Accounting for Backfat Thickness on the Prediction of Residual Feed Intake of Young Beef Bulls Influences Its Relationship to Breeding Soundness Examination Traits

During the last decade there have been great efforts to improve feed efficiency in beef cattle. Although, the use of residual feed intake (RFI) as indicator of net feed efficiency has been the measurement of choice, there are still some concerns about potential antagonistic related effects on reproductive traits, such as age at puberty and fertility traits in young animals. The objective of this study was to examine the relationship between two alternative RFI measures and breeding soundness examination traits in Angus young bulls. Twenty five animals were examined at the end of a 70-d feed efficiency performance test for sperm concentration (SPC) and percentages of motile spermatozoa (PM), progressive motility (PPM) and normal spermatozoa fraction (NSF). The semen sample was obtained by the electroejaculation method and analyzed by the computer assisted sperm analysis (CASA) system. The backfat thickness (BFT) was measured using an Aloka 500 ultrasound with a 17 cm linear transducer. The images were taken by a certified technician and then sent to the Centralized Ultrasound Processing Lab (CUP) for interpretation. The two RFI measures were: RFI_Koch, computed as the residuals from linear regression of DMI on mid-test body weight (BW^0.75) and ADG; and RFI_fat, which included the additional adjustment for BFT. Animals were classified into those with the greatest (least efficient, n = 8), intermediate (n = 9) and least (most efficient, n= 8) RFI values. Two separate one-way ANOVA were performed to evaluate the class effects of feed efficiency as determined by RFI_Koch or RFI_fat. For significant ANOVA results, a Tukey´s range test was run for means comparisons of efficiency groups. Under the RFI_Koch classification, SPC and NSF tended to be greater (P=0.10) for the least efficient group (418.2 ± 97.9 millions/ml and 89.6 ± 3.0 %) than for the intermediate (146.2 ± 92.3 millions/ml and 80 ± 2.8 %) and most efficient (245.8 ± 97.9 millions/ml and 83.2 ± 3.0 %) groups. Those differences disappeared when accounting for BFT under the RFI_fat classification. We concluded that inclusion of backfat in the model to predict RFI for feed efficiency evaluation statistically eliminates its antagonistic relationship with breeding soundness examination traits in young Angus bulls, but the study of potential biological implications deserves further research efforts.