Improving sow herd production levels is one of the most challenging aspects of pork production. Herd production levels depend on many related, interdependent factors that can frustrate even the best managers. Taken together, these factors often have a multiplier effect on total output. Small improvements in several measures can mean large economic returns.

Better Reproductive Selection

Selection for reproductive traits, such as prolificacy and fertility, is almost entirely in the genetic supplier's control. Commercial pork producers are not prepared to deal with the extra expenses incurred when intense selection for lowly heritable reproductive traits is practiced.

The genetic supplier's role will increase as genetic markers become more important in selection programs. The most important genetic decision for the commercial pork producer targeting improved reproduction is which maternal sow line to use. It is the genetic difference between sow lines that is important.

The genetic component of reproductive efficiency is only 10-20% of the variation within a genetic line. However, commercial producers can control some environmental factors, such as nutrition, herd health and housing, which account for 80-90% of the reproductive variation.

Most commercial producers require replacement females to meet some minimum individual standards for herd entry, whether they are purchased or produced internally. Since replacement gilts and gilt acclimation programs are expensive, only leg soundness and fertility standards are generally used.

Environmental Interactions

A source of producer frustration can be a genetic lines' sensitivity to environmental interactions. These may become more important when the within-line genetic component is small. This sensitivity can affect how a genetic line performs for different producers.

An extreme example would be ranking wild pigs vs. the improved maternal lines in commercial production. In the wild, it is best to have a few large pigs per litter rather than a large litter of smaller pigs. And, it is best to conceive only when the litter would be born in good weather. In the wild, these females are likely to raise more live offspring per year than their commercial counterparts. However, in a commercial setting, the improved maternal lines would surely outperform sows acclimated to the wild.

Genetic suppliers constantly face the challenge of determining the type of environment in which to raise and select their foundation lines. Their current practice is to provide a very high health, intensively managed production environment for foundation line animals.

Commercial producers may have difficulty achieving these same high herd health and management levels. Producers may also be tempted to decrease feed costs by using lower nutrient levels than recommended by their genetic suppliers.

Table 1. Gilt Loss During Growing/Development Period
Outcomes of gilts not taken to sow units at 165 days of age
Reason No. of gilts Percentage of gilts entered to wean-to-finish
Death 163 4.6
Umbilical hernia 49 1.4
Injury 19 0.5
Unthrifty 36 1.0
Other 9 0.025
276 7.75%


Table 2. Summary of Gilts Not Meeting Breeding Criteria
No. of gilts Percentage of gilts taken to sow units
Abnormal tracts 21 0.6
Quit cycling 52 1.6
Not cycling 242 7.3
Normal cycling 108 3.3
Pregnant 4 0.1
Unknown 6 0.1
Total gilts slaughtered, head 433 13.1


Table 3. Genetic Line Differences of Age at First Estrus and First Farrowing (Parity 1)
Gilt age at first estrus and first farrowing
Line No. of gilts % showing estrus Age at first estrus (days) % of gilts farrowing Age at farrowing (days)
American Diamond Swine Genetics 562 91 225 77 371
Danbred USA 541 87 222 77 366
Monsanto DK44 550 87 222 75 367
Monsanto GPK347 547 97 209 92 354
National Swine Registry 515 90 222 77 367
Newsham Genetics 568 88 223 78 368
3,283


Modern commercial maternal lines may have specific environmental sensitivities such as disease resistance, nutritional requirements and seasonal breeding cycles. These sensitivities are affected by the foundation genetics used to develop maternal lines and the type of internal selection program followed. All genetic lines trace back to a few superior animals. The unique differences between the foundation animals become properties of the resulting genetic line.

Therefore, successful sow management requires knowledge of how a specific sow genetic line interacts with the environment and procedures they will face in the commercial operation.

Gilt development programs, breeding herd efficiency and sow lactation management are all factors of herd production that can be adjusted to improve output of a specific genetic sow line.

Maternal Sow Line Program

The National Pork Producers Council conducted the National Genetic Evaluation Maternal Line Program (MLP) from 1997 through 2000, which provides examples of some interactions as well as parity effects that result from between-line genetic differences.

The goal of the checkoff-funded MLP was to evaluate six genetic sow lines for reproduction, growth, carcass and meat quality traits through four parities. Gilt grower/development records and breeding performance records were collected through six parities.

Genetic lines studied were American Diamond Swine Genetics, Danbred USA, Monsanto Choice Genetics (two lines: DK 44 and GPK 347), National Swine Registry Yorkshire x Landrace F1 crossbreds and Newsham Genetics. Each line was represented by about 600 females.

