Estimation of dominance variance in purebred Yorkshire swine

We used 179,485 Yorkshire reproductive and 239,354 Yorkshire growth records to estimate additive and dominance variances by Method Fraktur R. Estimates were obtained for number born alive (NBA), 21-d litter weight (LWT), days to 104.5 kg (DAYS), and backfat at 104.5 kg (BF). The single-trait models for NBA and LWT included the fixed effects of contemporary group and regression on inbreeding percentage and the random effects mate within contemporary group, animal permanent environment, animal additive, and parental dominance. The single-trait models for DAYS and BF included the fixed effects of contemporary group, sex, and regression on inbreeding percentage and the random effects litter of birth, dam permanent environment, animal additive, and parental dominance. Final estimates were obtained from six samples for each trait. Regression coefficients for 10% inbreeding were found to be -.23 for NBA, -.52 kg for LWT, 2.1 d for DAYS, and 0 mm for BF. Estimates of additive and dominance variances expressed as a percentage of phenotypic variances were, respectively, 8.8 +/- .5 and 2.2 +/- .7 for NBA, 8.1 +/- 1.1 and 6.3 +/- .9 for LWT, 33.2 +/- .4 and 10.3 +/- 1.5 for DAYS, and 43.6 +/- .9 and 4.8 +/- .7 for BF. The ratio of dominance to additive variances ranged from .78 to .11.     

Breed group effects on milk production of Brazilian crossbred dairy cows

Lactation records (n = 2362) of 1402 crossbred cows in 22 cooperating dairy herds in southeastern Brazil were evaluated. Cows were mixtures of Zebu (Gir, Guzera, and unknown) and European breeding (mostly Holstein). Lactation milk yields were expressed as total, 3050-d, or deviated 305-d yields, either adjusted or unadjusted for days in milk (DIM). Mean DIM was 280. Arithmetic means unadjusted for DIM were 1942, 1666, and 5 kg per record. Milk yields of daughters from sires of 6/8 and 7/8 European breeds were higher than yields of daughters from sires of 5/8 European breeds when data were either adjusted or unadjusted for DIM. The differences associated with breed group of sire were only slightly reduced when records were adjusted for DIM. There was no evidence of a decline in milk yield as the fraction of European breeding of the sire increased from 6/8 to 7/8. For a given breed group of sire, whether the grandsire was purebred or crossbred had no detectable effect. These results should be useful in determining strategies for crossbreeding of dairy cows in tropical areas, particularly when crossbred sires are used.     

Breed-specific adjustment factors for reproductive traits in Duroc, Hampshire, Landrace, and Yorkshire swine

Number born alive (NBA) and litter weaning weight (LWT) can be influenced by many factors, including environment, parity, age at farrowing, lactation length, and genetic merit as well as number of pigs after transfer (NAT) and weaning age (WNAGE) for LWT. The objectives of this study were to estimate adjustment factors for NBA and LWT using all effects in the model and to refine parity effects by including age of the sow in parity 1 (P1) and parity 2 (P2). The models used included fixed effects of contemporary groups and parity/age class, random direct genetic and permanent environment effects, as well as the fixed effects of NAT and WNAGE for LWT. A large effect due to age of the sow at breeding within P1 and P2 was found and new adjustments were found to differ from previous studies. In the Yorkshire population, for example, the average P1 adjustment was 1.0 pig in this study, compared to the current .69; however, this ranges from 1.46 for the youngest P1 females to .57 for the oldest. Similarly, in P2 the average adjustment was found to be .50, with an adjustment of .99 for the youngest P2 and 0 for the oldest. Also, age of dam was found to contribute variation to P1 litter records for LWT. A residual analysis showed nonsignificant differences (P > .60) across the age classes after using the new adjustments; however, significant differences (P < .01) remained after using the current adjustments.     

