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Positional, comparative candidate gene analysis and previous quantitative trait loci linkage mapping results were used to search for candidate genes affecting milk production and reproduction traits in dairy cattle. The protease inhibitor (PI) gene was chosen for examination, and 5 single nucleotide polymorphisms were detected in coding regions of the gene by direct sequencing of reverse transcription-polymerase chain reaction products from a wide range of cattle tissues. A total of 6 different intragenic haplotypes were identified in North American Holstein population, and these were examined for associations with milk production traits in 24 half-sib families comprising 1007 sons utilizing a granddaughter design. One common haplotype was associated with increased milk and fat yields, increased productive life, and decreased somatic cell score. Another common haplotype was associated with decreased productive life and increased somatic cell score. One rare haplotype was associated with decreased milk, fat, and protein yields and increased milk protein percentage; another rare haplotype was associated with decreased milk yield, increased protein percentage, and decreased productive life. The observation that the PI gene is associated with analogous traits in humans demonstrates the effectiveness of the positional comparative candidate gene analysis that utilizes information about genes present in chromosomal regions with conserved synteny in other species.

Suboptimal innate and immune mechanisms of host resistance during the peripartum period may contribute to increased incidence of mastitis. To evaluate associations between antibody response to ovalbumin and milk production during the peripartum period, 136 Holstein cows and heifers from three herds with known antibody response profiles, were evaluated for projected 305-d milk, protein, and fat yield. Using a previously described index (Wagter et al., 2000), cows were quantitatively classified based on their profile of antibody response to ovalbumin into high, average, or low antibody response groups. The single-effect antibody response group contributed significantly to variation in fat and protein yield, but not milk yield. The interaction between antibody response and parity significantly contributed to the variation in milk, fat, and protein yields; therefore the effects of group were reported on a within-parity basis. Among first-parity cows, low responders had a higher fat and protein yield than high or average antibody responder animals. Among older cows (parity 3 or greater) milk yield was significantly higher for those in the high antibody response group compared with average and low response groups. However, no significant differences in fat or protein yields were observed between high and low antibody response groups. These results suggest the possibility to select cows for enhanced immune response with no adverse effects on yield. That first-parity cows with low antibody response produce more fat and protein may be offset by the fact that mastitis occurrence was highest in this group in two out of three herds investigated. Selection for high immune response may prove beneficial to herd life by maintaining optimal yield, yet minimizing occurrence of disease.

We implemented statistical models of Bayesian inference that included direct and maternal genetic effects for genetic parameter estimation of categorical traits by Gibbs sampling. The estimation errors and variances of estimates of animal versus sire and maternal grandsire models, of linear versus threshold models, of single-trait versus multiple-trait models, and of treating herd-year-season as fixed versus random effects in the model were compared. The results indicated that linear models yielded biased estimates of genetic parameters for categorical traits. The animal model was improper for analysis of categorical traits using a threshold model and the Gibbs sampler. Moreover, linear versus threshold models and animal versus sire-maternal grandsire models resulted in larger Monte Carlo errors and increased auto-correlations among posterior samples. Treating herd-year-seasons as random effects in the threshold models decreased the Monte Carlo error, auto-correlations, and the variances of estimates. Efficiency of the single-trait threshold sire model, as measured by the variance of the estimates, was lower than for a multiple-trait model that included a correlated continuous trait, but both estimates were unbiased. Therefore, the threshold single-trait sire and maternal grandsire model is a feasible alternative to the multiple-trait model for analysis of variance components of categorical traits affected by direct and maternal genetic factors.

The objective of the study was to calculate phenotypic relationships between energy balance in early lactation and health and reproduction in that lactation. Data were 26,701 daily records of dry matter intake and milk production, periodic measures of milk composition and body weight, and all health and reproductive information from 140 multiparous Holstein cows. Daily energy balance was calculated by multiplying feed intake by the concentration of energy of the ration and subtracting the amount of energy required for maintenance (based on parity and body weight) and for milk production (based on yield and concentrations of fat, protein, and lactose). Six measures of energy balance were defined: mean daily energy balance during the first 20, 50, and 100 d of lactation; minimum daily energy balance; days in negative energy balance; and total energy deficit. Measures of health were the numbers of occurrences of each of the following during lactation: all udder problems, mastitis, all locomotive problems, laminitis, digestive problems, and reproductive problems. Reproductive traits were the number of days to first observed estrus and number of inseminations. Several significant relationships between energy balance and health were observed. Increased digestive and locomotive problems were associated with longer and more extreme periods of negative energy balance.

