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Timing of milk-based PAG pregnancy tests can make a difference

Timing of milk-based PAG pregnancy tests can make a difference

Tests require samples to be sent in to a lab for analysis

By Ryan Sterry

Identifying open cows in a timely manner is essential for successful reproductive management of the dairy herd. Fortunately, the options for doing so have been expanding.  In addition to palpation, ultrasound, and blood testing; milk testing is now being offered through DHIA testing centers.  Blood and milk tests both analyze for levels of pregnancy-associated glycoproteins, commonly referred to as PAG's. For accurate test results for either milk or blood PAG tests, cows need to be at least 60 days in milk and over 28 days post breeding before testing. 

Manufacturer recommendations indicate the milk PAG test should not be conducted earlier than 28 days after AI, and the results of this study agree with that recommendation.

Both the blood and milk PAG tests require samples to be sent in to a lab for analysis. Milk sample based PAG tests may offer a labor savings over blood based PAG tests, since milk samples used for milk component and somatic cell testing can also be used for the PAG test. Blood PAG testing may require cows to be handled an extra time to draw the blood samples. 

While the existence of PAG's has been known since the 1980s, it hasn't been until more recently that commercial testing has become feasible. PAG's have a long half-life and are at their highest levels near calving.  Thus the need to wait at until least 60 days in milk before testing, or PAG levels from the previous pregnancy may still be detectable. PAG levels are relatively low in early gestation, and can fluctuate before rising again in later in gestation. 

A recent University of Wisconsin-Madison study by Dr. Paul Fricke and his research team tracked changes in the level of milk PAG's through the first 100 days of gestation, and then compared milk PAG test results to ultrasound pregnancy exam results. Milk samples were collected once weekly, starting between 25 and 32 days after AI.  Ultrasound was used to determine pregnancy status at 32 days after AI. Cows identified as pregnant by ultrasound at day 32 were then re-checked weekly with both ultrasound and milk sample PAG's testing until 102 days after AI. Results of the milk PAG tests showed that PAG levels increased from day 25 to 32 of pregnancy, but then declined until days 53 through 67.  Day 74 through each weekly test until day 102 showed PAG levels gradually increasing again. 

The results of this study indicated that between days 32 and 39 after AI were the best times to take the initial milk PAG test. During this time frame the milk PAG's test did not identify any cows as open that were identified as pregnant by ultrasound, along with less than 5% of cows classified as "re-check" with the PAG milk sample.  Taking an initial milk sample at 25 days after AI resulted in the highest proportion of cows classified as a "re-check" (around 35%), and a higher than desired proportion of cows called open by the milk PAG test but later identified as pregnant by ultrasound.  Manufacturer recommendations indicate the milk PAG test should not be conducted earlier than 28 days after AI, and the results of this study agree with that recommendation.  Between days 46 to 67 the proportion of "re-checks" based on the milk PAG test increased to around 15% of samples taken, until declining to around 5% at day 74 through the end of the study. 

Based on these trial results, dairy farmers using milk sample PAG tests should attempt to take the first sample between days 32 and 39 after AI. Cows identified as pregnant should then be re-checked at day 74 or slightly later to identify cows experiencing early pregnancy loss.  For more details on this study please view the paper "Pregnancy Diagnosis using Milk PAG Testing" by Dr. Fricke, available online at the UW-Extension Dairy Team page: http://fyi.uwex.edu/dairy/     

Sterry is the St. Croix County Extension agriculture agent.

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