Maternal performance differences between porcine stress syndrome-normal and -carrier Landrace females

Differences between porcine stress syndrome (PSS) normal (NN) and carrier (Nn) Landrace dams were determined for adjusted number of pigs born alive, adjusted number of pigs at 21 d, adjusted 21-d litter weight, proportion of pigs surviving to 21 d, and farrowing interval. Data were analyzed from a total of 841 females, 623 normal (NN) and 218 carriers (Nn) having 2,231 and 869 records, respectively. Three susceptible (nn) females from two herds were dropped from the analysis because of their small contribution to the total number of records. Frequency of the recessive PSS allele ranged from .07 to .28 in the nine herds involved in this study. Data were adjusted using Landrace breed-specific adjustments and analyzed with mixed-model derivative-free REML procedures fitting the dams' PSS genotype as a fixed effect in the model. Only females having two or more successive parities were used in the analysis of farrowing interval, resulting in a reduction of total records analyzed to 2,201 (1,564 NN and 637 Nn records) from 632 females (445 NN and 187 Nn females). No differences between NN and Nn dams were observed for adjusted number of pigs born alive, adjusted number of pigs at 21 d, adjusted 21-d litter weight, proportion of pigs surviving to 21 d, and farrowing interval. The results of this investigation indicate no significant maternal performance differences between PSS NN or Nn Landrace dams.     

Accuracy of international conversions of elite sires and cows when conversion equations are based on linear regression

Conversion equations that are based on linear regression are used widely to transform estimated breeding values (EBV) of sires for production, type, health, and management traits from the genetic base, scale, and units of measurement of an exporting country to that of an importing country. One of the major deficiencies of these regression equations is that the accuracy of converted EBV of elite sires and cows, which are of primary interest in genetic selection programs, is lower than that for average animals. In this study, it is shown both mathematically and in practical examples that the standard error (SE) of prediction of elite dairy sires can be much larger than for average sires. When more than 100 sires are used to develop conversion equations, the SE of prediction for elite AI sires is up to 10% larger than for an average sire, and, when 50 sires are used to develop conversion equations, the SE of prediction is up to 25% larger than for an average sire. When fewer than 50 sires are used to develop conversion equations, the accuracy of converted EBV of elite sires is very poor, and SE can be 30 to 60% larger than for an average sire. Based on this study, it is recommended that international sire evaluations based on BLUP methodology (rather than linear regression) be made available as soon as possible for nonproduction traits in all countries and for production traits in countries that currently do not participate in routine INTERBULL (International Bull Evaluation Service) analyses.     

Genetics of limb development and congenital hand malformations

Differences in breeding values between dominance and additive models were examined theoretically and with field data. Data included 5.2 million records on stature from 3.0 million US Holsteins. The largest full-sib family had 29 animals, and 7% of all animals had at least one full sib. The dominance model, which accounted for dominance covariances, included the following effects: management, age, stage of lactation, permanent environment, animal additive, and parental dominance (one-quarter of dominance variance) as well as a regression coefficient for inbreeding percentage. Two reduced models were also assumed; in the first, the parental dominance effect was removed, and, in the second, the inbreeding regression coefficient was also removed. The correlations between breeding values in the three models were > 0.999, but breeding values of some animals from full-sib families changed > 5 standard deviations of parental dominance. The largest changes were observed for parents with large numbers of full-sib progeny, with limited information from parents, and without individual performance records. On average, the differences were up to four times larger for cows than for bulls and up to five times larger for dams than for sires. The greatest differences in breeding values between the dominance and the additive models were observed for dams with full-sib progeny, female full sibs, and low reliability bulls with full sibs in the extended family. Animals with large amounts of additive information as progeny-tested bulls were influenced little by the inclusion of dominance. Animals with a large proportion of information coming from animals with dominance relationships, such as cows originating via embryo transfer changed the most.     

A comparison of breeding value predictors for longevity using a linear model and survival analysis

A comparison was made among breeding values of sires for longevity that were obtained by different methods: phenotypic averages of daughters using only uncensored records, BLUP using only uncensored records, survival analysis using only uncensored records, and survival analysis using both censored and uncensored records. Two data files were used: one contained data from small herds, and the other contained data from large herds. The results from both data files were similar. Different methods of predicting breeding values resulted in different rankings of sires. The results obtained using phenotypic averages were weakly correlated (< or = 0.46) with those results obtained using the other methods of prediction. The REML BLUP had strong correlations (< or = -0.91) with the survival analysis predictor if the same data were used, and correlations weakened (< or = -0.60) when censored records were included in the survival analysis. The correlations are negative because the linear method analyzed longevity, and survival analysis measured the risk of being culled, which has an antagonistic relationship with longevity. The results from REML BLUP and survival analysis methods differed mainly because of the different data that were used (uncensored only versus both censored and uncensored).     

