Methods of pregnancy detection in dairy cattle

10 March 2022

Dr Emma Davies: IBERS, Aberystwyth University.

• Poor fertility is one of the most significant factors that limits the productivity of the dairy industry in the UK.
• Accurate detection of oestrus and pregnancy could improve the reproductive performance of dairy herds by improving conception rates and reducing calving intervals.
• Farmers need to work with their veterinarian to develop a herd health plan to incorporate pregnancy detection and the diagnosis of infertility. This will ensure prompt and appropriate action can be taken.

Introduction

The productive performance of a dairy herd can be significantly affected by cow fertility. Cow fertility can be influenced by a wide range of factors, including the use of reproductive management strategies. As such, there are a wide range of different key performance indicators that can be used to monitor dairy herd fertility. While the exact parameters of these indicators may differ according to the herd type (e.g., block calving, spring calving, all year round calving, indoor herds, outdoor herds, and herds of different breeds), data can be compiled to create benchmarks and set targets for herds of similar types. For example, the data of one study that compiled the key performance indicators of 500 separate Holstein/Friesian dairy herds in the UK is often used to benchmark other similar dairy herds. Some examples of the reproductive key performance indicators recorded in 2020 for these herds are included in Table 1.

Many reproductive performance indicators are indicative of a herd’s conception rate. For the 500 herds included in the aforementioned study, the herd conception rate by 100 days post calving was less than 40% for the average herd, and 42% for the top performing 25% of herds. Low conception rates such as these can significantly limit a herd’s fertility and overall productivity. A wide range of different factors, such as genetic capacity, anovulation, uterine infections, mastitis, lameness, and poor nutritional management can all cause poor conception rates in dairy cattle. However, low conception rates are also caused by the sub-optimal detection of oestrus (heat) and pregnancy, both of which can lead to the inappropriate timing of artificial insemination (AI). Therefore, the accurate detection of oestrus and early pregnancy are extremely important to help dairy herds monitor and improve their fertility.

Of course, when aiming to improve fertility, it is important for farmers to work alongside their veterinarian and conduct any reproductive management strategies in accordance with veterinary advice.

Table 1: A sample of key performance indicators of 500 UK Holstein/Friesian herds measured in 2020.

Achieving optimal reproductive performance is key to the overall productive success of a dairy herd.

Pregnancy detection methods

The oldest pregnancy detection method is a physical examination of the ovaries and uterus via transrectal palpation. Palpation can detect pregnancy as early as 35-42 days following AI, however, palpation is traditionally conducted no earlier than 40-60 days of gestation. The most common method of pregnancy detection is the transrectal ultrasound examination of the ovaries and uterus. This method can diagnose pregnancy from 25 days post-AI, however, the accuracy of pregnancy diagnosis increases when ultrasound examination is conducted 30 days post-AI. In practice, ultrasound examination is usually conducted 30-50 days post-AI. In addition to detecting pregnancy, ultrasound examinations can identify how many embryos are present and estimate the stage of the pregnancy, therefore assisting the management of twin pregnancies and pregnancies where the exact covering date is not known. While both techniques provide immediate results, the accuracy of these methods in early gestation relies significantly on the skill and experience of the person conducting them.

Pregnancy can also be diagnosed from progesterone levels, measured from either a blood or milk sample. Progesterone is produced from a structure formed within the ovary following ovulation, called the corpus luteum. If a cow is pregnant then her progesterone level will remain high 21-24 days after AI, as the corpus luteum remains intact to maintain pregnancy. If the cow is not pregnant then her progesterone level will decrease 21-24 days following AI, as the corpus luteum regresses and the cow returns to oestrus. However, as progesterone levels fluctuate throughout the cow’s normal oestrous cycle, the use of progesterone as a pregnancy detection method is limited. While a low progesterone level 21-24 days after AI can accurately diagnose a non-pregnant cow, a high progesterone level 21-24 days post-AI cannot accurately diagnose a pregnant cow. This is because factors other than pregnancy can cause progesterone levels to be high. For example, cows with an extended oestrous cycle, cows inseminated when they are not in oestrus, and cows experiencing a delayed return to oestrus due to embryonic loss will exhibit increased progesterone levels 21-24 days post-AI, resulting in a false positive test result. Studies have reported the accuracy of progesterone for pregnancy detection to be between 65-77%. Therefore, rather than for the detection of pregnancy, progesterone testing is often used for the confirmation of oestrus or non-pregnancy.  

Alternatively, pregnancy can be diagnosed by measuring levels of pregnancy-specific protein B (PSPB) from a blood sample. PSPB is a glycoprotein produced by cells within the trophectoderm of the embryo. This embryonic layer attaches to the uterine epithelium facilitating placental attachment. PSPB concentrations increase in cattle from day 15 of gestation and can be used to accurately diagnose pregnancy as early as 24-35 days post-AI. This method has a 98% sensitivity rate (ability to identify pregnant cows, i.e. true positives), and a 97% specificity rate (ability to identify non-pregnant cows, i.e. true negatives). PSPB blood tests are commercially available, however they require a person trained in blood sample collection, and results may take 2-3 days.

