Genetic parameters for stayability, stayability at calving, and stayability at weaning to specified ages for Hereford cows1

Survival analysis for estimating lamb survival up to weaning in Harnali sheep

The present study was carried out to estimate lamb survival (in days) from birth to weaning under survival analysis using data records from 2057 Harnali lambs born to 134 sires and 623 dams between the period from 2001 to 2020. The weaning age in resourced population was 90 days from birth. The hazard ratio in terms of risk of death up to weaning was determined using Cox proportional hazards model by subjecting some fixed factors such as year of birth, sex of lamb, birth weight (kg), dam's weight at lambing (kg) and dam's age at lambing (years). The overall survivability up to weaning among lambs was 91.59% and Kaplan-Meier estimates of mean survival time up to weaning was 85.77 days. Cox proportional hazard modelling revealed that the hazards of death up to weaning was higher in male lambs [1.66, 95% confidence interval (CI): 1.22-2.26] compared with female lambs [hazard ratio (HR) = 1.00]. It was also observed that the hazards of death (HR = 0.91, 95% CI: 0.88-0.94) had decreasing trends over years. For birth weight (kg), hazard rate was 0.34 (95% CI: 0.25-0.46), which indicated that the risk of pre-weaning mortality was lower as birth weight increases. The weight and age of dams at lambing did not influence the survival time of studied population. The present findings indicated that survival time increased in studied lambs over the years and it could be increased more by giving more emphasis on better litter weight and general health aspects at farm level.

Genetic analysis for sow stayability at different parities in purebred Landrace and Large White pigs

Genetic parameters for sow stayability were estimated from farrowing records of 10,295 Landrace sows and 8192 Large White sows. The record for sow stayability from parity k to parity k + 1 (k = 1, …, 6) was 0 when a sow had a farrowing record at parity k but not at parity k + 1, and 1 when a sow had both records. Heritability was estimated by using single‐trait linear and threshold animal models. Genetic correlations among parities were estimated by using two‐trait linear–linear and single‐trait random regression linear animal models. Genetic correlations with litter traits at birth were estimated by using a two‐trait linear–linear animal model. Heritability estimates by linear model analysis were low (0.065–0.119 in Landrace & 0.061–0.157 in Large White); those by threshold model analysis were higher (0.136–0.200 & 0.110–0.283). Genetic correlations among parities differed between breeds and models. Genetic correlation between sow stayability and number born alive was positive in many cases, implying that selection for number born alive does not reduce sow stayability. The results seem to be affected by decisions on culling made by farmers.

Impact of Censored or Penalized Data in the Genetic Evaluation of Two Longevity Indicator Traits Using Random Regression Models in North American Angus Cattle

Simple Summary

Cow longevity is a key driver of the beef cattle industry profitability that can be improved through genetic and genomic selection. Censored data are commonly dealt with in genetic evaluations of longevity, which can unfavorably impact the accuracy of breeding values and the rates of genetic progress. In this study, we evaluated alternative scenarios to define the best approaches for genetically evaluating longevity in North American Angus cattle in the presence of censored data.

Abstract

This study aimed to evaluate the impact of different proportions (i.e., 20%, 40%, 60% and 80%) of censored (CEN) or penalized (PEN) data in the prediction of breeding values (EBVs), genetic parameters, and computational efficiency for two longevity indicators (i.e., traditional and functional longevity; TL and FL, respectively). In addition, three different criteria were proposed for PEN: (1) assuming that all cows with censored records were culled one year after their last reported calving; (2) assuming that cows with censored records older than nine years were culled one year after their last reported calving, while censored (missing) records were kept for cows younger than nine years; and (3) assuming that cows with censored records older than nine years were culled one year after their last reported calving, while cows younger than nine years were culled two years after their last reported calving. All analyses were performed using random regression models based on fourth order Legendre orthogonal polynomials. The proportion of commonly selected animals and EBV correlations were calculated between the complete dataset (i.e., without censored or penalized data; COM) and all simulated proportions of CEN or PEN. The computational efficiency was evaluated based on the total computing time taken by each scenario to complete 150,000 Bayesian iterations. In summary, increasing the CEN proportion significantly (p-value < 0.05 by paired t-tests) decreased the heritability estimates for both TL and FL. When compared to CEN, PEN tended to yield heritabilities closer to COM, especially for FL. Moreover, similar heritability patterns were observed for all three penalization criteria. High proportions of commonly selected animals and EBV correlations were found between COM and CEN with 20% censored data (for both TL and FL), and COM and all levels of PEN (for FL). The proportions of commonly selected animals and EBV correlations were lower for PEN than CEN for TL, which suggests that the criteria used for PEN are not adequate for TL. Analyses using COM and CEN took longer to finish than PEN analyses. In addition, increasing the amount of censored records also tended to increase the computational time. A high proportion (>20%) of censored data has a negative impact in the genetic evaluation of longevity. The penalization criteria proposed in this study are useful for genetic evaluations of FL, but they are not recommended when analyzing TL.

