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SB1 (8-06)

Genetic Evaluation of Stillbirth

J.B. Cole
Animal Improvement Programs Laboratory, ARS-USDA, Beltsville, MD 20705-2350
301-504-8334 (voice) ~ 301-504-8092 (fax) ~ ~

Calving Ability Index || Bull Evaluations || Phenotypic and Genetic Trends || Model Validation || Acknowledgments

A new stillbirth (SB) evaluation has been developed for Holstein bulls, and will be available beginning in August 2006. The data set includes 6 million stillbirth records from calves born since 1980. The genetic evaluation model is described in detail in USDA Calving Ease and Stillbirth Evaluations and Genetic evaluation of stillbirth in United States Holsteins using a sire-maternal grandsire threshold model. The genetic analysis includes effects for herd-year, year-season, parity-gender, sire birth year, maternal grandsire (MGS) birth year, sire, and MGS. Service sire SB (SSB) measures the effect of the calf itself on stillbirth. Daughter SB (DSB) measures the effect of a particular cow (daughter) on stillbirths. The SB test run included 44,290 AI bulls that either were the service sire for a cow with a calving record (for SSB) or had a daughter with a calving record (for DSB). The SB evaluations are expressed as percent stillbirths (%SB), where stillbirth is defined as a calf born dead or that dies within 48 hours of birth. The genetic base for both SSB and DSB is 8%. Heritabilities of SSB and DSB are 3.0% and 6.5%, respectively. (Co)variance components were calculated as described in Stillbirth (co)variance components for a sire-maternal grandsire threshold model.

Calving Ability Index

Sire and daughter SB will be included in Lifetime Net Merit (NM$) as described in Net merit as a measure of lifetime profit: proposed 2006 revision. The four calving traits (SSB, DSB, and sire and daughter CE) will be combined into a Calving Ability Index (CA$) and receive 6% of the total weight in NM$. CA$ is calculated as:

–4(sire CE) – 3(daughter CE) – 4(sire SB) – 8(daughter SB).

For Brown Swiss, economic values are –6 for sire CE and –8 for daughter CE because separate SB evaluations are not available but CE values include the correlated response in SB. Breeds without CE or SB evaluations will be assigned CA$ of 0.

Bull Evaluations

Summary statistics of the sire evaluations for stillbirth are provided in Table 1. Mean PTA were 7.9 and 8.6 for SSB and DSB, respectively. Reliabilities averaged 45%, and are lower than those for CE traits. The heritability of SSB is much lower than that of sire CE, and bulls have fewer SB records, on average, than CE records. Reporting of calf livability scores for all births will result in improved reliabilities.

Table 1. Summary statistics of sire evaluations for stillbirth.
Mean SD Min Max Mean SD Min Max Mean SD Min Max Mean SD Min Max
Active 695 7.8 0.8 5.3 11.5 54 19 39 99 8.2 1.5 4.1 15.1 49 15 39 99
All 44,290 7.9 0.6 5.1 12.2 45 11 14 99 8.6 1.2 3.6 18.4 45 12 15 99

Phenotypic and Genetic Trends

Stillbirths averaged 10.5% and 5.9% in heifers and cows, respectively, from 1980 to 2005. The incidence of SB increased from 11.2% in 1980 to 12.0% in 2004 for heifers, and decreased from 5.8% in 1980 to 5.6% in 2005 for multiparous cows. The apparent decrease in heifer SB from 12.0% in 2004 to 11.1% in 2005 may reflect that not all records from 2005 have been forwarded to AIPL, or the wider availability of bulls with desirable CE. Year-to-year variation was more pronounced in heifers than cows. It is likely that the rate of increase in stillbirths has decreased as a result of assortative mating of bulls with low PTA for CE to heifers. Correlated responses to selection for improved CE are also possible; we estimated genetic correlations between direct CE and SB and MGS CE and SB of 0.67 and 0.63, respectively. Neither phenotypic nor genetic trends were significant.

Phenotypic trend for stillbirth, 1980-2005

The lack of genetic trend for SB can be seen in the graph of SSB and DSB PTA over time shown below. The increase in mean PTA for DSB in 2002 is attributable to a prominent sire of sons with extremely poor merit for DSB. Sire and MGS solutions are smaller than for CE, and the MGS range is larger, reflecting the larger MGS variance.

Genetic trend for stillbirth, 1980-2005

Model Validation

The evaluation model was validated by comparison against results from the Interbull stillbirth evaluation, which currently includes Denmark, Finland, Israel, Holland, and Sweden. The U.S. will provide data for the September 2006 test run, with the expectation that we will be a participant in the routine November 2006 run. Correlations among U.S. stillbirth solutions on the underlying scale with Interbull evaluations on foreign scales for bulls with at least 90% reliability in both countries ranged from 0.63 to 0.90 for SSB and 0.69 to 0.96 for DSB, indicating that results from this analysis are generally consistent with those in other countries. These ranges are similar to the range of correlations among Interbull participants. Correlations were not uniformly high due to differences in model and trait definition between countries. Results were also validated by Interbull genetic trend testing procedures, and met Interbull requirements for inclusion in the routine international evaluation for Holstein SB.

(Co)variance components estimates and solutions for effects in the model were compared against results in the scientific literature and found to be generally consistent with earlier reports.

Acknowledgments: The author thanks Paul VanRaden and George Wiggans for review and revision of this research report.