The overall reproductive performance of dairy herds is monitored by various measurements and indicators. In Sweden those measurements are computed in the Swedish Official Milk Recording Scheme (SOMRS). SOMRS is a voluntary service for dairy herds and is an equivalent of a dairy herd improvement program (DHI) in the USA. In Sweden 80% of the herds are enrolled to the SOMRS and they have regular access to the information on how they perform, both in relation to their own historical records and for benchmarking against other herds. Some herds consult an advisor to help them analyze and prioritize what should or could be done regarding to the information that they are given from the SOMRS. In Sweden there is a policy not to use oestrous synchronization and timed artificial insemination (AI) for dairy cows. Most cows (approx.85%) are subjected to AI and the herds practice year round calving.
When monitoring the reproductive performance, time interval measurements are frequently used as reproductive performance indicators and many are calculated in relation to the cow's individual calving date. Examples of such indicators are the calving interval (CI), days to first service (CFI) and days to conception or last insemination (CLI). The drawbacks with these indicators are that they can only be calculated for animals that either have a consecutive calving or have been inseminated and/or checked for pregnancy (depending on indicator), thus introducing possible selection bias. Cows that have not been inseminated or cows that fail to conceive or to calve again are never included in those kinds of indicators and therefore they are not completely representative of a herd's reproductive status. This problem can be alleviated by measuring the proportion of pregnant cows by specific intervals after their calving date or after a fixed time period and using survival analysis on the time-to-event data, where all information, also on animals without the event, is used. The 100-days in calf-rate (IC100) is an increasingly popular indicator that uses this methodology [1, 2].
However, most indicators do not consider the different management strategies applied at farm level, such as the herd's voluntary waiting period (VWP). The VWP is the time period between calving and when the management of the herd decides the cow is ready for breeding and it gives the cow some time to resume normal ovarian cyclicity. This time period might be decided in advance by the farmer's management strategy and in Sweden it is suggested by herd advisors to be 50-60 days in milk for the herd. An American study  found that the VWP, as reported by the herdsmen, for 673 American dairy herds had a mean of 55.6 ± 0.6 days with a range between 30 and 90 days. Similar numbers have also been found in Sweden where the VWP, as reported by the herdsmen, had a mean of 66.5 days with a range between 50 and 80 days . The VWP obviously varies between herds, but may also vary within a herd according to the cow's parity and milk yield . With such variable VWPs, the commonly used reproductive indicators are largely influenced by strategic or managerial decisions along with biological variation and it can be difficult to compare the reproductive efficiency between herds with different management strategies. Consequently, the commonly used indicators do not only reflect the biological reproductive performance of the cow. A way to reduce the variation caused by herd management and to better reflect the biological reproductive performance is to use an indicator that controls for VWP at the herd level. The variation in VWPs will also influence the genetic evaluation of dairy sires for daughter fertility. A proposed model  for the calculation of the American breeding value for daughter pregnancy rate includes both censoring of records from non-pregnant cows and calculated herd specific VWP.
Herd management and the reproductive physiology are reflected differently by various reproductive indicators. Depending on the target of the monitoring, i.e. the management level or the reproductive physiology, different performance indicators might be needed. The best indicator to measure the physiological reproductive performance is not necessarily the best indicator to measure the reproductive management level.
Evaluating reproductive performance indicators on data from actual herds may be problematic because the "true" status is never known and it is difficult to know what component and to what extent that component has influenced the reproductive status of that herd. The inherent natural variation of the components that influences the reproductive status makes it necessary to acquire a large amount of data to evaluate differences between herds. To control the setting one could perform large scale experiments, but they are both expensive and time consuming. Stochastic simulations have therefore been used to exemplify herds with different reproductive status [7, 8]. Simulation studies of economic consequences of postponed first insemination in herds with different reproduction management  and different times for the start of inseminations in herds with different culling rates  have been carried out previously. Successful reproduction, i.e. pregnancy, relies on complex physiological dynamics and is the result of a chain of events. The resumption of ovarian cyclicity, oestrus and ovulation are all events that need to precede conception and a failure at one stage results in a failure in the whole process . The animal and its organs are influenced by both internal (genotype, parity and milk production) and external (nutritional and management) factors. All these multiple factors can be integrated in mathematical modelling to characterize an animal's reproductive state efficiently .
The aim of this present study was to assess how two reproductive performance indicators, which are adjusted for the herd VWP and based on survival analysis, perform in a simulation of herds with different reproductive status and how they compare to traditionally used reproductive performance indicators that do not consider the VWP. The two indicators were percentage of pregnant cows in the herd after the herd voluntary waiting period plus 30 days (PV30) and percentage of inseminated cows in the herd after the herd voluntary waiting period plus 30 days (IV30).