Cow genotyping strategies for genomic selection in a small dairy cattle population

This study compares how different cow genotyping strategies increase the accuracy of genomic estimated breeding values (EBV) in dairy cattle breeds with low numbers. In these breeds, few sires have progeny records, and genotyping cows can improve the accuracy of genomic EBV. The Guernsey breed is a small dairy cattle breed with approximately 14,000 recorded individuals worldwide. Predictions of phenotypes of milk yield, fat yield, protein yield, and calving interval were made for Guernsey cows from England and Guernsey Island using genomic EBV, with training sets including 197 de-regressed proofs of genotyped bulls, with cows selected from among 1,440 genotyped cows using different genotyping strategies. Accuracies of predictions were tested using 10-fold cross-validation among the cows. Genomic EBV were predicted using 4 different methods: (1) pedigree BLUP, (2) genomic BLUP using only bulls, (3) univariate genomic BLUP using bulls and cows, and (4) bivariate genomic BLUP. Genotyping cows with phenotypes and using their data for the prediction of single nucleotide polymorphism effects increased the correlation between genomic EBV and phenotypes compared with using only bulls by 0.163 ± 0.022 for milk yield, 0.111 ± 0.021 for fat yield, and 0.113 ± 0.018 for protein yield; a decrease of 0.014 ± 0.010 for calving interval from a low base was the only exception. Genetic correlation between phenotypes from bulls and cows were approximately 0.6 for all yield traits and significantly different from 1. Only a very small change occurred in correlation between genomic EBV and phenotypes when using the bivariate model. It was always better to genotype all the cows, but when only half of the cows were genotyped, a divergent selection strategy was better compared with the random or directional selection approach. Divergent selection of 30% of the cows remained superior for the yield traits in 8 of 10 folds.     

Energy Nutrition and Metabolism of the Lactating Dairy Cow

The genetic trend for increasing milk yield in the dairy cow population is impressive evidence of the application of genetic principles in dairy cattle selection. Walton (31) recently predicted that by the year 2000, individual cow records will approach 31,800 kg of milk per year and 1,360 kg of milk fat. Top individual herd averages will be nearly 16,000 kg of milk and 635 kg of milk fat. If these predictions are close, the subject of energy nutrition and metabolism will remain a relevant topic for many years. It will be a continuing challenge to design feeding systems and diet formulations that support this high production. An appropriate question is what high genetic merit for milk production means to a nutritionist or physiologist. To me it means the physical capacity and the physiological drive for cows to eat large volumes of energy-dense feeds, to assume negative energy balance through much of the first trimester of lactation, and to partition most of the feed energy plus energy from body stores into milk energy.I will sketch a broad picture of energy nutrition and metabolism in the lactating cow and identify those areas in knowledge and understanding that constrain dairy scientist from assisting cows of superior genetic merit to produce milk more efficiently. I see five areas for potential improvement in use of energy by lactating cows: a) increased production, b) increased energy consumption, c) increased digestive efficiency, d) increased metabolic efficency, and e) changed energy flow to partition more dietary energy to mik and less to body reserves for a longer period during lactation. I will attempt to relate the symposium speakers’ presentations to some of these areas.     

Comparison of Culling Rates,Reasons for Disposal,and Yields for Registered and Grade Holstein Cattle

Survival of cows was investigated using data from the USDA files for daughters of 199 Holstein sires with at least 500 progeny each. Records included cows that first calved from 1965 to 1980 (n = 1,145,616). A smaller subset of data (n = 199,982) was summarized for age at first and last record, termination codes, Modified contemporary deviations of milk yield, and calving intervals. Grade cows leaving the herd were more likely sold for low production and less likely sold for dairy. Modified contemporary deviations in first lactation averaged 339 kg for grade cows and 331 kg for registered cows. Cows with only one record had lower yields in first lactation by 589 kg (grade) and 506 kg (registered). Compared with cows with more than two lactations, cows with two records had lower yields in first lactation by 199 kg (grade) and 259 kg (registered) and longer first calving intervals by 3.5 d (grade) and 5.0 d (registered). Grade and registered daughters of these sires left herds at different rates and for different reasons.     

