Relevance of exterior appraisal in pig breeding

In farm animals characterization of exterior is not a goal in itself but rather serves as an indicator of economically important traits, i.e. for pigs: growth performance, reproductivity and constitution. This indication might be of interest when these traits can not be measured on living animals (e.g. slaughter and meat quality) or when traits can only be measured on one sex or at later age (e.g. piglet production, longevity). Several methods are available to characterize exterior: direct measurements (e.g. width, height, circumference), photography and visual appraisal. Disadvantages of the last method are subjectivity and the relatively low accuracy of the characterization. Despite of those disadvantages visual appraisal is frequently used mainly because of the low costs and quickness of measuring.Since introduction of ultrasonic measuring equipment, prediction of slaughter characteristics of living pigs can be done more accurately by ultrasonic backfat measurement then by visual appraisal. Since then, the main relevance for appraisal of exterior traits is due to presumed relationships with constitution and reproduction. Leg and foot disorders are a main constitutional problem in pigs. These disorders are generally described by the term leg weakness. The proportion of tested nucleus boars culled for leg weakness varies from 15 to 50%. In sow herds 15 to 20% of the disposed sows are culled because of leg weakness. Economic loss due to leg weakness can arise in different ways: (1) reduced genetic improvement in performance traits at nucleus level due to involuntary culling, (2) premature disposal of breeding stock at nucleus and multiplier levels, (3) loss of sales at nucleus and multiplier levels, and (4) reduced reproduction and production efficiency at all levels. Next to the economic consequences, leg weakness has a negative impact on animal welfare and emphasizes the negative image of intensive husbandry systems.In pig breeding programmes emphasis is given to exterior traits mainly as a method to select for a better constitution. Very little, however, is known about the effectiveness of that selection. Responses to selection depend on heritabilities of exterior traits and on correlations between exterior traits and constitution, whereas application of modern techniquesfor breeding values estimation necessitates an appropriate way for exterior scoring.The aim of this study was to evaluate the importance of exterior appraisal in pig breeding. In the first chapter the function of exterior judgement in pig breeding and husbandry is reviewed. Special attention is given to relationships between exterior and the problem of foot and leg disorders.Investigations were set up to obtain the parameters that are relevant for the evaluation of the importance of exterior appraisal in pig breeding. Before investigations started, exterior traits to be investigated were chosen in deliberation with inspectors of the Dutch pig herdbook and three pig breeding companies. The choice of the traits to be evaluated was based on the expected economic value and the likelihood to have a relationship with constitution. Results and conclusions are based on appraisal of exterior traits of centrally tested boars and of gilts and sows on 24 commercial multiplier farms.Methods of exterior appraisal.A large number of exterior appraisals is needed for a sufficient reliability of heritability and correlation estimates. Ease, and time necessary for a judgement are therefore important criteria for the choice of the method for exterior characterization. Visual appraisal meets those criteria. In literature several advantages are mentioned for linear scoring of visually appraised exterior traits. For present investigations 19 distinct and two composite exterior traits are defined and described on a linear scale which ranged from 0 to 9 with 0.5 intervals. Traits considered refer to size and structure of the body (6), forelegs (3), rear legs and claws (8), movements (2), and a lifetime expectation score which should be an estimate of the animal's life time as interval between time of judgement and expected disposal. The evaluation of the linear scoring is described in chapter 2.The linear scoring system is evaluated in terms of: differences between inspectors, use of the 19 (0 to 9 with 0.5 increments) categories, accuracy of scoring and relationships between traits. For that purpose 40 boars were judged twice by each of 10 inspectors. Differences in mean and standard deviation for linearly scored exterior traits occurred between inspectors, with most inspectors showing a marked preference for whole number categories. On average, repeatability was 0.60 and reproducibility, indicating the correlation between repeated measurements done by different inspectors, was 0.30. Exterior traits could be summarized by principal component analysis to five new independent exterior variables, that are characterized as 1) side view of legs, 2) way of walking and lifetime expectation, 3) size, 4) harmony of the frame and 5) rear legs and claws.In present form, usefulness of application of described linear scoring system for exterior