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Reproductive Biology 101

Sifiting through the various causes of reproductive failure generally requires a better understanding of the biology of reproduction.

Sifiting through the various causes of reproductive failure generally requires a better understanding of the biology of reproduction.

Once the basics of reproductive biology are mastered, producers and managers can apply “critical analysis” principles to get to the root of low farrowing rates and small litters.

North Carolina State University swine reproductive specialist Billy Flowers is often called in to help solve reproductive shortfalls. Commonly, he starts by identifying where in the reproductive process things can go wrong.

On the boar side, Flowers focuses on spermatogenesis. That includes what happens in the testicle and sperm storage in the epididymis.

On the female side, he explores the fertilization process, as well as embryonic and fetal survival and development.

Boar Biology

Flowers turns to a schematic diagram of spermatogenesis for a review of the three basic steps of a boar's reproductive physiology (Figure 1).

Spermatogenesis works in a continuum; every 2-3 days a new group of sperm cells leaves the “resting pool” (testicles), moves on to the “developing pool”(testicles and epididymis), where the multiplication and maturation process advances over a 5- to 7-week period. In this ongoing process, matured sperm cells advance to the “storage pool” (epididymis proper) every 2-3 days. Semen for natural mating or collection for artificial insemination comes from there.

“Male reproductive physiology is a replenishing process,” explains Flowers. “Every time you have one cell leave the resting pool, one cell stays behind, and the others (advancing cells) actually start to produce more sperm cells. It replenishes itself so, really, you don't have to worry about the boars running out of the potential for producing sperm cells.”

Occasionally, a stress disrupts this process, which Flowers refers to as an “insult.”

“An insult may be something that you don't intend to do but it actually affects the boar's performance,” he explains.

For example, a boar is exposed to a very transient insult that occurs for just a few hours or a day. If severe enough, it will alter sperm production. But, because of the time it takes for sperm to mature, the impact of the insult will not be seen for a couple of weeks. Then, sperm quality gradually declines.

In the continuum, sperm quality gradually stabilizes, and then returns to normal, assuming the insult has been rectified. Normally, these insults have their greatest impact on developing sperm rather than the sperm in the resting or storage pools.

“In many cases, we have to rely on retrospective analysis to determine what happened,” Flowers says.

Prospective indicators, often seen as degrees of damage to the tails of the sperm cells, are shown in Figure 2. Curved or U-shaped tails often reflect a moderate insult has occurred; cooled or frozen semen often exhibit this tail position. Severe insults result in more sharply defined changes in the tail morphology, such as the tail folding back on itself.

Chronic insults, those less severe stressors that may occur over a long period of time, should also be checked. A good example is the gradual decline in the production of normal sperm cells as temperatures increase and stabilize. “With chronic insults, the boar copes with them for a certain period of time, then basically can't cope anymore,” he explains.

Female Biology

Equally important to the reproductive troubleshooting process is gaining a better understanding of female reproductive physiology, including the basics of maintaining pregnancy and the post-farrowing recovery period.

Flowers uses the most important reproductive event — breeding — as a reference point, then works backwards and forwards from there. A schematic diagram outlines specific reproductive events for his review (Figure 3).

Assuming an initial successful conception and farrowing, Flowers focuses on what must happen before the sow can be successfully bred again.

“Some things have to happen before she can resume a normal reproductive functioning,” he says. “Once the piglets are weaned, you remove the negative inhibition of lactation on the reproductive hormones.” What follows is a very rapid period of follicular growth, culminating in estrus, and then, hopefully, fertilization.

The next signal comes Day 12 to 14 after fertilization when estrogen signals the presence of embryos in the uterus. Another estrogen signal is sent between Days 17 and 28 days after breeding. The exact timing remains a subject of debate. A 12-week fetal development period follows.

Stumbling Blocks

“Reproductive performance is erratic and unpredictable in sows with short lactation lengths,” Flowers says. “Most sows probably cannot respond normally until Day 12 to 14, but there is significant variation amongst sows.

“There are two components of her reproductive system that need to recover — the brain (hypothalamus) and the uterus. The ovary is pretty much ready to go at farrowing, but the brain and the uterus actually lag behind.

“Here's what happens: the area called the hypothalamus — responsible for coordinating most all the things that the sow senses and then translates to an endocrine signal — loses its ability to respond normally to these (endocrine) signals until about Day 12 (post farrowing),” he explains.

