Growing concerns about sow herd fallout rates have commercial pork producers looking for answers to the sow longevity riddle.

From an economic standpoint, some research has indicated that improving a sow's productive life by one parity is equal to improving lean meat content by 0.5% in the finishing herd.

Other models have shown that production systems with the lowest replacement rates were most profitable. When replacement rates were high, replacement gilts were worth no more than 175% of market value. However, when replacement rates were low, replacement gilts were worth up to 450% of market value.

Similarly, sows with high lifetime production values, a trait closely related to longevity, reduced breeding cost/pig weaned by over $1.50, when compared to their low lifetime production counterparts. A higher proportion of high-parity females is a clear indication of a herd's lifetime reproductive performance. One economic study suggests that the optimal economic time to cull a sow is after her eighth or ninth parity.

These and other observations were gleaned from a review of the scientific literature examining the factors affecting sow longevity and sow mortality. The reported studies spanned over 200 scientific articles and 40 years of research. The majority of the studies were completed in the 1980s and 1990s.

While it appears the reasons for sows leaving the herd have not dramatically changed over the past four decades, the underlying reasons for their exit may have changed.

PigCHAMP record summaries report that average, annual sow replacement rates have averaged about 50% for the past five years.

Improving sow longevity has the potential to have a positive impact on a pork producer's profitability by reducing replacement gilt expenses and associated development, isolation and acclimation costs.

The bottom line is this — a replacement female must produce a sufficient number of offspring to offset her purchase price and maintenance costs. Any pigs produced above those costs are viewed as profit. This edge can make one operation more successful than another.

Sow Longevity Measures

Popular ways to evaluate female longevity in commercial operations include removal, culling and replacement rates; percent gilts in herd; mean parity of females in inventory; and mean parity at removal.

Pork producers tend to focus on culling and replacement rates as a measure of female retention rates. However, when making comparisons across herds or to other studies, culling and replacement rate values can differ based on methods used to calculate them. In many cases, some of these values may be difficult to measure or obtain.

For example, if replacement gilts are purchased and their birth dates are not provided, there is no way to accurately arrive at a length-of-life measure.

Reasons for Culling

Reproductive failure, which can encompass a variety of problems such as failure to cycle or inability to conceive, is clearly the single biggest reason sows are removed from the breeding herd.

Table 1, compiled from a variety of studies, lists the reasons sows were culled, by percentage. The literature reveals that the reasons for culling vary somewhat with parity.

For example, reproductive failure and feet and leg problems are the predominant reasons Parity 3 and younger sows are culled.

Several studies demonstrated that lifetime number of pigs born alive was lower in females culled for reproductive problems. This reinforces that young sows are being removed at higher rates for reproductive problems than older sows. Once sows reach the fourth parity, culling for reproductive failure becomes less of an issue, and age, poor performance, and even death are cited as reasons sows are removed.

Reports showed that sows removed for poor performance produced an average of 5.11 litters, while the average parity of sows removed for reproductive failure was only 2.37. In other words, producers rarely get the chance to cull young females for poor reproductive performance.

Getting younger females to reproduce seems to be the largest impediment to improving sow longevity. Table 2 shows the “ideal” parity distributions of breeding herd females from four research reports. Clearly, the goal must be to avoid culling younger females.

In addition, older sows have typically been exposed to the diseases present on a farm and, therefore, offer the benefits of being able to provide more immunity to their offspring.

Reasons for Sow Mortality

The research also suggests that the most common causes for sow deaths include torsion and other abdominal organ injuries, heart failure and cystitis. As with culling, locomotion or leg-related problems tend to be associated with the death of younger sows. As you might expect, sow deaths appear to be seasonally problematic, particularly during the hottest months.

Cystitis/nephritis, often associated with water intake problems, accounted for over 20% of sow mortalities. Frequency of feeding and changes in feeding practices also appear to contribute to gastric torsion problems. Some suggest that feeding gestating females three times daily can reduce the mortality risk, when compared to once or twice per day feeding.

Management factors that appear to lower the risk of mortality include weaning at 28 days or greater, smaller litter size at birth (12 pigs or less), reaching maximum daily feed intake before the 15th day of lactation, and having maximum daily feed intake at less than 17.5 lb.

Additionally, increasing herd size has been shown to adversely affect sow mortality rates. One report indicated that if herd size increased by 500 sows, mortality risk increased by almost one-half of one percent.

