Pork producers long have known the positive economic implications of a productive sow herd. Sow herd production goals focus on producing large numbers of high-quality pigs through multiple parities.

To date, pork producer attempts to improve reproductive performance have focused on management, nutrition and herd health programs that maximize the genetic potential of the sow herd.

The completion of the National Pork Producers Council (NPPC) National Genetic Evaluation Maternal Line Program (MLP) gives producers the opportunity to make valid genetic comparisons among available commercial lines. It also provides valuable information about how to manage these lines for maximum profit.

This large-scale genetic evaluation program involved six genetic lines through four parities. Table 1 shows the number of litters farrowed by each line, at the respective parities. Traditional reproductive traits associated with the sow and litter were measured and evaluated to quantify genetic differences among the lines and parities tested. The interaction between genetic line and parities also was analyzed.

Litter Traits Total number of pigs born, number born alive, total litter birth weight and live pig birth weight per litter, by genetic line and parity, are shown in Table 2. These traits have a large influence on the economic viability of a pork production enterprise.

Total pigs born per litter is an indication of the upper limit for genetic potential for number born alive. Total pigs born is a function of ovulation rate, number of eggs fertilized and embryonic survival. Total pigs born is often considered a primary indicator of female fertility.

Number of pigs born alive, that is the total pigs born minus the stillborn pigs, is an indicator of uterine environment in late gestation, farrowing time and of the farrowing manager's skills.

The Dekalb Monsanto MXP200 gilts excelled all other lines for total pigs born and number born alive with 1.76 and 1.45 more pigs per litter, respectively, than the lowest performing line. The Dekalb Monsanto DK44 line ranked second for total pigs born and number born alive, with a 0.53 and 0.48 pig advantage, respectively, over the third-ranked Danbred USA females.

Parity and genetic line averages for the litter birth traits, in Table 2, contains superscripts which help identify the differences between genetic lines and parities. Line and parity averages with common superscripts in a table indicate the figures are not statistically different.

A line-by-parity interaction, shown in Table 3, was found for both total pigs born and number born alive. This significant interaction shows that a genetic line's rank changed for total pigs born and number born alive, depending on which parity was observed.

In general terms, total pigs born and number born alive for each line decreased slightly from parity 1 to parity 2. This decline was followed by a slight increase for both traits in parity 3, then little or no change in the two traits in parity 4.

The exception was the Danbred USA line in which a significant decline in total pigs born and number born alive occurred in parity 2, followed by a significant increase in both traits in parity 3. But then, both traits declined significantly again in parity 4.

The Dekalb Monsanto MXP200 females consistently had the highest total pigs born and number born alive across the four parities.

Litter birth weights are considered important for piglet viability and growth, particularly in early wean systems. Heavier litter birth weights result from more pigs born per litter, heavier individual pig birth weights, or both. Assuming no complications during farrowing, larger litter weights often result in better pig viability at birth with positive effects on growth rate throughout life. Likewise, heavier birth weights often translate to fewer small or substandard pigs at weaning.

Genetic lines varied significantly for both total birth weight and live birth weight of litters. Danbred USA and Dekalb Monsanto DK44 females had the heaviest litter birth weights (Table 2).

Average individual pig weights, calculated from live pig birth weights and number born alive, were lowest for the Dekalb Monsanto MXP200 pigs (3.14 lb./pig) and heaviest for the American Diamond Genetics pigs (3.68 lb./pig). Since all sows were bred to the same types of boars and received the same care and management, the differences in pig birth weight are clearly a feature of the sow lines.

Total litter birth weight increased significantly from parity 1 to parity 2 and remained steady through parity 4 (Table 2). Similarly, live litter birth weights increased significantly from parity 1 to parity 2, then remained steady at parity 2 and 3, but declined significantly in parity 4.

When compared across parities, the calculated individual live piglet weight at birth increased from parity 1 to 2 and declined slightly in parities 2 through 4. Second-parity sows had fewer total born but no difference in number alive, resulting in bigger pigs than first-parity sows and reflecting a higher survival rate of second-parity pigs.

Table 4 shows pigs weaned per litter in the second parity was about the same or greater than first-parity litters for most lines.

American Diamond Genetics females contributed to a line-by-parity interaction for both total litter birth weight and live pig birth weight. These females went from having a low to intermediate live pig birth weight in parities 1 and 2, to the highest live pig birth weight and individual pig birth weight in parity 4. This trend was significantly different from the other lines in which live pig birth weights declined following parities 2 and 3.

Maternal Qualities The primary indicators of sow mothering and milking ability are the number of pigs weaned and litter weaning weight.

To make the line and parity comparisons for number weaned and litter weaning weight, a subset of litters was used to eliminate any biases caused by crossfostering. This subset consisted of only those females that nursed their birth litter (no crossfostered pigs). Number of pigs weaned and litter weaning weight were calculated after standardizing each line to their parity number born alive (Table 3), therefore parity differences were not tested. Line averages for weaning traits were standardized to a lactation length of 15 days.

The Dekalb Monsanto MXP200 line weaned the most pigs across the four parities, followed by the company's DK44 line (Table 4). The American Diamond Genetics sows weaned significantly fewer pigs than other lines across the parities but were superior to other lines for piglet survival, which averaged 93% (number weaned divided by number born alive) from birth to weaning within each parity.

The Dekalb Monsanto MXP200 line consistently weaned the most pigs, but it also had a lower percent survival (83-89%) as a proportion of pigs born alive. These differences in survival may be a function of sow milking ability and the lighter birth weights' effect on pig viability.

