Actions for Genomic evaluation of horn fly resistance and phenotypes of cholesterol deficiency carriers in Holstein cattle

Email RIS file
Bookmark
Report an Issue

Genomic evaluation of horn fly resistance and phenotypes of cholesterol deficiency carriers in Holstein cattle

Author
Basiel, Bailey
Published
[University Park, Pennsylvania] : Pennsylvania State University, 2020.
Physical Description
1 electronic document
Additional Creators
Dechow, Chad Daniel
Access Online

Availability

I Want It
I Want It

Finding items...

Graduate Program
Restrictions on Access
Open Access.
Summary
Artificial selection of Holstein cattle in North America has resulted in more productive dairy cows in each generation. However, selection for milk production has also led to increasing rates of inbreeding and selection for other economically-relevant traits such as fertility and health have only been implemented in recent years. Holsteins are the most prevalent dairy breed in the United States; their productivity makes them desirable cows in both conventional and organic herds and subsequently makes them popular show cattle. Inbreeding and selection decisions impact producers differently because there is no one type of cow that suits every herd. We set out to determine how a lethal recessive genetic disorder, cholesterol deficiency (CD), that arose in Holstein cattle from inbreeding impacts conventional and elite CD carriers. The CD causative mutation may result in lowered cholesterol of heterozygous animals indicating partially dominant inheritance. Serum samples (n = 70) collect on two dates from CD carriers (n = 22) and controls (n = 35) were analyzed for cholesterol concentration (mg/dL) and results were analyzed using mixed models. Daily phenotypic records of milk yield, milking time, milk conductivity, activity, and body weight were compared between carriers and controls. Additionally, the CD carrier status of elite cows that were sired by known CD carriers and had available genotypes was recorded. Chi-square tests for equal proportions were used to evaluate if CD carrier status impacted CD carrier-sired cows placing at World Dairy Expo over the last 13 years. Control animals had higher serum cholesterol (119.03±6.52 mg/dL) than CD carriers (92.36±7.56 mg/dL) and CD carriers had shorter milking times and lower body weights than their herd-mates. Further, there were more CD carriers in the top 5 and more carriers tended to place in the top 10 at World Dairy Expo when repeated placings were included. Our analyses support that CD is partially dominant and indicate that CD carriers display other phenotypic differences from their herd-mates which may give them a small advantage in elite cattle competitions. Another objective was to determine the genetic parameters of a trait that is not currently selected upon in dairy cattle, horn fly resistance, through genomic analysis. Horn flies contribute to major economic losses of pastured cattle operations, particularly in organic herds because of limitations on control methods that can be used. Observations of fly load were recorded from pastured organic Holstein cows using a novel scoring method that was validated by counting flies from photographs taken at the time of scoring on a subset of observations. Records of stayability were merged with observations to mitigate the effect of our data structure on potential selection bias effects on genetic parameter estimates. Genotypes were available from a subset of animals so that both a pedigree and genomic analysis could be performed. Genetic parameters were estimated with three-trait mixed models that included fly score, stayability, and a third phenotype. The animal effect was based either solely on pedigree or a blended pedigree and genomic relationship matrix. A genome-wide association study was performed by decomposing estimated breeding values from the genomic analysis into SNP effects to detect significant genomic regions associated with fly score. Genomic analysis estimated the heritability of fly score as 0.22 ± 0.04. Fly score was positively genetically correlated with milk and fat yield but higher fly infestation levels corresponded to lowered test-day milk and component yields. A significant genomic region on BTA 6 contained KIT which was subsequently identified as the primary candidate gene for fly resistance. KIT is required for melanogenesis and is associated with coloration in many species including cattle; further, white coloration was associated with lower fly scores while black coloration was associated with higher fly scores in our population. These findings suggest that we can select for resistance to horn flies. The impacts of CD carrier status should be further investigated with larger sample size before determining if the CD mutation should be further proliferated in carriers. Horn fly resistance should be investigated in other dairy breeds and fly scoring should be initiated on a larger scale so national evaluations of fly resistance can be performed. Information derived as we continue to understand the phenotypes of CD carriers and the availability to select for horn fly resistance each provide producers more opportunities to breed Holsteins that best fit the needs of individual herds.
Other Subject(s)
Genre(s)
Dissertation Note
M.S. Pennsylvania State University 2020.
Technical Details
The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
View MARC record | catkey: 32894628