When to breed and when not to breed

Roy Kirkwood, DVM, PhD

Royal Veterinary College, United Kingdom

If you have a herd with a high return rate, possibly with an unacceptable number of females with vulval discharges, check the breeding records. If >15% of females are being mated only once per service, the efficiency of estrus detection is suspect. If <5% of females are bred only once, or >25% of females are bred on day 3 of estrus, then breeding management is probably too aggressive, and these females are at a higher risk for urogenital disease.

Insemination (artificial or natural) during late estrus/early metestrus is a definite risk factor for initiating urogenital infection.1 It is not possible to breed a female without introducing some bacteria. However, during "good" standing heat, the urogenital tract of the female is still influenced by the high estrogen concentrations of early estrus. By virtue of its effects on uterine immune status (increased blood flow and permeability to leukocytes), the estrogen allows the uterus to more easily clear introduced bacteria, unless the degree of contamination is sufficient to overwhelm uterine defenses. By late estrus, the estrogenic effect has markedly decreased, the female has ovulated, progesterone concentrations are rising, and the uterus becomes more susceptible to infection.1 Late breeding may initiate infectious endometritis, expressed clinically as low farrowing rate and small litter size.2 The low farrowing rate may also be associated with a high incidence of vulval discharge when the cervix reopens at the return to estrus. If you see a purulent (usually copious) vulval discharge at the return to estrus, expect a farrowing rate in these females of <30% to subsequent rebreeding. My personal opinion is that treating these sows with antibiotics or prostaglandin is a waste of time, and they should just be culled.

So how can breeding records indicate that breeding management needs attention? Figure 1 shows the distribution of estrus onset-to-ovulation intervals in sows subjected to estrus detection twice daily or at 8-hour intervals.3,4 In this data set, 7%-19% of sows ovulated within 24 hours after estrus was detected. Unless still showing a good standing estrus, females that ovulate within 24 hours of estrus detection should not be bred on day 2 of estrus. Although the exact figure will depend on your assessment of the estrus detection management and expertise in each herd, it is reasonable to suggest that about 10% of females will be early ovulators. Single mating of these animals is not a problem, as it almost guarantees they will be bred during the optimal period for high fertility, which is the 24-hour window before ovulation.5 If >15% of females receive a single breeding, detection of estrus onset is probably inadequate. If < 5% of females receive a single breeding, some early ovulators are being bred in late estrus (or possibly diestrus).

Using the same datasets, 22%-24% of females ovulate more than 48 hours after the onset of estrus is detected. Breeding these females on day 3 should not be a problem, except that the cost of the first breeding, in terms of time and money, is probably wasted. If the breeding records show <20% of the females are bred on the third day of estrus, it is possible that some females are being bred too early, while >25% third-day breedings indicates that some females are being bred too late.

Your challenge is to develop a breeding management protocol for each herd based on your assessment of when ovulation is most likely to occur. Time of ovulation will be influenced by the apparent duration of the estrous period, which is affected not only by wean-to-estrus interval and possibly parity, but also by staff expertise at estrus detection. Good luck!


1. de Winter PJJ, Verdonck M, de Kruif A, Coryn M, de Luyker HA, Devries LA, Haesebrouck F. The relationship between the blood progesterone concentration at early metestrus and uterine infection in the sow. Anim Reprod Sci. 1996;41:51-59.

2. Rozeboom KJ, Troedsson MHT, Shurson GC, Hawton JD, Crabo BG. Late estrus or metestrus insemination after estrual inseminations decreases farrowing rate and litter size in swine. J Anim Sci. 1997; 2323-2327.

4. Knox RV, Lamberson WR, Robb J. Factors influencing time of ovulation in post-weaned sows determined by transrectal ultrasound. Therio. 1999;51:435.

5. Soede NM, Wetzels CCH, Zondag W, de Koning MAI, Kemp B. Effects of time of insemination relative to ovulation as determined by ultrasonography, on fertilization rate and accessory sperm count in sows. J Reprod Fertil. 1995;104:99-106.


3. Weitze KF, Wagner-Rietschel H, Richter L. Standing heat and ovulation in a sow herd. Proc IPVS. 1992; 2:460.