Effect of ignoring random sire and dam effects on estimates and standard errors of breed comparisons

Data were weights of F1 calves and weaning weights of top-cross progeny from sires and maternal grandsires of 13 breeds. Three analyses were performed on each trait to obtain estimates and standard errors of breed effects needed to calculate across-breed EPD and accuracies. Model (R) for records of F1 progeny contained fixed effects for birth year and date of birth, sex, age and breed of dam, and breed of sire, and a random residual effect. The second analysis included random effects for sires (RS), and the third analysis included random effects for sires and dams (RSD). In maternal analysis of top-cross progeny, model (Rm) contained fixed effects for cycle of experiment, age of dam, year of birth, sex, breeds of maternal grandam and grandsire, and breed of sire, and a random residual effect. In addition, the second and third analyses fit random effects for maternal grandsires (RSm) and for maternal grandsires and daughters of maternal grandsires (RSDm). Estimates of breed of sire effects changed only slightly for different models. Total variance increased in RSD and RS relative to R. Standard errors of breed of sire comparisons were underestimated with Model R, compared to Models RS and RSD. Standard errors of other contrasts were generally not affected. Variance components, breed effects, and standard errors followed patterns for Rm, RSm, and RSDm similar to those for R, RS, and RSD. Ignoring random variation due to sires and dams underestimated standard errors of breed of sire comparisons.     

Germplasm evaluation in beef cattle-cycle IV: birth and weaning traits

Gestation length, unassisted calving percentage, perinatal mortality, calf crop weaned (survival from birth to weaning), birth weight, and 200-d weaning weight of 2,597 calves born and 2,433 calves weaned are reported for F1 crosses resulting from matings of Angus, Hereford, Charolais, Gelbvieh, Pinzgauer, Shorthorn, Galloway, Longhorn, Nellore, Piedmontese, and Salers sires to Angus and Hereford dams (> or = 3 yr of age) in Cycle IV of the Germplasm Evaluation (GPE) Program at the U.S. Meat Animal Research Center. Hereford and Angus sires included 1) reference sires born from 1963 to 1971 used in previous cycles of the GPE Program, 2) sires born from 1982 to 1985 (1980s), and 3) sires born from 1983 to 1985 used in natural service clean-up matings. Effects of sire breed of calf were significant for gestation length, unassisted calving percentage, birth weight, and 200-d weaning weight. Gestation length was significantly longer for Nellore than for Charolais, Galloway, Longhorn, Piedmontese, and Salers, which were in turn longer than for Hereford-Angus and Shorthorn. Rankings for birth weight tended to be inversely related to those for calving ease, except for Shorthorn and Salers sires, which required low assistance relative to their heavy birth weights. Rankings for 200-d weight among AI-sired progeny were as follows: Charolais (231.3), Nellore (229.7), Salers (225.5), Shorthorn (223.8), 1980s Hereford-Angus (223.1), Piedmontese (220.0), Galloway (209.5), reference Hereford-Angus (210.1), and Longhorn (199.0); differences > or = 6.5 kg were significant.     

Effect of tryptophan and of glucose on exercise capacity of horses

A method based on discounted income and risk assessment was developed to aid in the selection of dairy sires. The discounted profit generated from milk production of daughters was proposed as the suitable composite selection criterion to combine estimates of predicted transmitting ability (PTA) for yields of milk, fat, and protein and estimates of sire evaluations for dystocia or expected progeny difference. Steps are described to derive discounted profit (defined as expected income) for a sire with known PTA and evaluation for dystocia. The derivation of profit considered costs for semen, dystocia, heifer raising, production and maintenance of the daughter, and income from milk. Variance of income from a sire depended on the reliability of his PTA and evaluation for dystocia. Total variance from a selected set of sires was defined as the risk. A quadratic programming procedure was developed to identify the best set of sires from a given pool of sires that met a desired expected income goal with minimum risk. Combinations of sires with minimum risk for all possible levels of expected income were defined by the expected income variance frontier. The set of sires at the maximum lower boundary for 95% confidence of the expected income variance frontier was defined as the optimum set of sires to be selected; the optimum set maximized the 95% guaranteed expected income. The quadratic programming procedure provided the optimum percentage of cows to be mated to each sire in the selected set.     