A quantitative approach was developed to classify Holstein cows and heifers based on phenotypic variation of serum antibody response and to determine associations with peripartum mastitis. Using an index, 136 cows and heifers were classified into high (Group 1), average (Group 2), or low (Group 3) antibody groups following immunization with ovalbumin at wk -8, -3, and 0 relative to parturition. The ranking of groups based on the quantitative index of serum antibody response to ovalbumin were similar for sera and whey antibody such that Group 1 > Group 2 > Group 3. Animals were also vaccinated with Escherichia coli J5 (Rhône Mérieux, Lenexa, KS) at wk -8 and -3 relative to parturition. The ranking of groups for E. coli J5 was similar to that observed for serum and whey antibody to ovalbumin. Serum and whey IgG1 and IgG2 concentrations were measured at wk 0, 3, and 6 but differences between groups were not significant. There was no occurrence of mastitis for Group 1 animals in two of the herds. In contrast, Group 1 animals from the third herd had the highest occurrence of mastitis; however, these cases all occurred in first-parity heifers. According to pooled data across all herds, Group 3 animals had the highest occurrence of mastitis. Heritability estimates of serum antibody response to ovalbumin varied between 0.32 to 0.64 depending on week relative to parturition. Heritability estimates of serum antibody response to E. coli J5 also varied between 0.13 to 0.88 depending upon week relative to parturition. These results indicate that high peripartum antibody may be beneficial in some herds.

Several approaches for analysis of survival in the first three lactations were compared using data from approximately 700,000 Canadian Holsteins. Two approaches (linear model and threshold model) were used to analyze a binary measure of survival. Other approaches were survival analyses to evaluate two measures of the number of days that cows were in milk during their first three lactations. One measure restricted days per lactation to < or = 305; the other was based on the actual number of days in milk without an upper limit on days per lactation. Variance components and breeding values (EBV) were estimated. Sire models were used almost exclusively, but one set of EBV was obtained using a linear animal model. Effects in the models were herd-year of calving, age at first calving, interaction of several factors related to herd, and production. Thus, all EBV were for functional herd life. Heritabilities were approximately 0.04, 0.07, and 0.10 from linear, threshold, and survival analyses, respectively. Correlations among sire EBV from all analyses using sire models were high, particularly for linear and threshold models (0.98). In contrast, correlations of EBV from sire models with EBV from the linear animal model were less than 0.90, regardless of the approach taken. In Canada, the current linear animal model remains in use for sire evaluation of herd life, but research with survival analyses will continue.

The objective of this study was to evaluate the importance of environment, management, physiological status, and genetics on semen quality (volume of the ejaculate, sperm concentration, sperm motility, number of sperm, and number of motile spermatozoa per ejaculate) of Canadian Holstein bulls. For this purpose, semen production data from 198 bulls were analyzed using mixed linear models. Young bulls (up to 30 mo old) and mature bulls (between 4 and 6 yr old) were analyzed separately. Semen characteristics generally improved significantly with age of young bulls. Season significantly affected all semen traits in young bulls but did not significantly affect volume and sperm motility of mature bulls. Performance was better in winter than in summer. The highest numbers of motile spermatozoa per ejaculate were obtained with intervals of at least 4 to 5 d between collections. Although the bull handler and semen collector caused less than 10% of the variance, the collection team significantly affected semen volume, number of sperm, and number of motile sperm per ejaculate for both growing and mature bulls. Heritabilities for volume, concentration, sperm motility, number of sperm, and number of motile sperm per ejaculate were, respectively, 0.24, 0.52, 0.31, 0.38, and 0.49 for young bulls and 0.44, 0.36, 0.01, 0.54, and 0.64 for mature bulls. Repeatability of semen traits varied from 0.41 to 0.64. Genetics, management, and environmental factors clearly contribute to semen production in Holstein bulls.

