Biomedical subjects

H D Norman

Publications and source records attributed to H D Norman.

At least 19 recordsLinked to original sources

PubMed2007

Within-herd heritability estimated with daughter-parent regression for yield and somatic cell score.

Estimates of heritability within herd (h(WH)(2) ) that were generated with daughter-dam regression, daughter-sire regression, and REML were compared, and effects of adjusting lactation records for within-herd heritability on genetic evaluations were evaluated. Holstein records for milk, fat, and protein yields and somatic cell score (SCS) from the USDA national database represented herds in the US Northeast, Southeast, Midwest, and West. Four data subsets (457 to 499 herds) were randomly selected, and a large-herd subset included the 15 largest herds from the West and 10 largest herds from other regions. Subset heritabilities for yield and SCS were estimated assuming a regression model that included fixed covariates for effects of dam yield or SCS, sire predicted transmitting ability (PTA) for yield or SCS, herd-year-season of calving, and age within parity. Dam records and sire PTA were nested within herd as random covariates to generate within-herd heritability estimates that were regressed toward mean h(WH)(2) for the random subset. Heritabilities were estimated with REML using sire models (REML(SIRE)), sire-maternal grandsire models (REML(MGS)), and animal models (REML(ANIM)) for each herd individually in the large-herd subset. Phenotypic variance for each herd was estimated from herd residual variance after adjusting for effects of year-season and age within parity. Deviations from herd-year-season mean were standardized to constant genetic variance across herds, and records were weighted according to estimated error variance to accommodate h(WH)(2) when estimating breeding values. Mean h(WH)(2) tended to be higher with daughter-dam regression (0.35 for milk yield) than with daughter-sire regression (0.24 for milk yield). Heritability estimates varied widely across herds (0.04 to 0.67 for milk yield estimated with daughter-dam regression), and h(WH)(2) deviated from subset means more for large herds than for small herds. Correlation with REML(ANIM) h(WH)(2) was 0.68 for daughter-dam and was 0.45 for daughter-sire h(WH)(2) for milk yield. The correlation between daughter-sire h(WH)(2) and REML(MGS) was greater than the correlation between daughter-dam h(WH)(2) and REML(MGS). Data adjustments had a minimal impact on breeding value bias. Within-herd heritability can be estimated rapidly using regression techniques with moderate accuracy, but adjusting lactation records for h(WH)(2) resulted in only a small improvement in the accuracy of genetic evaluations.

Animals↗

PubMed2006

Trends in calving ages and calving intervals for dairy cattle breeds in the United States.

Trends since 1980 for calving age and calving interval, 2 factors that influence herd life, were examined by parity for 5 breeds of US dairy cattle. Calving data were from cows with records that passed edits for USDA genetic evaluations and were in herds that remained on Dairy Herd Improvement test. First-calf heifers calved at progressively younger ages over time, but the age decline was less for later parities because of longer calving intervals. Breed differences for calving age were evident for all parities; current mean age at first calving ranged from 24 mo for Jerseys to 28 mo for Ayrshires. Mean calving age across all parities declined over time for all breeds, primarily because of increased turnover rate, and ranged from 48 mo for Holsteins to 54 mo for Ayrshires. Across parity, annual increase in calving interval was reasonably consistent (0.90 to 1.07 d/yr) for all breeds except Jersey (0.49 d/yr). Within parity, regressions of calving interval on year were generally similar to overall breed trend. Breed means for first calving interval across time ranged from 390 d for Jerseys to 407 d for Brown Swiss.

Aging↗

PubMed2006

Major advances in genetic evaluation techniques.

The past quarter-century in genetic evaluation of dairy cattle has been marked by evolution in methodology and computer capacity, expansion in the array of evaluated traits, and globalization. Animal models replaced sire and sire-maternal grandsire models and, more recently, application of Bayesian theory has become standard. Individual test-day observations have been used more effectively in estimation of lactation yield or directly as input to evaluation models. Computer speed and storage are less limiting in choosing procedures. The increased capabilities have supported evaluation of additional traits that affect the net profitability of dairy cows. The importance of traits other than yield has increased, in a few cases due to an antagonistic relationship with yield. National evaluations combined internationally provide evaluations for bulls from all participating countries on each of the national scales, facilitating choices from among many more bulls. Selection within countries has increased inbreeding and the use of similar genetics across countries reduces the previously available outcross population. Concern about inbreeding has prompted changes in evaluation methodology and mating practices, and has promoted interest in crossbreeding. In just the past decade, distribution of genetic evaluations has gone from mailed paper or computer tapes for a limited audience to publicly accessible, request-driven distribution via the Internet. Among the distributed information is a choice of economic indices that combine an increasing array of traits into numbers reflecting breeding goals under different milk-pricing conditions. Considerable progress in genomics and the mapping of the bovine genome have identified markers for some deleterious recessive genes, but broader benefits of marker-assisted selection are still in the future. A possible exception is the proprietary use of DNA testing by semen producers to select among potential progeny test bulls. The collection and analysis of industry-wide data to evaluate genetic merit will continue to be the most important tool for genetic progress into the foreseeable future.

