Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Animal Science

First Advisor

Doyle Wilson


Reproduction is economically important to the beef producer. The objective of this study was to determine if pregnancy rate, calving day (CD) and age at first calving (AFC) could be used to select for female fertility in Angus beef cattle. Producers from six herds in five states provided 3144 heifer records. A general linear animal model, using the relationship matrix, was fitted to the binary trait of pregnancy status by the software Matvec. The heritability of pregnancy percentage on the underlying scale was 0.13 +/- 0.07. Estimated breeding values, which are in units of underlying fertility, ranged from -0.48 to 0.80 for heifers, and from -0.56 to 0.70 for sires of heifers. These results indicated that, while lowly heritable, some improvement in fertility could be made by selecting on heifer pregnancy rate.;The second part of the study examined CD and AFC using records (n = 2082) from two herds. Data were analyzed by MTDFREML using a general linear animal model with and without a maternal effect. Heritability for CD and AFC using the direct model were 0.07 +/- 0.04 and 0.28 +/- 0.06. Average, minimum, and maximum estimated breeding values for sires of heifers were -0.7, -10.6, and 9.8 days for CD and -0.6, -46.6, and 45.9 days for AFC. When the maternal effect was added to the model, direct heritability for CD went down slightly, and direct heritability for AFC increased to 0.66 +/- 0.14. The maternal heritabilities were 0.08 +/- 0.05 for CD and 0.32 +/- 0.08 for AFC. The direct-maternal genetic correlations were -0.18 +/- 0.58 for CD and -0.85 +/- 0.06 for AFC. Heritabilities for CD and AFC increased slightly when growth traits were analyzed in two-trait models. The genetic correlations between CD and AFC and the growth traits were low to moderate and negative. Although AFC had a higher heritability and a wider range of breeding values than CD, the negative direct-maternal correlation indicated that selecting on AFC may favor heifers that are themselves born later in the season. Therefore, CD may be more useful in selecting for female fertility in beef cattle.



Digital Repository @ Iowa State University,

Copyright Owner

Jennifer Arleen Minick



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94 pages