Figure 1. Annual preg rate and number of pregnancies included in the preg rate model, calculated by DairyComp305®, every day over a 21-d period. Moderate conception rate, with 60% of breeding done on Thursday, resulted in fluctuations of 1% by day of the week the report is run, Sunday being the greatest. The difference is nearly all attributed to the inclusion or exclusion criteria for first breeding’s, as seen by the number of breeding included in the calculation. The number of pregnancies is highly correlated to the number of pregnant breedings included in the metric (R2=0.51).
Figure 2. Annual preg rate and number of pregnancies included in the preg rate model, calculated by DairyComp305®, every day over a 21-d period. High conception rate, with 90% of breeding done on Thursday, resulted in fluctuations of 3% by day of the week the report is run, Sunday being the greatest. The within week variance is nearly all attributed (R2=0.93) to the inclusion or exclusion criteria for first breeding’s, as seen by the number of breeding included in the calculation. Sunday resulted in the greatest preg rate, Mondays the lowest.
An interpretation of the these charts is that for farms with high first service conception rate and strict protocol adherence in terms of VWP and resynchronization procedure, the variance in pregnancy rate calculated by DairyComp305® is primarily derived from the exclusion criteria of the model. Thus conception rate should be the metric for evaluating reproduction as it does not exclude upwards of 20% of breedings (figure 2, day 10 to day 11 comparison). On farms with good levels of compliance model exclusion criteria does account for half the variability so both should be considered as independent and co-dependent metrics. Farms with natural heat detection by electronic monitor or otherwise and farms with poor protocol compliance the preg rate is the best metric to use in DairyComp305®. For any farm comparison using DairyComp305, it is recommended the report’s start date is always set to the day of the week two days prior to the highest frequency breeding date of the farm.
BoviSync uses a different methodology, whereby the cycle is calculated for each animal based on its history of breedings and VWP. The length of the cycle is allowed to be set by the user, with the suggestion it match the target interbreeding interval (21d for naturally cycling farms and longer for synchronization farms). Doing this is a more complex approach. But, in the opinion of the author, results in a more robust calculation. In simplified terms the cycles are centered around the pattern of breeding to best match that animal’s breeding history. If a 21 d cycle is used, breeding 2 days after the VWP and again 21 d later will result in 2 cycles with 2 breedings in an animal based calculation 350 370 390 410 430 450 470 28.0% 28.5% 29.0% 29.5% 30.0% 30.5% 31.0% 31.5% 32.0% 1 3 5 7 9 11 13 15 17 19 21 Preg Rate Day 59% 1st Service Conception Rate, 90% Th breeding 45%, subsequent conception rate Preg Rate # of Preg (BoviSync). In a calendar system (DairyComp305 and all other calculations to the author’s knowledge) results in 1 cycle and 1 breeding on some of days the report is run and 2 cycles and two breeding on other days. This is the basis for our objection to the calendar approach, the results should not change solely based on the day the report is run. The animal based method is designed to include nearly all the breedings. The model also removes the daily variance due to exclusion criteria because all breedings are included.
Increasing the cycle length can result in better understanding of on-farm trends, and will significantly increase the preg rate for the farm. However, for standardized comparison between farms the same number should be used.
Another primary point of difference is the use of the VWP. In DairyComp305® a “global” VWP is set for the entire report, where any breeding activity done before the VWP is excluded. In BoviSync, the VWP for every animal is recorded. So if the farm employs different VWP by lactation or there are changes to the VWP, the data is retained on that animal, allowing for a more accurate calculation. Another primary difference is that breedings occurring prior to the VWP are treated as the start of the VWP. The rational for this treatment is that a breeding that occurs before the VWP indicates the recorded VWP for that animal was incorrect and that the breeding date was the true VWP for the animal. This results in nearly all breedings being included in the calculation.
A consequence of this decision is that the preg rate can be over-inflated by having a late VWP and a significant number of breedings occurring before the VWP. This can result in less accurate comparisons between farms. The alternative of excluding all those breeding activities results in inaccurate numbers on the farm and not great comparison between farms either “cherry-picking” animals for insemination or not following the stated VWP. Thus, the reason for the alternative method adoption.
Farms engaged in cherry-picking followed by synchronization, may choose to use a late VWP that matches the synchronization program and animals that are bred are included. The method used by BoviSync results in a more accurate preg rate based on the farm’s actual activity, but will likely be much higher than all other preg rate model calculations.