Strategies for implementing genomic selection for feed efficiency in dairy cattle breeding schemes

Alternative genomic selection and traditional BLUP breeding schemes were compared for the genetic improvement of feed efficiency in simulated Norwegian Red dairy cattle populations. The change in genetic gain over time and achievable selection accuracy were studied for milk yield and residual feed intake, as a measure of feed efficiency. When including feed efficiency in genomic BLUP schemes, it was possible to achieve high selection accuracies for genomic selection, and all genomic BLUP schemes gave better genetic gain for feed efficiency than BLUP using a pedigree relationship matrix. However, introducing a second trait in the breeding goal caused a reduction in the genetic gain for milk yield. When using contracted test herds with genotyped and feed efficiency recorded cows as a reference population, adding an additional 4,000 new heifers per year to the reference population gave accuracies that were comparable to a male reference population that used progeny testing with 250 daughters per sire. When the test herd consisted of 500 or 1,000 cows, lower genetic gain was found than using progeny test records to update the reference population. It was concluded that to improve difficult to record traits, the use of contracted test herds that had additional recording (e.g., measurements required to calculate feed efficiency) is a viable option, possibly through international collaborations.     

Dairy Cattle Breeding Simulation Program: A simulation program to teach animal breeding principles and practices

A dairy cattle breeding simulation program (DCBSP v.4.9) has been developed to teach undergraduate and graduate students animal breeding principles associated with selection for multiple traits in dairy cattle. The current version of the program was written in FORTRAN 90, and a web-based interface was developed for the students to interact with the program in the teaching environment. This software simulates a population of dairy cattle herds and artificial insemination bulls through several generations by integrating students’ decisions about mating, culling, and selection of new heifers and bulls based on a multivariate animal mixed model evaluation and marker-assisted selection. All simulation parameters (e.g., number of herds and cows per herd, variance components, effect of genetic markers) can be defined by the administrator of the program in relation to the animal breeding course. During each running period, the program simulates the composition of each herd during a virtual year, generating new calves and new productive records and performing a genetic evaluation for all productive traits. A herd-specific productive summary of all demographic, productive, and genetic data is provided to the students at the end of each simulation period. After several running periods, the genetic trend can be evaluated, providing a realistic experience for the development of animal breeding skills that will be relevant to students with a basic knowledge of animal breeding. Earlier versions of this program have been used at several universities where it has proven to be a very useful teaching tool to illustrate the theoretical basis of animal breeding in livestock.     

Relative Efficiency of Selection Methods for Profit in Dairy Cows

Selection singly on milk yield and calculated profitability traits, selection indexes, and regression indexes were compared for the relative gains in expected profit to 72 mo of age or life time profit. Indexes consisted of five traits of the first lactation: milk, fat percent, days in milk, number of breedings, and age at first calving. The study was on 1806 Holstein cows sired by 404 bulls in 38 California herds. Genetic gain in total profit was 13 to 14% greater when selection was on estimated profit to 41 mo than on milk yield to 305 days in the first lactation. Selection on traits of estimated profit on three bases, to 305 days in first lactation, to end of first lactation, and to 41 mo of age was 2 to 14% more efficient for genetic gain in total profit than selection on corresponding traits of milk yield or selection indexes. Regression indexes were 3 to 19% more efficient than selection indexes for predicting subsequent cumulative profit. In contrast, selection indexes were 6 to 20% more efficient than regression indexes for genetic gain in total profit. Heritabilities for estimated profit and traits of milk yield in the first lactation ranged from .26 to .49 for cows with two or more calvings. Heritabilities for profit to end of first lactation and to 41 mo of age exceeded comparable milk yield traits by 15 to 27%. The economic value of a 1-mo decrease in age at first calving was equivalent to an increase of 138 kg milk yield to the end of the first lactation for gain in total profit to 72 mo of age. Similarly, the genetic effects, or relative selection value, of a 1-mo decrease in age at first calving is valued at approximately 471 kg milk.     

Breeding Practices on Selected North Carolina Dairy Farms

In 1978, 147 North Carolina dairy farms were surveyed concerning their breeding practices. Average herd size was 121 cows (range 24 to 440). Of 17,773 cows 78.5% and of 4,300 heifers 25.5% were bred by artificial insemination. Approximately 25% of dairymen indicated they had increased artificial insemination in the past 3 yr, whereas 12% reported less. A bull was used on 88% of farms. Herds having the same number of cows but using 100% artificial insemination on the milking herd averaged more days open (13.3), longer calving intervals (.44 mo), and more cows leaving the herd because of reproductive problems (8.3% vs. 5.8%) compared to herds using 80 to 99% artificial insemination. Herds using 0 to 75% artificial insemination were intermediate between the two. Herds using 100% artificial insemination and those using 80 to 99% averaged 7050 kg milk and 1.8 services per conception. Herds using 0 to 75% artificial insemination had lower milk production (480 kg). Larger herds had lower reproductive performance. Herds with higher average milk production had more services per conception. Average estimated transmitting abilities of 88 natural service sires were 371 kg milk, ?.05% fat and 10 kg fat.     