Sex ratio variation between ejaculates within sire evaluated by polymerase chain reaction, calving, and farrowing records

Ejaculates from sires were examined by polymerase chain reaction to determine percentage of sperm bearing the Y chromosome. Results were verified by examining the percentage of male calves per ejaculate used in artificial insemination (AI) and the percentage of male piglets per litter from a controlled mating program. Spermatozoal DNA was amplified by polymerase chain reaction with specific primers for the Y chromosome. Image analysis measured the fluorescent intensity of the 194-bp band. Ejaculates were compared with a pooled standard of spermatozoal DNA equated to a 50% Y-bearing sperm ejaculate. Calving data were obtained from information collected for the National Association of Animal Breeders for dystocia evaluation of cows bred to AI bulls. Breeding data were obtained from AI technician receipts. Calving and breeding data were merged on cow, sire, calving date, and breeding date. The percentage of males were calculated per sire, ejaculate, and herd combination. Farrowing data were evaluated for the percentage of male piglets per litter. Ejaculates within bulls contributed to variation (24 +/- 9.8% to 84 +/- 9.8%) in the percentage of sperm bearing the Y chromosome. Ejaculates from the same bull contributed to variation in the percentage of male calves (16.1 to 72.3%). Ejaculates from the same boar contributed to variation in the percentage of male piglets that ranged from 7.8 to 94.7%. These percentages and the results obtained by polymerase chain reaction analysis of ejaculates suggested that spermatozoa bearing X and Y chromosomes were unequally represented in ejaculates. The use of ejaculates screened by polymerase chain reaction could enhance production of the desired sex of calf.     

Influence of mating frequency on sow reproductive performance

Gilts and sows were bred one, two, or three times during a single estrous period in a commercial herd for evaluating the effect of mating frequency on reproductive performance. Estrus detection started at approximately 0630 daily by applying back pressure to females with the presence of a mature boar. Natural mating was used. Gilts detected in estrus were mated in the morning of d 1 (AM), the morning of d 1 and 2 (AM-AM), and the morning and afternoon of d 1 and morning of d 2 (AM-PM-AM) for mating frequencies 1, 2, and 3, respectively. Sows were bred in the AM, AM-AM, AM-PM-AM (1), and morning of d 1 and morning and afternoon of d 2 [AM-AM-PM (2)] for mating frequency 1, 2, 3 (1), and 3 (2), respectively. Breeding events in the morning and afternoon started at approximately 0730 and 1530. Females were randomly assigned to a mating frequency. Boars were randomly assigned to each breeding event. In total, 256 gilts and 766 sows were involved in the study. Gilts with a single mating (76.5%, P = .06) and triple matings (80.4%, P < .05) had higher farrowing rates than those with double matings (65.3%). No differences (P > .1) in the farrowing rates of sows were observed between mating frequencies 1, 2, 3 (1), and 3 (2). Double-mated gilts had more (P < .03) total born (9.7 vs 8.6) and pigs born alive (9.3 vs 8.2) than did single-mated gilts. There were no differences (P > .3) in total born and pigs born alive in sows between mating frequencies. We concluded that triple-mating gilts and sows did not improve farrowing rate and litter size compared with single and double matings. There were no differences in farrowing rate and litter size between double- and single-mated sows. Gilts with double matings had a larger litter size than those with a single mating.     

Association of sex of fetus, sire, month of conception, or year of foaling with duration of gestation in standardbred mares

OBJECTIVE: To determine whether sex of fetus, sire, month of conception, or year of foaling was associated with duration of gestation in mares. DESIGN: Epidemiologic retrospective cohort study. ANIMALS: 500 foalings for 296 Standardbred mares. PROCEDURE: Data for reproductive events from 1986 to 1992 were analyzed. Analyses were conducted to determine whether duration of gestation was associated with sex of fetus, sire, month of conception, or year of foaling. RESULTS: Mean duration of gestation was 343.3 days and was significantly greater for colt fetuses (344.4 days) than for filly fetuses (342.2 days). Sire was associated with duration of gestation; gestation after mating with certain sires was consistently less than 340 days in duration, whereas duration after mating with other sires was consistently more than 350 days. Duration of gestation was associated with month of conception, decreasing by about 2.5 d/mo for mares conceiving later during the breeding season. CLINICAL IMPLICATIONS: Duration of gestation is affected by mating to specific sires and by month of conception. Stallions associated with exceptionally prolonged gestations might be used to breed mares early in the breeding season, whereas stallions associated with shorter gestations might be desirable for mares bred later in the breeding season. Preparations for impending parturition should be more effective and efficient with improved information on expected date of parturition.     