Using Random Regression Models to Genetically Evaluate Functional Longevity Traits in North American Angus Cattle

Simple Summary

Cattle longevity is usually defined as the duration of life of a cow from first calving to death. In addition to a longer lifespan, it is crucial that cows are productive throughout their lives. Incorporating optimal indicators of productive longevity in breeding schemes will directly improve the economic profitability of the beef cattle herd and long-term sustainability of the industry. Thus, the impact of different longevity indicators in the selection of North American Angus cattle was evaluated and optimal parameters were defined to perform the evaluations.

Abstract

This study aimed to propose novel longevity indicators by comparing genetic parameters for traditional (TL; i.e., the cow’s lifespan after the first calving) and functional (FL; i.e., how long the cow stayed in the herd while also calving; assuming no missing (FLa) or missing (FLb) records for unknown calving) longevity, considering different culling reasons (natural death, structural problems, disease, fertility, performance, and miscellaneous). Longevity definitions were evaluated from 2 to 15 years of age, using single- and multiple-trait Bayesian random regression models (RRM). The RRM fitting heterogenous residual variance and fourth order Legendre polynomials were considered as the optimal models for the majority of longevity indicators. The average heritability estimates over ages for FLb (from 0.08 to 0.25) were always higher than those for FLa (from 0.07 to 0.19), and higher or equal to the ones estimated for TL (from 0.07 to 0.23), considering the different culling reasons. The average genetic correlations estimated between ages were low to moderate (~0.40), for all longevity definitions and culling reasons. However, removing the extreme ages (i.e., 2 and >12 years) increased the average correlation between ages (from ~0.40 to >0.70). The genetic correlations estimated between culling reasons were low (0.12 and 0.20 on average, considering all ages and ages between 3 and 12 years old, respectively), indicating that longevity based on different culling reasons should be considered as different traits in the genetic evaluations. Higher average genetic correlations (estimated from 3 to 12 years old) were observed between TL and FLb (0.73) in comparison to TL and FLa (0.64), or FLa and FLb (0.65). Consequently, a higher average proportion of commonly-selected sires, for the top 1% sires, was also observed between TL and FLb (91.74%), compared to TL and FLa (59.68%), or FLa and FLb (61.01%). Higher prediction accuracies for the expected daughter performances (calculated based on the pedigree information) were obtained for FLb in comparison to TL and FLa. Our findings indicate that FLb is preferred for the genetic evaluation of longevity. In addition, it is recommended including multiple longevity traits based on different groups of culling reasons in a selection sub-index, as they are genetically-different traits. Genetic selection based on breeding values at the age of four years is expected to result in greater selection responses for increased longevity in North American Angus cattle.

Genetic Polymorphisms in LTF/EcoRI and TLR4/AluI loci as candidates for milk and reproductive performance assessment in Holstein cattle

The aim of this study was to explore the genetic polymorphisms in LTF/EcoRI and TLR4/AluI loci and their association with milk and reproductive performance in Holstein cattle. A randomly selected 800 Holstein dairy cows from two dairy farms (400 animals each) in Egypt were used. Based on the two farm records, association between LTF/EcoRI genotypes and milk performance traits (order of lactation, daily milk yield, days in milk, corrected milk at 305 day and dry period) was carried out. Meanwhile, exploring of TLR4/AluI genotypes effect was done on data for reproductive performance (age at first freshening, calving interval, number of services per conception, ovarian rebound and days open). DNA was extracted from blood samples collected from Holstein dairy cows of the both farms and restriction analysis of 301-bp PCR products of LTF gene revealed two genotypes: AA genotype (301 bp) and AB genotype (301, 201 and 100 bp). Meanwhile, restriction analysis of 382-bp PCR products of TLR4 gene digested with AluI yielded two alleles (A and B) and three genotypes (AA, AB and BB). The A allele was indicated by two bands at 300 and 82 bp, and the B allele resulted in three fragments of 160, 140 and 82 bp. There was a significant association (p ≤ 0.05) between LTF genotypes and milk performance traits except for days in milk. The TLR4 genotypes had significant effects (p ≤ 0.05) on age at first freshening, calving interval, number of services per conception, ovarian rebound and days open. Ordinal logistic regression statistical model also revealed that it is possible to calculate high reproductive performance traits and to predict favourable dairy cows based on LTF and TLR4 genotypes. This research reveals the effectiveness of LTF/EcoRI and TLR4/AluI loci as candidates for reproductive performance assessment in Holstein cattle.