Animals↗

PubMed2006

Dry period length to maximize production across adjacent lactations and lifetime production.

The primary objectives of this research were to determine the dry period lengths that maximize production across adjacent lactations and also dry period length that maximizes lifetime yield. Effect of days dry (DD) after lactations 1 through 3 were determined separately for both adjacent lactation sums and lifetime yield. Field data, collected through the Dairy Herd Improvement Association, on US Holstein cows first calving between January 1997 and January 2004 were utilized. Lifetime records were restricted to cows first calving no later than December 1999. Actual lactation yields, in contrast to standardized records, were used to calculate lactation sums and lifetime records. Herds were required to be on test for the entire period to avoid partial records. Another important edit was that actual calving dates had to agree with expected calving dates, based on reported days open, within 10 d. This edit ensured that the producer knew, at least at one point in time, when the cow was going to calve. Cow effects were corrected for in both the adjacent lactation and lifetime analyses. The minimum DD to maximize production across adjacent lactations depended on parity. For yield across first and second lactations, there was little loss in production with a minimum of 40 to 45 DD. Longer dry periods (55 to 65 DD) were required after second and third lactations however, presumably due to the lower persistency of second and later lactation cows. Lifetime production was maximized by 40 to 50 DD after first lactation and 30 to 40 DD after second and later lactations. Fewer DD were required to maximize lifetime yield than adjacent lactation yield because cows with fewer DD also had more lifetime days in milk. Although dry periods of 30 to 40 d can be used after second and later lactations without cost in lifetime yield, their benefit to lifetime production is minimal. Dry periods shorter than 30 d or longer than 70 d are costly to lifetime yield and should be avoided. Dry periods longer than 80 d are even more costly than dry periods less than 30 d.

Animals↗

PubMed2006

Invited review: Culling: nomenclature, definitions, and recommendations.

Replacing cows on a dairy is a major cost of operation. There is a need for the industry to adopt a more standardized approach to reporting the rate at which cows exit from the dairy, and to reporting the reasons why cows are replaced and their destination as they exit the dairy. Herd turnover rate is recommended as the preferred term for characterizing the cows exiting a dairy, in preference to herd replacement rate, culling rate, or percent exiting, all of which have served as synonyms. Herd turnover rate should be calculated as the number of cows that exit in a defined period divided by the animal time at risk for the population being characterized. The terms voluntary and involuntary culling suffer from problems of definition and their use should be discouraged. Destination should be recorded for all cows that exit the dairy and opportunities to record one or more reasons for exiting should be provided by management systems. Comparing reported reasons between dairies requires considerable caution because of differences in case definitions and recording methods. Relying upon culling records to monitor disease has been and will always be an ineffective management strategy. Dairies are encouraged to record and monitor disease events and reproductive performance and use this information as the basis for management efforts aimed at reducing the need to replace cows.

Abattoirs↗

PubMed2006

Survival rates and productive herd life of dairy cattle in the United States.

Survival rates and productive herd life were examined for 13.8 million US dairy cows that calved from January 1, 1980, through March 2, 2005. Cows that left the herd for dairy purposes or were from herds that discontinued Dairy Herd Improvement testing were excluded from any calculations to prevent underestimation of population longevity. Mean lactation length for cows without subsequently recorded lactations ranged from 205 to 235 d across breed-parity subsets and were 4 to 29 d longer for parities 2 through 7 than for parity 1. Mean survival rates were 73% to parity 2; 50% to parity 3; 32% to parity 4; and 19, 10, 5, and 2% to parities 5 through 8, respectively. The mean number of parities for Holsteins declined from 3.2 for those first calving in 1980 to 2.8 for those first calving in 1994. Mean numbers of parities for other breeds first calving in 1994 were 2.9 for Ayrshires and Brown Swiss, 2.4 for Guernseys, and 3.2 for Jerseys. Breed means for productive herd life (through parity 8) ranged from 28 to 36 mo. All regressions of mean number of parities or mean productive herd life on year were negative. The trend for decline of many of those indicators of longevity slowed or ended after the early 1990s. Between 31 (Jersey) and 39% (Guernsey) of herds were made up of first-calf heifers.