Genotype by environment interactions in fertility traits in New Zealand dairy cows

New Zealand's seasonal dairy farming system entails a condensed calving pattern with cows required to conceive within approximately 12 wk of the planned start of calving. This has resulted in strong selection for fertility through culling of nonpregnant cows and relatively strong emphasis on fertility in Breeding Worth, the national breeding objective that drives sire selection. Despite this, average herd-level fertility is highly variable across New Zealand dairy farms. We studied genotype by environment interaction in fertility-related traits, with the goal of improving selection decisions in different fertility environments. We used data from the New Zealand national dairy database, which contains records on 3,743,862 animals. Herds were classified into high-, mid-, or low-fertility categories or environments based on herd average fertility performance, and data were analyzed in 2 different ways. First, we estimated genetic parameters when the fertility trait was defined specifically for each fertility environment to determine the extent to which genetic correlations between high- and low-fertility environments differed from 1 and the extent of changes in genetic variance across environments. Second, we used simple regression to evaluate the impact of ancestral genetic merit for fertility on cow fertility phenotypes to compare the effect of changes in genetic merit on phenotypic performance between fertility environments. The genetic standard deviations of fertility-related traits were 1.5 to 3.6 times higher in low-fertility herds than in high-fertility herds, and the genetic correlations between the same fertility-related traits between the high- and low-fertility environments were moderate to high, albeit with high standard errors. The high standard errors of the correlations reflected the low heritabilities of the traits and potential problems of culling bias, particularly for traits expressed in later parities. Regression analysis revealed that the bottom 30% of herds (in terms of fertility) could achieve more than twice the benefit from selection for fertility than the top 30% of herds. Although our analyses do not support separate genetic evaluations of fertility in the different environments, they indicate that low-fertility herds could benefit more from targeted selection of sires with higher fertility estimated breeding values than from selection based solely on the multitrait national index. Conversely, high-fertility herds could focus their sire selection on traits other than fertility, provided they avoid very low fertility sires.     

Reproductive Practices and Results in Dairies Using Owner or Professional Inseminators

In study 1, information on breeding practices of owner-inseminators in 234 herds of Holstein cattle was obtained by mail survey. Improved timing of insemination during estrus and convenience were reasons dairy farmers gave for performing insemination themselves. Many also used natural service bulls or professional inseminators to breed some cows. Cows in less than half of the herds were observed specifically for estrus at scheduled intervals. Cows were frequently inseminated more than once per estrus; 65% of the inseminations were within 2 h of milking. Mean services per conception for cows in 135 herds responding to a second questionnaire were 1.7 and mean calving interval was 12.8 mo. Study 2 involved only herds and associated records in Dairy Herd Improvement. Herds using only professional inseminators were randomly selected to match similar owner-serviced responding and nonresponding tested herds in Study 1. Reproductive efficiency varied greatly among herds with a slight decrease as the size of the herd increased. The 1.70 services per conception achieved by professional technicians was only slightly better than 1.74 services per conception for direct service personnel. Dairy farmers in large herds who did not respond to a special questionnaire handled more cows per worker, had more services per conception, and had a slightly longer calving interval.     

Effect of Business and Dairy Herd Management Practices on the Variable Cost of Producing Milk

A Cobb-Douglas-type function was used to study the effect of several business and dairy herd factors on the variable cost of production per 45.4 kg milk in 410 New York State dairy herds. The model used was a recursive system of equations with milk sold per cow per year and total variable cost associated with milk production as endogenous variables. Solutions were obtained using the two-stage least squares procedure. Relationships were essentially linear between variable production cost per 45.4 kg of milk and average age of the herd, percent days in milk, average age at first calving, average body weight, average days dry, fat test, and kilograms of concentrates fed. For average days open, herd size, number of cows per worker, tillable acres per cow, percent cows leaving the herd, and capital investment per cow, relationships were curvilinear. A lower variable production cost was associated with those herds having fewer days open, younger age at first calving, higher percent days in milk, lower percent cows leaving the herd, and heavier cows. Average body weight of all cows, average days dry, cows per worker, and tillable acres per cow had very little relationship with the variable production cost.     

