Same Trait, Different Result. How Can This Be?

Kristen Parker Gaddis CDCB Geneticist

What if you had your blood pressure checked twice in the same hour – and received different results? It’s certainly plausible, as conditions change. Different machines function differently and use slightly different formulas to calculate the systolic and diastolic readings. How would you determine which reading is more accurate?

Dairy producers now face a similar situation with the expanding number of genetic evaluations for similar traits, available from different sources. The best examples currently are health traits. A producer may see multiple genetic evaluation results related to a trait like mastitis. If the evaluations are different, which should you believe?

In April 2018, the CDCB released official genetic evaluations for resistance to six health events: hypocalcemia, displaced abomasum, ketosis, mastitis, metritis and retained placenta. These health evaluations joined several others available from various industry sources.

Some health evaluations are proprietary to a specific company, meaning the specific data or methodologies used in calculations is not readily available. The TransitionRight index is calculated by ABS and incorporates mastitis, metritis and ketosis. CRV calculates the Better Life Health index that includes foot health problems, clinical and subclinical mastitis, and ketosis using data primarily from Dutch herds. Genex produces the Ideal Commercial Cow index, or ICC$, which includes three proprietary traits related to health: subclinical ketosis, metritis and foot health. Several other evaluations are also available, which Progressive Dairyman detailed earlier in 2018.

 

Alternatively, some sources of health evaluations provide more technical details on the management and analysis of data. Zoetis calculates genetic evaluations for nine health traits (mastitis, lameness, metritis, retained placenta, displaced abomasum, ketosis, calf pneumonia, calf scours, and calf survival) with CLARIFIDE® Plus. The health evaluations through CDCB and Zoetis’ CLARIFIDE Plus have published information, with methodologies described in peer-reviewed scientific articles, making it possible to better understand why differences may occur. Therefore, this article focuses on the similarities and differences between the CDCB disease resistance evaluations and the Zoetis CLARIFIDE Plus wellness trait evaluation.

There are several reasons why evaluation results may differ, even when the traits appear the same. Let’s start with the data itself. The data source for evaluations may be completely different. The CDCB data is collected from producer-recorded data on U.S. dairies through the Dairy Herd Information (DHI) system. The data used for Zoetis’ CLARIFIDE Plus similarly come from producer-recorded data on U.S. dairies that may or may not be a part of the data reported to DHI. As the data going in is different, the information coming out (the genetic evaluation) may also differ. If there is less data in common, the differences in the genetic evaluations can be greater.

Next, editing the data involves several decisions that can produce different evaluation results. What time span (of a reported health event) will be used? How will it be determined that a herd is reporting a health event consistently? Many of these decisions do not have a clear right or wrong answer; decisions are made based on knowledge of the current dataset. Along with editing, there are data analysis decisions. The CDCB health evaluations and Zoetis’ CLARIFIDE Plus evaluations each use a different modeling approach to analyze the data. Again, there is no clear right or wrong choice, and differences will impact the evaluations.

Evaluation results can be further obscured by differences in presentation. Both CDCB and Zoetis health evaluations are presented with positive values being more desirable. However, CDCB health evaluations are presented as percentage points of disease resistance above or below the breed average. Zoetis CLARIFIDE Plus health evaluations are standardized with a value of 100 representing the average expected disease risk. Standardized transmitting abilities (STA) greater than 100 represent lower expected average disease risk. Selecting for a higher STA will apply selection pressure for reduced genetic risk of disease.

With health traits, there are additional considerations. Available data on direct health traits are much more limited than a trait such as somatic cell score that has been collected for decades. Less data, along with factors such as low heritability, results in lower reliabilities for health traits than longer-standing traits such as milk yield. Reliabilities for health evaluations will increase as more data are accumulated. At present, however, we should expect re-rankings among animals between evaluation systems due to the low average reliabilities. In general, the lower the reliability of the trait in the two evaluations, the greater the difference will be in their predictions.

 

Additional features set the CDCB health evaluations apart from other sources. CDCB evaluations are carried out independently such that animals from all controllers are evaluated on the same basis. CDCB results are universally and publicly distributed, allowing the entire industry to continuously monitor the properties and accuracy of the evaluations. Lastly, CDCB’s mastitis resistance evaluation is submitted to Interbull for international comparisons and validation, meaning both the methodologies and the results are peer-reviewed.

In Summary

Differences between evaluations are not surprising given the limited data, lower reliability and differences in data and evaluation methods. Important questions remain as to how to interpret results from multiple evaluations. Are the evaluations coming from an unbiased source and based on sound science? What traits are being evaluated? How are the traits presented? Are the economics of the traits appropriately considered? The information available to aid in deciding which evaluation to use will depend greatly on the source.

Regardless, it is important to not focus selection pressure on only a few traits – especially when considering lower-reliability traits. The availability of genetic evaluations for direct health traits is an important step forward; however, all traits of economic importance – from production to fertility to health – must be considered together. CDCB health evaluations add six more traits to consider in an animal’s overall merit, allowing identification of animals with increased or decreased resistance to common health problems. The Animal Genomics and Improvement Laboratory (AGIL) and CDCB collaborate to research and provide indices that appropriately weight traits based on their heritability, reliability, correlations between traits and economic importance. Use of Net Merit – or Fluid Merit, Cheese Merit or Grazing Merit – will provide the best strategy to make optimal genetic progress in all economically-important traits and maximize overall profitability.

Kristen Parker Gaddis is a Geneticist at Council on Dairy Cattle Breeding (CDCB). Dr. Parker Gaddis studied at North Carolina State University, where she received her Bachelors and Doctorates degree in animal science, and completed her Post-doctorate at the University of Florida and USDA- AGIL. At CDCB, she has focused on improving the development of the genomic evaluations, researching health traits, and incorporating health records into the CDCB system.