Genetic architecture of fertility traits in hormonally synchronized and heat detected dairy cows
Ovulation synchronization is becoming a popular alternative to estrus detection in the dairy industry. Accurate heat detection (HD) can be challenging and because of this, ovulation synchronization in combination with artificial insemination, known as timed artificial insemination (TAI), has become a management tool for producers. In addition to overcoming reproductive inefficiencies, TAI can be used reduce the interval between calving and conception. Previous research has shown that using TAI affects accuracy of genetic evaluations for fertility traits. Moreover, bulls ranked differently for fertility traits under TAI and HD scenarios, suggesting that selection for fertility traits without differentiating between breeding methods might lead to potential bias within genetic evaluations. The objectives of this study were to estimate genetic correlations between fertility traits measured under TAI and HD in Canadian Holstein cows. Lactanet provided data containing 3 842 breeding protocol descriptions, of which 2 002 were classified as TAI and 1 840 as HD. First parity cows were included in the dataset, excluding heifers due to a low frequency of TAI usage in this category. Calving to first service (CTFS), first service to conception (FSTC), and days open (DO) were the fertility traits considered. The final dataset included 228 744 records from 152 104 cows. The genotype dataset included 6 985 genotyped cows with records for FSTC and DO and 7 220 genotyped cows with records for CTFS. Variance components were estimated using a Bayesian single-step GBLUP multiple-trait animal model adapted from models used by Lactanet in the genetic evaluations for fertility traits. The heritability estimates were slightly different between HD and TAI (FSTC (0.02, 0.03), CTFS (0.02, 0.01), and DO (0.03 and 0.04), with all the posterior standard deviation (PSD) values < 0.003) and were in the range of those in the literature. The additive genetic correlation (± PSD) between HD and TAI were 0.73 ± 0.04, 0.89 ± 0.03 and 0.91 ± 0.01 for FSTC, CTFS and DO, respectively. The genetic correlations less than unity suggest the phenotypic expression of two different traits. In the next step, we will explore the potential different genetic backgrounds between HD and TAI traits by performing genome-wide association studies.
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