The science behind the stick

How a drop of milk becomes a breeding decision.

The hormone

Progesterone drives the cycle, behaviour is the expression. A healthy cow runs on a ~21 day reproductive cycle involving six different hormones.¹˒² Progesterone (P4) is the one that tells the clearest story: it raises after ovulation, maintains through the luteal phase and drops sharply at oestrus “heat”.¹˒² That drop is the biological event, “in heat” is, at the hormone level.

Standing heat, the traditional behavioural marker, is only visible for short windows throughout the day,⁴ and cows can stand to be mounted when they’re not in heat at all.⁴˒⁵ False bulling is common,⁵ and some cows don’t express outward signs. A silent heat is a missed heat.⁶

This is why measuring the hormone, rather than relying on behaviour as the indicator is the most reliable way to know where a cow is in her cycle.²

When progesterone drops, shes in heat.

The signal
Why it works in milk

Progesterone circulates in the blood, but it also passes into milk in proportion to blood concentration.¹˒³ Meaning a milk sample is a valid proxy for a blood sample, without needing specialist expertise or a needle. The relationship between blood P4 and milk P4 has been characterised in the dairy science literature since the 1970’s,² and milk P4 assays have been used as research tools for decades.² What’s changed is the format, move the assay out of the lab and onto the farm.

Milk-based progesterone testing enables you to do three this behavioural methods can’t reliably do:

  • Confirm true heat before insemination.⁷

  • Catch silent heat - up to 30% of cycling cows show no visible signs.⁶

  • Detect non-pregnancy early - a low P4 results at ~21 days post AI means shes open, so you can re-inseminate sooner rather than waiting for a PAG or scan.¹⁰

Milk P4 has always been the gold standard for cycle tracking. What's more recent, is the ability to accurately measure it cow-side.

87-97%

Agreement between cow-side milk P4 testing and reference lab assay for oestrus detection and non-pregnancy.

Peters, 2022 - Kenyan dairy field study, n=500+.⁷

700+

Jersey cows on a timed-AI programme where on-farm milk P4 reliably identified cows that were not pregnancy, enabling earlier reservice.

Omontese et al, 2020.¹⁰

Over 40 years

Peer-reviewed literature established milk P4 as a reliable indicator of ovarian activity.

Reviews: Wathes & Diskin 2025; Muasa et al 2017.²

The format
From lab to on-farm

Lab-based P4 assays are accurate but slow as the samples get sent away, results returned and by then the breeding window is closed. What P4 HeatRead does, is take the same underlying immunoassay chemistry, and deliver it in a lateral flow test.

Milk is applied to a sample pad and wicks across the strip. A test line and a control line develop over 10 minutes. Whether a test line appears, tells you whether P4 is high ‘NOT Low / not in heat. or Low, in heat.

Same chemistry, same accuracy, just quicker.

References

    • Wathes DC, Diskin MG. Reproduction, events and management: reference overview of bovine reproductive management including progesterone monitoring. 2015.

    • Friggens NC, Chagunda MGG. Prediction of the reproductive status of cattle on the basis of milk progesterone measures: model description. 2005.

    • Peters A, Wong, Williams, Muasa B, Makoni, Ngige, et al. Cow-side P4 Rapid lateral flow test as an aid for oestrus detection and non-pregnancy in Kenyan dairy cows. 2022.

    • Jahedi, Mostafalou, Moslemipour, Firouzeh. Accuracy of Rapid P4 milk kit for oestrus detection and early pregnancy in Holstein cows (Iran). 2019.

    • Järvinen M, Kajava S, Ruuska S, Mononen J, Lindeberg H. Reliability of P4 Rapid milk progesterone test for dairy cows — assessment of cyclic activity detection post-calving. 2014.

    • Back PJ, Martin NP, Purchas G, Susnjar M, Yapura J. Validation of a visual estrus scoring system for use in grazing dairy herds. 2012.

    • Mehmedi, Gecaj, Kastrati, Heltai, Turmalaj. Practical application of P4 Rapid in milk for early pregnancy detection at day 21 post-AI. 2021.

    • Omontese BO, Gomes, Santos, et al. Qualitative on-farm milk P4 lateral flow test to predict non-pregnancy in Jersey cows under timed AI. 2020.

    • Omontese BO, Santos AR, Silva LG, Merenda VR, Bisinotto RS. Use of milk progesterone data to predict non-pregnancy in dairy cows subjected to timed AI — preliminary findings. 2017.

    • Turner A. Literature review: can milk progesterone assay predict embryo survival post-AI? Review of P4 concentration thresholds on days 4–7. 2014.

    • Ridgeway Science / P4Gold Team. Progesterone profiling of three dairy cows using P4Gold and ELISA post-AI. 2021.

    • Ingenhoff, Hall, House. Diagnostic sensitivity and specificity of P4 Rapid milk assay for corpus luteum detection in NSW farms. 2016.

    • Waldmann, Raud. Comparison of lateral flow milk P4 test with enzyme immunoassay for reproductive status in dairy cows. 2016.

    • Muasa B, Chagunda MG, Onyango, Peters A. P4 Rapid test with activity monitors (CowAlert) in tropical and temperate dairy systems. 2025.

    • Bonev G. Application of P4 Rapid test and exogenous progesterone in prevention of early embryonic death in repeat-breeder Holstein dairy cows. 2021.

    • Muasa B, Chagunda MG, Ashworth C, Peters A. Cow-side reproductive performance monitoring technologies in dairy cows — a review. 2017.

    • Ball P, Rogerson R, Maconochie G, Peters A, Hollingshead M, Fisher M. The use of a cow-side lateral flow milk progesterone test (P4Rapid) to aid reproductive management in a dairy herd: a case study. 2014.