The 3,600 gilts were delivered at 10-20 days of age and grown in wean-to-finish barns before being placed in two new, 1,600-sow, breeding-farrowing-lactation facilities at about 165 days of age (150 days in wean-to-finish).

The attending veterinarian, wean-to-finish unit manager and NPPC program manager jointly evaluated gilts for health and abnormalities at approximately 165 days of age. Culling of gilts prior to delivery to the breeding site was limited to the following criteria:

  • Chronic illness or injury as determined by the attending veterinarian.

  • Extreme light body weight for age that indicated a chronic problem (three standard deviations from genetic line mean).



About 3% of the gilts were culled for umbilical hernias, chronic illness and severe injuries. Gilts were not culled for growth or backfat thickness. The number and percentage of gilts culled for various reasons are shown in Table 1.

To enter the test, gilts had to conceive on their second or later observed estrus, after they were 205 days old and before 300 days of age. The 3,283 gilts entering the two sow facilities were exposed to vasectomized boars daily from 165 to 300 days of age. All matings were done by artificial insemination (AI) using fresh semen from one unrelated sire line. Gilts not meeting the breeding standard were slaughtered.

Reproductive physiology specialist Don Levis of the Ohio State University evaluated the reproductive tracts of culled gilts. Most of the gilts were not having estrous cycles at slaughter age, which was at least 330 days (Table 2).

Table 3 shows the genetic line differences in age at first estrus and age at first farrowing. The GPK 347 line was youngest at both first observed estrus and first farrowing. Likewise, in addition, a higher proportion of GPK 347 gilts were mated and conceived. The combination of these fertility traits reduced open days in the sow herd.

An important design requirement of the MLP was that no gilt or sow could be culled for poor reproductive performance (small litters, poor milking, etc). The only way that a female could leave the program was death, injury or failure to conceive within 50 days of weaning. Fertility problems were the most important cause of involuntary culling.

The amount of daily gestation feed given each sow was determined by her postweaning weight and desired weight gain before next farrowing. Litters were crossfostered within 48 hours of birth, standardizing the number of pigs nursed at about 10.

All sows were fed three times daily during lactation in an effort to maximize feed intake. Daily lactation feed disappearance for each sow was recorded. Average weaning age was 15.4 days.

Results of the MLP show that the Monsanto GPK 347 females ate less feed in lactation and lost more weight and backfat thickness during lactation. General observation would suggest this line would perform at a lower reproductive level. However, as the results in Tables 4 and 7 show, the reverse was true. The GPK 347 females actually produced more pigs born per litter and per sow lifetime. In the confinement environment of the MLP, the GPK 347 had very few fertility problems at Parity 1 and 2 (P1 and P2). That single factor gave the line a tremendous advantage in longevity. Differences between other sow lines were not as large.

Parity Management

The MLP results in Table 5 show the challenges and opportunities for managing genetic lines and parities differently.

Parity 1 females had as many pigs born as Parity 3 (P3) and Parity 4 (P4) sows, were smaller at farrowing, lost more weight and backfat during their first lactation and ate less feed.

However, not all genetic lines had a significant drop in P2 performance. A few lines had differences between Parities 3 and 4, American Diamond increasing and Danbred decreasing in number of pigs born per litter.

Extending gilt development programs through mating for P2 litters may offer significant benefits. Clearly, if young females are to grow and reproduce successfully, they must meet certain nutritional and health goals. Even though the MLP females had increased gestation feed allotted for growth, these sows did not return to the same body composition at P2. The sows were larger but had less backfat depth. There may be an advantage to feeding a P1 gestation diet to bred gilts that increases longevity.

Table 4. Total Number of Pigs Born per Litter, Genetic Line by Parity Interaction Means
Genetic Line Pigs born Parity 1 Pigs born Parity 2 Pigs born Parity 3 Pigs born Parity 4
American Diamond Swine Genetics 10.09 9.90 10.16 10.72*
Danbred USA 11.10 10.74 11.52* 10.77*
Monsanto DK44 11.59 11.44 11.56 11.66
Monsanto GPK347 12.19 11.86 12.03 11.82
National Swine Registry 11.08* 10.23* 10.79* 10.37
Newsham Genetics 10.61 10.37 10.50 10.61
*Indicates statistical difference (P<.05) between parities for total pigs born.


Table 5. Differences in Performance Between Parities
Parity Total pigs born Sow weight at farrowing, lb. Sow lactation weight loss, lb. Sow backfat at farrowing, in. Sow lactation backfat loss, in. Litter birth weight, lb. Litter wean weight, lb.
1 11.08 453 53.2 0.82 0.07 34.9 100.8
2 10.79 493 47.0 0.77 0.05 37.8 112.3
3 11.14 516 45.8 0.74 0.04 37.8 111.1
4 11.08 533 46.1 0.71 0.01 37.3 107.2
Means with a common superscript are not different (P<.05).