Animals↗

PubMed2005

Effectiveness of national and regional sire evaluations in predicting future-daughter milk yield.

National and regional bull evaluations were compared for ability to predict standardized milk yield of future daughters. Correlations between evaluations and first-, second-, and third-parity yields of future daughters were calculated within herd-year-month group. Mean correlations with predicted yield of future daughters across the United States were higher for national (0.109, 0.111, and 0.082 for first, second, and third parities, respectively) than for Northeast (0.098, 0.085, and 0.061) Holstein evaluations; corresponding correlations for future Northeast daughters were similar. Bull evaluations based on the first 5 parities of daughters that first calved through 1991 from either California, North Central, Northeast, or Southeast regions as well as from the entire United States were compared with standardized milk yields of daughters that calved later. Correlations with first-, second-, and third-parity yields of future daughters were higher (from 0.001 to 0.011) for national than for regional evaluations. National evaluations were better predictors of future-daughter yield, especially for California and the Southeast. Evaluations based on only first parity were slightly better than those based on the first 5 parities in predicting first-parity yield for 3 of 4 regions but were far less useful in predicting second-or third-parity yield regardless of region. Regional evaluations included fewer bulls because of limited numbers of daughters in each region. The top 100 bulls for genetic merit for milk yield based on regional rankings were inferior to the top 100 bulls based on national ranking by 25 to 173 kg. Reliance on regional rather than national evaluations would reduce current US genetic gains.

Animals↗

PubMed2005

Characterization of days dry for United States Holsteins.

This research characterized days dry (DD) for modern US Holsteins. This included investigation of factors influencing DD, summarizing variation in herd means as well as within-herd variation, and estimation of parameters related to DD. Data consisted of records initiated before 2003 from cows first calving after 1996. A total of 459,075 records from 295,067 cows in 3527 herds were included. The overall mean DD was 60.5 d, and standard deviation among herd means was 5.8 d. Approximately 91% of all herds had mean DD between 50 and 70 d. There were 41 herds that had mean DD of < or =45 d and 2 herds with mean DD of < or =30 d. Some herds (125) had unusually long mean DD (exceeding 71 d). Variation across regions of the United States was minimal, although southwestern herds did show somewhat longer DD than other regions. Days open (DO) accounted for the most variation in DD with longer DO leading to longer dry periods. Heritability of DD was 7% and repeatability 12%. Error correlations indicated that longer dry periods benefit both milk yield and somatic cell score in the subsequent lactation. Genetic and permanent environmental correlations were large, which indicated that 1) bias would result if genetic evaluations for milk yield included an adjustment for DD, and 2) correction for cow effects is warranted in estimation of DD effects on performance. The relationship between DD and DO is due primarily to permanent cow effects. Improved fertility will enhance the feasibility of shortened dry periods.

Animals↗

PubMed2005

Accuracy and stability of national and international somatic cell score evaluations.

Somatic cell score (SCS) evaluations have been published in the United States since 1994 and international evaluations have been available through Interbull since May 2001. The accumulated data provides an opportunity to investigate the accuracy and stability of SCS evaluations. United States domestic evaluations from January 1995 through August 2004, for 21,500 Holstein bulls were considered, over time and sequentially within bull, for changes to the November 2004 evaluation. On average, predicted transmitting ability (PTA) SCS increased (worsened) by 0.002 from earlier evaluations to November 2004. Although bias was small, PTA changes were more than expected based on change in reliability. When looked at sequentially, bulls' earlier evaluations were generally lower (i.e., merit was overestimated) relative to November 2004. Differences were small, and PTA SCS increased steadily with the addition of second-crop daughters. All 524,081 evaluations were considered pairwise providing over 8,000,000 pairs of bulls' evaluations for analysis of PTA differences relative to change in reliability. Agreement of observed and expected SD improved for larger changes in reliability. The November 2004 US and Interbull PTA were matched with US and Interbull PTA from May 2001 (US04, IB04, US01, and IB01, respectively) for 14,652 Holstein bulls. For bulls having only US daughters in IB01, correlations were similar for US01 and IB01 with US04, and IB01 with IB04. Corresponding regressions were all nearly 1.00. For bulls also having nonUS daughters in IB01, correlations with yield deviations calculated for later daughters (used as source of independent data) were higher (0.747 vs. 0.714) for IB01 than for US01. For bulls with added US daughters, correlation with US04 was also higher for IB01 than US01, showing that inclusion of foreign data improved predictive value of SCS evaluations.

