Cardiac function and feed efficiency: Increased right-heart workload in feed inefficient beef cattle

Abstract

Heart rate (HR) during rest and stress as a beef cattle feed efficiency proxy (residual feed intake; RFI) has potential to reduce environmental impact and feed costs. A broader understanding of the association between the cardiac system and feed efficiency is needed before commercial application of heart rate as a feed efficiency proxy can occur. Objectives of this study were to assess the relationships between cardiovascular function and structure traits, their relationships with RFI and carcass composition, and the variation in RFI they explained. Residual feed intake, overnight HR, transport HR, abattoir HR, low-frequency power (LF), stroke volume indicator (SVI), cardiac output indicator (COI), blood volume (BV), oxygen-carrying capacity measures, heart structure measures, histomorphometry measures, actomyosin Mg2+ATPase activity measures, myofilament protein phosphorylation measures and carcass composition measures were determined in heifers, bulls and steers (n; 165, age; 390 ± 34 days, weight; 526 ± 127 kg). Correlations amongst traits and with RFI were determined and RFI-dependent multiple regression models containing linear and quadratic effects were selected, where a regression coefficient represented the effect of each trait on RFI. Positive and negative effects of HR and LF within overnight, transport and abattoir recordings on RFI were observed. Mean corpuscular volume trended towards a positive effect on RFI (P = 0.09). Carcass composition measures were only correlated with heart structure measures. Blood volume, SVI, COI and myofilament protein phosphorylation measures were not correlated with RFI. Right ventricle thickness had a positive correlation with RFI (P = 0.005) and myocyte width (P = 0.07) and actomyosin Mg2+ATPase activity measures trended toward positive correlations with RFI (P < 0.10). Positive effects of BV (P = 0.01), right ventricle thickness (P = 0.002), myocyte width (P = 0.05), actomyosin Mg2+ATPase activity at submaximal calcium (P = 0.05) and myosin light chain 2 phosphorylation (P = 0.09) on RFI and negative effects of SVI (P = 0.05) on RFI were observed. Heart structure measures explained the largest amount of RFI variation (Adj. R2 = 0.145; P = 0.002). Results indicate that high-RFI cattle have an increased right heart workload, potentially due to differences in activity elicited by higher brain centers, suggesting selection for low-RFI cattle will not negatively impact cardiovascular structure and function.