Application of canonical transformation with missing values to multitrait evaluation of Jersey type

A multitrait animal model was used to calculate predicted transmitting ability and reliabilities for final score and 15 linear type traits of 225,632 US Jersey cows. Records were adjusted for age and stage of lactation before analysis. The model contained effects for interactions of herd and date scored; year scored, parity, and age; and herd and sire; effects of permanent environment and additive genetics were also included. Of the 381,511 records included, some observations were missing for final score (8%), body depth (43%), and teat length (33%). The evaluation system used a canonical transformation, included several random effects, and estimated missing values with each iteration. Inbreeding was considered in the computations. Convergence was achieved in approximately 50 rounds of iteration. Correlations between animal and sire model predicted transmitting ability ranged from 0.56 to 0.95 and generally were higher for bulls than for cows and for more recent birth years. Genetic trend was strongly positive for dairy form, final score, and rear udder traits (height and width) and negative for udder depth. For other traits, genetic trend was small. This methodology should improve the accuracy of genetic evaluations for type traits of US Jerseys.     

Comparison of sire breed solutions for growth traits adjusted by mean expected progeny differences to a 1993 base

Records on growth traits were obtained from five Midwestern agricultural experiment stations as part of a beef cattle crossbreeding project (NC-196). Records on birth weight (BWT, n =3,490), weaning weight (WWT, n = 3,237), and yearling weight (YWT, n = 1,372) were analyzed within locations and pooled across locations to obtain estimates of breed of sire differences. Solutions for breed of sire differences were adjusted to the common base year of 1993. Then, factors to use with within-breed expected progeny differences (EPD) to obtain across-breed EPD were calculated. These factors were compared with factors obtained from similar analyses of records from the U. S. Meat Animal Research Center (MARC). Progeny of Brahman sires mated to Bos taurus cows were heaviest at birth and among the lightest at weaning. Simmental and Gelbvieh sires produced the heaviest progeny at weaning. Estimates of heritability pooled across locations were .34, .19, and .07 for BWT, WWT, and YWT, respectively. Regression coefficients of progeny performance on EPD of sire were 1.25+/-.09, .98+/-.13, and .62+/-.18 for BWT, WWT, and YWT, respectively. Rankings of breeds of sire generally did not change when adjusted for sire sampling. Rankings were generally similar to those previously reported for MARC data, except for Limousin and Charolais sires, which ranked lower for BWT and WWT at NC-196 locations than at MARC. Adjustment factors used to obtain across-breed EPD were largest for Brahman for BWT and for Gelbvieh for WWT. The data for YWT allow only comparison of Angus with Simmental and of Gelbvieh with Limousin.     

Genetic evaluation for herd life in Canada

Methods were developed for the national genetic evaluation of herd life for Canadian Holstein sires. The genetic evaluations incorporate information from survival (direct herd life) and information from conformation traits that are related to herd life (indirect herd life) after adjustment for production in first lactation to remove the effect of culling for production. Direct genetic evaluations for herd life were based on survival in each of the first three lactations, which was analyzed using a multiple-trait animal model. Sire evaluations thus obtained for survival in each of the first three lactations were combined based on their economic weights into an overall sire evaluation for direct herd life. Sire evaluations for indirect herd life were based on an index of sire evaluations for mammary system, feet and legs, rump, and capacity. A multiple-trait sire model based on multiple-trait across country evaluation methodology was used to combine direct and indirect genetic evaluations for herd life into an overall genetic evaluation for herd life. Sire evaluations for herd life were expressed in estimated transmitting ability as the number of lactations and represent expected differences among daughters in functional herd life (number of lactations); the average functional herd life was set equal to three lactations. Estimated transmitting abilities were normally distributed and ranged from 2.31 to 3.43 lactations.     

The effect of the Hurnik-Morris (HM) system on sow reproduction, attrition, and longevity

Sows were randomly selected and assigned to either the Hurnik-Morris system (HM; n = 59) or a conventional gestation crate system (GC; n = 90) before breeding. The HM system provides housing for sows in small groups. All sows were bred to Duroc x Hampshire commercial boars. A common farrowing barn with 18 farrowing crates was used for the HM and GC sows. The HM sows had less (P<.001) backfat at farrowing. Average parity per sow (P<.046), lifetime number of pigs born per sow (P<.02), lifetime number of pigs born alive per sow (P<.02), and lifetime number weaned per sow (P<.045) were higher for sows housed in the HM system. The total number of sows removed from the GC system was higher (P<.025) than the number leaving the HM system. The higher parity level and lifetime production exhibited by the sows in the HM system indicates that this system supports greater longevity and may reflect the superior effect that this housing system has on animal well-being.     