Genetic Effects

It is clear that crossbred females can withstand the rigors of commercial breeding operations better than purebred or pureline females. Studies showed that crossbred sows produce nearly one litter more than their pureline counterparts, on average.

Many commercial operations are moving toward internal gilt multiplication systems. It is important to realize that the purebred or pureline females used to produce parent females for commercial production will likely produce fewer litters than sows in the commercial breeding herd. And, higher culling and mortality rates may be expected with the great-grandparent or grandparent internal multiplication systems.

Stayability, a measure some use to describe longevity, is the ability of a sow to produce an additional litter. The term, often used in the cattle industry, has a genetic basis in swine.

Like other reproductive traits, stayability is considered a lowly heritable trait (5-10%). Researchers who have studied longevity or lifetime prolificacy have found heritabilities ranging from 10 to 27%.

The most telling information might be the genetic correlations of stayability with measures of sow longevity and other economically important production traits like average daily gain and backfat. These associations are generally unfavorable and, thus, the selection emphasis traditionally placed on growth and backfat may have had a negative impact on the traits affecting sow productivity for an extended number of parities.

Certainly, it appears that sow longevity differs between genetic lines, with differences approaching one full parity. Producers should ask genetic suppliers for production information on lines they are considering. It is important that the data be collected in production systems similar to those where the animals will be placed.

Remember, when an internal gilt multiplication program is used, the commercial producer is now responsible for a portion of the genetic improvement. A key challenge is making sure enough replacement gilts are produced. Producers must plan months in advance because the number of gilts needed may vary considerably (50 to 80%).

The replacements, whether raised or purchased, require a development, isolation and acclimation period. And, feet and leg structure must be critically evaluated to ensure animals are structurally sound. Short change any of these items, and sow longevity will surely suffer.

Impact of Gilt Development

Some production advisors suggest there is a relationship between body composition at mating and a female's longevity, particularly with today's lean genetic lines.

When weight and condition loss are minimized in lactation, there may be no association between live weight or backfat depth at first successful breeding and subsequent reproductive performance. However, not every modern, lean, breeding female is placed in an “ideal” nutrient and housing situation, so some relationship between backfat and production likely exists.

Some minimum level of backfat is likely needed on replacement gilts if they are to maximize the lifetime number of pigs born alive. One research project found that gilts with 0.70 to 0.87 in. of backfat, averaging 330 lb., averaged 7.2 more pigs born live over five parities when compared to gilts with 0.55 to 0.63 in. of backfat.

Gilt Pool Management Effects

A major factor contributing to poor sow longevity may be related to gilt pool management. Many operations breed gilts early so that breeding targets are met, thereby allowing them to remove sows of questionable productivity.

Breeding gilts at virtually any age so that production flow is not hindered may be a major reason why breeding herd females do not remain productive in intensely managed herds.

Gilts bred at early ages may not have had sufficient time to build adequate body reserves or acquire the necessary immunity to keep them healthy throughout a long, productive life.

Age At First Breeding

A common notion among pork producers is that if gilts reach puberty at an early age, their longevity and/or lifetime reproductive performance will be improved.

And, having reached puberty, there is also a perception that increasing age of gilts at first successful breeding will improve sow longevity.

Both beliefs were supported by several studies.

One study indicated that the total number of pigs produced per sow and the parity number at culling significantly increased in gilts that began cycling younger, but were not bred until a subsequent estrus.

In another study, gilts that cycled and were bred at an older age had a significantly shorter expected herd life when compared to gilts that cycled and were bred at younger ages.

The easiest way to think about age at puberty vs. age at first farrowing is that you want gilts to cycle as quickly as possible, because earlier cycling females tend to stay in the herd longer. Conversely, gilts that reach puberty later will most likely leave the herd early. The latter supports the adage: once a problem breeder, always a problem breeder.

Delayed breeding of gilts that cycle early generally allows gilts time to mature and build body reserves needed to remain in the breeding herd for a longer period of time.

Lactation Length

Weaning ages have become progressively younger over the past 15 years. This trend maximizes facility throughput and improves the health of the weaned pig. However, early weaning also creates some new challenges that will ultimately affect sow longevity.

Research data clearly showed that weaning at early ages, particularly less than 16 days of age, can adversely affect sow longevity through increased culling due to reproductive failure — the number one reason sows are culled.

