Detecting Pregnancy with DHI Test Day Milk Samples: Kings County Dairy Sees Value During Recent Demonstration Project
Adapted by Carol Collar, UCCE Kings County and Dr. Alex Souza, UCCE Tulare & Kern Counties
Pregnancy diagnosis in dairy cattle has traditionally been accomplished by rectal palpation. This method allows large numbers of cows to be checked per hour, and also enables action immediately after the examination depending on the outcome. Drawbacks of rectal palpation are the need to lock up cows in headlocks, technician skills that can influence accuracy of the exam, time to diagnosis (usually 35 days post breeding; longer than some other pregnancy detection methods), and biosecurity issues related to palpating multiple cows with the same palpation sleeve. Available alternatives to rectal palpation for detecting pregnancy include ultrasound, blood pregnancy tests and most recently, milk pregnancy tests.
Ultrasound technology can be used to detect pregnancy starting at 28 days post breeding, a week earlier than is possible by rectal palpation. Another benefit of ultrasound is that it enables visualization of fetal heartbeats to check for calf viability. Blood tests for pregnancy diagnosis have been on the market for a few years. These tests can detect pregnancy related proteins as early as 28 to 30 days post breeding and they are reported to be as accurate as ultrasound. Last year milk based pregnancy tests became available and preliminary studies indicate that they are accurate to detect pregnancy starting 30 days post breeding. As with the blood test, the milk test measures proteins that are produced by pregnant cows. These proteins, also known as PAG (pregnancy associated glycoproteins) are not produced in open cows. The milk based method has clear advantages over other methods, because the same milk sample that is routinely collected for SCC and milk components on DHIA test day can be used. No additional handling of animals is necessary.
How accurate is the milk pregnancy test? Very accurate for confirming pregnant cows, but only moderately useful for finding open cows according to a recently published field study (LeBlanc, 2013, J. Dairy Sci. 96:2345-2348). In this study, DHIA test day milk samples from cows that were 60 or more days pregnant were used to validate the performance of the milk test under field conditions on 8 different dairies in Canada. The same day that milk samples were collected, rectal palpation was performed by a veterinarian to confirm pregnancy status. There were 661 pregnant and 22 open cows diagnosed by rectal palpation and these results were compared to the milk test results to determine sensitivity and specificity of the milk test. Sensitivity is defined as the % of pregnant cows that are correctly classified as pregnant, and specificity is the % of open cows correctly classified as open. The milk test had a very high sensitivity (99.2%), which implies that users could have high confidence that a milk test result of pregnant is accurate. The value of the test in predicting a positive outcome (pregnant) was calculated to be 99.8%. The specificity of the milk test was lower (95.5%). The negative predictive value (open) of the milk test was only 81%. About 1 in 5 milk test results of “open” was incorrect. The author of the field study concluded that cows classified as open by the milk test should NOT be injected with prostaglandin or inseminated without further confirmation of pregnancy status.
Closer to home, a collaborative field demonstration project was recently undertaken by Kings DHIA (Hanford, CA), Animal Profiling International (Portland, OR), Eden Vale Dairy (Lemoore, CA) and UC Cooperative Extension. The objective was to observe farm and DHIA laboratory experiences in implementing the milk pregnancy test (EasyPreg, API). On three consecutive DHIA test days during September through November 2013, milk samples from selected cows were shipped from Kings DHIA to API where the milk pregnancy test was performed. Cows were selected in three categories post breeding: 35-60 days (first pregnancy diagnosis), 60-95 days (confirmation of pregnancy) and 185-215 days (reconfirm pregnancy before dry off). Results of the milk test were compared to rectal palpation by the herd veterinarian within 1 to 6 days of the milk sample collection. A total of 516 milk samples were submitted for milk pregnancy testing over the three DHIA test day periods. The results of the demonstration project were similar to those reported in the Canadian study. Sensitivity was high (99.2%), and specificity was lower (87.8%). These findings indicate that the milk test is very good at finding pregnant cows, but only moderately good at finding open cows, just like in the Canadian study. Follow up evaluation of cows diagnosed “open” with the test is recommended. It is important to note that this was not a carefully controlled research study, but rather an opportunity to observe and record the experiences of one dairy producer.
Logistically, we learned a few lessons. It is important to keep breeding records in the herd data file up to date to avoid unnecessary costs associated with testing cows that have a recent breeding. About 30 samples were selected for milk pregnancy testing from cows that should not have been included for this reason. Another lesson relates to sample handling. At the DHI lab, routine samples are identified with a unique sample number that cross references to cow ID. Before sending samples to API, we learned that cow ID must be included with samples shipped, otherwise API reports results by DHI sample number which has little value for the dairy producer. To be most helpful, samples for shipment should be labeled and ordered by cow number within string. With regards to sample shipment, the first month samples were shipped overnight from Kings DHIA via the United Parcel Service (UPS) at a cost of about $1.00 per sample. The United States Postal Service priority mail was utilized for subsequent shipments at a much lower rate of $0.35 per sample. Among DHI labs, Kings DHIA has one of the fastest turn-around times for sample processing and reporting; less than 2 days. But the interval between DHIA sample collection and arrival of those milk samples at the API lab was 5 or 6 days. The milk pregnancy test is rapid and the API lab reported results for the milk tests the same day that the samples were received. Transit time and cost are clearly issues that need to be addressed. In general, the dairyman was satisfied with the performance of the milk test in finding pregnant cows. He especially liked the convenience of using DHIA milk samples for the 60-95 day pregnancy confirmation without needing to hold cows in lock-ups. A more detailed summary of the demonstration project is available from UC Cooperative Extension or Kings DHIA.
More field studies to investigate implementation strategies and farm logistics of the milk pregnancy tests as they relate to DHIA milk samples are needed. In particular, a closer look at strategies to improve turn-around time would be helpful. Further study of the potential for carryover of milk PAG in meters (cross contamination) or sampling errors in the milking parlor are also needed. Most importantly, additional research to improve the negative predictive value of the milk pregnancy test is necessary to improve the usefulness of this technology for dairy reproductive management. We look forward to leading controlled studies in collaboration with DHIA and other dairy industry stakeholders in the near future.
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