Attempts to rebreed sows before the brain has fully recovered often result in the formation of cystic follicles. Follicles continue to grow because the sow can't elicit the normal luteinizing hormone (LH) surge needed for ovulation.

The uterus also needs about 14 days to recover after farrowing. “This is actually the physical replenishment of the uterine endometrium (lining), and the regeneration of some of the things that will actually support the next pregnancy,” he adds.

Beyond Fertilization

“In pigs, fertilization is a very efficient process,” Flowers says. “If everything is handled properly, we see fertilization rates at 90% or more.”

For a pregnancy to be maintained, signals from the embryos are essential. “If the sow does not receive the signal at Day 12 (post breeding), that means the embryos are not producing enough estrogen or there are no embryos there. Then, the sow will actually return to estrus in 20-21 days, as if she was never bred,” he says.

“Based on work done by Phil Dzuik (University of Illinois) back in the mid-'80s, we believe we need at least 4-6 embryos at Day 12, and again at Day 28, for the sows to receive the (estrogen) signals (needed to maintain pregnancy). If a sow doesn't get this second burst of estrogen, she will usually come back into heat somewhere between Day 28 and Day 40,” he adds.

Still, if everything is done perfectly, it is fairly common to lose between 25% and 40% of the embryos between fertilization and implantation. “This part of reproduction is a highly inefficient process,” Flowers says.


Given that brief reproductive biology lesson, Flowers sorts through scenarios producers can use to pinpoint reproductive problems. For example:

High embryonic mortality, low ovulation rates — check lactation length, lactation management (feed, etc.), plus anything that happens to the females the first two weeks after breeding.

Fertilization shortfalls — check breeding management, breeding techniques and semen quality; consider additive effects of insults to boars.

“When I visit with boar studs that are having problems, we make a list of things that possibly could influence sperm output, then we break them down into acute insults and chronic insults,” Flowers explains.

Acute insults include:

  • Transient temperatures above 85∞ F. (consider humidity);

  • Transient health problems (scours);

  • Vaccinations;

  • Changes in housing (moving boars around within the stud);

  • New collection techniques (change technicians);

  • Changes in collection frequency.

Chronic insults include:

  • Nutritional changes (type or formulation of diet);

  • Changes in types of housing (inability to adapt to new environment over a long period of time);

  • Long-term exposure to temperatures in upper end of thermoneutral zone.

Recognizing that not all of these insults are a problem in every operation, they are recorded during winter and summer months to build a frequency distribution. The frequency distribution, by stud, reinforces that multiple causes can have an impact on sperm output and quality.

All Boars Are Not Equal

Heterospermic insemination (blending of ejaculates from multiple boars) is a common attempt to overcome the possibility that a single boar's fertility might be low. Flowers has tested this philosophy in the laboratory through competitive insemination tests using equal numbers of sperm cells (1 billion each) from three boars to breed sows and genetically fingerprint the offspring.

Table 1. Results of Competitive Fertilization
Stud No of piglets Proportion sired by Boar 1 Proportion sired by Boar 2 Proportion sired by Boar 3
A 640 75%* 20% 5%
B 700 66%* 25% 9%
C 750 57%* 25% 18%
*different from Boar 2 and Boar 3 (p<.05)

“We never see a third, a third, a third,” he says. “These boars are of equal morphology, equal sperm production, equal motility, but we never, ever see a distribution of 33% across the boards. There's always one boar that sires most of the pigs in a given litter (see Table 1).

Critical Analysis of Reproductive Shortfalls

North Carolina swine reproductive specialist Billy Flowers uses the “biology of reproduction schematic” (Figure 3, page 13) when diagnosing reproductive problems in a sow herd. His “critical analysis” focuses on three measures — farrowing rate, return-to-estrus interval and litter size — information that is readily available in most recordkeeping programs.

Flowers reinforces that reproductive performance is situation dependent, influenced by genetics, environment, nutrition, etc. He suggests comparing performance among farms with similar genetics, management and production environment.

For his case study discussion (below and Table 1), he uses the reference points drawn from actual farm records he has worked with (Table 2).

Table 1. Critical Analysis of Reproductive Problems — Five Case Studies
Case Farrowing Rate Return Interval Litter Size
1 Bad < 22 days Bad
2 Bad > 28 days Bad
3 Good Bad
4 Bad < 22 days Good
5 Bad > 28 days Good

Case Study No. 1

Poor farrowing rate, poor litter size with normal return to estrus.

