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Understanding Why Sows Are Culled

Sow culling rates have a direct correlation to the economic efficiency of a breeding herd. Some sows are culled voluntarily in an effort to improve overall herd performance. These sows may have weaned small litters, savaged their pigs or had a poor disposition. Other sows are culled involuntarily. These sows may have failed to breed, failed to conceive after two estrous cycles or simply died. Most

Sow culling rates have a direct correlation to the economic efficiency of a breeding herd.

Some sows are culled voluntarily in an effort to improve overall herd performance. These sows may have weaned small litters, savaged their pigs or had a poor disposition.

Other sows are culled involuntarily. These sows may have failed to breed, failed to conceive after two estrous cycles or simply died.

Most recordkeeping programs offer mechanisms to record the reason(s) why individual sows are culled.

Cull Sow Study

In an attempt to more specifically identify the reasons sows are culled, Iowa State University researchers, with funding from the National Pork Board, investigated the physical and reproductive conditions of 3,158 cull sows delivered to midwestern sow harvest plants.

Two plants were chosen because they harvest sows considered representative of the range of sows — thin or “boner” sows to well-conditioned or “fancy” sows — culled from North American sow herds.

Body condition, feet and legs, shoulders, teeth, respiratory system, digestive and reproductive tracts were visually evaluated for the presence (or absence) of lesions and abnormal conditions in the sows.

A National Swine Improvement Federation certified Real-Time ultrasound technician measured backfat, loineye area and loin muscle depth from a cross-sectional image taken at the 10th rib of sows prior to slaughter. A visual assessment of body condition was made immediately postmortem, evaluating each sow's condition and scoring them on a 1 (thin) to 5 (fat) scale, much as a producer would do as sows reenter the gestation barn.

Front feet, rear feet and dewclaw condition were evaluated and recorded by a trained technician. Both hooves (toes) and dewclaws were evaluated for various abnormalities, including sidewall lesions, cracks in the white line and toes, cracks in the soft heel of the toe, abscesses, cuts, abrasions and bruising. Overgrown and missing toes and/or dewclaws were also recorded.

Shoulders were evaluated for the presence of lesions in the form of abscesses, abrasions and wounds. Shoulder lesions were categorized as follows:

  • No lesion;

  • Abscess (open or closed);

  • Abrasion; or

  • Wound (open wound/healing open wound).

Teeth were evaluated and recorded by a trained technician. Top and bottom teeth were counted and scored for severity of wear as follows:

  • Minimum wear — sharp points present on molars and incisors;

  • Moderate wear — points on molars and incisors present but smooth; and

  • Severe wear — no points present on molars and incisors.

Harvest plant personnel removed all thoracic and abdominal viscera, including the respiratory, reproductive and digestive organs from the carcass. Immediately after removal from the carcass, U.S. Department of Agriculture personnel and a veterinarian visually inspected the viscera.

From the macroscopic appearance of the ovaries, the sows were classified as normal, cystic or acyclic (non-cyclic). Pregnancy status was determined and fetal tissues (if present) were classified as normal, decomposing or mummified.

The research veterinarian visually evaluated the thoracic and abdominal cavity and organs for clinical signs of disease. The presence or absence of peritonitis, pleural adhesions, and pneumonia were recorded. If pneumonia was visually diagnosed, an estimate of the percentage of lung involvement was made. Lesions other than those previously mentioned were noted when visually present.

Early Findings

Preliminary evaluation of the data is underway. The most common, observable disorders among harvested cull sows were rear and front pad lesions, found in 67.5% and 32.9% of sows, respectively (Table 1).

Rear pad lesions were associated with body condition scores (BCS). As BCS increased, sows had more rear pad lesions. One possible explanation is that as BCS increases, sow weight generally increases. This added weight might increase pressure and thereby cause damage to the rear pad.

The second most common observable disorder was front and rear cracked hooves, found in 22.6% and 18.1% of sows, respectively.

Again, both front and rear cracked hooves were associated with BCS. But in this case, the incidence of cracked hooves increased as sow body condition became poorer (thinner).

Previous research has shown flooring type influences the presence of cracked hooves. Sows housed in confinement housing (stalls, tethers) with partially slotted flooring generally had fewer cracked hooves when compared to sows housed in loose housing systems with partially slotted concrete flooring. The same study reported that sows housed in a loose housing arrangement that included deep bedding were associated with minimal claw lesions.