Genetic parameters for fertility and production traits in Red Angus cattle

Heifer pregnancy (HPG) and Stayability (STAY) are female reproductive traits that have EPD reported by the Red Angus Association of America. Challenges arise when making genetic predictions for these traits. Specifically, HPG and STAY phenotypes can only be collected on females retained in the breeding herd and have low heritability estimates. Additionally, STAY is measured late in an animal's life. The objective of this research was to investigate the genetic relationships between HPG or STAY and 13 other traits, which included measurements of growth, carcass, ultrasound, and scrotal circumference. For STAY relationships between mature weight (MW), body condition score (BCS), teat score (TS), and udder suspension score (US) were also evaluated. Data from 142,146 and 164,235 animals were used in the analyses for HPG and STAY, respectively. Genetic relationships were investigated using a series of 2 trait animal models and a REML procedure. In all analyses, the appropriate contemporary groups were included as a fixed effect, and direct genetic as a random effect. Additional fixed effects included as follows: sex for weight, carcass and ultrasound traits, age of dam for weight traits, and age of measurement for ultrasound, carcass, BCS, udder traits, and MW. Maternal genetic effects for preweaning gain (Pre-WG), weaning weight (WW), and yearling weight (YW) were also modeled. Permanent environmental effects of the dam were modeled for the traits Pre-WG and WW. Permanent environment of the individual for the traits MW, BCS, TS, and US was also included. Heritability estimates were 0.12 ± 0.01 and 0.10 ± 0.01 for HPG and STAY, respectively. Heritability estimates for direct genetic effects of production traits were moderate to high in magnitude, maternal heritability estimates were low, and permanent environmental effects accounted for 0.00 to 0.18 of the total variation. The strongest genetic correlations were those among Pre-WGD (0.24 ± 0.08), WWD (0.18 ± 0.08), YWD (0.20 ± 0.07), ultrasound rib eye area direct (0.16 ± 0.08), and ultrasound backfat direct (0.14 ± 0.08) and HPG. The highest genetic correlations were between STAY and WWM (0.54 ± 0.05), YWM (0.36 ± 0.07), backfat (0.53 ± 0.20), marbling score (0.40 ± 0.20), UREA (0.19 ± 0.07), ultrasound backfat (0.37 ± 0.07), TS (0.30 ± 0.11), and US (0.23 ± 0.11). Relationships between HPG or STAY and other traits were minimal. These results suggest that genetic relationships exist between HPG or STAY and other more densely recorded traits.

Genome-wide association study for stayability measures in Nellore-Angus crossbred cows

Beef cow stayability is a complex, economically important trait often used as an indicator of a cow's potential lifetime productivity. Stayability is defined as capability of a cow to maintain a perfect record up to 6 yr of age. This age is commonly cited as a financial break-even point, where initial costs of cow development and maintenance are recovered by her cumulative net income from yearly calf receipts. Later-maturing Bos indicus-Bos taurus crossbred cows may experience reproductive difficulty early in life but have a high potential for a long reproductive life span. It was the objective of this study to identify genetic variants associated with measures of beef cow stayability. A population of B. indicus-B. taurus crossbred cows (n = 305) from central Texas was used. Phenotypes for various measures of stayability to 6 yr of age were produced by artificially imposing five different culling criteria on data from the population. Cows were scored either as a 1 (indicating a perfect record through 6 yr) or a 0 (indicating failure at or before 6 yr), under each criterion. Genome-wide association studies (GWAS) were conducted for each criterion using univariate procedures and prefitting the fixed effect of cow contemporary group. SNP associations for two criteria surpassed the false discovery threshold of 0.15, when a cow was scored as 0 upon her first failure to wean a calf, regardless of reason, through 6 yr (criterion 2), and when a cow was scored as 0 upon her first failure to give birth to a calf, through 6 yr (criterion 3). Associated SNP were found on bovine chromosomes (BTA) 1, 2, 5, 9, 18, and 21 for criterion 2 and on BTA 1, 5, 11, 15, and 24 for criterion 3. A critical region on BTA 5: 43-50 Mb was identified for each criterion. Due to the similarities to prior work, the tendency for B. indicus cattle to experience reproductive difficulties early in life, and due to the large proportion of cows that left the herd at an early age under these criteria, these results suggest that the associations are likely driven by an early life trait such as age at puberty or rate of heifer development.

