Random regression analysis to infer the genetic plasticity in superovulatory responses of donor cows by altering temperature-humidity index

To achieve future sustainable high-quality Wagyu beef production, it is important to provide a novel approach to genetically improve Japanese Black cattle. This study assessed the possibility of genetic evaluation for adaptability to thermal environment changes in this breed by using random regression model with temperature-humidity index ( THI ) information. A total of 30 491 records of the total number of embryos and oocytes recovered ( TNE ) and the number of good embryos ( NGE ) per flush collected from 2,006 Japanese Black donor cows during 9 January 2008 and 29 January 2024 were analysed. First, on-farm temperature and humidity data were recorded using a data logger for 1 231 days during 9 April 2021 and 30 November 2024. Daily mean ambient dry-bulb temperature at the location of the Zen-noh Embryo Transfer Center during 1 January 2007 and 29 January 2024 were estimated using meteorological observation records from weather stations. Quadratic regression analysis was performed to provide an equation to estimate on-farm daily mean THI using estimated temperature. Estimated THI were calculated using the equation obtained with the coefficient of determination of 0.972 and estimated temperature during 1 January 2007 and 31 December 2024, resulting estimated THI ranged from 29.0 to 82.2. Next, multiple regression analyses of TNE and NGE were conducted to identify a suitable day for considering the effect of THI as a quadratic regression form. A day 13 days before embryo collection was identified as a suitable day to consider the effect of THI and the estimated curve was convex upward within the range of estimated THI for both traits. Finally, a two-trait random regression model with estimated THI at the day detected as a suitable day was fitted to TNE and NGE records to estimate variance components and predict breeding values. Given the range of estimated THI as 30 to 80, estimated heritability using the random regression model ranged from 0.22 to 0.33 for TNE and from 0.17 to 0.25 for NGE. Genetic correlation across estimated THI within trait was estimated to be generally very high but sometimes lower than 0.8 between THI <70 and THI >70. Estimated genetic correlation between the traits ranged from 0.09 to 0.65. In conclusion, this study could provide the genetic evaluation model for thermal environment adaptability in Japanese Black cattle reared in Japan using meteorological observation records and superovulatory response performance records, as well as providing fundamental insight into improving heat-tolerance in cattle.