Post-Lactation Body Condition

Thin sows at weaning have been problematic ever since intensive management of the sow herd became commonplace. Poor body composition contributes to poor reproductive performance.

Many producers score body condition of sows after weaning and periodically during gestation. Unfortunately, the relationship between body condition score and body composition may not be very reliable.

Researchers have correctly pointed out that changes in condition are related to both backfat content and body weight. When backfat is evaluated alone, without assessing body weight change, it can be misleading. However, producers may find it difficult to estimate body weight. A heart girth tape may be useful in estimating body weight.

Other Factors

Certainly, there are a variety of other factors that can influence sow longevity, alone or in concert with other factors. Season can influence sow herd culling and mortality rates. Different sow housing systems, like crates and pens, can impact culling rates.

People and/or management factors can influence culling rates. Common mistakes include inaccurate pregnancy checking — open sows misdiagnosed as pregnant, pregnant sows misdiagnosed as open. The ability to identify females in heat and then successfully inseminate them is another. These are worker mistakes, not a deficiency in the sows' reproductive ability.

Finally, diseases like porcine reproductive and respiratory disease (PRRS), pseudorabies and others can have an immediate, long-term impact on sow replacement rates.

Table 1. Research Composite of the Reasons Sows Are Culled (by percentage)1

Study2 Reproductive Failure Poor Performance Old Age Feet, Leg & Locomotion Disorders Farrowing Problems Health & Disease Milking Problems Death
Pomeroy, 1960 21.4 22.4 17.1 NR 2.0 13.3 6.1 NR
Jones, 1967 8.8 NR 2.2 9.4 NR 2.4 5.6 10.1
Svendsen et al., 1975 28.8 10.0 3.9 15.0 NR NR NR NR
Dagorn & Aumaître, 1979 39.2 8.4 27.2 8.8 4.0 NR NR 6.5
Pattison et al., 1980 37.5 13.8 24.4 11.8 NR NR NR NR
Joo & Kang, 1981 32.6 15.7 16.7 9.7 NR NR NR NR
Muirhead, 1981 35.4 NR 28.2 10.8 2.8 NR 5.0 4.6
Stone, 1981 12.9 20.6 33.4 11.0 1.6 4.2 8.9 NR
Friendship et al., 1986 23.7 14.5 19.2 11.8 2.3 2.5 9.0 3.0
D'Allaire, 1987 32.4 16.8 14.0 8.9 7.2 1.6 NR 11.6
Dijkhuizen et al., 1989 34.2 20.1 11.0 10.5 NR NR NR NR
Stein et al., 1990 29.6 9.4 17.9 11.0 5.0 0.8 8.8 10.7
Cederberg and Jonsson, 1996 29.0 1.0 8.0 14.0 NR NR 13.0 7.5
Kangasniemi, 1996 28.2 14.4 16.8 13.5 2.4 1.4 1.9 3.2
Paterson et al., 1996 21.3 2.3 7.2 9.3 NR 3.5 1.6 5.0
Pedersen, 1996 34.5 4.6 18.8 6.1 NR NR NR 12.3
Sehested & Schjerve, 1996 28.7 4.8 11.3 10.2 1.9 4.9 0.9 4.2
Boyle et al., 1998 29.8 11.1 31.3 11.3 NR 7.4 NR 6.6
Lucia et al., 2000 33.6 20.6 8.7 13.2 NR 3.1 NR 7.4
1Portions of this table have been adapted from D'Allaire and Drolet, 1999.
2All of the studies reviewed did not report results exactly in the same categories. When this occurred, the authors attempted to summarize the study and place results in the appropriate classification.
NR = not reported.
Table adapted from the Pig News and Information article published in June 2004.


Table 2. Ideal Parity Distribution Recommendations (by percentage)1

Parity
Study 0 1 2 3 4 5 6 7 >7
Straw, 1984 20 18 17 16 15 14
Parsons et al., 1990 30 23 19 14 10 5 2 1 0
Muirhead & Alexander, 1997 17 15 14 13 12 11 10 5 3
Morrison et al., 2002 19.1 16.5 16.9 14.1 10.2 8.2 5.1 4.9 4.9
1Values by parity within a row indicate the percentage of females that should be in each classification for an ideal distribution.
Table adapted from the Pig News and Information article published in June 2004.