A simulation model including ovulation rate, potential embryonic viability, and uterine capacity to explain litter size in mice: I. Model development and implementation

Litter size in mice was studied using a model including ovulation rate, potential embryonic viability, and uterine capacity. Simulated results were compared with experimental results from a selection experiment with mice. The four criteria of selection were selection on number born (LS), selection on an index of ovulation rate and ova success (IX), selection on number born to unilaterally ovariectomized females (UT), and unselected control (LC). Comparisons were made to statistics of the base generation and to responses after 13 generations of selection. Phenotypic and genetic statistics for uterine capacity were generated so that simulations produced the experimental means, standard deviations, and correlations between left and right litter size, as well as responses in number born using the LS, IX, and UT criteria. Statistics for the simulated data generally agreed with observed values. Simulated heritability in the base generation for uterine capacity was .065. Experimental and simulated responses per generation in litter size through 13 generations of selection were .15 and .16, .17 and .18, and .10 and .11 for LS, IX, and UT, respectively. Simulated responses in uterine capacity after 13 generations were 2.19, 1.60, and 3.40 for LS, IX, and UT, respectively. Simulated means for the base generation were 13.22 and 16.30 for ovulation rate and uterine capacity, respectively. Uterine capacity was an important component of the variability in litter size; however, ovulation rate was the more limiting component.     

Covariance components and prediction for additive and nonadditive preweaning growth genetic effects in an Angus-Brahman multibreed herd

Estimates of covariances and sire expected progeny differences of additive and nonadditive direct and maternal genetic effects for birth and weaning weights were obtained using records from 1,581 straightbred and crossbred calves from the Angus-Brahman multibreed herd at the University of Florida. Covariances were estimated by Restricted Maximum Likelihood, using a Generalized Expectation-Maximization algorithm applied to multibreed populations. Estimates of heritabilities and additive genetic correlations for straightbred and crossbred groups were within the ranges of values found in the literature for these traits. Maximum values of interactibilities (ratios of nonadditive genetic variances to phenotypic variances) and nonadditive correlations were somewhat smaller than heritabilities and additive genetic correlations. Sire additive and total direct and maternal genetic predictions for birth and weaning weight tended to increase with the fraction of Brahman alleles, whereas nonadditive direct and maternal genetic predictions were similar for sires of all Angus and Brahman fractions. These results showed that it is feasible to evaluate sires for additive and nonadditive genetic effects in a structured multibreed population. Data from purebred breeders and commercial producers will be needed to accomplish the same goal at a national level.     

Genetic influences on reproductive performance

The potential for genetic improvement of reproductive performance in pigs is great. Improvement can be achieved by increased commercial use of F1 hybrid females produced by crossing lines with excellent reproductive performance and by selection within nucleus populations of these lines for improved reproduction as well a for growth and carcass characteristics. The value of incorporating reproductive traits in selection programs is critically dependent on the relative economic values of fat (or lean) and of extra pigs produced. There is widespread agreement that litter size is the first choice as both the selection objective and the criterion to improve reproductive performance, on economic, genetic, and ease of measurement grounds. Although there are few examples of successful response to selection for increased litter size in pigs (exceptions being the University of Nebraska selection experiment and results achieved by hyperprolific selection schemes), overall results do not suggest that selection for litter size is impossible, only that it is difficult with limited resources. Artificial insemination is being used increasingly by pig breeding companies to accelerate rates of genetic improvement and to improve dissemination. Combining data from genetically connected herds and analyzing with BLUP computer programs to make efficient use of information on relatives makes selection for litter size feasible while maintaining selection pressure for growth and carcass traits. Although selection for lifetime productivity is impractical today, the knowledge and data accumulated from successful breeding programs for litter size may help make it the selection objective of the future. Meanwhile, the search continues for useful indirect criteria for selection, from testis size to molecular markers, and scientists are working with highly prolific breeds of Chinese pigs to better understand the physiologic and genetic basis of large litter size.