Genetic analysis of swine production traits

Transcription

1 Genetic analysis of swine production traits Item Type text; Thesis-Reproduction (electronic) Authors Ramos-Castillon, Francisco, Publisher The University of Arizona. Rights Copyright is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 25/07/ :00:46 Link to Item

2 GENETIC ANALYSIS OE SWINE PRODUCTION TRAITS by Francisco Ramos-Castillon -A Thesis Submitted to the Faculty of the DEPARTMENT OF ANIMAL SCIENCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA

3 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: Professor of Animal Sciences

4 PREFACE Sonora, a state situated in the northwest part of Mexico bordering Arizona, has always been the main producer of agricultural products in Mexico. Recently, a change in the agricultural policy toward a better utilization of the agricultural resources by means of crop diversification and the development of the high-yield varieties of grain has brought about the development of the swine industry as an excellent outlet for grains and agricultural by-products in this area.. There were only three important swine enterprises in Sonora before 1968, two of these being integrated operations. After 1968, several farms began to turn to commercial swine production, encouraged by the advantages that this business offers: fast turnover of capital investment; utilization of local agricultural products and by-products; favorable climate; improved managerial and technical personnel; and a very favorable market for pork products. Characteristics of swine such as high prolificacy, fast rate of growth, efficient feed conversion, and year-round breeding make its exploitation an optimistic resource to meet the growing demand for food supply in Sonora and Mexico. Furthermore, countries like Japan have shown an interest in the importation of beef and swine carcasses from Mexico. This would mean a strengthening of the Mexican economy in the world market and an increase in its per capita income and sources of employment.

5 iv These promises that the future of the swine industry offer show the necessity of developing a highly organized industry according to the most modern technology in order to meet the requirements of the producers and consumers. The Federal Government of Mexico through the Consejo Nacional de Ciencia y Tecnologia supported all my graduate work and made possible the accomplishment of this study, I want especially to thank Dr, Carl B. Roubicek for his guidance9 encouragement and valuable suggestions during this study and the subsequent preparation of this thesis. Dr. Donald E. Ray made useful suggestions during the planning of the statistical analysis and suggested improvements in the thesis. I thank Mr. Epifanio Salido, Ing. Francisco Trujillo, and Ing. Arturo Madrid for their help during the collection of the data used in the present thesis. Appreciation is given to my mother and relatives for their patience and encouragement.

6 TABLE OF CONTENTS Page LIST OF TABLES... vi ABSTRACT vii INTRODUCTION LITERATURE REVIEW MATERIALS AND M E T H O D S... 8 RESULTS AND DISCUSSION General Considerations REFERENCES v

7 LIST OF TABLES Table. Page 1, Distribution of class and subclass numbers for the analysis of variance , Over-all means with standard deviations and heritability estimates with standard errors of litter performance traits , Mean squares from analyses of variance for birth traits * 17 4» Mean squares from analyses of variance for weaning traits » Least squares deviation for breed of sire, sires within breed, breed of sire x breed of dam, parity, and months e 20 6» Genetic, phenotypic, and environmental correlations for traits considered # ««««23 vi

8 ABSTRACT. Data from 919 litters from crossbred sows of the Hampshire5 Yorkshire, and Duroc breeds, sired by nine purebred boars of the three breeds were used to estimate genetic parameters from sire variance components, Heritability estimates obtained were 1,7% for number of pigs born alive (L.S.), 5,3% + 5,6 for litter weight at birth (L,Wt,), 0,7% +2,7 for litter size at weaning at 32 days (L.S.Wn,), 1,4% + 3,1 for litter weight at weaning (L.Wn.Wt.), and -0.2% + 2,2 for average pig weight at weaning (Pig Wt,), Parity and month of farrowing showed consistent significant effects on all traits studied. Breed of dam and interaction of breed of dam by breed of sire were not statistically significant for any trait. Breed of sire was only significant for L,Wt, Differences between sires within the Duroc breed were significant in birth traits. Differences between sires within the Hampshire breed were highly significant (p <.01) in L.S.Wn, Least squares deviations suggested a superior general combining ability of the Yorkshire breed. Correlations for litter size and litter weight were positive at all ages, however the correlation between L.S.Wn. and Pig Wt, was negative. vii