The National Swine Registry, Danbred USA and Newsham Hybrids USA sows were generally intermediate for number weaned but showed significant variation in rank across the parities. For example, Danbred USA females had the lowest number weaned average in parity 1 and 2 but were not different from the Dekalb Monsanto MXP200 sows for parity 3.

Litter weaning weights were also significantly different by line, within parity (Table 5). American Diamond Genetics, Danbred USA and National Swine Registry sows weaned heavier litters at each parity while the Dekalb Monsanto MXP200 sows weaned the lightest litters at each parity. The Newsham Hybrids USA and Dekalb Monsanto DK44 sows were generally intermediate in litter weaning weight, but were also the most variable lines with regard to rank within a parity.

As an example, the Dekalb Monsanto DK44 line was not statistically different from highest producing lines in parity 1 and 2, but then it recorded the poorest litter weaning weight in parities 3 and 4. These findings suggest that peak milking ability, as expressed by litter weaning weight, may occur at different parities for some lines.

The differences described for number weaned and litter weaning weight indicate that mothering ability differences exist in the genetic populations studied. Higher number weaned is a function of increased number born alive and improved piglet survivability from birth to weaning. Litter weaning weight is a function of milking ability and survival of pigs from birth to weaning.

The Dekalb Monsanto MXP200 line showed superior performance for number weaned but weaned the lowest percentage of pigs born alive and had the smallest individual pig weight and litter weight at weaning. The opposite effect is shown for the American Diamond Genetics sows as they weaned the highest percentage of piglets born alive, had the heaviest average pig weights and the heaviest litter weight at weaning. The smaller litter weaning weight of the Dekalb Monsanto MXP200 sows is certainly influenced by their smaller piglet birth weights.

Sow Traits Sow weight and last rib backfat depth were measured as gilts and sows entered the farrowing room and again when they were weaned. These measurements were collected for each parity to provide data on sow weight and body condition changes that may affect performance.

Table 6 shows sow weight and backfat depth averages at farrowing, as well as the average backfat loss during lactation. Total sow feed intake during lactation is also presented.

Significant farrowing weight differences were observed with a range of 82 lb. between the two Dekalb Monsanto lines, MXP200 and DK44.

The weight and backfat variation observed is likely an indicator of line differences in mature size. The Dekalb Monsanto MXP200 sows were significantly smaller than sows of the other lines. Composition differences were also observed with a 0.25-in. range in backfat from the fattest, American Diamond Genetics, to the leanest, Danbred USA.

Changes in body weight and condition during lactation are related to appetite as measured by voluntary feed intake under ad libitum feeding situations. Often in production settings, sows that lose large amounts of weight and backfat, due in part to low lactation feed intake, have poorer weaning weights and extended wean-to-service intervals. This data set provides a unique opportunity to describe how these traits interact for the genetic lines tested.

Genetic line averages for lactation feed intake, weight loss in lactation, and backfat loss are presented in Table 6.

Feed intake varied by 20 lb. (171 vs. 191 lb.) over a standardized lactation length of 15 days. The Dekalb Monsanto DK44 and American Diamond Genetics lines, which were also the heaviest, ate the most feed in lactation. The Dekalb Monsanto MXP200 sows, which were the lightest, ate the least feed in lactation. The other lines were intermediate for lactation feed intake. Feed intake increased each parity for all lines.

Evaluation of lactation feed intake, weight loss, backfat loss (Table 6) and wean-to-service interval (Table 7) data shows that the lines performed differently for these traits.

The Dekalb Monsanto MXP200 lines provide an unusual combination of performance for these traits. This sow line was the smallest in each parity and ate the least amount of lactation feed. In turn, this resulted in the highest absolute amount and percentage of body weight loss during lactation and the greatest backfat loss within each parity. Still, the Dekalb Monsanto MXP200 sows had the shortest wean-to-estrus interval following each weaning and the largest number of total pigs born per litter in subsequent litters. This goes against conventional production scenarios, in which poor feed intake, excessive weight loss and large compositional changes result in longer wean-to-service intervals and poorer subsequent litter size. The only negative impact from the lower feed intake in this line appears to be the lighter litter birth and weaning weights.

In contrast, the American Diamond Genetics and Dekalb Monsanto DK44 sows, both lines that were large at farrowing with high lactation feed intake, had smaller weight losses during lactation. However, both lines had longer wean-to-service intervals at each parity.

These results reinforce that more investigation is needed to discover how these physiological factors relate sow farrowing weight, backfat and feed intake with wean-to-service interval, subsequent litter size and litter weight.

Parity also affected lactation feed intake, weight at farrowing, backfat loss and weight loss.

Sow weights increased significantly with each parity, as expected, as sows continued to grow toward maturity. Lactation weight loss decreased from parity 1 to 2 and remained the same for parities 2 through 4.

Lactation feed intake increased from 155 lb. in parity 1 to 200 lb. in parity 4, with each parity significantly different from the previous parity.

Backfat and backfat loss declined with parity, indicating that as sow weight increased, the proportion of fat decreased and the weight loss occurring during lactation was more a function of fat mobilization in early parities than in later parities.

What We've Learned The message is clear - know your sow line's genetic performance potential so you can fine-tune your management. Nearly all the important economic traits showed interactions with parity. That means producers have a 'different animal' to deal with at each parity.

This can be viewed as a problem for some or as an opportunity for skilled producers to increase sow herd efficiency.

The MLP results provide a sound basis for comparing available commercial lines. In order to achieve the most value from these results, producers must recognize the strengths and weaknesses of their operation.

Reproductive traits are very responsive to management skills. Once these areas are identified, an economic analysis that accounts for specific production system economic values for the traits will show the farm economic values of available sow genetic lines.

Results of the MLP are applicable and will provide economic opportunities for all producers wishing to improve the productivity of their operation.