PSI-36 Phenotypic Relationship of Feed Efficiency and Fertility in Angus and Red Angus Bulls

Abstract

Abstract Recent advances in measuring feed intake improve selection for efficient beef bulls; however, such selection must not decrease bull fertility. Objectives herein were to determine the phenotypic relationship between feed efficiency and fertility traits in growing Angus (n = 14) and Red Angus (n = 14) bulls. Two feeding trials were conducted at the CSU, Chico University Farm in 2017 (87 d) and 2018 (84 d). Each year, bulls were randomly assigned to 7 x 18 m pens equipped with GrowSafe feed intake units and allowed ad libitum access to water and feed (CP: 15.6%, TDN: 56.22%, NDF: 52%, DM basis). Performance traits included: feed conversion ratio (FCR) and residual feed intake [RFI; measured as the difference between predicted dry matter intake (PDMI) and actual dry matter intake (DMI)], start weight (SWT), end weight (EWT), metabolic mid-weight (MMWT), residual gain and average daily gain (ADG). Breeding soundness exams (BSE) were conducted at 18 mo of age and included traits: scrotal circumference (SC), motility (MT), and morphology (MP). Residual feed intake was calculated by regressing DMI on ADG and MMWT. The RFI group was categorized as low (0.5 SD above the RFI mean), marginal (± 0.5 SD about the RFI mean), and high (0.5 SD above the RFI mean) by year. Data were analyzed using ANOVA, blocking on year-pen and fitting breed and RFI group as independent factors. Tukey pairwise contrasts were run. Motility among breeds and RFI group were analyzed using chi-square. Pearson correlations were calculated between growth, feed efficiency, and BSE measures of SC and MP. There were no differences (P > 0.05) among breeds nor RFI groups for SWT, EWT, MMWT, residual gain, SC, and MT. No interaction was detected between breed and RFI group with exception of MP (P = 0.049); specifically, low RFI Red Angus bulls had poor MP compared to marginal and high RFI groups (66.94 ± 6.87 vs. 87.43 ± 3.77 and 93.74 ± 5.48 %, respectively). Angus bulls were superior to Red Angus bulls in ADG (1.23 ± 0.04 vs. 1.06 ± 0.05 kg/d; P = 0.01), FCR (7.98 ± 0.31 vs. 9.18 ± 0.38; P = 0.04), and RFI (-0.24 ± 0.09 vs. 0.09 ± 0.11 kg DMI/d; P = 0.04). High and marginal RFI groups had greater DMI compared to low RFI group (10.40 ± 0.26 and 9.64 ± 0.18 kg/d vs. 8.31 ± 0.32 kg/d; P < 0.001). There were no significant phenotypic correlations between growth and efficiency measures with SC and MP (P > 0.05). Cumulatively, results indicate that SC and MT were not affected in efficient bulls.