9 INTRODUCTION Seventy-five percent of all the production costs of a swine enterprise are represented by feed. The remaining 25 percent of production costs are largely determined by housing and management practices. Therefore, the economic returns from a swine enterprise depend largely upon such factors as litter.size, livability, growth rate, efficiency of gain, and carcass desirability. The application of modern technology to the health care, feeding and management of swine can bring about larger, faster growing and healthier litters. However, these improvements are only environmental, consequently they are not genetically transmitted to succeeding generations. Heredity plays an extremely important role in determining an animal s performance. Thus, records of performance are necessary in order to identify animals that are genetically superior in order to select them as replacement breeding stock. Although Sonora excels other states in Mexico in animal production, genetic studies have not been made to identify superior breeding stock, or to determine the genetic parameters of performance traits under these conditions. Knowledge of genetic and environmental factors that affect the animal s performance would be of great help in establishing optimum breeding and management systems. 1

10 2 The objective of the present work was to analyze genetic and environmental factors that affect productive traits in swine from birth to weaning. Traits of economic importance in swine from birth to weaning are litter size at birth, litter size at weaning, and litter weight at weaning. Number of pigs at birth is a measure of the prolificacy of the sow, while number of pigs at weaning and weight of the litter at weaning are measures of the sow's ability to nourish and care for the litter as well as of the pig's own ability to survive and to use the available food supply.

11 LITERATURE REVIEW A wide range of heritability estimates of litter size in swine have been reported. These estimates show that the proportion of additive genetic variance is apparently quite low9 thus significant estimates of heritability have been difficult to obtain (Boylan, Rempe1 and Comstock 1961;' Urban et al. 1966; Edwards and Omtvedt 1971), Heritability estimates vary for different traits and from one experiment to another for the same trait. The average of several experiments, however, tells us something about the amount of progress we can generally expect to make by direct selection (Lasley, Day and Tribble 1970), The authors published average values for the heritability estimates of different economically important traits in swine taken from many studies made in different parts of the world. The average values were 0.15, 0.19, and 0.17 for number of pigs farrowed, number of pigs weaned, and weight of the litter at weaning, respectively. The review of swine breeding investigations made by Craft (1958) reveals numerous heritability estimates for various performance traits in swine. The average heritability estimates and their ranges reported were 0.15 ( ) for number of pigs farrowed, 0.12 ( ) for number of pigs weaned^ and 0.17 ( ) for litter weight at weaning. In an earlier report, Craig, Norton and Terrell (1956) working with records of performance of Hampshire swine, obtained heritability 3

12 estimates for birth and 21 day weight of 0.07 and 0.05, respectively, from combined regression values; while estimates of 0.28 and 0.30 were obtained from sire variance components. Craig et al. (1956) indicated that the latter estimates are likely to include larger errors than those from the other methods used since any error in the sire component of variance in these inbred litters would be multiplied by the factor of 4.26 in estimating heritability. Shelby (1967), using records of performance of Duroc swine calculated heritability estimates, derived from sire variance components, of 0.31, 0.17, and 0.19 for number of pigs, at birth, number of pigs at weaning, and for litter weight at weaning, respectively. Estimates of heritability of 0,09 ± 0,04, ,04, 0, , and of for the traits litter size at birth, litter size at one day, litter size at 56 days, and total litter weight at 56 days, respectively, were obtained by Urban et al. (1966), The finding that these estimates for litter size were significantly different from zero at the five percent level does not support the hypothesis that low estimates represent random fluctuations of a non-heritable (narrow sense) trait. Louca and Robison (1965) analyzed genetic parameters from records of performance of purebred and crossbred animals of the Duroc and Yorkshire breeds of swine. Heritability estimates from paternal half-sib correlations were essentially zero for birth weight. Heritability estimates computed from daughter-darn regressions were 0.05 and 0,19 for litter size at 0 and 56 days, respectively.