Genotype by environment interaction for stayability of Red Angus in the United States

Bulls are used across a wide variety of environments through the use of artificial insemination. However, not all bulls rank the same for genetic merit in all environments. Sire selection could be more accurate via improved methods of characterization. The objective of this study was to evaluate the presence of genotype by environment (GxE) interaction for stayability in Red Angus in the United States. Environments were defined as nine regions within the continental United States with similar temperature-humidity indices. Stayability was defined as having a calf at age 4 given that the cow had a calf at age 2. A probit sire model was used to determine the heritability on the underlying scale. The percentage of females that calved at age 2 that also calved at age 4 ranged from 32.9 to 58.5% across regions and was 55.0% for the national data set. The heritability of stayability ranged from 0.10 to 0.57 across regions and was 0.12 for the national data set. Genetic correlations were estimated for stayability between all pairs of regions. An estimate of less than 0.80 indicates GxE at a level for concern. Genetic correlations between regions ranged from 0.32 to 0.87 and were <0.80 for 29 of 36 region pairs.

Genetic parameters for stayability to consecutive calvings in Zebu cattle

Longer-lived cows tend to be more profitable and the stayability trait is a selection criterion correlated to longevity. An alternative to the traditional approach to evaluate stayability is its definition based on consecutive calvings, whose main advantage is the more accurate evaluation of young bulls. However, no study using this alternative approach has been conducted for Zebu breeds. Therefore, the objective of this study was to compare linear random regression models to fit stayability to consecutive calvings of Guzerá, Nelore and Tabapuã cows and to estimate genetic parameters for this trait in the respective breeds. Data up to the eighth calving were used. The models included the fixed effects of age at first calving and year-season of birth of the cow and the random effects of contemporary group, additive genetic, permanent environmental and residual. Random regressions were modeled by orthogonal Legendre polynomials of order 1 to 4 (2 to 5 coefficients) for contemporary group, additive genetic and permanent environmental effects. Using Deviance Information Criterion as the selection criterion, the model with 4 regression coefficients for each effect was the most adequate for the Nelore and Tabapuã breeds and the model with 5 coefficients is recommended for the Guzerá breed. For Guzerá, heritabilities ranged from 0.05 to 0.08, showing a quadratic trend with a peak between the fourth and sixth calving. For the Nelore and Tabapuã breeds, the estimates ranged from 0.03 to 0.07 and from 0.03 to 0.08, respectively, and increased with increasing calving number. The additive genetic correlations exhibited a similar trend among breeds and were higher for stayability between closer calvings. Even between more distant calvings (second v. eighth), stayability showed a moderate to high genetic correlation, which was 0.77, 0.57 and 0.79 for the Guzerá, Nelore and Tabapuã breeds, respectively. For Guzerá, when the models with 4 or 5 regression coefficients were compared, the rank correlations between predicted breeding values for the intercept were always higher than 0.99, indicating the possibility of practical application of the least parameterized model. In conclusion, the model with 4 random regression coefficients is recommended for the genetic evaluation of stayability to consecutive calvings in Zebu cattle.

BEEF SPECIES SYMPOSIUM: Can we build the cowherd by increasing longevity of females?