Otherwise well-researched definitions of breeding objectives and selection criteria may never be used in practice if those definitions do not take into account the perceptions and wishes of the breeders for whom they are designed. Finding selection criteria that are widely accepted and implemented is a daunting task that requires considerable time and full interaction between the scientists and the industry. We review various aspects of this process and, in particular, how scientific principles can be used to ensure that the outcome best meets both the perceptions and needs of the users while remaining as close as possible to the technical economic optimum. Alternative methods of presenting and delivering selection indexes, such as index expression, index formulation, focus on response to selection rather than on index weights, construction of component indexes, and the use of direct accounting for costs of constraints rather than rescaling methods can all help in improving acceptance of an index. Development and implementation of selection criteria also involve consideration of the selective mating decisions that form an integral part of selection decisions in the field. The technical basis of factors that foster emphasis on individual mating decisions in the field are discussed in relation to formulation of the breeding goal and selection index and in relation to nonlinear economic and genetic parameters. Strategies that focus on use of a linear index for the selection of sires and dams followed by selective mating of selected parents have the greatest potential for implementation in the industry. We focus on examples taken from the Canadian dairy industry, but principles apply generally.

A total of 4368 first lactation records for Holstein cows from 30 herds was used to estimate genetic parameters for yield, conformation traits, and the binary coded disease traits of udder edema, milk fever, retained placenta, metritis, displaced abomasum, ketosis, cystic ovary, mastitis, and lameness. Data on health, parentage, and yield came from an on-farm program for record keeping and management. Test day production data were obtained from British Columbia DHI. Type classification data were received from the Holstein Association of Canada. Heritabilities of disease traits were low ranging from 0 to 0.05. Exceptions were lameness (0.16) and ketosis (0.39). Correlations of disease traits with 305-d milk yield and of selected type traits with retained placenta, displaced abomasum, mastitis, and lameness were estimated. Phenotypic correlations did not substantially differ from 0 except for the correlation between lameness and rear leg set (0.37). Genetic correlations between disease traits and milk yield were mostly positive (0.02 to 0.44). Only retained placenta had a negative genetic correlation with milk yield (-0.28). Genetic correlations ranged from 0 to 0.37 between udder conformation traits and mastitis, from -0.38 to 0.09 between leg conformation traits and lameness, and from -0.11 to 0.38 between rump conformation and retained placenta. The results suggest that selection based solely on yield may increase the incidence of disease. Selection on conformation traits can help reduce the incidence of disease, although genetic correlations are low.

Scores for clinical lameness from two separate studies were combined, and genetic parameters were estimated based on linear and threshold models. Cows were from 24 herds in Minnesota, Wisconsin, and Virginia. To evaluate clinical lameness, cows were observed walking and were assigned a score between 0 and 4 (where 0 = no observable problems to 4 = inability to walk). Data included 1624 records on 1342 cows. The models included fixed effects of herd visit, parity, and stage of lactation. Random effects were additive genetics, permanent environment to account for repeated records, and residual. Estimates of heritability were 0.10 and 0.22 from the linear and threshold models, respectively. The correlation between ETA from linear and threshold models based on all animals was 0.974. Deregressed ETA of sires and REML were used to estimate genetic correlations between clinical lameness and conformation traits. Among the type traits, foot angle, rear legs (rear view), and rump width had strongest associations with clinical lameness; absolute values for genetic correlations between these traits and clinical lameness were approximately 0.65. Low foot angle, hocking in, and wide rumps were associated with increased clinical lameness. Correlations with strength and body depth ranged from 0.20 to 0.43, indicating that heavier cows were more prone to clinical lameness.