Table 6. Parity-to-Parity Loss of Females by Line
Genetic Line Parity 1 %* Parity 2 % Parity 3 % Parity 4 % Parity 5 % Parity 6 %
American Diamond Swine Genetics 23 13 7 7 10 10
Danbred USA 23 14 7 8 9 13
Monsanto DK44 25 10 8 7 11 7
Monsanto GPK347 8 9 6 7 7 12
National Swine Registry 24 13 6 5 9 11
Newsham Genetics 22 13 6 7 9 13
*Percentage of entered gilts that never farrowed a litter


Management skill is a key factor in recognizing and maintaining gilt fertility. Some managers are better at recognizing gilts in heat. Having these managers work only with gilts may greatly increase the farrowing rate of gilt matings.

Litter birth weights and weaning weights were smaller for P1 females. Pigs weaned from P1 litters are smaller and may have less maternal immunity to disease.

Another management opportunity may be to segregate P1 pigs through nursery and finishing periods. Extending gilt development programs to P1 weaning or P2 mating makes P1 pig segregation easier.

Sow Longevity Barriers

Results of Table 6 show that the greatest barriers to sow longevity are gilt and post-first-parity sow losses due to infertility. The major component of sow loss prior to 450 days of age is reproductive failure.

Gilts that are never serviced or fail to conceive reinforce the difference between the GPK347 line and others prior to 300 days of age.

Many producers fail to account for the cost of days on feed and facility space occupied by these gilts that do not have breeding records. The National Pork Board's Production and Financial Standards require entering all gilts into the sow herd when they are delivered or selected. Litters and/or pigs per sow year ratios will be lower due to increased open gilt development days when these standards are implemented. A parity-segregated management system will include all gilt development days in its reports.

Table 7. Production of 130 Cohorts Through Six Parities
Genetic line Total sow days Avg. sow life, days Total pigs born Total pigs born live Live pigs/sow day Total litters born
American Diamond Swine Genetics 14,138 566 835 758 .054 79.7
Danbred USA 13,632 545 860 767 .056 76.9
Monsanto DK44 14,009 560 944 843 .060 80.5
Monsanto DK347 17,197 688 1,312 1,172 .068 109.0
National Swine Registry 14,033 561 871 790 .056 80.6
Newsham Genetics 14,230 569 870 790 .055 81.4


Table 8. Six-Parity Output Among the 130 Maternal Line Program Cohorts
Trait Best cohort Worst cohort Goal
Total litters born 127 40 150
Live pigs born 1,347 397 1,575a
Avg. sow life, day 781 358 900b
Total sow days 19,542 8,957 22,500c
a10.5 live pigs born per litter
bSow life days start when gilt is entered to gilt developer, 170 days of age.
c900-day life times 25 sows


Keeping management records of groups of females (cohorts) entering the sow herd on the same dates will account for all bred and open sow days used. A cohort is “a group of animals that share a common event within a defined period of time.”

Cohort management allows animals to be grouped several ways. The MLP gilts were randomly assigned to 25 gilt cohorts by genetic line-sow unit-age group when delivered to sow units. The MLP cooperator herds allowed NPPC to follow the sows through P5 and P6 reproduction measures. All days in sow units are included, from gilt entry at 165 days of age, until weaning of P6 litters.

Table 7 shows the large differences in herd output for the GPK 347 line, which was superior in age to first farrowing, proportion of gilts conceiving, proportion of P1 females conceiving and number of pigs born per litter. Together these traits give a great advantage in total pigs born and sow life.

Sow Herd Potential

There is tremendous biological potential for increased sow herd output. This may not be good news for hog market prices, but pork producers have no choice but to continue improvements to remain competitive.

Table 8 shows the large range in cohort production in a carefully recorded, side-by-side comparison of genetic lines. Improvements of 20-30% more pigs from a sow herd are suggested by these data. Individual producer management and cost structures are needed to find the most profitable herd production level.

Choice of a maternal sow line is the major genetic decision for commercial producers. The next decision is how to maximize that sow line's potential in the production environment at all sow ages.

Extension of gilt development through first parity may give “multiplier” benefits to the mature sow herd if gilt fertility and sow longevity are improved.

Segregation of P1 litter pigs in nursery and finishing may also increase net returns.

Rodney Goodwin was director of the Maternal Line Program for the National Pork Producers Council.