Increasing longevity of beef cows by decreasing the proportion culled due to reproductive failure provides an efficient process to rebuild a cow herd and can reduce number of replacements needed to sustain a constant herd size. Rate of reproductive failure varies due to cow age, where failure in cows 2 to 4 yr of age is often greater than in cows 5 to 7 yr of age. In addition, BW of cow and calf at weaning increase as cows advance from 2 to 5 yr of age. The cumulative effect of increasing retention of young cows is improved production efficiency through decreased replacement rate and changing age structure of the herd resulting in a greater proportion of cows at maximal production potential for calf BW at weaning and cow BW at time of culling. Calculations from cow age-specific culling and BW data from commercial and research herds indicated that reducing replacement rate from 18% to 14% resulted in a 23% increase in calf BW weaned and a 2% increase in cull cow BW per pregnant replacement heifer going into the herd. Although improving longevity increases production efficiency, genetic advancement in sustained reproductive function is challenging, as it is the sequential culmination of the annual repetition of numerous discrete physiological processes, each ending in a qualitative response. Successful completion of one process is prerequisite to evaluating subsequent processes. These physiological processes are subject to nutritional threshold requirements that may vary due to genetic potential for other production traits such as milk, growth, and mature size resulting in genetic-by-nutrition interactions. This is in contrast to most traits for which EPD exist, where genetic-by-environment interactions are not considered to be significant. Extensive research concerning impact of limited nutrition on reproduction has led to recommendations that heifers and cows be fed to a threshold BW or BCS to ensure reproductive success; a process that masks nutritional interactions that might otherwise result in reproductive failure. This management approach minimizes selection for animals capable of sustained reproductive function under limited nutritional environments. Rearing and managing cows under nutritionally limited environments may lead to adaptations that result in relatively high levels of reproductive success under lower input levels. Such adaptation may improve chances for longer retention in their offspring in nutrient-limited environments.

Genetic relationship among reproductive traits in Nellore cattle

In order to achieve improvements in production efficiency in livestock, herds of high sexual precocity and good fertility are needed. These traits increase the availability of animals in herd, either for sale or selection, allowing both greater selective intensity and greater genetic progress. This study aimed at estimating genetic parameters for reproductive traits measured directly in females in order to verify whether they could be used as selection criteria for genetic improvement in Nellore cows, as well as estimating the genetic relationship among these traits and scrotal circumference (SC), the traditional selection criterion for sexual precocity in cattle. In addition to SC, stayability (STAY), number of calvings at 53 months (NC53) and heifers rebreeding (HR) were studied. The (co)variances and genetic parameters were estimated using Bayesian inference. STAY, NC53 and HR were analyzed assuming a threshold model, whereas SC was analyzed with a linear model. Heritability estimated for NC53 was 0.22, and this trait was strongly and positively correlated with STAY, meaning selection for NC53 would improve productive longevity of Nellore cows. Correlations estimated between HR and STAY (≈0.97) and between HR and NC53 (≈0.99) allow an improvement on HR rates if selection was applied to traits related to longevity. Genetic correlations among SC and female reproductive traits were positive but weak, suggesting the need to use reproductive traits directly measured in females in order to obtain greater improvements in sexual precocity and longevity.

Genome-wide association studies of female reproduction in tropically adapted beef cattle

The genetics of reproduction is poorly understood because the heritabilities of traits currently recorded are low. To elucidate the genetics underlying reproduction in beef cattle, we performed a genome-wide association study using the bovine SNP50 chip in 2 tropically adapted beef cattle breeds, Brahman and Tropical Composite. Here we present the results for 3 female reproduction traits: 1) age at puberty, defined as age in days at first observed corpus luteum (CL) after frequent ovarian ultrasound scans (AGECL); 2) the postpartum anestrous interval, measured as the number of days from calving to first ovulation postpartum (first rebreeding interval, PPAI); and 3) the occurrence of the first postpartum ovulation before weaning in the first rebreeding period (PW), defined from PPAI. In addition, correlated traits such as BW, height, serum IGF1 concentration, condition score, and fatness were also examined. In the Brahman and Tropical Composite cattle, 169 [false positive rate (FPR) = 0.262] and 84 (FPR = 0.581) SNP, respectively, were significant (P < 0.001) for AGECL. In Brahman, 41% of these significant markers mapped to a single chromosomal region on BTA14. In Tropical Composites, 16% of these significant markers were located on BTA5. For PPAI, 66 (FPR = 0.67) and 113 (FPR = 0.432) SNP were significant (P < 0.001) in Brahman and Tropical Composite, respectively, whereas for PW, 68 (FPR = 0.64) and 113 (FPR = 0.432) SNP were significant (P < 0.01). In Tropical Composites, the largest concentration of PPAI markers were located on BTA5 [19% (PPAI) and 23% (PW)], and BTA16 [17% (PPAI) and 18% (PW)]. In Brahman cattle, the largest concentration of markers for postpartum anestrus was located on BTA3 (14% for PPAI and PW) and BTA14 (17% PPAI). Very few of the significant markers for female reproduction traits for the Brahman and Tropical Composite breeds were located in the same chromosomal regions. However, fatness and BW traits as well as serum IGF1 concentration were found to be associated with similar genome regions within and between breeds. Clusters of SNP associated with multiple traits were located on BTA14 in Brahman and BTA5 in Tropical Composites.

