Feet and leg unsoundness is an on-going concern for the swine industry, and has been back in the spotlight lately with concerns expressed by show pig exhibitors debating the issue of feeding PayleanTM. This article reviews causes of structural unsoundness and offers suggestions for improving it. Some of the information is available in more detail in Pork Industry Handbook 101, which also contains several excellent diagrams that illustrate conformation differences.
Structural unsoundness in pigs includes buckling of the knees, bowed legs, splayed legs, post leggedness, weak pasterns, uneven toe size, pigeon toes, and sickle hocks. Pigs may also exhibit swollen joints, goose stepping, and a general lack of freedom in movement. Surveys indicate that only reproductive failure results in more sow culling than lameness. Poor rear leg structure may prevent a boar from successfully staying mounted on a female during mating. Unsound front legs may limit a boar's desire to mount a female. For young breeding stock, estimates of culling levels because of unsoundness range from 10 to 40 %. In total, this problem results in a significant economic loss for producers.
Feet and leg soundness problems may occur in the front and rear legs in all ages and sex classes of pigs. Boars are generally evaluated to be the poorest in leg structure. Breeds can differ in soundness, and crossbred pigs tend to be more sound than purebreds. Many producers used to consider unsoundness to be caused by modern housing systems. Over the past few decades, however, it has been shown that pigs can adapt to concrete floors and indoor production, and cases of severe unsoundness have decreased dramatically. Other factors that may affect structural soundness include genetics, level of production, sex, nutrition, disease, floor surface, equipment location, and space available for exercise. Unfortunately, in recent years emphasis has been placed on other economically important traits, to the detriment of feet and leg soundness. Consequently, there has been a rise in the number of animals exhibiting some degree of structural unsoundness.
Conformation is of course the primary determinant of how sound an animals will be. An unsound pig tends to be too straight on its front and/or rear legs, resulting in an arched back, short stride, and unwillingness to move. A sound pig tends to have a flatter top, more level rump, and higher tail setting. From a side view, the front legs slope from the shoulder, resembling a curved sickle blade. The rear leg joints are properly angled to allow the hip, stifle, and hock joints to absorb pressure equally. The pasterns are sloping and long to provide a cushioning effect, and toes rest squarely on the floor surface.
In addition to structural differences, several other characteristics need to be considered to improve soundness in pigs.
Extremes of muscle development that impair free movement or cause abnormal stance should be avoided. The desired muscling as viewed through the ham should be long and thick to facilitate free movement and desirable stance of feet and legs.
Extremely tall, flat, deep-sided breeding animals have difficulty surviving on concrete floors. Visual selection for moderate length of leg combined with appropriate body length in boars and gilts should be used to avoid extremes in skeletal size.
Lack of mobility can contribute to the presence of concrete blisters, calluses, or abrasions on leg joints because such pigs have a tendency to dog-sit.
Toe Size is another factor to consider. The ideal foot should include two fairly even-sized toes that are big and slightly spread to improve ease of movement and stability. The outside toe is normally slightly wider and longer than the inside toe, but differences greater than 0.5 inches should be avoided.
A number of nutritional factors have been linked to unsoundness in pigs, but normal, balanced diets that meet NRC specifications will prevent such problems. Of greatest importance is level and ratio of calcium and phosphorus, and the use of trace mineral and vitamin mixes formulated for swine. In regard to the feeding of PayleanTM, it is recommended that potential breeding stock not be fed the additive, and that market show pigs be selected carefully for soundness if they are to be fed PayleanTM. It is important to follow label directions at all times, and to feed an appropriate diet to pigs receiving PayleanTM.
Several bone disorders can cause serious structural soundness problems, although a well-balanced diet will prevent most of them. Osteochondrosis (OC), however, is a different story. It is an abnormality of bone and cartilage that occurs in young animals whose bones are growing rapidly. There is no inflammation present, and OC lesions can occur in sound pigs as well as unsound pigs. OC alone cannot be the primary causes for bucked forward knees and uneasy movement, but it may play a role in the total syndrome of leg unsoundness. OC is often ignored by producers, perhaps because the condition is best diagnosed by necropsy, but it may be one of the major reasons for culling young breeding stock. The best approach for dealing with OC is prevention. Selection for soundness and conformation, adequate exercise on non-slippery floors, and rations adequate for proper development of bones and joints are recommended.