Invited review: Beef-on-dairy—The generation of crossbred beef × dairy cattle

Because a growing proportion of the beef output in many countries originates from dairy herds, the most critical decisions about the genetic merit of most carcasses harvested are being made by dairy producers. Interest in the generation of more valuable calves from dairy females is intensifying, and the most likely vehicle is the use of appropriately selected beef bulls for mating to the dairy females. This is especially true given the growing potential to undertake more beef × dairy matings as herd metrics improve (e.g., reproductive performance) and technological advances are more widely adopted (e.g., sexed semen). Clear breed differences (among beef breeds but also compared with dairy breeds) exist for a whole plethora of performance traits, but considerable within-breed variability has also been demonstrated. Although such variability has implications for the choice of bull to mate to dairy females, the fact that dairy females themselves exhibit such genetic variability implies that “one size fits all” may not be appropriate for bull selection. Although differences in a whole series of key performance indicators have been documented between beef and beef-on-dairy animals, of particular note is the reported lower environmental hoofprint associated with beef-on-dairy production systems if the environmental overhead of the mature cow is attributed to the milk she eventually produces. Despite the known contribution of beef (i.e., both surplus calves and cull cows) to the overall gross output of most dairy herds globally, and the fact that each dairy female contributes half her genetic merit to her progeny, proxies for meat yield (i.e., veal or beef) are not directly considered in the vast majority of dairy cow breeding objectives. Breeding objectives to identify beef bulls suitable for dairy production systems are now being developed and validated, demonstrating the financial benefit of using such breeding objectives over and above a focus on dairy bulls or easy-calving, short-gestation beef bulls. When this approach is complemented by management-based decision-support tools, considerable potential exists to improve the profitability and sustainability of modern dairy production systems by exploiting beef-on-dairy breeding strategies using the most appropriate beef bulls.     

A Field Study of Grouping Cows by Nutrient Requirements for Feeding

A field study was designed to evaluate the practicality of a system of grouping cows based on nutrient requirments for concentrations of CP and TDN. This cluster system was computerized and implemented in 10 commercial dairy herds feeding total mixed rations. Grouping recommendations based on DHIA records were provided for 12 mo. The final modified computer program was applied in retrospect to the 12 mo of production records of the herds. The cluster procedure, when compared with a milk only grouping with all other constraints identical, placed in the high groups cows 7 mo younger and 16 kg lighter, requiring .9% more TDN and .6 kg less DM. The cluster system placed in the low groups cows 9 mo older and 22 kg heavier, requiring 1.2% less TDN concentration and .8 kg more DM. Intragroup correlations for the cluster and milk systems were .59 and .41 for TDN, .65 and .57 for CP, and .19 and .48 for intake of DM, indicating that cows in clustered groups were more homogeneous in nutrient requirements than cows grouped by milk. Analysis of predicted feed cost per cow per day indicated similar feed costs for each grouping system.     

Herdmates versus Contemporaries for Evaluating Progeny Tests of Dairy Bulls

Distributions of various kinds of herdmates, defined as nonpaternally related cows calving in the same moving 5- mo herd-year-season, were studied on 113,386 first and 70,044 second lactations initiated 1966 to 1968. These data were all the lactations available from 6,428 herds and 5 major dairy breeds. From 6 to 23% of progeny in first lactation sired by bulls in artificial insemination and 9 to 25% of the naturally sired progeny with a herdmate of some age did not have contemporary herdmates in first lactation. Similar values for second lactations were 10 to 28% and 15 to 29%. Only 1 to 5% did not have herdmates of some age. For those with herdmates, the average number of contemporaries ranged from 3 to 15 but were 5 to 10 for most groups. Numbers of herdmates ranged from 12 to 37, but most groups averaged over 20. Values for second lactation were lower. Biases against AI sired first lactations caused by comparing them to selected older cows were small in three breeds (+7 to ?10 kg) but were larger for Jerseys and Brown Swiss (?86 and ?115 kg). Biases against non-AI sired first lactation cows were of similar magnitudes. Larger biases resulted from comparing first lactations to only their first and second lactation herdmates. Sire summaries based on first lactation vs. first lactation herdmates would have a larger sampling variance (about 5 to 40%) but would not be biased by culling. Sampling variances would be lower when first lactations are compared to herdmates of all ages, but biases resulting from older cows being the survivors of culling for yield would be present. A modified contemporary comparison that uses all cows with at least one herdrnate of any age but that is unbiased by culling is presented.     