Evolutionary demography and quantitative genetics: age-specific survival as a threshold trait

Researchers must understand how mutations affect survival at various ages to understand how ageing evolves. Many models linking mutation to age-specific survival have been proposed but there is little evidence to indicate which model is most appropriate. This is a serious problem because the predicted evolutionary endpoints of ageing depend upon the details of the specific model. We apply an explicitly quantitative genetic perspective to the problem. To determine the inheritance of dichotomous traits (such as survival), quantitative genetics has long employed a threshold model. Beginning from first principles, we show how this is the most defensible mutational model for age-specific survival and how this, relative to the standard model, predicts delayed senescence and mortality deceleration at late age. These are commonly observed patterns of ageing that heretofore have required more complicated survival models. We also show how this model can be developed further to unify quantitative genetics and evolutionary demography into a more complete conceptual framework for understanding the evolution of ageing.

Genetic parameters for stayability in Murrah buffaloes

In order to contribute to the breeding programmes of Asian water buffalo, the aim of this study was to analyse the influence of genetic effects in the stayability of Murrah dairy buffaloes. The stayability trait (ST) was defined as the female's ability to stay in the herd for one (ST1), two (ST2), three (ST3), four (ST4), five (ST5) or six years (ST6) after the first calving. The same trait was also considered as continuous and was designated stayability in days up to one (STD1), two (STD2), three (STD3), four (STD4), five (STD5) or six years (STD6) after the first calving. Data from 1016 females reared in nine herds located in the State of São Paulo, Brazil, were analysed. Statistical models included the additive genetic effect of the animal and the fixed effects of the buffalo breeding herd, birth year and birth season. Additive effects for ST were estimated by approximate restricted maximum likelihood using a threshold model, while for STD, the additive effects were estimated by restricted maximum likelihood. Heritability estimates were lower for ST, except for ST1, (0·11±0·07, 0·17±0·06, 0·23±0·06, 0·16±0·08, 0·14±0·09 and 0·16±0·10 for ST1, ST2, ST3, ST4, ST5 and ST6, respectively) when compared with STD (0·05±0·06, 0·18±0·08, 0·40±0·10, 0·49±0·11, 0·41±0·11 and 0·30±0·13, for STD1, STD2, STD3, STD4, STD5 and STD6, respectively). Considering the values of heritability and owing to the serial nature of STD to a specific age, selection for STD3 should have a favourable influence on STD to other ages.

Confirmation of quantitative trait loci using a low-density single nucleotide polymorphism map for twinning and ovulation rate on bovine chromosome 5

Traditional genetic selection in cattle for traits with low heritability, such as reproduction, has had very little success. With the addition of DNA technologies to the genetic selection toolbox for livestock, the opportunity may exist to improve reproductive efficiency more rapidly in cattle. The US Meat Animal Research Center Production Efficiency Population has 9,186 twinning and 29,571 ovulation rate records for multiple generations of animals, but a significant number of these animals do not have tissue samples available for DNA genotyping. The objectives of this study were to confirm QTL for twinning and ovulation rate previously found on BTA5 and to evaluate the ability of GenoProb to predict genotypic information in a pedigree containing 16,035 animals when using genotypes for 24 SNP from 3 data sets containing 48, 724, or 2,900 animals. Marker data for 21 microsatellites on BTA5 with 297 to 3,395 animals per marker were used in conjunction with each data set of genotyped animals. Genotypic probabilities for females were used to calculate independent variables for regressions of additive, dominance, and imprinting effects. Genotypic regressions were fitted as fixed effects in a 2-trait mixed model analysis by using multiple-trait derivative-free REML. Each SNP was analyzed individually, followed by backward selection fitting all individually significant SNP simultaneously and then removing the least significant SNP until only significant SNP were left. Five significant SNP associations were detected for twinning rate and 3 were detected for ovulation rate. Two of these SNP, 1 for each trait, were significant for imprinting. Additional modeling of paternal and maternal allelic effects confirmed the initial results of imprinting done by contrasting heterozygotes. These results are supported by comparative mapping of mouse and human imprinted genes to this region of bovine chromosome 5.