The oestrous cycle across species
Phases and hormones of the oestrous cycle, how cattle, sheep, pigs, horses, dogs and cats differ, heat detection, and a herd pregnancy rate sum.
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A cow that is not detected in heat, or is inseminated at the wrong time, costs her owner three weeks of lost milk and a later calf. A ewe flock that is not cycling in spring will not lamb at the planned time. The oestrous cycle, the recurring series of reproductive changes in female mammals, governs when animals can conceive. Its length, the timing of ovulation and its seasonality differ greatly between species, and good reproductive management depends on knowing them.
The oestrous cycle and its control are core topics in veterinary physiology, animal reproduction and livestock management exams.
Phases of the cycle
- Pro-oestrus: follicles grow under follicle-stimulating hormone (FSH), and rising oestradiol prepares the reproductive tract.
- Oestrus ("heat"): the female accepts the male; oestradiol peaks and triggers the luteinising hormone (LH) surge, which causes ovulation.
- Metoestrus: the ruptured follicle becomes the corpus luteum (CL), which begins to secrete progesterone.
- Dioestrus: the longest phase; progesterone from the mature CL dominates, preparing the uterus for pregnancy.
If there is no pregnancy, the uterus releases prostaglandin F2α (PGF2α), which destroys the CL (luteolysis); progesterone falls, and a new cycle begins. In ruminants, the conceptus signals its presence by secreting interferon-tau, which blocks luteolysis; in pigs, the embryos secrete oestrogens.
The follicular phase (pro-oestrus and oestrus) is short; the luteal phase (metoestrus and dioestrus) is long. Anoestrus is a period without cycles, as after calving, during poor nutrition or outside the breeding season.
The hormones
| Hormone | Source | Main role |
|---|---|---|
| GnRH | hypothalamus | stimulates FSH and LH release |
| FSH | anterior pituitary | follicle growth |
| LH | anterior pituitary | ovulation, CL formation |
| Oestradiol | growing follicles | heat behaviour, LH surge |
| Progesterone | corpus luteum | maintains pregnancy, suppresses heat |
| PGF2α | uterus | luteolysis |
| Melatonin | pineal gland | signals day length in seasonal breeders |
Differences between species
| Species | Cycle length | Oestrus duration | Ovulation | Pattern |
|---|---|---|---|---|
| Cow | about 21 days | about 12–18 hours | about 10–14 h after end of oestrus | polyoestrous all year |
| Buffalo | about 21 days | about 12–24 hours | after end of oestrus | polyoestrous, more breeding in cooler months; many silent heats |
| Ewe | about 17 days | about 24–36 hours | near end of oestrus | seasonal, short-day breeder (autumn) |
| Doe (goat) | about 21 days | about 24–48 hours | near end of oestrus | seasonal in temperate breeds |
| Sow | about 21 days | about 48–72 hours | about two-thirds into oestrus | polyoestrous; returns to oestrus about 4–7 days after weaning |
| Mare | about 21 days | about 5–7 days | 24–48 h before end of oestrus | seasonal, long-day breeder (spring and summer) |
| Bitch | intervals of about 6–7 months | about 9 days | early in oestrus; eggs need time to mature | monoestrous |
| Queen (cat) | about 2–3 weeks when cycling | several days | induced by mating | seasonally polyoestrous |
Seasonal breeders respond to day length through melatonin: sheep and goats breed as days shorten, horses as days lengthen. Breeding outside the season can be induced with light control, progesterone devices and the ram effect.
Detecting heat
Signs in cattle include standing to be mounted (the most reliable sign), mounting others, restlessness, bellowing, clear mucus discharge, a swollen vulva, and a drop in milk yield. Detection is improved by observing at least two or three times a day, tail paint or chalk, heat-mount detectors, activity monitors (pedometers, collars) and teaser animals. Poor heat detection is one of the most common causes of low fertility in dairy herds.
Timing insemination
The AM–PM rule for cattle: a cow first seen in heat in the morning is inseminated that evening; one first seen in the evening, the next morning. Sows are often inseminated twice, 12 to 24 hours apart.
Controlling the cycle
Synchronisation programmes use PGF2α (to end the CL), progesterone devices, and GnRH to bring groups into heat or ovulation together, enabling fixed-time artificial insemination without heat detection, such as the Ovsynch protocol in dairy cows.
Heat detection and herd pregnancy rate
The 21-day pregnancy rate = heat detection (submission) rate × conception rate.
A dairy herd detects 50 per cent of eligible cows in heat each 21 days, and 40 per cent of inseminations result in pregnancy:
- Pregnancy rate = 0.50 × 0.40 = 20 per cent per 21-day cycle.
If better observation and activity monitors raise heat detection to 70 per cent, with the same conception rate:
- Pregnancy rate = 0.70 × 0.40 = 28 per cent.
The proportion of cows still open after four cycles falls from (1 − 0.20)4 = 41 per cent to (1 − 0.28)4 ≈ 27 per cent, shortening the calving interval and increasing milk sold per cow per year.
Key points
- Phases: pro-oestrus, oestrus, metoestrus, dioestrus; anoestrus when cycles stop.
- GnRH → FSH and LH; oestradiol drives heat and the LH surge; progesterone from the CL; PGF2α ends the CL.
- Cycle about 21 days in cows, buffalo, goats, sows and mares; about 17 days in ewes.
- Sheep and goats are short-day breeders, horses long-day breeders; cats are induced ovulators; dogs are monoestrous.
- Heat detection and correct insemination timing (AM–PM rule) are vital; synchronisation enables fixed-time AI.
- 21-day pregnancy rate = heat detection rate × conception rate.
What the exams ask
- School-leaving and entrance papers: phases of the oestrous cycle, and signs of heat.
- Degree and research-fellowship entrances: hormonal control, cycle lengths and oestrus duration by species, seasonality, maternal recognition of pregnancy, and synchronisation protocols.
- Veterinary and professional examinations: managing reproductive failure, anoestrus, heat detection programmes and herd fertility indices.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
What is silent heat?
Ovulation without visible heat signs, common in buffalo, high-yielding dairy cows and animals under heat stress or poor nutrition. Activity monitors, teaser bulls and hormone tests help detect it.
Why do cows stop cycling after calving?
The reproductive tract must recover, and in early lactation many cows are in negative energy balance, which delays the return of ovulation. Good nutrition and body condition shorten this period.
Why are cats called induced ovulators?
They do not ovulate spontaneously at a fixed point in the cycle; mating stimulates an LH surge that triggers ovulation. Rabbits and camels behave similarly.
Sources
- Senger, P. L. Pathways to Pregnancy and Parturition, 3rd edition. Current Conceptions, 2012.
- Hafez, E. S. E. and Hafez, B. (eds) Reproduction in Farm Animals, 7th edition. Lippincott Williams and Wilkins, 2000.
- Noakes, D. E., Parkinson, T. J. and England, G. C. W. (eds) Veterinary Reproduction and Obstetrics, 10th edition. Elsevier, 2019.
- Pursley, J. R., Mee, M. O. and Wiltbank, M. C. "Synchronization of ovulation in dairy cows using PGF2α and GnRH." Theriogenology 44, 915–923 (1995).
- Roelofs, J. et al. "When is a cow in estrus? Clinical and practical aspects." Theriogenology 74, 327–344 (2010).
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