A variety of factors linked to reproductive physiology, sow management and herd size appears to have triggered a sharp rise in sow mortality in U.S. herds.

The cause of rising breeding female mortality seems to be from two groups, replacement gilts rushed into a breeding herd andsows kept past their prime.

This issue is a real problem and an increasing expense on most sow farms, relates John Deen, DVM, University of Minnesota.

Losing a sow through culling or death is a cost that can be measured several ways. One is the cost of replacement with a gilt. But, if gilts are in short supply, the cost may be measured against the use of a sow that should have been culled. If a sow dies during gestation, there are opportunity costs due to loss of pigs produced.

"Replacement costs plus opportunity costs can easily equal $400-500 per sow that dies," says Deen. Plus, there is the affect of sow mortality on employee morale and animal welfare concerns.

Probing Sow Losses To study this growing concern, a team of veterinary researchers, including Deen, collected 3.6 million sow parity records from U.S. herds for 1996-98.

In the study, specific causes of sow mortality weren't recorded because of the wide variety in reasons between farms and production systems, explains Deen. Rather, researchers collected and analyzed "hard data" such as the effect of seasons, stage of reproduction and parity.

Researchers in the sow mortality study noted a big jump in sow losses in all systems, without exception, stresses Deen. And the rate of losses was much higher than what was being reported in the scientific literature.

At PigCHAMP, sow mortality has been monitored for years using the recordkeeping service's database, says Yuzo Koketsu, DVM, director, database and research. From 1993-98, sow mortality in North America (includes Canada) increased from 4.1% to 5.6%. For 1999, sow mortality reached 6.7%, he says.

In the sow mortality study, losses varied from 7% to 8% in early 1996 to a high of near 15% in mid-1998.

Reasons For Sow Mortality Summer sow mortality is widely seen as an industry problem, but it was difficult to pinpoint the cause of sow deaths in systems surveyed.

Survey data does show a link between season and stage of reproductive cycle as a risk factor in sow mortality, notes Deen.

About half the post-farrowing sow deaths occurred during the first three weeks after farrowing. That figure shot up to 65% for summer farrowings.

Some 27% of the mortality in the survey occurred in breeding females that never farrowed.

Gilt Problems According to the survey, first-parity females are the group most at risk for mortality, Deen points out.

Proper gilt development has all but been forgotten on some farms, according to North Carolina State University's (NCSU) Monte McCaw, DVM. "We've forgotten to feed them separately, forgotten to not house them too tightly (like finishers) and forgotten to upgrade management when we bring them in much younger," he stresses.

Says McCaw, "Sometimes when we start bringing gilts in younger, people who aren't used to raising gilts are forced into raising gilts and don't focus on getting the job done right." It can result in feet and leg problems, gastric ulcers or out-of-condition sows.

That lack of proper condition contributed to an increase in recorded sow deaths at time of farrowing in the survey, says Deen.

And poor sow condition is directly related to the fact that average backfat thickness for incoming gilts has dropped from 1.0 in. down to 0.6 in. or less in the '90s, points out Kevin Rozeboom, Extension specialist, swine reproductive physiology, NCSU.

"When the question is raised why we have greater sow mortality, I suggest just looking at the condition these sows are in," he stresses. "We have super-producing sows that, when they go into a farrowing crate, are not eating as much as they used to. Coupled with the lack of fat reserves, they just get worn out," he says.

On some farms, it's not the gilts but higher-parity sows that are most at risk, often due to culling practices. If gilt pool size is low when sows are weaned and a marginal sow is kept, she is more likely to die during the next parity because she was probably retained too long, Deen points out.

Sows that have more stillbirths are also more likely to die. The odds increase by 24% when one or more stillbirths occur.

The large expansion of herd size in the U.S. seems to have coincided with the start of increased sow mortality, says PigCHAMP's Koketsu. He suggests that workers may be hard-pressed to manage all the animals on large farms. Unlike in the sow mortality survey, chronic diseases may play a role that smaller herds don't face. (Infectious swine diseases were not considered a factor in the sow mortality study.) See Figure 1 for PigCHAMP data on the impact of herd size on sow mortality.

Barb Straw, DVM, Michigan State University, says realistic targets for sow mortality should take into account size of operation. The target should be 3% or less for herds of 150 sows or less and 5% for herds with 150 sows or more.

Pushing Sow Mortality There are several practical factors pushing sow mortality to double-digit levels, says Paul Armbrecht, DVM, Lake City, IA.

One factor is producers can no longer sell a sow with locomotor problems. The only choice is to euthanize her, he says.

Second, cull sows have become byproducts. Revenue from a sow is so small that it's become almost worthless to try and market them, it's cheaper to euthanize them.

Conclusion There were no clear-cut reasons for sow mortality in the survey conducted. But Deen says there were a number of risk factors involved with changes in sow management and pig production. Veterinary researchers will continue to track those factors in search of solutions.