Animals↗

PubMed2005

Impact of maturity rate of daughters on genetic ranking of Holstein bulls.

If genetic evaluations are calculated with a single-trait repeatability model, evaluation changes may be attributed in part to bulls that have daughters that deviate considerably from the typical response to aging. Differences in maturity rate of bull daughters were examined to determine whether they influence change in bull evaluations. Standardized milk records for Holsteins that first calved between 1960 and 1998 were used to calculate 12 tailored predicted transmitting abilities (PTA) for each bull. Predicted transmitting abilities were tailored from combinations of 4 annual cut-off dates and 3 parities. Date screening selected cows first calving before January of 1996, 1997, 1998, or 1999. Parity screening selected milk records from the first 1, 2, or 3 parities. Therefore, 4 evaluations (PTA1) included only first-parity records available for daughters and contemporaries prior to the respective years designated. Four more evaluations (PTA(1,2)) included the records from the first 2 parities for cows first calving prior to those same year cutoffs; likewise, the last 4 evaluations (PTA(1,2,3)) included records from the first 3 parities. Stability of bull evaluations (standard deviations of differences as well as correlations between bull evaluations) across time was compared. Bulls born after 1984 with > or =500 daughters were of interest because of the high precision of evaluations and recent activity. Tailored PTA of those bulls had more uniformity across years in mean records per daughter than did official USDA PTA. Standard deviation of differences in PTA1, PTA(1,2), and PTA(1,2,3) for milk between evaluation years 1996 and 1997 were 28, 28, and 27 kg compared with 63 kg for official evaluations; similarly, between 1996 and 1999, SD were 36, 32, and 32 kg compared with 80 kg. Results suggested that a modification to the current evaluation model to account for maturity rate should reduce fluctuations in individual bull PTA across time and may improve accuracy of evaluations.

Aging↗

PubMed2005

Impact of estimated genetic correlations on international evaluations to predict milk traits.

The Interbull procedure for combining dairy bull evaluations uses estimated genetic correlations between countries. It is important to know whether the resulting difficulties from differences in ranking in each country are justified by improved accuracy relative to a system assuming unity correlations. Data submitted for the May 2001 yield and somatic cell score (SCS) Interbull evaluations were processed once with the usual estimated genetic correlations (E01) and again assuming these correlations to be essentially unity (0.995; U01). The 2 sets of resulting evaluations were compared with August 2004 national evaluations (N04) for bulls not having local evaluations used in the 2001 evaluations. Thus, the examination was of Interbull evaluations from foreign data in predicting national evaluations. Countries in the study for yield were Australia, Canada, France, Germany, Great Britain, Ireland, Italy, The Netherlands, New Zealand, and the United States. Countries included for SCS were Canada, France, Germany, Great Britain, The Netherlands, and the United States. For most countries' evaluations, standard deviations of differences between E01 or U01 and N04 were smaller for E01 by about 5 to 7% and correlations between E01 and N04 were higher by 0.01 or the same as for U01 and N04. Although use of estimated correlations tended to improve prediction, the advantage was small. A previous study had concluded no difference in accuracy for yield but did not include Australia and New Zealand, countries with the lowest correlations with other countries. Excluding bulls from those countries produced results for the other 8 countries more like the previous study, but still favoring E01 slightly. Those 2 countries were not in the SCS data. Estimated genetic correlations improved the prediction of future national evaluations slightly in most countries but more substantially for the evaluations and bulls of Australia and New Zealand.

Animals↗

PubMed2004

Performance of dairy cattle clones and evaluation of their milk composition.

Genetic and phenotypic performance of U.S. Holstein embryo-split and nuclear-transfer clones was documented for yield and fitness traits. For cows, mean genetic superiority based on pedigree was 186 kg of milk, 9 kg of fat, and 7 kg of protein for embryo-split clones and 165, 10, and 8 kg, respectively, for nuclear-transfer clones compared with the population for the same birth year; pedigree advantage for male clones generally was slightly greater. Estimates of genetic merit that considered a clone's own performance as well as pedigree merit were slightly lower for embryo-split cows than for their full siblings for yield but not for milk composition (fat and protein percentages), mastitis resistance (somatic cell score), longevity (productive life), or cow fertility (daughter pregnancy rate); no corresponding genetic differences were found for nuclear-transfer cows or for cloned bulls regardless of clone type. For bulls, estimated genetic merit based on daughter yield was more similar for clone pairs with apparent identical genotype than for pairs from the same biotechnology but nonidentical as confirmed by blood typing. Yield deviations were lower for clones than for their full siblings. Milk composition (total solids, fat, fatty acid profile, lactose, and protein) also was compared for nuclear-transfer clones (Brown Swiss, Holstein, and Holstein-Jersey cross) with non-cloned cows and literature values; no differences were found for gross chemical composition of milk. No obvious differences were evident between cloned and non-cloned animals or for the milk that they produced.

