (ECM) on 92 DIM (58.6 vs. 70.1 % for cows with higher and lower than the median daily milk yield [i.e. 39.9 kg ECM] respectively). The proportion of cows whose estrus was detected within 46 d after the end of the VWP was greater in G120 (88.9 %) and G180 (90.8 %) compared to G40 (70.4 %). These effects were more apparent in cows with high ECM yield. Cows from both groups with an extended VWP had greater overall first service conception rates (G120: 48.9 % G180: 49.6 %) and a lower number of services per pregnant cow (G120: 1.56; G180: 1.51) compared to G40 (36.6 % and 1.77 respectively). No difference in pregnancy loss or in the proportion of cows culled up to 305 DIM was observed between the 3 groups. The number of Ovsynch protocols per 1 000 000 kg of ECM was reduced by 75 % in G180 and by 74 % in G120 compared to G40 (5.9 vs. 7.1 vs. 25.1). Cows with an extended VWP produced daily numerically but not statistically significant less milk and ECM per feeding day in comparison to cows from the control group. Primiparous cows performed better during the extended phase of the lactation (i.e. from day 305 onwards) in terms of daily milk and daily ECM production in comparison to pluriparous cows. The extended lactation groups showed greater persistency as the rate of decline based on a Wilmink function was lower for G120 (c = −0.063 and −0.045 for milk and ECM respectively) and G180 (c = −0.061 and −0.047) compared to G40 (c = −0.071 and 0.056). No significant differences were found between the 3 groups regarding somatic cell count incidence of mastitis and days off milk due to mastitis. Significantly more cows in G180 (7.9 %) were culled due to low productivity compared with G40 (0.7 %); a statistical trend was found comparing G40 (0.7 %) with G120 (2.8 %). Moreover cows of G180 showed higher median BCS at the time of dry off compared with cows of both G40 and G120 (G180: 3.50; G120: 3.25; G40: 3.25) At the time of dry off G180 cows also had greater BFT (25.0 mm) compared to G40 (22.2 mm) and G120 cows (21.6 mm). In conclusion the extension of the voluntary waiting period of high yielding cows can improve substantially main reproductive measurements. A VWP of 120 d had no adverse effects regarding milk production involuntary culling udder health or BCS gain. Based on these results a calving interval of at least 15 months can be recommended for high yielding dairy cows.