Genetic parameters of subjectively scored milking speed and somatic cell score were estimated using REML and a sire model. Approximately 250,000 records were used. Heritabilities were 0.15 for milking speed and 0.14 and 0.16 for lactation mean somatic cell score (SCS) for first and second lactations, respectively. Genetic correlations between milking speed and SCS were 0.41 and 0.25 for first and second lactations, respectively, indicating that faster milking was associated with increased SCS. Genetic parameters for milking speed, SCS, and udder conformation were estimated using REML and an animal model. Records from approximately 120,000 cows were used. Genetic correlations were greatest for udder depth (-0.26) with SCS and for width of rear udder attachment (-0.24) with milking speed. An udder health index for use in sire selection was developed for an aggregate genotype that included subclinical mastitis in lactations 1 and > or = 2, clinical mastitis in lactations 1 and > or = 2, and milking time. Respective economic weights were -$12, -$31, -$15, -$59, and -$11 per genetic standard deviation. Traits in the selection index were milking speed, udder conformation, and SCS in first and later lactations. Standardized weights for a simple index for sires based on estimated breeding value from 50 daughter records were 5.5, -1.2, 3.5, -3.9, and -8.7 for udder depth, front teat length, milking speed, and SCS for first and later lactations, respectively. The accuracy of the index was 0.776, an increase of 15% over an index with only SCS.

Substantial evidence indicates that innate and acquired defense mechanisms are lowest from 3 wk precalving to 3 wk postcalving. This lowered responsiveness includes aspects of systemic and mammary gland immunity that may account, at least in part, for the increased incidence of peripartum disease. The physical and metabolic stresses of pregnancy, calving, and lactation may contribute to this decrease in host resistance and the subsequent increase in disease incidence. However, variation among cows in their host resistance mechanisms suggests that genotype and phenotype may possibly be used to identify cows that are able to mount beneficial immune responses over the periparturient period. Our own studies suggest that cows may be categorized as high or low responders based on the peripartum antibody responses to ovalbumin and Escherichia coli J5. Low responders were hyporesponsive to these test antigens and had a higher incidence of peripartum diseases, particularly mastitis. In many species, a functional link exists between the immune and endocrine systems, and, during periods of stress or physical injury, neuropeptides and neuroendocrine hormones function as immunomodulators. Initial investigations of peripartum cows reveal positive relationships between growth hormone kinetics and profiles of antibody response. Whether hormone fluctuations during the periparturient period are responsible for the alterations observed in immune responsiveness remains uncertain.

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.

Relationships between measures for reproductive performance and net revenue were studied using data that had been generated by a stochastic model in a computer simulation program for Ontario dairy herds. These measures included projected calving interval, involuntary culling rate, and adjusted calving interval. Adjusted calving interval was calculated by dividing the projected calving interval for pregnant cows by the fraction of the cows that were not culled for reproductive failure. The regression of adjusted calving interval on net revenue had an R2 of 0.72, which was higher than the R2 of 0.59 obtained by the regression of projected calving interval on net revenue. Hence, the estimation of financial losses from suboptimal reproductive performance was more accurate when adjusted calving interval was used as a measure of this performance than when projected calving interval was used. This difference is because projected calving interval did not consider cows that were culled for reproductive reasons, but those cows contributed to a reduction in profit because of suboptimal reproductive performance. The highest R2 (0.78) was obtained with a model that included projected calving interval and involuntary culling rate. However, use of that model might not be practical because herd operators differ in their ability to distinguish between involuntary and voluntary culling. The mean reduction in net revenue from a 1-d increase in adjusted calving interval was estimated at $4.7 (Canadian) per cow. This economic value was not significantly affected by level of adjusted calving interval, carcass price, feed costs, cost of a replacement heifer, or milk price.

Effects of the interaction between type and production on two measures of functional herd life were examined for Canadian Holsteins. Data were records of survival through first lactation for 1,153,706 cows and number of lactations initiated (maximum of five lactations) for 705,930 cows. Survival data were regressed on ETA for type traits of the sire of each cow after the cows were assigned to groups with low, medium, or high production. Survival through first lactation was analyzed with a threshold model. Factors in the model included herd-year-season; age at calving; month of calving; interaction of registry status, change in herd size, and season; fat and protein production; and linear regressions of sire ETA for type within each production class. Numbers of lactations were analyzed with a linear model that also included month of last calving. Overall conformation and udder traits had the largest effects on survival through first lactation. Effects on number of lactations for feet and leg traits were about the same as for udder traits. Interactions were significant. Type traits were relatively unimportant for herd life of low producing cows. Few differences were observed in the relationships between herd life and type for medium versus high producing cows, indicating no need to increase the emphasis on type in response to current trends for greater production.

