Estimates of correlation between estrus behavior and estradiol concentrations during estrus
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Estrus traits have economic value in dairy production systems and can potentially be incorporated into genomic selection. Three studies were performed to further understand selection responses. Study one and two explored the relationship between rump touches and number of steps during estrus. Holstein-Friesian cows (n = 1197; Study 1) across five pasture-based grazing dairy herds were fitted with a capacitive touch sensing (CTS) device on the rump (FlashMate; Farmshed Labs Limited, Hamilton, NZ). The number of times touched and the sum of the touch duration were used to compare farms and estimate the intra-class correlation (repeatability). For Study 2, postpartum Holstein (n = 85) and Guernsey (n = 5) cows in a confinement-style dairy were used. Cows were fitted with an IceQube accelerometer (IceRobotics Ltd., Edinburgh, UK) to measure steps taken per hour and a CTS device was applied to both rumps. The inter-class correlation for the number of rump touches and number of steps taken during estrus was calculated. Study 1 had an intra-class correlation (repeatability) for rump touches during estrus was approximately 0.2. For Study 2, the number of steps and the number of rump touches during estrus increased in a synchronous manner. The inter-class correlation (r) for rump touches and steps was approximately 0.45. Experiment 3 focused on the association between circulating concentrations of estradiol and overt phenotypes for estrus [greater activity and (or) rump touches (mounts, chinrests, etc.)] that can be easily observed on farm. We also tested the effect of lactation on the estrus traits that we measured. Cows (n = 11 lactating and n = 9 nonlactating) were treated with PGF2[alpha] to synchronize estrus. The jugular vein was cannulated to collect blood every 2 h for plasma estradiol measurement. Plasma LH was measured during the periestrual period to determine the time of the LH surge. Cows were fitted with an IceQube accelerometer to measure activity (steps per h) and a CTS to measure the number of rump touches and total touch time. The correlation between plasma estradiol concentrations and overt signs of estrus ranges from near 0 for a cow coming into estrus to [greater than] 0.5 for a cow going out of estrus. Lactating cows had shorter estrus periods because the interval from the onset of estrus activity to the LH surge was shorter. Selection for a longer estrus period (based on activity) could potentially increase the interval from the onset of activity to the LH surge and provide for a longer estrus. In conclusion, the repeatability for rump touches during estrus was approximately 0.2 and this suggests that the maximum heritability for this estrus trait is 20 [percent]. Selection for the number of rump touches during estrus, therefore, should increase overt signs of estrus that include rump touches in dairy cows. The correlation between rump touches and the number of steps taken during estrus was 0.45. Selecting cows for increased activity should increase the number of rump touches (mounts, chin rests, etc.) because based on the correlation at least 20 [percent] (r[superscript 2]) of the variation in the number of steps was explained by the estrus cow walking in response to other cows interacting with her rump. Likewise, selecting cows for rump touches at estrus using a CTS device (or similar) should increase the number of steps during estrus. These data can be used to support large-scale phenotyping projects of cows with known genotypes to perform genome-wide association studies (GWAS) that can be used to identify genetic makers for estrus expression. These genetic markers for estrus expression can be incorporated into genetic selection indices to improve estrus expression in dairy cows.
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