The incidence of overgrown toes and/or dewclaws on the rear feet of all sows evaluated was higher than expected (21.1%). Additionally, the incidence of overgrown rear toes and/or dewclaws increased as BCS decreased. Whether this is attributable to some genetic, nutritional, environmental or some unknown factor is not clear.

Previous reports have indicated that pigs housed on plastic slats had a greater prevalence of overgrown toes when compared to pigs housed on concrete slats. This may be attributable to decreased wear on plastic compared to concrete floor surfaces.

Yet another study reported that gilts reared on higher feeding levels had longer dewclaws later in life. This indicates a farm nutritional component to the formation of overgrown dewclaws.

Toe lesions were more frequently observed on the rear toes when compared to the front toes. One possible explanation may be uniformity of toe size. It has been demonstrated that front toes are more uniform in size than the rear toes. Furthermore, toe lesions are known to increase as the difference in size between the medial and lateral toes becomes larger.

Housing conditions may also have an impact. Toes on the rear feet of sows housed in crates may experience wetter conditions when compared to the front feet because of urination.

The presence of severe teeth wear was found in 42.5% of sows. However, neither minimum, moderate, nor severe teeth wear scores and teeth numbers were associated with BCS.

The occurrence of shoulder abrasions was recorded in 12.5% of sows. The incidence of shoulder abrasions increased as the BCS decreased. Of the sows with a BCS of 1, 21.6% had shoulder abrasions. By comparison, only 3.9% of sows with a BCS of 4 or 5 had shoulder abrasions.

Shoulder lesions commonly occur during lactation, because sows spend a greater percentage of the time lying on their side. Good lactation management and improved body condition prior to farrowing can reduce the risk of shoulder lesions.

The presence of pneumonic lesions was found in 9.7% of sows. The majority of these pneumonia lesions had less than 10% of lung involvement.

The incidence of pneumonia lesions increased as BCS decreased among the sows evaluated. This study was designed to detect an association between BCS and pneumonia lesions, but couldn't determine if the pneumonia led to poor condition or, conversely, poor condition led to greater susceptibility to pneumonia.

The presence of acyclic ovaries was identified in 9% of sows. The incidence of acyclic ovaries increased as BCS decreased.

Previous reports have demonstrated that acyclic ovaries were associated with protein loss. Because protein loss was unknown in this study (sows were not evaluated at the farm), it is not clear if it contributed to the incidence of acyclic ovaries.

What Have We Learned?

The current study suggests that cull sows evaluated at U.S. harvest facilities have a relatively high presence of foot disorders. Additionally, these cull sows had shoulder, teeth, respiratory and reproductive lesions.

Reproductive failure is the most commonly cited reason for culling, but the question remains: “Why are sows with normal ovaries culled for reproductive reasons?”

The incidence of reproductive tract problems was substantially less than the percentage of sows culled for reproductive failure by commercial recordkeeping services.

It is important to distinguish between gross lesions and functionality. It is possible that sows with ovaries that appear grossly normal still fail to exhibit behavioral estrus reliably. If ovaries appear functional on gross examination, environmental factors on the farm would more likely explain poor reproductive performance by these sows.

Table 1. The Incidence of Abnormalities Among Sows Evaluated at Two U.S. Cull Sow Harvest Facilities
Trait Frequency or Count (n=) Incidence, %
Front feet, n= 3,1171
Heel lesions 1,024 32.9
Cracked hooves 703 22.6
Digital overgrowth 109 3.5
Abscesses 20 0.6
Missing dewclaws 4 0.1
Hind feet, n= 3,058
Heel lesions 2,064 67.5
Digital overgrowth 644 21.1
Cracked hooves 552 18.1
Missing dew claws 152 5.0
Abscesses 134 4.4
Ovaries, n= 3,062
Acyclic 277 9.0
Cystic 192 6.3
Fetuses, n= 3,070
Pregnant 180 5.9
Normal 157 5.1
Mummified 15 0.5
Decomposed 8 0.3
Disease processes, n= 3,083
Pneumonia 1- 10%2 153 5.0
Pneumonia >10%3 145 4.7
Pleural adhesion 174 5.6
Peritonitis 54 1.7
Detritus 11 0.4
Shoulder lesions, n= 3,146
Abrasions 394 12.5
Open 150 4.8
Abscesses 12 0.4
1n= number of sows with recorded data
25.0% of sows had pneumonia with 1-10% lung involvement.
34.7% of sows had pneumonia with >10% lung involvement.