13 Results quite similar to these were obtained by Edwards and Omtvedt (1971), heritability. They compared different procedures to estimate Estimates obtained from regression of individual offspring on mid-parent were intermediate between those from regression on sire, and regression on dam. The reported values of heritability estimates for number of pigs per litter at birth and weaning, and litter weight at birth and weaning, based on daughter-darn regression were 0,01 ± 0,14, 0,24 ± 0,15, 0,27 + 0,15, and 0,29 ± 0,16, respectively, Edwards and Omtvedt (1971) pointed out that pig weight at birth was largely a reflection of maternal environment. In a recent study by Revelle and Robison (1973), the linear regression of daughter on dam resulted in a heritability estimate of ,06 for litter size at birth. However, the heritability estimate from granddaughter-granddam regression was 0,28 + 0,26. The first estimate agrees with the figures reported previously by Boylan et al, (1961) and Urban et al. (1966), while the latter estimate indicates a negative maternal effect on litter size, A negative maternal environmental effect on litter size would result in low heritability and ineffective selection for increased litter size because females from larger litters are unable to express their genetic superiority (Revelle and Robison 1973). However, it has been reported in the literature that selection for litter size has given positive results. Siler and Fiedler (1972) reported an annual genetic increase of litter size of 0.14 pig, and of 0.43 kg for litter weight at

14 6 three weeks of age. This was estimated on the basis of selection differentials and response during seven years of selection. In the case of litter size in swine, one heritability estimate for all sizes of litters seems to give only a partial indication of what might be the true genetic situation (Urban et al, 1966, Revelle and Robison 1973), A major obstacle to the estimation of real genetic parameters is the effect of various environmental influences on individual records of an animal s performance (Shelby 1967), Urban et al, (1966) reported that age of the sow showed a highly significant quadratic effect, while Shelby (1967) indicated that age of the dam has a curvilinear effect on litter size at birth. Urban et al, (1966) also found a cyclic effect in parity (first and third litters better than second and fourth) because of condition of the sow, Shelby (1967) reported that litter size increased rapidly with age of sow from 10 to 24 months and then less rapidly until a peak was observed at 36 months. Thus, fertility, mothering ability and milk production change rapidly with an increase of age of dam when the sows are young, but change only slightly at older ages. Reddy, Lasley and Mayer (1958) and O Ferrall et al. (1968) reported that sire breeds do not have an effect on litter size and litter weight. However, the results of Johnson and Omtvedt (1973) indicated that sire breeds do differ in their effect on litter size and litter weight, but that this effect depends on the specific breed of dam involved in the mating. This suggests specific combining ability for these traits among the breeds involved.

15 Johnson and Omtvedt (1973) found that breed of dam had a signifi^- cant effect on all the traits considered except for number of pigs at 7 birth. Yorkshire dams had larger litters at each age of the litter and the survival rate of pigs from Yorkshire dams was approximately 12 percent higher than the survival rate of pigs from Duroc and Hampshire dams* With regard to the effect of season on litter size, Shelby (1967) reported differences between litters farrowed in the spring and in the fall. This may be the result of general changes in weather, in rations, or in management. Urban et al. (1966) observed that the effect of season became substantially more important at weaning than at birth. Louca and Robison (1965) obtained the genetic covariances between weight and litter size. The covariances were positive in all groups (birth to 154 days), suggesting a positive genetic association between the two traits.

16 MATERIALS AND METHODS The data used In this study were compiled from the records of a commercial swine enterprise located five miles north of Hermosillo, Sonora, Mexico, The foundation animals in the breeding herd were obtained from the major established swine producer in Sonora (1000 sow unit). All of the purchased gilts were crossbred animals from a three breed rotational crossbreeding program involving the three most popular breeds (Yorkshire, Hampshire, and Duroc) which have been commonly recommended to complement each other in economically important traits such as reproduction, growth, and carcass quality. All the boars used in the breeding herds were purchased from registered purebred herds in Texas and Nebraska, in the United States, Selection is applied only on the female side. The following standards were used to select the replacement gilts: eight or more pigs raised per litter, a minimum of 12 evenly-spaced teats, and 90 kg or more in weight at six months of age. There was no selection for carcass traits. Sows which did not breed in three consecutive heat periods or farrowed less than five pigs alive were culled. No selection pressure was undertaken on the male side unless there was a physical defect that impaired reproductive performance. Litter performance from birth to weaning were the only records obtained. Because of frequent personnel changes these records were not

