Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Animal Science

First Advisor

Albert E. Freeman

Second Advisor

Rohan L. Fernando


For use in genetic evaluation, records of yield traits (milk, fat, and protein) are adjusted to a 305-day lactation length, to a 2 times a day (2x) milking, and to a mature equivalent basis. If these adjustments are not accurate, the PTAs will be biased and the genetic progress will be less than expected. Adjustment factors are usually estimated from solutions of a linear model. Some covariance components are needed to use in an animal model. However, the effect of different variance component estimates when estimating adjustment factors has not been studied. The goals for this study were: (1) estimate covariance components to be used in mixed model equations; (2) test for the effect of interactions of milking frequency with parity, covariates of days in milk, and covariates of days pregnant; (3) estimate adjustment factors to correct yield records from 3x to 2x; (4) evaluate the effect of covariance component estimates on estimating adjustment factors to correct yield records from 3x to 2x. Test-day data from 10 states of the USA and a random regression model were used for estimation of adjustment factors. Three sets of covariance components were estimated from three samples of the data. Adjustment factors were estimated for each set of covariance component estimates which were used in the mixed model equations. There were three sets of adjustment factors estimated for milk, fat and protein yield. Covariance component estimates did not change the estimates much of adjustment factors obtained from the solutions of the mixed model equations. All contemporary group effects (herd-test-date, herd-year-season, and parity-calving-season-state) showed significant effects. The interactions of management group with parity, covariates of days-in-milk, and covariates of days pregnant, also, showed significant effects. A set of adjustment factors estimates, obtained by using a pool of three sets covariance component estimates in the mixed model equations, is recommended for use in dairy industry.



Digital Repository @ Iowa State University,

Copyright Owner

Nerilson Terra Santos



Proquest ID


File Format


File Size

186 pages