Most dairy farms use behaviour as the indicator of heat or oestrus, whether increased activity associated with oestrus is detected automatically by collars or pedometers, or by observation of typical behaviour.

Observation of changes in activity depends upon making sufficient time available, and depends on cows exhibiting the behaviour. A number of factors can reduce behavioural signs; including some types of housing and high yield, despite the cow cycling - leading to the term “silent oestrus”. Activity is controlled by hormone activity, with the presence of progesterone (P4). It is possible to assess levels of progesterone to detect heat, either by automatic systems such as; Herd Navigator, or using manual systems such as P4 Rapid.

Oestrus cycle

Whilst progesterone measurement can be used in numerous ways, the main use is to identify heat or oestrus. This is a period of time lasting two to three days within the cycle (typically 21 days), when the cow has low progesterone. Before and after this period, progesterone levels fall and rise respectively, rapidly reaching a high plateau level for approximately 18 days between heats. The diagram below shows the typical levels of the different hormones throughout the oestrous cycle.

Using progesterone as a tool for heat detection

After calving there is a period of anoestrus as the reproductive tract returns to a non-pregnant state. Throughout this time the hormones do not cycle and progesterone remains low. Anoestrus lasts for approximately 24-60 days and may be prolonged by; suckling; high milk yield; poor nutrition; poor body condition; time of year and uterine recovery. After the voluntary waiting period is over, weekly assessment of P4 levels over a three week period should show one low and two high (in any order) if the cow has started cycling. Assessment of P4 levels from 18 days after the low was detected will permit pinpointing of the next heat event. Artificial insemination (AI) on the second or third day of low P4, results in better conception rates than insemination on day one of low P4.

21–24 days post-insemination return to oestrus can be assessed using P4 status: low P4 indicates return to oestrus having not held to the AI. Even when pregnancy has been established, there are still follicular waves at intervals, which can result in oestrus-like behaviour. Of course, AI during pregnancy is ill-advised and can result in infection or even abortion. True return to oestrus can be distinguished from follicular wave activity as the former is associated with low P4, whilst pregnancy has high P4.

There are various ways of using P4 as a tool to assist heat detection. In a recent study conducted by Bridgit Muasa, at the SRUC, P4 Rapid used three times weekly to detect heat had an 89.91% sensitivity and a 86.02% specificity, compared to 11.91% sensitivity and 92.69% specificity for Estrotect, a heat mount detector used according to manufacturer’s instructions.

P4 assessment can complement other heat detection methods, either to confirm P4 status where a probable heat event has been detected using activity or behaviour monitoring, or in the population of cows within the herd where oestrus behaviour is poorly shown or absent.

P4 assessment can provide a method of heat detection that does not rely on overt behavioural changes. It can be used as a stand alone or in combination with other systems.

Reference
Muasa, B., 2017. Cow-side Reproductive Performance Monitoring Technologies in Dairy Cows. BSAS Conference, April 2017 Chester, UK.