Ultrasound versus milk hormone test

It has never been more important to know the gestational state of our dairy cows as early as possible.

A positive diagnosis of pregnancy is good news, but determining the negatives is crucial. The time a non-pregnant cow is empty must be kept to a minimum.

For many years manual palpation was the only way to diagnose pregnancy and experienced veterinarians performed it with good accuracy from 35 days post-service.

The first hormonal assays, detecting progesterone, allowed testing via milk. However, this had major drawbacks including false-positive results, requiring sequential sampling for better accuracy.

The ultrasound machine improved the manual technique, allowing earlier and more accurate diagnosis. Hormonal testing reached a new level when the progesterone test was replaced with testing for alternative pregnancy-specific hormones.

The need to establish pregnancy for herd production efficiency is unquestionable. Farmers can choose between an accurate hormonal testing package and an ultrasound examination. In some cases, a combination of both may be the answer.

Ultrasound method

A portable ultrasound machine operating between 3.5MHz and 9MHz is used to diagnose pregnancy.

Many machines with different frequencies are in use, with the soundwaves conducted through either a linear or sector array of crystals to produce the picture. The quality and style of the picture influences the accuracy of the diagnosis.

For this article I will assume diagnosis is made using a machine with a quality image and conducted by an experienced operator.

An accurate ultrasound diagnosis is possible as early as day 28 after insemination¹. The first visible changes appear between 13 and 17 days, but diagnosis cannot be determined at this point. By day 21 the fetal heartbeat can be visualised, confirming a possible pregnancy², but it is still too early to confirm.

Trans-rectal ultrasonography is routinely performed from the 28 days gestation point, with accuracy of 98 per cent to 100 per cent in the hands of the experienced operator.

Ultrasonographic diagnosis also provides information on ovarian structures, identifies twins, determines fetal viability and determines the age and fetal gender³.

The instant result and the ability to examine the reproductive tract to find any problems, allows the negative cow to be treated without delay.

Hormonal testing

A molecule able to determine bovine pregnancy has attracted extensive research. The goal is an in-line milk test for the molecule, giving an instant result to a computer system.

Many molecules have been and are being investigated. These include the hormones progesterone, oestrone sulphate, early conception factor and pregnancy associated glycoprotein (PAG). PAG is the most promising so far; a test is commercially available.

PAG is produced from the trophoblastic cells in the placentomes. A sample is sent to a laboratory, with the results known inside three days.

Its accuracy was determined in an experiment by Paul Fricke, a professor of dairy science in Wisconsin^4,5. Based on PAG profiles in milk samples collected weekly, the best time to conduct a first pregnancy diagnosis is 32 days to 39 days after timed artificial insemination (TAI), when milk PAGs are at an early peak in pregnant cows. They could not determine the earliest day testing was accurate, so they recommended following the manufacturer’s guidance of ≥28 days after AI.

By contrast, conducting the milk PAG test during the temporal nadir, from 46 days to 67 days after AI, resulted in a lower overall accuracy of 75 per cent to 80 per cent – and the possibility of aborting pregnancies if prostaglandin F2α is administered based on “not pregnant outcomes”.

Finally, they recommended pregnant cows should be checked again at 74 days after AI or later, when PAG profiles have rebounded from their nadir.

The study supported milk PAG testing 32 days to 39 days after TAI when levels in pregnant cows are at an early peak, with pregnancy outcomes approaching 100 per cent accuracy. PAG levels detected by these tests had a half-life in maternal circulation, resulting in a delay of 7 to 14 days in identifying cows undergoing pregnancy loss based on plasma or milk PAG levels.

PAG levels remain high for up to 70 days post-calving, preventing its use for pregnancy diagnosis at this stage.

Comparing methods

Table 1 compares the ultrasound and milk test methods.

It is difficult to compare the costs because the cost of an ultrasound varies.

The milk test costs around £3 to £3.50 and the ultrasound cost is similar when few cows are tested. However, the ultrasound cost is favourable when larger groups are examined. The ultrasound cost per pregnancy diagnosis is determined by the efficiency of the handling facility. An experienced vet and efficient handling facilities enables 45 to 64 cows per hour to be examined, giving a cost range of £2.44 to £1.70 per cow.

In addition to the direct cost, there are indirect cost benefits for each method.

The main advantage of the hormonal test is its ease of sample collection. Samples are taken during milking, eliminating the need for separation and handling of the cow. The only extra time required is to prepare and post the sample. This can be done at the same time as milk recording samples, but this restricts the days they are collected – reducing the advantage of the earlier diagnosis.

In addition, restricting the taking of the samples to milk sample day will preclude some cows that may fall outside the early accurate testing window.

Ultrasound diagnosis is instant, so the positives can be recorded and negative cows can be treated immediately.

If a routine herd visit is conducted weekly, the negatives are diagnosed quickly and treated on average two to five days sooner, this being the time it takes between taking the milk sample and obtaining the result.

Other indirect advantages of the ultrasound include the detection of twins, fetal viability, gender determination, reproductive tract health and very accurate detection of the stage of the pregnancy.

To get the best from the ultrasound, it is imperative an experienced veterinarian performs the diagnosis. Once the negative diagnosis has been determined the cow can be treated with hormonal regimes or advice strategies, which can quicken its rebreeding and conception times compared to a blanket re-treatment regime. Treatment of the negative can take many choices, determined by the structures seen on the ovary.

Milk testing has a role in small herds where the frequency of the routine visit and the number of cows to be examined are small; therefore, the cost of ultrasound diagnosis becomes higher.

It is also of benefit where ultrasound may not be an option, or in countries where the ultrasound machine is expensive and experienced operators are not available.

Both methods can be used for testing at a later stage (more than 74 days) and for the routine retesting of all cows just before drying off to confirm they are still in calf. Ultrasound diagnosis has a slight advantage for this due to the possibility of the “false positive” scenario of the PAG levels persisting after fetal loss. Ultrasound can also reconfirm the stage of pregnancy where the cow could have been rebred at a later date than recorded.

Both methods will continue to develop, offering more choice and continued benefits. As the trend for larger herds continues, the cost benefits of both methods will develop as their delivery methods evolve.