Realized genetic selection differentials in Canadian Ayrshire,Jersey, and Brown Swiss dairy cattle populations

Estimated breeding values of a selection index, production, durability, health, and fertility traits from Canadian Ayrshire, Jersey, and Brown Swiss bulls and cows were used to study genetic selection differentials (GSD). The bulls and cows were born from 1950 and 1960, respectively. The GSD for the 3 Canadian dairy populations were studied along the 4-path selection model: sire-to-bull (SB), dam-to-bull (DB), sire-to-cow (SC), and dam-to-cow (DC) pathways. We also determined the variations in realized GSD due to herd and herd × year of conception in addition to the effects of some environmental factors on realized GSD of the SC and DC paths. The mean realized GSD of the DB were higher than those of other paths and were increasing for lifetime performance index, 305-d milk yield, 305-d fat yield, and 305-d protein yield in all 3 dairy cattle populations. We observed no clear trends in realized GSD for type traits in all 3 dairy cattle breeds except for the apparent increasing trends in realized GSD of mammary system, dairy strength, and feet and legs in the DB and SC paths of the Ayrshire breed. No clear patterns were observed in the realized GSD of daughter fertility in the SB, DB, and SC paths of all dairy cattle breeds. Realized GSD for somatic cell score showed increasing and favorable trends in the 3 most influential selection paths (SB, DB, and SC). Year of conception influenced realized GSD of artificial insemination bulls in Ayrshire, Jersey, and Brown Swiss dairy populations. Selection emphases for the SC path generally increased with time. There was considerable variation among herds in selection pressures applied in the SC and DC pathways but no clear association with housing system or region. This study demonstrates that variations exist among herds of minor dairy cattle breeds in their selection for economically important traits. These variations offer opportunities for further improvements in these populations.     

Length of Productive Life of Dairy Cows. 1. Justification of a Weibull Model

The length of productive life of 39,683 grade Holstein cows milked in 150 large herds in New York state between 1981 and 1986 was analyzed by modeling their hazard, which is a measure of their probability of being culled. Animals still alive when the analysis was performed were assigned a “censored” record equal to the current value of their length of productive life. The concept of hazard allows an adequate statistical treatment of these censored records. The proportional hazards models considered involve a baseline hazard function and log-linear time-dependent explanatory variables affecting culling rate. These include a herd × year effect, a stage of lactation × lactation number effect, and a within herd and level of milk production effect (normalized rank based on 305 mature equivalent milk yield).A semiparametric analysis, for which the baseline hazard function is completely unspecified (Cox's regression), showed that the assumption of proportional hazards is appropriate, that all the effects in the model are highly significant, and that the baseline hazard function can be closely approximated by a Weibull hazard function of the form λ(t) = λp(λt)^p−1. Such an approximation greatly simplifies computations and facilitates further genetic and nongenetic studies on longevity of dairy cows.     

Genetic parameters for conformation traits in herds that differ in mean final score and completeness of pedigree and performance data

The objectives of this study were to assess differences in the heritability of type (conformation) traits between herds that differ in mean final score and completeness of pedigree and performance data and to estimate genetic correlations among these environments. Measurement of subjective characteristics, such as conformation traits, may be more difficult in herds with poor management conditions, and genetic evaluation of sires using data from such herds could lead to inaccurate selection decisions. Furthermore, missing pedigree data is a significant problem in many herds, and a lack of phenotypic data from maternal relatives may reduce the effectiveness of animal model evaluation systems. These hypotheses were examined using type classification scores of 1,728,836 first-parity Holstein cows (from 54,223 sires) that calved from 1993 to 2002 in 24,207 US dairy herds. These data included 480,927 records from progeny test daughters, but only 254,891 (47%) were from dams that had valid sire identification, and only 132,953 (28%) were from dams that had also been classified. Herds were grouped into quartiles by mean classification score, percentage of known maternal grandsires, and percentage of classified dams. Estimated heritability of final score was 0.20 in herds with mean score <74.5, 0.17 in herds with <25% known maternal grandsires, and 0.19 in herds with <18% classified dams. Conversely, estimates were 0.39 in herds with mean score >78.7, 0.35 in herds with 100% known maternal grandsires, and 0.37 in herds with >71% classified dams. Estimated genetic correlations between quartiles ranged from 0.86 to 0.95. Based on this study, it appears that improvements in animal identification and data collection in progeny test herds would lead to greater accuracy and stability of genetic evaluations for conformation traits in US Holstein cattle.     

