Twenty years after Rodger Johnson began a selection experiment focusing on the components of litter size, the resultant Nebraska Index Line is the talk of the pork industry.
Housed at the University of Nebraska research farm, the genetic line achieved its notoriety when the results of the National Pork Producers Council (NPPC) National Genetic Evaluation Maternal Line Program (MLP) were reported last spring (see "Maternal Line Genetics" National Hog Farmer Blueprint, April 15, 2000).
The MLP test was designed to measure maternal line performance from gilt development through four parities. The line that came out shining was from a mating of Nebraska Index Line (NIL) boars to Dekalb Monsanto Genepacker gilts. Reported as Dekalb-Monsanto MXP200 line, the 592 F1 gilts averaged more than one more pig/litter than their contemporaries from five other lines. The MXP200 line has since been renamed the Dekalb Choice Genetics GEPK327.
Birth of a Line Johnson's two decades of selection efforts that led to the unique NIL actually surfaced during a 1980 road trip with two colleagues, Kreg Leymaster and Larry Young from the USDA Agricultural Research Service in Clay Center, NE. The trio of swine geneticists began speculating why reproductive trait selection experiments were more successful in mice than in pigs. They agreed - pig researchers had not defined the best selection methods to get the job done.
"Litter size is really an index of ovulation rates, embryo survival and uterine capacity," notes Johnson. "We felt that if we could get the components of those traits, we could probably design a better index than the natural litter size index (number farrowed)."
Six months later, Johnson had an experimental plan. But, hampered by limited facilities at the research farm, he needed to gain some selection efficiency.
"I realized that if I could measure litter size on pregnant females before they farrowed, I could breed more gilts in each generation, but only select the number that I had farrowing spaces for," he relates. His plan was to breed 150 gilts, identify those carrying the largest litters, then farrow only the top one-third.
On paper, the pre-screening method should produce a 30% greater response than direct selection for litter size farrowed naturally, he explains.
The pre-screening procedure, called laparotomy, requires a 4-in. incision made midway between the aitch bone and the navel of a bred gilt at 50 days of gestation. The entire reproductive tract is brought outside the body to palpate the uterus for the number of fetuses and count the corpora lutea on the ovaries for ovulation sites. The reproductive tract is then carefully replaced in the body cavity. Naturally, gilts are anethesized.
The base genetic line Johnson started with traced back to PIC Large Whites and Landrace brought into the University herd between 1976 to 1978. The two lines were crossed in 1979 to make the base population and then randomly mated for a couple of years "to mix those genes up really well," he explains. Then the herd was permanently closed to outside genetics.
In 1981, the herd was randomly split into a "control" group, receiving absolutely no selection and the "select" or index line that would undergo the pre-screening and intense selection. A combination of ovulation rate and embryo survival became the selection index criteria for the NIL line. Conservatively, the project would take 10 years - longer if it was successful.
Five-Year Review The first analysis was done after five generations of selection. The laparotomy inspections revealed a significant increase in ovulation rates (compared to the controls) and litter size increased pretty close to his predictions. The results were very encouraging, so Johnson committed to another five years of research.
10-Year Analysis "At the end of 10 generations of that selection (protocol), it was very clear that we had made a significant change in reproduction, and we had a line that was quite unique," Johnson says. "The mean ovulation rate was up around 21 (eggs) compared to 13.5 (eggs) in the control line - something like a 55% increase.
"The litter size (in the select line) was bumping up around 12.5-13 pigs; the control line was 9.5-10 pigs," he adds.
Satisfied they were on the right track, Johnson abandoned the laparotomy procedure after the 11th generation.
"A female carrying 20 pigs in her uterus is quite different than one carrying 10 pigs," he explains. "Twenty pigs gets to be a very large mass of material. I was causing a fair number of abortions in those (large) litters - and those are the ones you want."
Therefore, beginning with the 12th generation, selection emphasis was shifted to litter size, and it has remained the primary focus through the 17th generation gilts that farrowed last summer.
Johnson also has added number born alive and litter weights to the traits measured. Pigs from inbred parents and/or larger litters often have lower birth weights, he explains. Johnson estimates the inbreeding coefficient on the closed line is about 22%.
Now, as Johnson selects the next generation, he concentrates on litter size, then selects the largest pigs within those litters to represent the index line. He's hoping some selection emphasis on birth weights will improve survival rates, at least a little.
Progress to Date Johnson outlines the response seen in the select line from 1981 to 2000:
- Higher ovulation rates - about 7-7.5 more eggs/cycle than the controls.
- Larger litters - approximately 4-4.5 more pigs/litter. In the last two generations, the select line averaged around 13 live pigs/litter; controls averaged 9-9.5 pigs/litter.
- More stillborns - roughly one more/litter than controls. Johnson attributes that to greater inbreeding, larger litters and, possibly, lower birth weights (2.3-2.4 lb. average) in the select line.
- Earlier puberty - age at puberty averages about 10 days younger in the select line, consistently around 170-175 days of age.
Here's the "Catch" Because there has been no selection for performance traits in either line, growth and carcass characteristics are comparable to the 1980 model pig. "They grow slower and are fatter - significant disadvantages for the line," he acknowledges. "And, their food intake is lower, which was also reflected in the MLP test."
That said, however, Johnson believes the select line genetics have a place. Combining the select line with another modern, maternal line to produce F1 replacement females shows great promise.
"I would not recommend that anybody have more than 50% of this (select) line in the sow herd. And, that female needs to be crossed with a terminal sire that has very good growth and carcass characteristics," he advises. "Send the pigs to market. Don't keep the gilts," he cautions.
Johnson is currently conducting performance tests on terminal cross pigs (F1 gilts bred to terminal boars) to learn more about their market value. The pigs are running 52-53% lean, 0.85-0.90 in. average backfat, he reports.
He also believes the "pure" select line genetics are best left in the hands of trained geneticists, used in controlled nucleus herds. "It's a tender line; it's not as easy to manage as the highly heterotic lines that producers are used to working with."
Benefactors The buzz surrounding the release of the MLP data had many commercial breeding companies clamoring to obtain a genetic sample of a superior reproductive line. DeKalb Monsanto and Newsham Hybrids were the first. Others followed, including Cotswold (Heartland Pork), Premier Swine, Waldo Farms, NGT, a couple of independent producers, Rezende Alimentos of Brazil, and Hermitage Exports Ltd., an Irish company.
In June, Monsanto Co. announced the purchase of a 10-year agreement for exclusive rights to the Nebraska Index Line. The exact terms of the agreement were not disclosed; however, the royalties are tied to the number of animals and the amount of semen the company sells through Dekalb Choice Genetics.
Being a state-supported, public institution, the University insisted on one exception to the agreement - that Nebraskans still would have access to the line.