Animals↗

PubMed2004

Development of a national genetic evaluation for cow fertility.

A national fertility evaluation was developed based on pregnancy rate, which measures the percentage of nonpregnant cows becoming pregnant within each 21-d opportunity period. Data for evaluation are days open, which are calculated as date pregnant minus previous calving date. Date pregnant is determined from last reported breeding or from subsequent calving minus expected gestation length. Success or failure of last breeding can be confirmed by veterinary diagnosis or a report that the cow was sold because of infertility. Data are adjusted for parity and calving season within geographic region and time period and evaluated. Fertility records are considered complete at 250 d in milk, and lower and upper limits of 50 and 250 d are applied to days open. For calculation of genetic evaluations, days open are converted to pregnancy rate by the linear formula pregnancy rate = 0.25 (233 - days open). Evaluations are expressed as predicted transmitting ability for daughter pregnancy rate, and calculation is done with an animal model. Genetic correlations among several fertility measures and other evaluated traits were estimated from 3 large data sets. Correlation with days open was less for nonreturn rate than for days to first breeding, probably because nonreturn rate had lower heritability. Cow fertility was negatively correlated with yield but is a major component of longevity. Thus, recent selection for longevity may have slowed the long-term decline in fertility. Direct selection for fertility could halt or reverse the decline.

Agriculture↗

PubMed2004

Relationship of test-day somatic cell score with test-day and lactation milk yields.

To determine the relationship of test-day (TD) somatic cell score (SCS) to TD and lactation milk yields, 1,320,590 records from Holstein first and second calvings from 1995 through 2002 were examined. All lactations had recorded yield and SCS for at least the first 4 TD. Least square analyses were conducted for yields on TD 2 through 10 within herd and cow. The model included regressions on current TD SCS and mean SCS of all previous TD with separate estimates by parity; effects for parity and calving year were included as well as regression on days in milk on TD 1. Corresponding analyses were conducted without regression on current SCS. An analysis of lactation yield was performed with a similar model and regression on all TD SCS. The SCS was highest most often on TD 1 for parity 1 (22.5%) and on TD 10 for parity 2 (18.5%). Regression of TD milk yield on mean of previous TD SCS was highest during the latter half of lactation (maximum of -0.346 kg/SCS unit on TD 9) for parity 1 and during TD through 7 (maximum of -0.366 kg/SCS unit on TD 4) for parity 2. Regression of TD yield on current TD SCS tended to be larger for later lactation. Regression of lactation yield on TD SCS was negative and important for TD 1 through 6 for parity 1 and for all TD for parity 2. To minimize milk loss, mastitis control is most important immediately pre- and postcalving for parity 1 and throughout lactation for parity 2.

Animals↗

PubMed2004

Performance of Holstein clones in the United States.

Phenotypic and genetic performance of US Holstein clones from embryo splitting (ETS) and nuclear transfer (ETN) was documented for yield and fitness traits. Holstein Association USA has registered 2319 ETS and 215 ETN clones. The number of male clones has decreased. Animals selected for cloning were slightly superior genetically to population mean for yield traits. For females, mean superiority of pedigree merit of ETS clones was 186 kg for milk, 9 kg for fat, and 7 kg for protein compared with the population for the same birth year; for ETN clones, superiority to the population was 165, 10, and 8 kg. Advantage in pedigree merit for male clones generally was slightly greater. The small pedigree advantage for female clones of <1 standard deviation above breed mean indicates that selection of animals to clone was not based primarily on yield. Yield deviations were lower for ETS clones than for their full siblings, which indicates a possible impact of the technology on performance. Yields were lower for ETN clones than for their noncloned full siblings, but differences were small and based on small numbers of clones. Milk composition for cloned cows was not different from that for the population. Estimated genetic merit based on daughter yield was more similar for male clone pairs with apparent identical genotype than for clone pairs from the same biotechnology but nonidentical as confirmed by blood typing. For ETS clones with confirmed identical genotypes, identical genetic merit should be assigned.

