For anyone interested in learning more about the genetic components of litter size and how they affect reproductive efficiency in swine, the road to that knowledge will likely lead through the East Campus of the University of Nebraska. It is there that swine geneticist Rodger Johnson has spent over 30 years teaching animal breeding and researching the genetic and physiological components of sow productivity and, more recently, the genetics of disease resistance.

Johnson grew up on a 1,250 acre, predominantly grain and cattle farm in northwest North Dakota where “pigs were never a big deal,” he remembers. With two brothers and four sisters, what was a big deal was making sure he and his siblings pursued a higher education.

When he graduated from high school in 1961, a drought-stricken wheat crop tapped the Johnson family dearly, but a local banker's advice echoed in his father's ears. “You've got to find a way to educate those kids; it's the single most important thing you can do,” he urged.

“Dad told me — get a job, go to college, find a way to get through. If I have any money to help you, I will, but I can't promise anything,” Johnson recalls.

Parental lessons in hard work and commitment took hold and inspired Johnson to pursue a bachelor-of-science degree in animal science at North Dakota State University (NDSU). Within two months after receiving his degree in 1965, Uncle Sam drafted him for a two-year hitch in the Army.

Returning from Vietnam in 1967, Johnson took an assistant county agent position in Barnes County, North Dakota, where the focus was crops and cattle. One day his NDSU undergraduate advisor, Merle White, contacted him with news that a graduate level assistantship was available at Oklahoma State University (OSU), and he nudged Johnson to take a look.

“I think Merle recognized something in me that I probably didn't recognize in myself,” Johnson explains. The catch was, Joe Whiteman, chairman of the animal science graduate committee at OSU, insisted on a face-to-face meeting with all applicants.

Johnson made the trek to Stillwater, secured the assistantship and spent the next couple of years studying carcass composition in beef cattle. As part of the master's degree program, Johnson took an advanced animal breeding course, which introduced him to what would become a career in swine genetics, and a person who would become a treasured mentor throughout his academic career — OSU swine geneticist Irv Omtvedt.

Johnson completed the master's degree program in 1971, and stayed on to get a Ph.D. Despite a slight preference for cattle, Johnson was drawn to Dr. Omtvedt's reputation as an excellent teacher, advisor and mentor. “I chose the person to work with, rather than the species to work with,” he explains.

From that point forward, the swine genetics die that has carried Johnson through nearly 40 years of teaching animal breeding and swine-based research was cast.

Crossbreeding Trials

At OSU, Omtvedt had set up a swine crossbreeding research program designed to help commercial producers understand the merits of crossbreeding. “There were a lot of questions about the strengths and weaknesses of the pure breeds and how they combined together,” Johnson explains.

Duroc, Hampshire and Yorkshire pure lines were evaluated in all possible combinations, using them as sire lines and replacement gilt/maternal lines, to gain estimates of heterosis (hybrid vigor), and to identify which combinations performed best. Results were the focus of Johnson's Ph.D. dissertation.

Soon after, Omtvedt left to take an associate dean position at Auburn University, leaving a vacancy that he recommended Johnson fill.

Johnson accepted and spent the next five years teaching animal breeding classes and advancing the swine crossbreeding project. He added purebred Spots to the mix, trying various crosses, back-crosses, and tracking of crossbred sire data on about 400 litters/year.

Johnson's five years of crossbreeding research had not gone unnoticed by his mentor, who assumed responsibilities as the animal science department head at the University of Nebraska in 1978. Omtvedt recruited his former student to fill the swine breeding position vacated by P.J. Cunningham. Johnson joined the university with a 50:50 research and teaching appointment.

Maternal Line Focus

Anxious to do something different on the research side, Johnson teamed up with well-known swine reproduction specialist Dwane Zimmerman. “There really wasn't a lot of information about the genetics of reproduction, and Dwane was a good person to team up with to do a physiology/genetics research program,” he says.

“I've always thought that if you're in research, you should try to do something that is forward-looking,” Johnson explains. By design and necessity, he has since tackled two of the pork industry's greatest challenges — improving the reproductive efficiency of the sow herd and attempting to understand the heritability of disease resistance in pigs.

Genetic research is a slow process requiring patience and perseverance, both attributes Johnson has mastered over the years. “It takes a while in a selection experiment to understand the nature of the response. If you just look at one generation at a time, you get lots of variation. So you have to stay with an objective for a while before you can evaluate it.”

“Litter size components of reproduction weren't all that well understood and the heritability estimates for litter size were pretty low, indicating that there may not be much opportunity to select for (improved) reproduction in the pig,” he says. “But I wasn't convinced that you couldn't make progress.”

Litter size is a reflection of three components — ovulation rate, embryonic survival and uterine capacity. Master those pieces of the puzzle and you can probably design a selection index to improve litter size.

In his early work, one experiment focused on direct selection for litter size, another on decreasing age at puberty and a third on selecting for ovulation rate.

“Direct selection for ovulation rate didn't change litter size greatly, so I was trying to figure out why. We had females that ovulated more eggs, but litter size was not improving, so it had to have something to do with uterine capacity,” he speculated.