The importance of early pregnancy detection

One of the most common methods for detecting ovulation in dairy cows, is the observation of oestrus behaviour (heat behaviour). Therefore, when a cow has been inseminated, one of the methods that farmers commonly use to assess whether the cow has conceived is to observe either an absence of oestrus behaviour, or a return to oestrus behaviour. If a cow shows oestrus behaviour following AI, it is often assumed that she has not conceived, and she is about to ovulate again. As a result, many cows showing oestrus behaviour will be inseminated again.

However, relying on the observation of oestrus behaviour alone, without corroboration from other oestrus or pregnancy detection methods can be detrimental to cow fertility. This is because up to 10% of pregnant cows actually show oestrus behaviour within 21 days of conception. Such behaviour is identical to that exhibited by non-pregnant cows and may include restlessness, being mounted, mounting other cows, mounting the head of other cows, or showing standing heat.

In addition to being financially expensive, the insemination of pregnant cows significantly increases the rate of late embryonic loss that occurs between 21-42 days of gestation. One study reported that late embryonic loss due to the AI of pregnant cows can be as high as 24%, significantly greater than the average rate of spontaneous pregnancy loss for the same period, which ranges from 7-12%. In this particular study, embryo loss following the AI of pregnant cows increased the average number of days between parturition and conception from 106 to 157. Therefore, early pregnancy detection is very important to reduce the number of unnecessary inseminations of pregnant cows and to reduce embryonic losses.

When combined with breeding and insemination records, accurate pregnancy detection can provide an estimation of the cow’s calving date. This can inform important management practices, including the inducement of the dry period, the management of dry period length and the management of parturition.

In contrast, having early confirmation that a cow is not pregnant can confirm if repeated AI is necessary. If the cow is to be inseminated again, the cow can be prioritised for the observation of oestrus. Alternatively, an early negative pregnancy test result can inform AI management decisions, such as deciding whether a dairy or beef sire is used for the subsequent insemination, or whether a bull will be used to naturally cover the cow.

An early confirmation of non-pregnancy can also assist the management of cows that do not show any behavioural signs of oestrus. A lack of oestrus behaviour, also known as a silent heat, is common in heifers and primiparous cows. This means a failed insemination can go undetected and opportunities for subsequent inseminations can be easily missed. Therefore, the early confirmation of a failed insemination can also be used to decrease insemination intervals in animals not showing oestrus behaviour.

Therefore, the use of an accurate early pregnancy test that is able to inform reproductive management decisions can increase productivity by increasing conception rates, enabling timely culling, and assisting in the planning of replacements.

Pregnancy tests are important tools to confirm the success of artificial insemination, as although a return to heat is generally considered confirmation of non-pregnancy, some pregnant cows also display oestrus behaviour.

While early pregnancy detection methods can improve reproductive management by reducing the number of inseminations, methods such as palpation, ultrasound examination and progesterone levels can only guarantee pregnancy status at the time of diagnosis. This means that an early pregnancy diagnosis can be confounded by subsequent pregnancy loss. The rate of pregnancy loss is greater in early gestation and decreases as gestation progresses. In a summary of 14 studies, late embryonic loss between days 27-50 post-AI averaged 13%. Therefore, if early methods of pregnancy detection, such as physical examination and progesterone levels are used, additional pregnancy tests to reconfirm pregnancy later in gestation are required to detect those cows that have lost a pregnancy. If these animals go undetected the time between parturition and conception increases, increasing the calving interval, and reducing reproductive performance.

Subsequent pregnancy testing could be conducted simply repeating any of these pregnancy detection measures. However, one study that measured the percentage change in PSPB levels between 17- 24 days post-AI reported that low levels of PSPB on day 24 post-AI was predictive of embryo loss by day 60 post-AI. The study concluded that low levels of PSPB at this time could alert farmers and veterinarians to cows that are at risk of pregnancy loss.

Today, the most common method of pregnancy detection on dairy farms is ultrasound examination, followed by palpation, non-return to oestrous, and finally progesterone and PSPB testing. However, the practical on-farm application of PSPB testing is being increasingly investigated by both researchers and farmers alike. In Wales, one European Innovation Partnership project, involving four dairy farms, aimed to compare the use of PSPB testing with transrectal ultrasound examination for pregnancy detection. The project found that the accuracy of PSPB testing and transrectal ultrasound examination was comparable. PSPB testing and ultrasound examination had 94% and 95% sensitivity rates (ability to identify pregnant cows, i.e., true positive rate) respectively. PSPB testing and ultrasound examination had 87% and 86% specificity rates (ability to identify non-pregnant cows, i.e., true negatives) respectively. The project concluded that the early identification of infertility led to consultation with the farm’s vet, facilitating the use of appropriate fertility treatments.

Summary

Early pregnancy detection methods including physical examinations and PSPB testing can provide accurate pregnancy diagnoses. While high progesterone levels can be indicative of pregnancy, progesterone levels need to be monitored over an extended period of time or be used in conjunction with other testing methods to diagnose pregnancy. In contrast, low progesterone levels can accurately detect a non-pregnant cow and indicate oestrus, informing the optimal time for artificial insemination. If used appropriately, early pregnancy detection methods can improve the productivity and profitability of dairy farms by helping them achieve the benchmarks of good reproductive performance.

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