Genetic parameters of different feed efficiency traits of Charolais young bulls

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إنجليزي. Genetic selection on feed efficiency could increase the profitability of French beef farms. The BEEFALIM 2020project aims to study the genetic determinism of this trait measured from 2016 to 2019 on 588 young Charolais bullsin four experimental farms. After weaning in commercial farms, the young bulls entered the fattening barns at 303(±26) days of age and were fed ad libitum either with grass or corn silage. During the 29 (±3) weeks of the test period,dry matter intake (DMI) was recorded daily and animals were weighted every 14 days. Average daily gain (ADG)and final body weight (FW) were calculated using regression of body weight on time. Three feed efficiency criteriawere used: residual feed intake (RFI), residual gain (RG) and feed conversion efficiency (FCE). Genetic parameterswere estimated using WOMBAT software with an animal model containing the contemporary group as fixed effect,age at the start of the test as covariate and farm origins × year, genetic and residual as random effects. During thetest period, DMI averaged 9.65 (±1.13) kg/day, ADG 1.51 (±0.29) kg/day and FW 698 (±74) kg. The three traitswere closely correlated phenotypically: 0.67 between FI and ADG, 0.77 between FW and ADG and 0.82 betweenFI and FW. Feed intake and ADG were more heritable (0.34) than FW (0.20). The three feed efficiency traits weremoderately heritable (0.22 for RFI and RG; 0.18 for FCE). No genetic correlation was found between RFI and RGor FCE while a strong genetic correlation of 0.99 was estimated between RG and FCE. Residual feed intake wasgenetically correlated with DMI (0.82), ADG (0.52) and FW (0.49). Both RG and FCE were closely correlated withADG (0.79 and 0.75, respectively) and moderately with DMI (0.31 and 0.23, respectively) or FW (0.33 and 0.25,respectively). Even if RFI and RG were phenotypically independent with traits used in both regressions, geneticrelationships were found between them. Genetic improvement is possible through a selection index combining bothfeed efficiency and production traits.