Random regression models have been proposed for the genetic evaluation of dairy cattle using test day records. Random regression models contain linear functions of fixed and random coefficients and a set of covariates to describe the shapes of lactation curves for groups of cows and for individual cows. Previous work has used a linear function of five covariates to describe lactation shape. This study compared the function of five covariates with a function of only three covariates in three random regression models. Comparisons of estimates of components of variances and covariances, as well as comparisons of EBV and their prediction errors for milk yield, were made among models. Small practical differences existed between models in all respects. The model using regressions with five covariates had a slight advantage for comparison of prediction error variances of daily yields.

A model for analyzing test day records that contains both fixed and random regression coefficients was applied to the genetic evaluation of first lactation data for Canadian Holstein dairy cows. Data were 5.1 million test day records with milk, fat, and protein yields from calvings between 1988 and 1995 from herds in four regions of Canada. Each evaluated animal received five predictions for each trait representing the random regression coefficients. From these solutions, a range of estimated breeding values for various parts of the lactation could be calculated. Three genetic measures of persistency were compared. Bulls could be selected for both yields and persistency of their daughters in whatever combination was desirable. Test day analyses could result in monthly genetic evaluations for yield traits.

A semi-stochastic model for the simulation of genetic improvement in a dairy cattle population was used to evaluate and optimize progeny-testing programs for AI firms that operate in a competitive market for semen from progeny-tested bulls with regard to number of bulls sampled and size of progeny groups. The population was serviced by four firms. The competition for market share and semen sales was determined by the relative rank of progeny-tested bulls from a firm based on EBV for a trait with a heritability of 25%. For a fixed total number of daughters from young bulls for an AI program (test capacity), optimal size of the progeny groups was highly dependent on the objective to be maximized. The rate of genetic gain was maximized with a progeny group of 57 to 61 daughters per bull, but was relatively robust to changes in size of progeny groups. The number of marketable bulls was maximized with progeny groups between 20 and 40 daughters, depending on the test capacity. However, when a relationship between price per dose of semen and EBV of marketable bulls was considered, returns from semen sales were maximized at 49 and 82 daughters per bull, respectively, for linear and quadratic functions for semen price. The critical objective, net returns from semen sales, subtracting costs of sampling bulls, was maximized for progeny groups of between 95 and 105 daughters. Optimal size of progeny groups was robust to changes in economic parameters and the breeding programs of competitors. For economic parameters that were typical for Canadian AI firms, net returns per annual cohort of young bulls were 40% higher for the optimal size of the progeny groups than for sampling with 60 daughters per bull.

An empirical stochastic model for monthly bulk milk SCC was used to determine the impact of bulk milk SSC on the probability and cost of penalties under the milk quality program of Ontario, Canada. Under this program, milk price is reduced by $1 (Canadian)/hl each time bulk milk SCC exceeds 500,000 cells/ml three times in a 4-mo period. The model included herd average bulk milk SCC, effects of season, five autoregressive terms for bulk milk SCC in previous months, and a random variable with a standard deviation that depended on herd average SCC. Probabilities of receiving a penalty were 20, 13, 7, 4, 1.5, and 0.4%/mo for herd average bulk milk SCC of 450,000, 400,000, 350,000, 300,000, 250,000, and 200,000 cells/ml, respectively. Associated average penalty costs were $0.81, 0.47, 0.17, 0.04, 0.002, and 0.000/hl, respectively. Marginal return functions for reducing herd average linear somatic cell score were derived also. Marginal values were minimal for herd average linear scores < 3. For the population, the marginal value of reducing average linear score by one unit from a mean of 3 was $19.60/yr per cow. Results indicated that efforts to reduce bulk milk SCC resulted in substantial extra milk revenues. A bulk milk SCC target of 250,000 cells/ml was advocated.