From 1979 to 1981, Johnson focused on establishing a genetic line that could be utilized through a multi-generational selection experiment. In 1981, the herd was permanently closed to outside genetics and the population was randomly split into two groups — a “control” group (no selection for litter size) and a “select” or index line that would undergo prescreening and intensive selection for litter size. A combination of ovulation rate and embryo survival served as the selection index criteria for what was to become the Nebraska Index Line (NIL).

Johnson reasoned that if he could measure litter size during gestation, he could simultaneously select for ovulation rate and uterine capacity. He turned to laparotomy, a procedure that requires a small incision near the navel of a bred gilt at 50 days of gestation. The reproductive tract is brought outside of the body cavity so researchers can palpate the uterus and count the number of corpa lutea on the ovaries for ovulation sites. The reproductive tract is carefully replaced and females are allowed to farrow naturally.

After five generations of selection, it was clear that ovulation rate was heritable because ovulation rates and number of fetuses at 50 days of gestation were increasing. But litter size lagged. “Something was happening after 50 days,” Johnson observes.

“Back then, physiologists thought most of the embryonic and fetal death loss occurred between 30 and 50 days of gestation. That may be the case in low-ovulating populations, but it clearly wasn't the case in our select line. When ovulation rate exceeds uterine capacity by a fair bit, then you start to get losses in late gestation,” Johnson explains.

Complicating the theory of uterine crowding is the difficulty in measuring uterine size. “Measuring a uterus in a non-pregnant female is not a very good predictor of what happens during pregnancy. And, if you measure the uterus during pregnancy, it's largely a function of how many fetuses are present,” he says.

The selection index was modified to place more emphasis on embryo survival, and by the 10th generation, ovulation rate had climbed from 13.5 in the control line (no selection) to 20 in the select line.

Litter size in the select line averaged about three more pigs per litter (12.5-13.0 vs. 9.5-10). Johnson felt he was on the right track, so the laparotomy procedure was abandoned in the 11th generation. In the 12th generation, he shifted more selection emphasis to number of pigs born alive. “Litter size just took off. And when it did, number of stillborns and mummified pigs declined,” he relates.

Johnson points out that the heritability of litter size in the select line was twice the popular estimate of 10%. “I think it is because ovulation rate is not a limiting variable. Females in the select line all had high ovulation rates, so what we were measuring was the heritability of uterine capacity.”

Selection for increased litter size continues, with the 30th generation born this spring. He shifted for a time to a two-stage selection program — selecting for ovulation rate, and then for large litter size among those with high ovulation rates. “The response went up sharply, especially in a line derived from the control line that had lower ovulation rate. Selection in recent generations has been for live pigs per litter, and numbers continue to go up,” Johnson reports.

The knock on the Nebraska Index Line, reported in the National Pork Producers Council (NPPC) Maternal Line Program (see: in 2000, was that the pigs were born too small, grew too slow, and had too much backfat. Johnson acknowledges those concerns, but reminds that the line was closed in 1981 and no selection pressure was placed on post-weaning performance.

In the last 10 years, the single reproductive trait in the selection index is live-born pigs/litter, plus he has added selection for increased growth rate and lower backfat levels. “Of course, they're not close to the industry's pigs yet, but the select lines now average about 25 lb. heavier at 180 days with about 0.25 in. less backfat than the control line. And those lines have been a really valuable resource for molecular genetics work that tries to find genes that control reproduction,” he adds.

Sow Longevity Study

A natural extension of Johnson's genetics of reproduction work is a gilt development and sow longevity study. “We are looking at different gilt development options, controlling how fast they grow; how much backfat they have, using nutritional manipulation; then tracking their reproductive performance through four parities,” he explains. (See http://nationalhogfarmer. com/genetics-reproduction/sow-gilt/farming_effects_energy_intake/). The study is nearly two-thirds complete.

Although he hasn't pinpointed the most common reasons sows fall out of the breeding herd, he has learned some things that are not related. For example, some argue that gilts must weigh at least 300 lb. and have a minimum backfat level. In his study, restricted feed intake held gilts to 220-230 lb. at breeding, but some have gone on to produce 60 pigs in four parities, he explains. Conversely, some gilts mated at 340 lb. never had a litter. “I think there is a positive association between those things, but perhaps not nearly as strong as some people believe,” he adds.

“Early puberty is probably the variable that is most highly correlated with lifetime productivity,” Johnson declares. He believes producers, and some previous researchers, begin boar exposure too late — 170-180 days of age. “They aren't measuring puberty; they are measuring response to when they start boar exposure.”

In the Nebraska trials, gilts are first exposed to boars at 140 days of age. Both boars and gilts are moved to a central area separate from where they were raised. Boars are exposed to groups of 10-15 gilts for about 15 minutes, every day. “When they hit puberty, those gilts lock right up,” Johnson says. “I'd like to see our nucleus breeders record age at puberty, but it will be hard to get them to do that because it's a costly and labor-intensive trait to measure.”