Breed of cow and herd productivity affect milk composition and modeling of coagulation,curd firming,and syneresis

Milk coagulation properties (MCP) have been widely investigated in the past using milk collected from different cattle breeds and herds. However, to our knowledge, no previous studies have assessed MCP in individual milk samples from several multi-breed herds characterized by either high or low milk productivity, thereby allowing the effects of herd and cow breed to be evaluated independently. Multi-breed herds (n = 41) were classified into 2 categories based on milk productivity (high vs. low), defined according to the average milk net energy yielded daily by lactating cows. Milk samples were taken from 1,508 cows of 6 different breeds: 3 specialized dairy (Holstein-Friesian, Brown Swiss, Jersey) and 3 dual-purpose (Simmental, Rendena, Alpine Grey) breeds, and analyzed in duplicate (3,016 tests) using 2 lactodynamographs to obtain 240 curd firming (CF) measurements over 60 min (1 every 15 s) for each duplicate. The 5 traditional single-point MCP (RCT, k₂₀, a₃₀, a₄₅, and a₆₀) were yielded directly by the instrument from the available CF measures. All 240 CF measures of each replicate were also used to estimate 4 individual equation parameters: RCT estimated according to curd firm change over time modeling (RCT_eq), asymptotic potential curd firmness (CF_P), curd firming instant rate constant (k_CF), and syneresis instant rate constant (k_SR) and 2 derived traits: maximum curd firmness achieved within 45 min (CF_max) and time at achievement of CF_max (t_max) by curvilinear regression using a nonlinear procedure. Results showed that the effect of herd-date on traditional and modeled MCP was modest, ranging from 6.1% of total variance for k₂₀ to 10.7% for RCT, whereas individual animal variance was the highest, ranging from 32.0% for t_max to 82.5% for RCT_eq. The repeatability of MCP was high (>80%) for all traits except those associated with the last part of the lactodynamographic curve (i.e., a₆₀, k_SR, k_CF, and t_max: 57 to 71%). Reproducibility, taking into account the effect of instrument, was equal to or slightly lower than repeatability. Milk samples collected in farms characterized by high productivity exhibited delayed coagulation (RCT_eq: 18.6 vs. 16.3 min) but greater potential curd firmness (CF_P: 76.8 vs. 71.9 mm) compared with milk samples collected from low-productivity herds. Parity and days in milk influenced almost all MCP. Large differences in all MCP traits were observed among breeds, both between specialized and dual-purpose breeds and within these 2 groups of breeds, even after adjusting for milk quality and yield. Milk quality and MCP of samples from Jersey cows, and coagulation time of samples from Rendena cows were better than in milk from Holstein-Friesian cows, and intermediate results were found with the other breeds of Alpine origin. The results of this study, taking into account the intrinsic limitation of this technique, show that the effects of breed on traditional and modeled MCP are much greater than the effects of herd productivity class, parity, and DIM. Moreover, the variance in individual animals is much greater than the variance in individual herds within herd productivity class. It seems that improvement in MCP depends more on genetics (e.g., breed, selection) than on environmental and management factors.     

Association of Herd Average Genetic and Environmental Milk Yield with Dairy Herd Improvement Variables

Two hundred fifteen Virginia Holstein herds were used to evaluate the relationship of genetic and environmental herd averages for mature equivalent milk yield with DHIA measures from 1976 to 1980. Herd breeding value was the average of twice the USDA Cow Index for individual cows. Means (±SD) were +243 kg (±107 kg) for genetic value and +685 kg (±757 kg) for environmental value. Herds progressed 31 kg/yr genetically and 122 kg/yr environmentally. Regressions of DHIA variables on genetic and environmental averages were within herd and year. Holding herd and year constant at +250 kg genetic milk, a 600 kg increase in environmental milk resulted in increases of 559 kg herd average milk, 19 kg fat, 13 kg genetic SD, and .08 services per conception and decreases of .64 d dry, 1.6 d open, .48 mo age at first lactation, and .05 mo calving interval. At +700 kg environmental milk, a 150 kg increase in genetic milk was associated with increases of 134 kg milk, 1.9 kg fat, and .05 services per conception; and decreases of .59 d dry, 4.8 d open, .65 mo age at first lactation, and .05 mo calving interval. Within herds, environmental improvement tended to be associated with improvement in reproductive measures, and genetic improvement was not detrimental to most measures of herd performance.     