The present study estimated variance components for test day records of somatic cell score and production traits. Data consisted of 235,100 test day observations recorded between 1986 and 1994 on lactations 1 to 3 of 15,922 Holstein cows from 143 herds. Records were considered as repeated observations within a lactation and as different traits across lactations. The multiple-trait animal model for analysis included random animal additive genetic and permanent environment effects by lactation. Fixed effects included herd test date and a set of four covariables for days in lactation, estimated by parity, age, and season, which accounted for the shape of the lactation curve. Gibbs sampling chains were carried out separately for somatic cell score and milk production and fat and protein yields. Heritabilities of somatic cell score for lactations 1 to 3 were .09, .09, and .11, respectively. Genetic correlations between lactations were high (.88, .79, and .95 between lactations 1 and 2, 1 and 3, and 2 and 3, respectively). Correlations between permanent environment effects were smaller (.29, .19, and .46 between lactations 1 and 2, 1 and 3, and 2 and 3, respectively). Heritabilities and correlations between permanent environment effects were higher for production traits than for somatic cell score. Genetic correlations between lactations for production traits were similar to those for somatic cell score. Variances between lactations differed significantly, indicating that observations from different lactations should no be considered as repeated observations of the same trait.

A multiple lactation model for test day data was applied to predict genetic merit for somatic cell scores of Canadian Holsteins. The model for genetic evaluation included a fixed effect for herd test date, fixed regressions on functions of days of lactation, random effects of permanent environment within lactation, random genetic effects on animal, and residual effects. Records from the first three lactations were used and treated as different traits. Procedures for this model, developed for national genetic evaluation for somatic cell score in Canada, were found to be practical. Use of starting values from the previous genetic evaluation reduced the number of rounds necessary to reach convergence. Test day models were compared with several single-trait models based on lactation average of somatic cell score in terms of computing efficiency and ranking of animals. Differences between EBV from the test day model and EBV from a repeatability model for lactation average were small for bulls with many daughters, but differences were large with EBV from a single-trait model for first lactation average of somatic cell count. Association were desirable for EBV for somatic cell score with EBV for some udder conformation traits, but undesirable for EBV for milk and protein yield.

This study evaluated the relationships between immune response, disease resistance, and bovine leukocyte antigens, BoLA, in Holstein cows following intramammary challenge with Staphylococcus aureus. This investigation was to determine whether immune responsiveness differed between these cows and whether differences were related to expression of class I BoLA antigens, which might explain the increased resistance or susceptibility to S. aureus mastitis. Antibody responses to S. aureus in milk and serum, total IgG1 in milk, and blastogenic responses with and without concanavalin A were evaluated. The CA42 allele, previously associated with increased risk of infection, was relatively uninformative for the immune response parameters examined. Other alleles, such as W3, were associated with higher milk antibody responses to S. aureus and higher milk IgG1 postchallenge. Alleles W7, W4, and W26 were associated with lower milk IgG1 and lower antibody titers in serum postchallenge. The association reported between allele CA42 and increased risk of S. aureus mastitis did not relate to lower antibody or blastogenic responses by these cows; however, indicators of innate resistance were not examined. In addition, the different associations detected between milk and serum emphasize the importance of considering mucosal immunity and unique immunological compartments when searching for relevant genetic markers of disease resistance.

A total of 65,491 lactation means of log2-transformed SCC measures were analyzed from first lactation Holstein cows in Ontario. Effects of inbreeding on SCC were estimated by a nonadditive sire and dam model that included additive, dominance, and additive by additive genetic effects and regression of lactation somatic cell score on inbreeding coefficients of the cows. Variance components were estimated using the tildehat approximation to REML. Solutions were by iteration on data. Estimates of heritability for lactation somatic cell score in the narrow sense were .165 and in the broad sense were .203. The additive by additive component (2.5% of the total phenotypic variance) was almost twice as large as the dominance component (1.3%). The regression coefficient of lactation somatic cell score per 1% increase of inbreeding was .012. The average increase of the population mean of lactation somatic cell score caused by a 10% increase of inbreeding coefficient was estimated to be 10.5% of the original phenotypic standard deviation of 1.153. The inbreeding depression was thus relatively low, but, on average, inbred animals tended to have higher lactation somatic cell score. This study provides preliminary evidence that inbreeding is related to disease prevalence in large purebred dairy populations.