Disease Resistance

Reinforcing his commitment to forward-looking research, about 10 years ago Johnson began looking at the genetic implications of the two biggest profit robbers in the pork industry — porcine reproductive and respiratory syndrome (PRRS) and porcine circovirus.

“There seems to be genetic variation for just about everything, so I thought there probably is genetic variation for response to PRRS virus,” he explains.

A trial was set up to challenge PRRS-negative pigs to a specific strain of the virus. “Pigs really responded differently. Every pig became viremic (replicating the virus), but within four days, some pigs had just a little spike in their temperature, a little blip in their growth, then cleared the virus and started to gain again. Others got sick and stayed sick for 2-3 weeks. Some never recovered,” he reports. “The nature of that response differed between two distinct, genetically different populations. That's evidence that there is underlying genetic variation.”

“Although I'm sure you could be effective in selecting for a PRRS-resistant population, you wouldn't have many pigs to sell,” he points out.

That's where genomics comes in. A national project is underway to identify the genetic markers associated with resistance to PRRS. The goal would be to map the gene frequencies of the pigs that got really sick and those that didn't. “It's a really good application of genomic technology, but funding is hard to come by,” he says.

Circovirus is Tougher

Circovirus is doubly difficult to work with in a genetic research environment because it exists everywhere and is hard to grow in a laboratory setting.

The percentage of non-vaccinated pigs that show phenotypic signs of circovirus in the Nebraska herd is about 15%, but it's just 4-5% in some herds.

“So we're vaccinating everything to protect a fairly small percentage of susceptible pigs. The (circovirus) vaccines are good, and they work well, but at $1.50/pig, it's costing over $150 million to protect a pretty small number of pigs. Strictly from an economic standpoint, if we had resistance to circovirus, it would save pork producers a lot of money,” he says.

Handling ‘the Pig Part’

Funding and complexity of disease resistance work remains an obstacle. Infecting pigs with a virus destroys their salvage value; the phenotypic data and viremia profile is important and expensive to collect; necropsy is costly, but essential; and there's the added cost of genomics. “But the motivation for this research is that we'd like to produce pigs with fewer pharmaceuticals,” he notes.

In the waning years of his career, Johnson feels his role is to design the experiments, and collect the phenotypic data and tissues — handling the pig part — and leave good data for the molecular geneticists to explore.

A Gratifying Change

One of the most satisfying changes Johnson has seen in his swine genetics career is the broad adoption of artificial insemination.

“In the '70s, we were trying to convince pork producers that crossbreeding had value, heterosis is important, and specific crosses had merit. Today, no one even questions that. Thirty-five years ago, no one would think about using a boar without ever seeing him. Now, they just call up their supplier, order the semen they need, inseminate the sows and know it will work; they're happy,” he reflects.

Industry Challenges

Johnson cites environmental and welfare-type regulations as the biggest challenges pork producers face going forward. Initially, he felt those driving these initiatives were simply meddling.

“It sort of made me mad, but when I stopped and thought about it as a consumer, I, too, want to know how products are made and want to feel good about what I am buying. Why would we think that the consumers of pork should be any different?” he asks.

The challenge is that the general public no longer can drive past a farm and see the pigs. “That creates a bit of a black-box effect, so they are concerned about how their food is being raised,” he observes. “People who buy pork want to know that it is a safe product and it has been raised humanely.”

Another ongoing challenge Johnson sees is producers' ability to handle new technology. “I'm sure they will be faced with sexed boar semen, as well as the possibility of delivering pharmaceuticals in their feeds, such as through modified corn hybrids that stimulate the immune system.

“These are some of the issues I think pork producers will face, and they will have to evaluate the cost-benefit ratio of that technology, which means they will have to be well educated,” Johnson notes.


Johnson is grateful for the work ethic instilled in him by his parents. He lists Joe Whiteman, Oklahoma State sheep geneticist, and Irv Omtvedt as the two most influential teachers and mentors.

He's most proud of the Irvin & Wanda Omtvedt Professor of Animal Science appointment he received in 2003, a position he currently holds. Also high on his list of awards is an OSU Outstanding Alumni award in 1999, the Darrell W. Nelson Excellence in Graduate Student Advising award in 2005, and being named an American Society of Animal Sciences fellow in research in 2008.

The best advice he can give students interested in the swine industry is to do an internship in a modern pork production system. “Learn as much as you can about the industry; understand the biology of pig production, connect with producers, get involved,” he says.

Johnson is a big fan of internship programs. “Many students have a vision of pig production that is outdated — smelly, dirty, high labor. The modern, updated units are clean and well-run, and the animals are well managed. That's why the internship experience is so important. They get a different feeling from the quality of that work experience.”

Johnson has traveled to over 20 countries, many states, and has consulted for numerous breeding stock firms. Looking back over his career, he says: “I don't ever regret having gone to pigs. I still enjoy cattle, but from a genetic standpoint, there are more traits that are important in pigs, they are a litter-bearing animal, and their generation interval is shorter. In cattle, it takes 15 years to do what we do in 3-4 years in pigs. I feel good about my career in pig genetics,” he adds.