17 always complete. All of the animals were kept under complete confinement, Sows and gilts were hand-mated with a sire of a different breed than the sow, placing more emphasis on the so^s hair color in determining the next breed to which she was going to be bred than to the "percentage of blood" of each breed that she had. However, the producer tried to keep a rotational crossbreeding system in the following order: females sired by a Hampshire boar were bred to a Yorkshire boar. The crossbred gilts from these matings were bred to a Duroc boar. Then the crossbred gilts from these latter matings were bred to a Hampshire boar, and so on, taking advantage of the hybrid vigor in both sows and litters Sometimes, when there were too many sows in heat to be bred to a particular sire, boars were then used according to their availability, Thi did result in some cases of backcrossing instead of rotational crossbreeding. Gilts were bred to farrow initially at 12 to 14 months of age. Litters were farrowed year-round in order to maximize the use of the farrowing crates. Bred females were kept in the gestation barn and taken to the farrowing crates about the 110th day of gestation. After farrowing the sows remained in the crates with their litters until the pigs were about four weeks of age or older, unless the sow's condition indicated that weaning the litter sooner was desirable. The most commonly recommended feeding practices were carried out taking advantage of the technical assistance provided by the nutrition* ists of the different feed companies of Sonora, Also, suitable sanitation programs were used to prevent development of major disease problems

18 10 Pigs were given access to a creep during the suckling period. After weaning they were fed ad libitum in the growing and finishing barns with complete balanced rations varying according to the animals, weight until they reached the market weight of 90 kg. Boars and pregnant sows are hand-fed in order to keep them in good breeding condition. Data up to the fifth parity (beginning in the fall of 1970 and continuing to the summer of 1973) of 240 sows of the foundation stock as well as their replacements were included in this study. Records were transferred to key-punched cards for analysis. Sows were identified by number and by the breed of the boar which sired them. Thus there were three groups of dams: Hampshire (H), Yorkshire (Y), and Duroc (D), Litter performance was recorded from birth to weaning for the following traits: number of pigs farrowed alive (L.S«), weight of the litter at birth measured to the nearest kilogram (L.B.Wt,), number of pigs weaned (L.S.Wn), litter weight at weaning (L.Wn.Wt.), and average pig weight at weaning (Pig Wt.), Litter records that were incomplete for any of these traits were not used in the analysis. A total of 919 litter records of performance from nine sires were available for the statistical analysis. The litter performance traits considered in the analysis were those indicated above. Age at weaning varied from 18 to 45 days depending upon the sow's condition. The data were analyzed by least squares methods to estimate heritability by means of paternal half-sib correlation analysis, as outlined by Harvey (1960), Several models for the statistical analysis were used. Main effects estimated were breed of sire (H, Y, or D),

19 breed of dam (H, Y, or D), parity (first to fifth), sires within a breed 11 (three sires in each breed), and month of farrowing (12 months). An interaction was fitted for breed of sire by breed of dam. A continuous partial regression for age of the litter at weaning was included in the analysis of weaning traits. Two of the linear additive mathematical models used for analysis of data with unequal and disproportionate subclass number were: Model 1: Yijkl = ^ + BS^ + SWB.. +?k + where: Y ^ k i is the observed value of a given trait for the ijkl-th litter. (j, is an effect common to all observations and is the mean of the population when equal numbers exist in the subclasses. BS. SWB_ij is the effect of the i-th breed of sire. is the effect of the j-th sire within the i-th breed of sire.?k is the effect of the k-th parity of the dam. E^jki is the random error associated with the ijkl-th litter. Model 2: Y.. = u, + BS. + SWB.. + BD, + P, + (BS. x BD. ) m + ijklmno i ij k 1 i k MTn + Eijklmno

20 12 where: ub is an effect common to all observations and is the mean of the population when equal numbers exist in the subclasses. BS^ SWBj,j BD^ is the effect of the i-th breed of sire. is the effect of the j-th sire within the i-th breed. is the effect of the k-th breed of dam. is the effect of the 1-th parity of a dam. (BS. x BD, ) is the effect of the m-th interaction between the i k m i-th breed of sire and k-th breed of dam. MT^ is the effect of the n-th month of farrowing. ^ijklmno *'s t*ie random error associated with the ijklmno-th litter. \ In these analyses it was assumed that the E o 1s were normally ij and independently distributed, had an expectation of zero, and the same variance.

