CALIFORNIA DEMO PROJECT REPRODUCTIVE ANALYSISPREGNANCY RATE:

Pregnancy Rate is a measure of how fast cows get pregnant once the breeding period begins. It is based on the number of pregnancies that occur per 100 eligible heat cycles. An eligible heat cycle is any cycle that occurs after the end of the voluntary wait period. This particular definition of pregnancy rate is sometimes referred to as a 21-day pregnancy rate.

21-day Pregnancy Rate = (No. Pregnancies / No. Eligible Heats) x 100

Eligible heats are defined as sequential 21-day periods following the end of the voluntary wait (VWP) period, until such time as the cow becomes pregnant, is coded as “do not breed” (DNB), or culled. In many herds, cows are bred and conceive prior to the end of the stated VWP policy. In this situation, the VWP is defined by how many days in milk the cow with the earliest pregnancy is when she conceives.

One example of how this formula works in practice is to consider the pregnancy rate for a single cow (Cow A). If Cow A is in a herd in which the voluntary wait period is 50 days and she becomes pregnant on day 120 then the calculation is as follows:

No. eligible days = 120 - 50 = 70 daysNo. eligible heat cycles = 70 / 21 = 3.33 cycles whichmeans she became pregnant in her 4 th eligible cycleNo. pregnancies = 1

21-day Pregnancy Rate = 1 / 4 = 25%
A pregnancy rate is also estimable for an animal that is not pregnant. Consider Cow B in the same herd is 90 days in milk and not yet been bred.

No. eligible days = 90 - 50 = 40 daysNo. eligible heat cycles = 40 / 21 = 1.9 cycles which isrounded to 2 heat cycles (round to the nearest wholenumber for open cows)

No. pregnancies = 0

21-day Pregnancy Rate = 0 / 2 = 0%

An average pregnancy rate for a group or herd of animals is simply calculated by counting all pregnancies and dividing by the sum of all eligible heat cycles across all cows in the group and multiplying by 100. Notice this is NOT the same as estimating a pregnancy rate for each cow and then averaging pregnancy rates across cows. This would cause an under estimate of the true pregnancy rate. Using cows A and B as an example, the correct average 21-day pregnancy rate is estimated as follows:

Total eligible heat cycles = 4 from Cow A + 2 from Cow B = 6

Total pregnancies = 1 (Cow A) + 0 (Cow B) = 1Average 21-day Pregnancy Rate = (1 / 6) *100 = 16.7%rounded to the nearest percent is 17%.Summary of Demonstration of POSILAC on California Dairy Herds Project Results

Across the seven herds in the Demonstration of POSILAC on California Dairy Herds project, pregnancy rates varied. In some herds, pregnancy rates for POSILAC supplemented cows appear to be greater than for cows not supplemented. In other herds the reverse appears to be true, and in other herds there is no apparent difference. Pregnancy rate is affected by heat detection and breeding efficiency. Any differences that may occur in herds using POSILAC are small and manageable. The key to sound reproduction is good heat detection and skillful breeding technique regardless of whether POSILAC is used or not.

DAYS OPEN:

Average Days Open is a more traditional measure of reproductive efficiency. It is often considered as a measure of how fast cows get pregnant. There are several inherent weaknesses in using only days open to monitor reproductive performance.
One issue is inconsistency in the definition of which animals should be included in the average. Should open cows be included? What about cows that have not yet made it through the VWP? If open cows are included, there is always the question of how to count days open for cows that are not yet pregnant. Is it the days from calving to current days in milk, or from the end of the VWP to current days in milk? No matter how it is counted, it changes every day.

Another issue with using days open as a sole monitor of reproductive efficiency is that no matter what definition is used; it is subject to being counter-indicative of what is occurring in the herd. For example, if only pregnant cows are included in the average days open estimation, there are times when success occurs, only to make the average days open to look worse. This happens when a long days open cow, say 250 days, gets pregnant. If average days open was 120, now including this cow in the average will cause it to increase despite a successful event – an additional pregnancy.

Another issue is that management decisions in other realms of the dairy operation can affect days open. One way to make days open look better is to cull open cows. This is a drastic decision indicative of poor reproductive management, yet the culling decision makes the reproductive monitor look better. There are other issues such as momentum, lag, and issues inherent in how dairy records may be managed that also may negatively impact the usefulness of average days open as an effective monitoring tool for a reproductive management program.

Average days open for the purpose of the demonstration project includes days open for pregnant cows only. Days open is simply calculated for each pregnant cow by determining how many days in milk a cow is when she conceived. Average days open is the simple average of days open over all pregnant cows in a treatment group. Therefore it is only a measure of how fast pregnant cows conceived and not necessarily indicative of the effectiveness of the total reproductive management program.

Summary of Demonstration of POSILAC on California Dairy Herds Project Results

There is variability in overall days open from herd to herd in the seven herds of the Demonstration of POSILAC on California Dairy Herds project. However when comparing days open between cows supplemented with POSILAC and control cows within herds, any differences are very small and inconsistent. Overall there appears to be no difference in average days open for pregnant cows between cows supplemented with POSILAC as compared to control cows in the same herd under the same management. This suggest s again that the same sound reproductive management principles apply regardless of if cows receive POSILAC.

PERCENT PREGNANT:

Percent Pregnant is simply a measure of the proportion of eligible cows that become pregnant. Often this measure is called a pregnancy rate. However, percent pregnant is a more accurate name for the calculation. It is calculated as follows:

Percent Pregnant = (No. Pregnancies / No. Eligible Cows) x 100

Percent pregnant is a function of how fast cows get pregnant (pregnancy rate) and how long a period they are given to get pregnant. In this case how long they are given the opportunity to get pregnant is directly related to how long a period the demonstration project covered. In other words, for two herds with identical pregnancy rates, the herd with the longer demonstration period will have a larger percent of eligible cows become pregnant. For this reason, unless cows in herds being compared have the same opportunity to get pregnant, percent pregnant cannot be compared across herds. When cows in different herds or groups have the same opportunity to get pregnant, than differences in percent pregnant is simply a function of differences in pregnancy rate.

Summary of Demonstration of POSILAC on California Dairy Herds Project Results

Differences of percent cows pregnant between herds in the demonstration project are a function of both differences in pregnancy rate and various lengths in duration of each herd project. Therefore, no comparisons should be made across herds. Within herds comparing cows supplemented with POSILAC to control cows, the results are very similar to differences in pregnancy rate. The differences appear to be small and inconsequential. And again, the focus on factors, which influence sound reproduction, should be the same regardless of whether POSILAC is used.

Source: Monsanto