Checking the reproductive biology schematic, Flowers suspects the problems are occurring between Day 0 (rebreeding) and Day 12, because it appears the sows are not receiving the signals to continue pregnancy. “That's why you have a low farrowing rate, low litter size and return-to-estrus intervals less than 21 days,” he says.

From a reproductive biology standpoint, complete fertilization failure could be caused by breeding problems or poor semen quality, or short lactations and their effect on follicular development. High embryonic death losses before Day 12 and low ovulation rates should be considered.

Flowers assumes five embryos are needed to maintain pregnancy at Day 12, fertilization rates at 90% or above, and embryonic mortality rates at 40% — the upper end of normal (25-40% is normal).

Focusing on low ovulation rates, Flowers works backwards from an ovulation of 14 eggs (calculation: 5 embryos divided by .90, divided by .40 = 14 embryos). A herd averaging less than 14 ovulations/female likely will not be successful in maintaining a high percentage of pregnancies because a high percentage of hormone signals will be missed at 12 and 28 days, he says.

Case Study No. 2

Poor farrowing rate and litter size, return-to-estrus interval is greater than 28 days and irregular.

Flowers suspects short lactations could be affecting follicular development, thereby impacting farrowing rate and litter size.

It is unlikely that the irregular returns to estrus are being caused by fertilization problems or embryonic deaths in the first two weeks, because there are obviously enough embryos to send the first signal (Day 12). Therefore, focus on the causes of embryonic loss between Days 12 and 28.

Table 2. Reference Points from Farm Records in Southeast States
Rating Farrowing Rate % Litter Size
Bad < 80 < 10
Can Improve 81-85 10-11
Good >86 >11

Case Study No. 3

Good farrowing rates (therefore no concerns about return-to-estrus intervals), but average litter size is lower than expected.

Low ovulation rate could be a problem, but more likely something is happening during fetal development (between Days 28 and 114) to reduce litter size. Flowers reasons: “Obviously there are enough embryos to push sows past Days 12 and 28, the first and second signals to maintain pregnancy. Farrowing rate is good because the females are staying pregnant, but we're actually loosing live pigs.

“In this type of analysis, it's important to look at mummies and stillborns per litter. If it's a stillborn problem, it's happening toward the end of the fetal development period; with a mummy problem, it's probably happening after Day 60 or 70,” he says.

Case Study No. 4 and No. 5

Both herds have poor farrowing rate, good litter size, but No. 4 has returns to estrus under 22 days, while No. 5 has returns to estrus after 28 days.

Situations where one measure is good and one is bad can benefit from subset analyses. The more obvious subsets to examine are seasonal effects, and the percentage of first- and second-parity sows versus older sows.

“Because we have situations where litter size is really good, but farrowing rate is poor, we know that the sows that do farrow have normal litters. So, it's probably a subset (of females) that isn't farrowing.

“For most of the herds that have these scenarios, breeding regimens should be checked first. There could be a subset of sows which aren't receiving semen close enough to ovulation to achieve good fertilization. A simple adjustment in the timing of matings corrects the problem,” he notes.

In No. 4, consider that some animals have high embryonic mortality during the first two weeks — high enough that the pregnancy doesn't continue and they return to estrus within 21 days.

Case Study No. 5 is similar to No. 4 in that the animals that stay pregnant have normal size litters. “Something is happening to a subset of animals at around 28 days of fetal development. Consider the high embryonic mortality after Day 12 in the subset. This is typically what we would think of as animals being ‘pseudopregnant’ or ‘not in pig,’” he says.

Ultrasound can help diagnose this problem, however, you have to change your focus. “Normally you would have a normal pregnancy check with Real Time ultrasound (at) about Day 28, then you should come back around Day 60 or Day 70. This is important because it's after this time where most of the cartilage and bone in the piglet gets laid down, so there's a nice solid image,” he explains.

With a pseudopregnancy, the use of ultrasound at Day 70 will reveal large open areas — a sign that the uterus is full of fluid but no live embryos. When pigs are present, you should be able to see the outline of the skeleton and backbone at Day 70, he says.

“It's really important to train technicians, to look for solid objects instead of the dark circle. Or, you can use a higher frequency probe to get a better image. In ultrasound diagnosis, the low frequency probe gives you depth but you lose resolution. With high frequency probes (7.5 MHz), you get better resolution,” he explains.