Growing Beef Newsletter
July 2025, Volume 16, Issue 2
Heritability and genetic correlations estimated for heifer pregnancy and carcass traits using the ISU McNay Angus herd
Sarah Phelps, animal breeding and genetics graduate student, Iowa State University
Heifer pregnancy is an economically relevant trait to cow calf producers, as revenue is generated through calf sales. Open heifers have a lower chance of recouping their input costs if they are kept and rebred the following breeding season compared to bred heifers Typically it is more cost-effective to cull open heifers in the first breeding season. Identifying factors that influence heifer reproductive performance is crucial to improving the reproductive performance of a herd and the overall profitability of an operation.
The relationship between carcass trait and reproductive performance has been a relevant topic of discussion. The use of expected progeny differences (EPDs) to genetically select for increased carcass trait performance has been successful and has resulted in larger ribeye areas, greater intramuscular fat percentages, and increased backfat at the time of slaughter. However, the indirect consequence of selecting for increased carcass traits on genetic selection for female reproductive performance is not well understood.
The genetic relationship between carcass traits and heifer pregnancy were estimated using pedigree and phenotypic information from the Iowa State University McNay Memorial Research and Demonstration Farm’s Angus herd. Phenotypic records included heifer pregnancy (HP) (evaluated as pregnant or open), weaning weight (WWT), and carcass information records for ribeye area (REA), marbling score (IMF), and backfat thickness (FAT). An important practice when evaluating animals across different years and groups is to use contemporary grouping (CG) to only compare animals within the same group who experienced the same environmental influences (i.e. diet, bull exposure, etc.). Animals of different management groups and years face difference conditions, and CGs accounts for this. Across all years, the average heifer pregnancy rate was 72%. The average measurements for carcass traits and weaning weights were as follows: 12.6 in2 REA, 0.5 in FAT, 5.5 IMF score, and 430 lbs WWT.
Pedigree information and CG were used to determine the heritability of each trait as well as genetic correlations between the traits. The heritability of a trait refers to the amount of variation in a trait that is controlled by genetics. Traits with a heritability of less than 0.2 are considered lowly heritable, 0.2 to 0.4 are moderately heritable, and above 0.4 are highly heritable. The higher the heritability, the quicker genetic improvement on a trait can be made through selection. The genetic correlations indicate whether the same genes that affect one trait also affect another trait and in what manner selection for one trait impacts the other trait. These values are measured on a scale of -1 to +1. Positive values for genetic correlations indicate that genetic selection for one trait will result in similar changes in the other trait. As one trait increases, the other genetically correlated trait will also increase. Negative values indicating that genetic selection for one trait is antagonistic of another. In these cases, as selection occurs to increase one trait, the other correlated trait will decrease.
Heritability estimates were 0.09 for HP, 0.38 for REA, 0.39 for FAT, 0.48 for IMF, 0.34 for HCW, 0.14 for WWT direct and 0.08 for WWT maternal. Weaning weight direct measures the calf’s genetic influence on its own weaning weight while weaning weight maternal measures the dam’s genetic influence on the weaning weight of her calf. These results indicate that HP is a lowly heritable trait, and genetic progress can be made but at slow rate. The heritability of carcass traits was moderate to highly heritable, suggesting genetic improvements in these traits can be made quickly through genetic selection.
Table 1 reports the heritability estimates and genetic correlations from the overall model. Negative genetic correlations were reported between HP and both carcass and weaning traits. These correlations with HP were -0.26 for REA, -0.21 for FAT, -0.09 for IMF, and -0.12 for WWT direct. The negative genetic correlations between carcass traits and HP demonstrate an antagonistic relationship between the traits. Genetic correlations between carcass traits and WWT are also reported in Table 1.
This study was performed on a single angus herd. The genetic composition of this herd may not represent the average genetic makeup of the entire Angus population. Therefore, if this study is conducted using information from the entire Angus population, the results may differ. The phenotypic association between these traits may be positive if there is a management or an environmental influence that promotes both increased carcass and reproduce performance.
In conclusion, the heritability estimate for HP indicated slow genetic progress can be made through genetic selection of this trait. Carcass traits heritability estimates were moderate to high, indicating fast genetic progress can be made from genetically selecting for these traits. Unfavorable genetic relationships were found between HP and carcass traits in this study. These results emphasize the importance of considering the EPD values of all economically relevant traits when selecting replacement heifers and to avoid selecting individual traits as this could unintentionally lead to downward selection of negatively correlated traits. Understanding how a herd performs genetically can help producers make educate decision when deciding which EPD values to prioritize and which values only need to be acceptable. Selection indexes use this concept of balancing the selection of multiple and are a great way to simultaneously select for multiple traits, even if these traits are unfavorably related.
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