21 RESULTS AND DISCUSSION Table 1 indicates the distribution of class and subclass numbers for the statistical analysis of model 2, deviations for number of pigs born alive Over-all means and standard (L.S.), litter birth weight (L.S/Wt.), litter size at weaning (L.S.Wn.), litter weight at weaning (L.Wn.Wt,), and average pig weight at weaning (Pig Wt.), as well 2 as their respective estimates of heritability (h ) obtained in this study are presented in Table 2. Tables 3 and 4 summarize the analyses of variance for the five traits considered. The estimates of heritability obtained from this, study were essentially zero for all the traits considered (Table 2). These results are in agreement with those reported previously in the literature (Boylan et al. 1961, Craig et al. 1956, Louca and Robison 1965, Edwards and Omtvedt 1971). This means that genetic improvement for these traits would be very slow by direct selection. Thus only a limited amount of direct selection for litter size in swine can be justified. Dickerson et al. (1954) pointed out that it was difficult to determine whether selection required to maintain litter size is more than will occur naturally because larger litters will, on the average, leave more members in the breeding herd than small ones. The low heritability also means that more than 90 percent of the variation in these traits is due to non-additive genetic factors and to 13

22 14 Table 1. Distribution of class and subclass of variance.* numbers for the analysis Identification of Independent Variable Number of Observations Breed of Sire: Breed of Dam: Parity: (H) 313 (Y) 273 (d) 333. (H) 237 (Y) 460 (D) 222 1st 305 2nd 228 3rd th :j th 66 - Month of Farrowing: January 67 February 128 March 159. April 133 May 89 June 61 July 73 August 83 September 35 October 22 November 29 December 40 Sires within Breed: (H)

23 15 Table Continued Identification of Independent Variable Number of Observations (Y) (D) Breed of Sire x Breed of Dam: (H x H) 17 (H x Y) 195 (H x D) 101 (Y x H) 162 (Y x Y) 10 (Y x D) 101 (D x H) 58 (D x Y) 255 (D x D) 20 * Total number of observations (litters) = 919.

24 16 Table 2. Over-all means with standard deviations and heritability estimates with standard errors of litter,performance traits. Traits Mean - S.D. Heritability (%) " S.E. L. S L.S.Wt.3 (kg) b c L.S.Wri L Wn.Wt.C (kg) 54 a s Pig Wt.C (kg) *Model 1, Table 3. partial regression was fitted for weaning age at 32 days. CModel 2, Table 4.

25 17 Table 3. Mean squares from analyses of variance for birth traits. Source of Variation df Litter Size at Birth Litter Weight at Birth (kg) Breed of Sire ,0a Parity lb 116.lb Sires within Breed: (H) (Y) (D) lb 205.lb Residual ap <.05 bp <.01

26 18 Table 4. Mean squares from analyses of variance for weaning traits.. Source of Variation df Litter Weight (kg) Litter Size Avg. Pig Wt. (kg) Breed of Sire Breed of Dam Parity a 11.ob. 8.0b Month b 4.6a 7.3b Sire within Breed: (H) b 1.9, (Y) (D) Breed of Sire x Breed of Dam c Regression lb b Residual ap <.05 bp <.01 ^Continuous partial regression of dependent variable on age at weaning.

27 19 environmental factors such as feeding, management, and disease. Therefore, more attention to environmental factors that affect these traits is needed in order to improve them (Lasley et al. 1970)e Month of farrowing was significant (p <.05) for all traits studied except for litter weight at birth (Tables 3 and 4). The least squares deviations for the main effects considered in the analysis of variance are presented in Table 5. Winter months showed the highest positive least squares deviations for birth traits indicating fall as the season of highest fertility. On the other hand, the.highest negative deviations were for litters born during the fall months, suggesting an adverse effect on fertility during the summer breeding season. For other seasons, the deviations were intermediate. The effect of parity on the traits considered was always significant (p <.05). Litter weight at birth increased from the first to the third parity and then decreased slowly. Litter weight at weaning-was the highest at the first parity and then decreased slowly to the fifth parity. Litter size at weaning was higher at the first and second parity than at third and fourth parity increasing up again at the fifth parity. Average pig weight at weaning showed a cyclic variation, being higher at the first and third parity than in the second and fourth parity. These results are similar to those reported by Shelby (1967) and by Strang (1970). Breed of dam had no significant effect on any trait considered. Breed of sire was only significant for litter weight at birth (Table 3). Johnson and Omtvedt (1973) reported that sire breeds had a significant