Genetic parameters for atypical reproductive patterns in dairy cows estimated from in-line milk progesterone profiles

Our aim was to estimate genetic parameters of atypical reproductive patterns and estimate their genetic correlation with milk production and classical fertility traits for commercial dairy cows. In contrast with classical fertility traits, atypical reproductive patterns based on in-line milk progesterone profiles might have higher heritability and lower genetic correlation with milk production. We had in-line milk progesterone profiles available for 12,046 cycles in 4,170 lactations of 2,589 primiparous and multiparous cows (mainly Holstein Friesian) from 14 herds. Based on progesterone profiles, 5 types of atypical reproductive patterns in a lactation were defined: delayed ovulation types I and II, persistent corpus luteum types I and II, and late embryo mortality. These atypical patterns were detected in 14% (persistent corpus luteum type II) to 21% (persistent corpus luteum type I) of lactations. In 47% of lactations, at least 1 atypical pattern was detected. Threshold model heritabilities for atypical reproduction patterns ranged between 0.03 and 0.14 and for most traits were slightly higher compared with classical fertility traits. The genetic correlation between milk yield and calving interval was 0.56, whereas genetic correlations between milk yield and atypical reproductive patterns ranged between ?0.02 and 0.33. Although most of these correlations between milk yield and atypical reproductive patterns are still unfavorable, they are lower compared with the correlations between classical fertility traits and milk yield. Therefore selection against atypical reproductive patterns may relax some constraints in current dairy breeding programs, to enhance genetic progress in both fertility and milk yield at a steady pace. However, as long as the target trait for fertility is calving interval, atypical reproductive patterns will not add additional value to the breeding goal in the near future due to the low number of available records.     

Quantification and at-risk period of decreased fertility associated with exposure to bluetongue virus serotype 8 in naïve dairy herds

The detrimental effect of bluetongue virus serotype 8 (BTV-8) on fertility was quantified in seroconverting cows. Although the effect on individual cows provides information regarding the potential biological burden of infection, losses at a herd level are also dependent on the proportion of infected cows within the herd. The objectives of this study were to quantify the average effect of BTV-8 exposure in field conditions on the fertility of dairy cows in previously naïve herds, and to determine the at-risk period of decreased fertility related to the date of detection of the disease in the herd. The effect of BTV-8 exposure on fertility was assessed using the 90-d-return-to-service rates after the first artificial insemination (AI) calculated for cows in exposed herds (during the 2007 epizootic in France) and compared with that for cows in unexposed herds. Only herds with a confirmed detection that were reported after clinical suspicion were included. To determine the at-risk period of decreased fertility, variations of fertility in exposed herds were quantified according to the time interval between the date of AI for individual cows and the date that disease was detected in the herd. Survival analyses were used to assess the risk of decreased fertility associated with BTV-8 exposure, adjusting for the main factors known to influence fertility. The episode at risk for decreased fertility depended on the month of disease detection in the herd. For herds detected early in the epizootic, fertility was decreased for cows inseminated from 1 mo before to 1 mo after the date of disease detection in the herd. Depending on time interval between the date of AI of cows and the date of detection in the herd, the increase of return-to-service rate associated with BTV-8 exposure varied from 8 to 21 percentage points of 90-d return to service. The episode of decreased fertility is likely due to a combination of the effect of the infection at different stages of conception and early pregnancy and the delayed exposure of cows due to the spreading of the virus within herds. For herds detected during the second half of the epizootic, fertility was decreased for cows inseminated more than 2 mo before detection, which suggests a delay in the detection of clinical signs following virus introduction in the herd. No correlation was observed between the effect of BTV-8 exposure on fertility and the incidence of BTV-8 in the local geographical area. Given the duration of the period that cows were at risk for decreased fertility and the magnitude of the effect, the average BTV-8 exposure in naïve herds led to major losses.