Animals↗

PubMed2004

Stability of genetic evaluations for active artificial insemination bulls.

Genetic evaluations for milk, fat, and protein from 1995 through August 2003 for 17,987 Holstein bulls in active artificial insemination (AI) service were examined for changes to the November 2003 evaluation. Evaluations for active AI bulls at each of 31 evaluation dates showed mean declines to November 2003. No evidence was seen of a worsening situation over time. Bulls' early evaluations with active AI status showed much larger declines, but this overevaluation diminished and essentially disappeared after 3 yr. The bulls with first active AI evaluations since 1995 were the primary focus of the study. The influx of second-crop daughters did not appear to cause a decline in evaluations for these bulls, attesting to the successful modification to the genetic evaluation system by expanding the genetic variance of short records. Mean declines and the variation of those differences were generally similar by bull sampling organization. A change from active to inactive AI status was generally concurrent with a decline in predicted transmitting ability (PTA). Bulls coded as having standard AI sampling declined less than bulls coded as having other sampling, but the differences were much less than in previous reports. Larger increases in reliability were generally associated with greater declines in PTA, and the magnitude of these changes decreased over time (increasing evaluation number). Change in reliability underpredicted the variance of change in PTA, indicating that other important factors contribute or that the assumptions for the calculation of the expected change in PTA are not met. Declines in estimated merit over time are not sufficient to alter present genetic selection programs, but reasons for the declines continue to elude explanation.

Animals↗

PubMed2004

Accuracy of foreign dairy bull evaluations in predicting United States evaluations for yield.

Combining foreign daughter data with domestic information in dairy bull genetic evaluations has been shown to improve prediction of future domestic evaluations for US bulls. This study focused on the accuracy of Interbull evaluations of bulls with only foreign daughters, in predicting the latest domestic evaluations (based on US daughters). August 2003 USDA evaluations based only on US daughters were matched with the most recent February or August Interbull evaluations without US daughters. A minimum reliability of at least 80% for yield and 70% for somatic cell score (SCS) was required in both evaluations. This provided pairs of evaluations based on different daughters (foreign or US) for 286 bulls (60 bulls for SCS). Mean Interbull reliabilities on the US scale were 88% for yield and 84% for SCS, and the mean US reliability for the current evaluations was 91% for yield and 80% for SCS. Correlations between the Interbull and domestic evaluations were 0.90, 0.87, 0.90, and 0.87 for milk, fat, protein, and SCS respectively. Expected correlations were 0.89 for yield and 0.82 for SCS. Mean differences between the Interbull and current domestic evaluations were near zero. These foreign bulls had graduated from progeny test programs (selected for positive Mendelian sampling) before being marketed in the United States. Thus, parent average was a substantial underestimate of merit. The small average differences between evaluations from foreign and US daughters and high correlations indicate that Interbull evaluations based solely on foreign daughters are useful predictors of the US evaluations for yield and SCS, providing accuracy in agreement with reliabilities and much better estimates than the alternative, parent averages.

Animals↗

PubMed2004

Duration of herd participation in dairy herd improvement milk recording in the United States.

Participation in milk-recording programs that provide data for national genetic evaluations of dairy cattle in the United States is voluntary, but the effectiveness of the evaluation system increases with the number of herds that contribute data. To investigate patterns of herd participation in Dairy Herd Improvement (DHI) testing, periods of continuous testing were computed based on the year that a herd initiated or terminated testing and by geographical region. Continuous testing was defined as at least one test per 6-mo period. Some herds discontinued testing and then re-enrolled. Across all years (1960 through 2002), 65% of herds had one period of continuous testing (no testing lapse). The percentage of herds with testing lapses decreased as the number of lapses increased and as the initial test year became more recent; overall, only 1.5% of herds had more than 6 continuous testing periods. For herds that terminated DHI testing from 1960 through 2002, 64% were on continuous test for <3 yr. In general, herd frequencies decreased as continuous test period increased except for continuous testing of > or =20 yr, which increased to 13% for years 2000 to 2002. Herds with more recent termination dates had remained on continuous test longer, and one-third of herds that were still on test after June 2002 had been on test for at least 20 yr. The duration of herd participation was longest for the northeastern and mideastern United States and shortest for the southeastern United States. Multiple periods of testing with lapses of >6 mo between test periods represent a loss of data that could have enhanced the study and evaluation of genetic characteristics of US dairy cattle.

Animals↗