28 20 Table 5* Least squares deviation for breed of sire, sires within breed, breed of sire x breed of dam, parity, and months. Dependent Variable Independent Variable L.S. L.Wt. L.S.Wn. ( D * (2) (3) L.Wt.Wn. (4) Pig Wt. (5) d of Sires; (H) <Y) (D) s within Breed: (H) > (Y) (D) Parity: 1st nd rd th th Breed of Sire x Breed of Dam: (H x H) (H x Y) (H x D) (Y x H) (Y x Y) (Y x D) (D x H) (D x Y) (D x D)

29 21 Table 5, - Continued Dependent Variable Independent Variable L.S. L.Wt. L.S.Wn. L.Wt.Wn. Pig Wt. (1) (2) (3) (4) (5) Month: January February March , April May June 0.2; July August September October November ,5 0.1 December ' -0.3 * (1) = litter size at birth, (2) = litter weight at birth (kg). (3) = litter size at weaning. (4) = litter weight at weaning. (5) = average pig weight at weaning (kg).

30 effect on litter size and litter weight, and indicated that this effect 22 depends on the specific breed of dam involved in the mating. However, from the present study it might be concluded that the breed of sire effect on litter birth weight was due to the high positive effect of sire #22 of the Duroc breed (Table 5)s As shown in Table 5, sire #22 of the Duroc breed sired litters with a positive deviation from the corrected least squares means of 0,5 more pigs for litter size at birth and of I.5 kg more in weight of the litter at birth. For weaning traits, sires within the Hampshire breed had a highly significant effect (p <,01) on litter size at weaning. This effect was due to sire #63 of the Hampshire breed as it is shown on Table 5, Interactions between breed of sire by breed of dam were not significant at any level for any trait (Table 4). However, the least squares deviations (Table 5) consistently showed a positive effect whenever the Yorkshire breed was involved in matings with the other two breeds. These results are in agreement with those reported by Johnson and Omtvedt (1973) which provides evidence of an excellent combining ability of the Yorkshire breed for birth and weaning traits in swine. Genetic, phenotypic and environmental correlations among the various traits are shown in Table 6, The analysis of covariance indicates positive genetic, phenotypic, and environmental correlations between litter size and litter weight at birth and at weaning (Table 6), Similar results were reported by Louca and Robison (1965), suggesting an association of these traits. As expected, correlations between litter size at weaning and average pig weight at weaning were negative.

31 23 Table 6. Genetic, phenotypic, and environmental correlations for traits considered. Trait Trait Genetic Correlation Phenotypic Correlation Environmental Correlation Litter Size at Birth Litter Size at Weaning Litter Size at Weaning Litter Weaning Weight Litter Weight at Birth Litter Weaning Weight Average Pig Weight at Weaning Average Pig Weight at Weaning General Considerations In spite of the low heritabilities of these traits, replacement stock should still be selected insofar as possible from the largest and heaviest litters. Craft (1958) indicated that a group of gilts selected because they averaged one pig more per litter than the average of the herd from which they are chosen should average about 1/6th more pigs in future litters than if no attention were paid to size of their litters in choosing which replacements to retain.

32 24 The selection of the gilts should be based on the following points. 1. Sound underline--12 or more prominent teats. 2c From families with high fertility (larger litter size) and early sexual maturity. 3. Heavy muscling and rapid rate of growth, with good feed efficiency. 4. Structurally correct, with sound feet and legs. 5. Select at approximately four months of age or 70 to 90 kg. To improve accuracy of selection, make adjustments for those environmental factors that affect the animals1 performance, allowing the producers to select their replacements on a more unbiased basis. In order to identify superior breeding stock, a very careful record-keeping system must be used, starting with a suitable method of individual animal identification. All weights and measurements must be accurately taken and recorded. For weaning traits, the best months of farrowing were those of fall and spring. In farrowing crates with controlled environment from birth to weaning, seasonal variation is still an important consideration. Selection of gilts from the largest and heaviest litters born during the summer months also selects for increased adaptability. To overcome negative environmental effects due to inclement weather, it is necessary to improve the care and management practices of the breeding animals, during adverse seasons.