Arthritis caused by infectious agents (streptococcus, erysipelas, mycoplasmas, hemophilus, and actinobacillus) is a major factor in clinical lameness of pigs. Foot and leg lesions resulting from trauma (injuries) also result in lameness. Occasionally infection occurs in the spine, resulting in lameness and eventual paralysis. Most of these causes can be prevented and/or treated and a veterinarian should be consulted. If lameness persists more than 4 weeks, culling should be considered.
Larger bone size is reputed to be important for durability, and tends to be correlated with heavier muscling. Larger bone should be preferred, but not at the expense of structural correctness.
Type of flooring within housing facilities can affect feet and leg soundness. Pigs grown on total slats have more problems than those on partial slats. Wider slats with rounded edges result in fewer problems than narrower slats with sharp edges. Aluminum and some other bare metal slats produce more lameness problems than plastic, concrete, or coated metal slats. The prevalence of footpad lesions increases on rough as compared to smooth concrete. Extremely smooth, wet flooring presents areas for potential injury to feet and legs due to slipping accidents. Older animals kept on soft, resilient surfaces like plastic-coated metal can develop overgrown hooves, which can lead to soundness problems because pigs tend to compensate by shifting their weight and throwing their bones off the correct alignment. Although difficult to do, trimming of the hooves is recommended in such circumstances, or a change of flooring should be considered. Researchers have also observed that exercise will increase muscle tone and coordination. A long, narrow pen (length:width of 2.5:1) affords more exercise opportunity and should be considered at least for breeding boars.
Genetic control of structural soundness has been studied by several researchers. Feet and leg soundness is measured subjectively, using a scoring system such as that recommended by the National Swine Improvement Federation. These scores are moderate in heritability. Thus, selection for feet and leg soundness combined with selection for other important economic traits will result in improved performance of a herd over time, although more slowly than if soundness is ignored. With the current emphasis on other economically important traits like backfat and muscling, many breeders have allowed less sound animals to remain in the breeding herd, leading to an increase in the number of unsound animals in commercial herds and a higher culling rate of breeding animals. These results are not surprising given the results of a divergent selection study conducted at Iowa State University. The realized heritability after 5 generations of selection for and against soundness score was .29 for improved soundness, but .42 for increased unsoundness.
Average daily gain is weakly but favorably correlated with feet and leg soundness, so fast-growing pigs that will stay sound can be produced. Unfortunately, the genetic correlations between backfat thickness and leg soundness, and between loin muscle area and soundness are not favorable. These findings indicate that fatter, less muscled pigs are more likely to be sound on their feet and legs than are leaner, meatier pigs. These unfavorable genetic correlations can cause problems in selecting for leaner, meatier, and more sound pigs. Careful and persistent selection practices that include the use of EPD indexes, along with visual appraisal and scoring of tested pigs for feet and leg soundness, in selecting both boars and gilts for breeding purposes, is strongly recommended.
Suggested guidelines for improving feet and leg soundness in pigs include:
1. Using a structure and movement scoring system to evaluate all breeding herd candidates at a standard age or weight in the selection process;
2. Purchasing sound breeding stock from breeders with a soundness scoring-culling system and with facilities that are similar to the ones to which the animals are going;
3. Avoiding the use of adverse environmental conditions like rough floors and crowded pens;
4. Feeding nutritionally balanced diets;
5. Providing consistent, best management practices on a daily basis to all pigs;
6. Separating lame or injured pigs from the group and placing them in a well-bedded pen if at all possible, consulting a veterinarian, and culling animals when necessary;
7. Recognizing that it may not be possible to select animals with the best soundness scores in all cases. Structural soundness is just one trait that must be considered in the selection process; compromises must often be made to achieve genetic improvement in productivity.
The National Swine Improvement Federation soundness scoring system uses a scale of 1 (poorest) to 5 (best) for both the front and rear legs. Front and rear scores are summed for the final evaluation. Pigs scoring 1-3 points total are unacceptable: they have severe structural problems that will restrict the animal's ability to reproduce. Scores of 4-7 points are assigned to animals with slight structural and/or movement problems. Scores of 8-10 points are excellent. Animals in this category show no obvious structural or movement problems, have even toe size, adequate length of stride, good flexion of hock, pastern cushion, and trueness and freedom of movement. From a practical standpoint, potential breeding stock should be ranked using a performance index, and only animals with a combined soundness score of 7 or higher should be chosen.
By Cindy M. Wood, Animal & Poultry Sciences, VA Tech