Calving ease is of economic importance in dairy cattle and should be considered in breeding programs. Economic values of direct and maternal calving ease were derived based on cost-benefit analysis and gene flow methodology. Marginal returns from dystocia reduction in primiparous dams were -$43 per phenotypic standard deviation compared with $300 for production. For an average mate, numbers of discounted expressions for sires to breed replacements were .58, .44, and .63 for direct and maternal calving ease and production. Discounted expressions for direct calving ease were higher when sires were mated to primiparous versus multiparous dams. Three alternative breeding strategies were compared: 1) separate selection of sires as mates of primiparous and multiparous cows, based on their respective optimal indexes for direct and maternal calving ease, 2) selection based on the optimal index for an average mate, and 3) selection on direct calving ease only. Strategies 2 and 3 allowed for assortative mating of sires with favorable direct calving ease to primiparous females subsequent to selection. Strategy 2 resulted in maximal economic response. Optimal standardized index weights under Canadian circumstances were approximately 100:9:7 for production:direct calving ease:maternal calving ease. Inclusion of calving ease traits had a minor effect (< .5%) on selection responses and efficiency but would provide proper guidelines to producers. Greatest benefits of genetic evaluations for calving ease were obtained through assortative mating.

Linear and nonlinear relationships between functional herd life and conformation traits were investigated by regression of functional herd life of 63,602 Quebec Holstein cows on the estimated transmitting abilities of their sires for conformation traits. Relationships with functional herd life were obtained by inclusion in regression models of covariants for the deviations of cows in production of milk and fat. Individual conformation traits had low to moderate genetic relationships with functional herd life. Relationships were strongest for traits associated with udder conformation and feet and legs, and relationships were stronger for registered than for grade herds. Nonlinear relationships were present for some traits. Indexes to predict estimated transmitting ability of sires for functional herd life, based on combinations of estimated transmitting abilities of sires for conformation traits, explained .3 to .8% of phenotypic variation in functional herd life, which, for a heritability of functional herd life of .08, amounts to 15 to 40% of genetic variation in functional herd life. For registered herds, unlike grade herds, composite traits provided significant information on functional herd life and linear conformation traits. Correlations between indexes of conformation traits to predict estimated transmitting abilities of sires for functional herd life for registered versus grade herds were low to moderate. Selection for functional herd life based on conformation traits is possible but has moderate efficiency because of low to moderate relationships between conformation traits and functional herd life.

Test day SCC records were obtained from the Ontario DHI and converted to somatic cell score using a logarithmic transformation. Adjustment factors for stage of lactation and calendar month were obtained for first and later lactations. Effects of stage of lactation were significant and followed a systematic pattern. Seasonal effects were small. Sire estimated breeding values for lactation mean adjusted test day somatic cell scores were obtained from an animal model based on first, later, and all lactations, assuming a heritability of .11 and repeatability of .27. Mean accuracy of estimated breeding values for sires with at least 30 daughters was .64 for first lactation, .76 for second to fifth, and .86 for all lactations. Genetic trends for somatic cell score were not significant. The correlation of breeding values estimated from first lactations with estimates based on later lactations was .62 for sires with at least 50 daughters, which resulted in an approximate genetic correlation of .72. Correlations of sire estimated breeding values for somatic cell score from first lactation with estimated breeding values for milk, fat, protein, fat percentage, protein percentage, and milking speed were .12, .05, .11, -.09, -.02, and .20. Correlations between sire estimated breeding values for somatic cell score and type traits were generally small, but favorable with mammary system, -.13, and fore udder, -.16, and unfavorable with dairy character, .24. Somatic cell score should be considered as an auxiliary trait in dairy cattle breeding programs.