traits for selection purposes highly depends on the persons who judge the animals. When selecting and training appropriate people, a satisfactory scoring accuracy can be achieved for most traits.Heritabilities of exterior traits and relationships with growing performance.In chapter 3 genetic relationships between growth performance and 20 linearly scored exterior traits are estimated from data of 2792 boars of several strains. Boars are tested with ad libitum feeding from approximately 25 to 105 kg at central test stations of the Dutch herdbook and three breeding companies. Heritability estimates of exterior traits ranged from 0.01 to 0.38. Traits describing body size and harmony (trait I to 6) are moderately heritable (h 2=0.15-0.3) except length of the hams (h 2=0.06). Of the forelegs, only side view of the pastern shows a significant heritability estimate. Heritability estimates of posture of rear legs ranged from 0.22 to 0.30. Movements and constitution score have a small genetic component (h 2-0.15). Close genetic correlations (r g =0.4-0.5) are found between daily gain and side view of foreleg pastern, side view and dryness of the hock joint. Slower growing animals have steeper foreleg pastern, less liquid at the hock joint and more sickle hocked rear legs. The genetic correlation between feed conversion and rear view of the rear legs indicates that boars with better feed conversion have more O-shaped rear legs.A two-stage selection strategy with selection in the first stage on gait pattern (movements) only and on growth performance in the second stage, reduces genetic improvement of performance traits. When assuming a nihil economic value for gait pattern and rejecting 25 or 50% of the boars in the first stage, then economic merit is reduced by 13 or 24%, respectively.The accuracy of the estimated breeding value for growth performance, based on an own performance index with the index traits: daily growth, backfat thickness, and daily feed intake, shows a minor increase when exterior traits describing size and structure of the body are included in the index.Exterior traits and constitution.In practice most interest for exterior traits is because of the expected correlations between exterior traits and longevity as determined by the animals constitution. Over a two-year period, several thousand gilts between 7 and 9 months of age and sows from weaning till about 30 days post weaning, were repeatedly judged for exterior on 24 commercial multiplier herds. In chapters 4 and 5 relationships of linearly scored exterior traits with reasons for disposal and longevity are studied. Some exterior traits judged at gilt stage, showed to be relevant for disposal for leg weakness. Sows culled for leg weakness before 4th parity were longer and broader at gilt stage, had less straight rear legs and more tubercles at the rear legs, they walked slower and twisted more with the hind quarters compared to animals that had produced at least 4 litters. Sows culled before fourth parity because of leg weakness, had a lower score for constitution at gilt stage than sows that produced ≥4 litters.For most traits concerning the posture of the legs, both low and high scores were negatively correlated with longevity. The effect of a change of one standard deviation unit (s.d.) from the average exterior score on length of productive life (LPL), as a measure for longevity, is considerable for a number of exterior traits. Broader hindquarters are unfavourably related to LPL (-53 days per s.d.) while with one s.d. thinner hindquarters LPL increases 44 days. LPL increases with decreasing swelling at the hock joint. A more flexible gait pattern, less twisting of the hindquarters and a higher lifetime expectation score coincide with a higher LPL (67, 44 and 42 days per s.d. respectively). Selection on exterior traits with main attention for gait (movements) and body width can increase average length of productive life and so decrease the annual involuntary disposal of sows.Main conclusions.From the investigations presented in this thesis, the following main conclusions can be draw:- A satisfactory accuracy of linear scoring of exterior traits in pigs can be achieved. When judgements are made by several persons, regular deliberation and training are recommendable.- Exterior traits describing size and structure of the body are not relevant for growth performance estimation.- Selection only on growth performance has an unfavourable effect on locomotory ability of boars.- Broad back, loin and hams at gilt stage are unfavourably related with sow lifetime. An increased herd life, which is of interest for multiplier farms, might be achieved by an increased exterior selection in gilts. A broad body, slow gait (movements) and extreme leg postures are the main exterior criteria on which gilts should be culled.- Selection for constitution in dam lines should be concentrated on the exterior traits: gait (movements) and body width, whereas animals with extreme posture of the legs, especially buckled knees, bowed and steep rear legs, should be discarded for breeding purposes. For a long lifetime, pigs with an easy and quick gait, and a relatively narrow sized body are desirable.