33 25 An increase in the culling standards for sows would also be necessary. Sows which do not wean a litter of at least seven pigs at second parity should be culled if the size of the herd permits it; otherwise^ six pigs at weaning should be the goal of the producer. This standard would give a second opportunity to the gilts that were selected as replacements. The mating system is another tool of the breeder. In this case, females should be selected from the largest litters sired by a Yorkshire boar with outstanding records of performance in order to take advantages of the high fertility, mothering ability, milk production, and general combining ability all traits in which this breed excels. On the sire side, a linebreeding program using outstanding boars such as sire number 22 of the Duroc breed which showed a consistent advantage of 0.5 pigs more in litter size at birth and 1.5 kg heavier litter weight at birth, should improve the herd performance for these traits. Whether the superiority of this boar is due to adaptation to the environment prevailing in this region or to his genetic merit could be determined from the comparative performance of his selected daughters. An advantage of the low heritabilities of the fertility traits is that they would permit the breeder to place more emphasis on direct selection for other traits such as growth, efficiency of gain, and carcass desirability, whose additive genetic variation have been reported to be greater.(lasley et al. 1970). Nowadays with the increased demand for carcass quality, the producer should be concerned with selection for growth and carcass traits.

34 REFERENCES Boylan, W. Jo, W» E, Rempe1 and R, E* Comstock Heritability of litter, size in swine. J. Anim. Sci. 20:566. Craft, W. A Fifty years of progress in swine breeding. J. Anim. Sci. 17:960. Craig, J. V., H, W. Norton and S. W. Terrill A genetic study of weight at five ages in Hampshire swine. J. Anim. Sci. 15:242. Dickerson, G. E., C. T. Blunn, A. B. Chapman, R. M. Kottman, J. L. Krider, E. J. Warwick and J. A. Whatley, Jr Evaluation - of selection in developing inbred lines of swine. Mo. Agr. Exp. Sta. Res. Bull Edwards, R. L. and I. T. Omtvedt Genetic analysis of a swine control population. II. Estimates of population parameters. J. Anim. Sci. 32:185. Harvey, W. R Least-squares analysis of data with unequal subclass numbers. USDA ARS Johnson, R. K. and I. T. Omtvedt Evaluation of purebreds and two-breed crosses in swine: Reproductive performance. J. Anim. Sci. 37:1279. Lasley, J. F., B. N. Day and L. F. Tribble Improvement of swine through breeding. Mo. Agric. Expt. Sta, Publ. C875. Louca, A. and 0. W. Robison Heritability and genetic correlations in swine. (Abst.) J. Anim. Sci, 24:850. O'Ferrall, G. J. More, H. 0. Hetzer and J. A. Gaines Heterosis in preweaning traits of swine. J. Anim. Sci. 27:17. Reddy, V. B.,J. F. Lasley and D. T. Mayeri, Genetic aspects of reproduction in swine. Mo. Agr. Expt. Sta. Res. Bull. 666, Revelle, T. J. and 0. W, Robison An explanation for the low heritability of litter size in swine,. J. Anim. Sci, 37:668, Shelby, C. E Genetic aspects of the production registry program. J, Anim. Sci. 26:5.. 26'

35 Siler, R. and J. Fiedler Estimation of genetic gain in reproductive characters in Czechoslovakian Improved White pigs. (Abst.) A.B.A. 40:3415. Strang, G. S Litter productivity in Large White pigs. I. The relative importance of some sources of variation. Anim. Prod. 12:225 (A.B.A. 38:2740). Urban, W. E., Jr., C. E. Shelby, A. B. Chapman, J. A. Whatley, Jr., and V, A. Garwood Genetic and environmental aspects of litter size in swine. J. Anim. Sci. 25:

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