This study evaluated relationships among intramammary challenge with Staphylococcus aureus, SCC, and susceptibility to infection and determined the genetic contribution to susceptibility of infection with special emphasis on bovine lymphocyte antigens. Intramammary challenge with S. aureus Newbould 305 was used as a model for natural infection. A total of 124 cows were challenged in all quarters with 600 cfu of S. aureus Newbould 305 in 2 ml of PBS solution. Quarter milk samples for bacteriology and SCC were taken at three times both before and after challenge. Lymphocytes were separated from whole blood and tested with a standard microlymphocytotoxicity test to determine the presence of bovine lymphocyte antigens class I alleles. One hundred and thirteen cows were negative for S. aureus at challenge, and 11 cows exhibited positive S. aureus counts in one or more quarters at challenge. Fifty-three percent of quarters from the 124 cows challenged became infected after challenge. Nineteen cows had no quarters infected; 19 cows, one; 26 cows, two; 25 cows, three; and 24 cows had four quarters infected after challenge. Average SCC before challenge was 181,000 and after challenge was 758,600. The SCC prior to challenge of cows that resisted infection were higher than prior SCC of cows that became infected (282,000 and 91,000, respectively). The reverse was true after challenge. Presence of the bovine lymphocyte antigens class I allele CA42, instead of EU28 (based on gene substitution), increased susceptibility to S. aureus infection. Heritability estimates of S. aureus susceptibility after experimental challenge were low and unstable.

Type classification records of Canadian Holsteins were investigated for evidence of heterogeneous variance across herds. Data consisted of records for 1,139,104 cows from 20,226 herds with classifications on 26 conformation traits collected from 1982 through 1992 and 338,046 cows from 9600 herds with classifications on 2 additional traits from 1990 through 1992. Phenotypic standard deviations of herd-round-classifier were fitted to a mixed model that included round, classifier, and region as fixed effects, herd size as a covariant, and herd as a random effect. Estimates of the variance components, solutions of fixed effects, and BLUP estimates for herd were obtained by maximum likelihood procedures. Repeatability of within-herd standard deviation across rounds ranged from 1.4 to 10.3% for the 28 traits. Type classification data were subsequently standardized for phenotypic standard deviations of herd-round-classifier that were derived from estimates of the fixed effects and the BLUP estimate of the herd effect. Genetic evaluations for cows and bulls were produced from adjusted and unadjusted data. Correlations between 3754 sire and 1,142,782 cow estimated transmitting abilities obtained from unadjusted and adjusted data were essentially unity. Although some evidence of heterogeneous variance existed across herds for 28 conformation traits, standardization of the classification records had only a minor effect on genetic evaluations.

Potential rates of genetic progress are limited by biological constraints, which along with genetic parameters determine the structure of breeding programs to be employed for maximum genetic improvement. The objective here is to determine whether current progeny-testing programs in dairy cattle, which have been dictated and constrained by low female reproductive rates, need to be changed to capitalize on new reproductive technologies and how these changes should be implemented. Many differences between breeding programs diminish when selection on animal model genetic evaluations across all age and population groups is adopted as a strategy. Progeny-testing schemes then evolve toward dispersed open nucleus breeding schemes when multiple ovulation and embryo transfer is used on bull-dams. Nucleus breeding schemes have been advocated to capitalize on embryo transfer technology. In nucleus breeding schemes utilizing high reproductive rates, inbreeding, rather than reproductive rate, poses a limit to genetic progress, and strategies that maximize response to selection while limiting inbreeding need to be employed. One strategy is mating each dam to several sires rather than only one sire. In vitro embryo production techniques can be used to facilitate such mating strategies. Large-scale in vitro embryo production programs, in which large numbers of embryos per female are tested in the commercial population, offer the greatest potential for genetic gain with low rates of inbreeding. Cloning has an impact mainly on methods for dissemination of genetic improvement. Breeding herds, genetically inferior to marketed clones, are needed for continuous genetic gain. Reproductive technologies offer the potential for genetic improvement. Whether new breeding programs require changes in population structure, e.g., by creation of nucleus breeding herds, depends mainly on logistics and on quantity and quality of field information.

The relationship between the relative emphasis of type and production in the sire selection decision and net farm income per cow are examined for a sample of Ontario dairy farmers. A significant positive correlation between farm profits and the genetic indices for type and production traits of selected bulls exists, but the direction of causality cannot be determined. Net farm income per cow was found to have a significant inverse association with the relative weighting between type and production in the average sire selected. In contrast, the emphasis on type relative to production was found to increase with an increase in milk yield and herd size. The results of this study show that there may be benefits to collecting the breeding, production, and economic information of selected herds. The information gathered over time would be useful for determining what factors influence breeding decisions and for determining the causality in those relationships.