Protein metabolism, body chemical composition, carcass traits and meat quality of Nellore steers (Bos indicus) as a function of their residual feed intake / Metabolismo protéico, composição corporal, características de carcaça e qualidade de carne de novilhos Nelore (Bos indicus) em função de seu consumo alimentar residual

Rodrigo da Costa Gomes

Residual feed intake (RFI) is a feed efficiency trait that is independent of growth rate and mature weight. Genetic improvement in RFI may reduce the costs of feeding cattle, however a better understanding of biological processes underlying variation in RFI is necessary. Moreover, associations between RFI and carcass quality have been poorly investigated in Zebu breeds. Therefore, this study aimed to evaluate protein metabolism, body composition, carcass traits and meat quality in high- and low-RFI Zebu cattle. In addition, the hypothesis that there are interactions between RFI and harvest body weight for carcass traits and body composition was investigated. Seventy-two Nellore steers (16 to 21 month-old, 334±19 kg initial body weight [BW]) were fed a finishing ration (74.5% TDN, 14.3%CP) on an ad libitum basis, for 70 days. Daily dry matter intake (DMI) and body weight gain (ADG) were measured individually. The 12 lowest and the 12 highest RFI steers were classed as low- and high-RFI groups, respectively (selection phase), and were fed until reaching slaughter BW of 460, 490, 520 and 550 kg (finishing phase). Before slaughter, total urine was collected for determination of daily 3-methylhistidine excretion and myofibrillar protein breakdown rates. Body chemical composition was estimated by the isotope dilution method using deuterium oxide. Objective tenderness and Ca+2-dependent protease activities were measured on Longissimus muscle. In the selection phase, low-RFI steers had lower DMI, feed:gain, RFI, and ultrasound rump fat thickness gain than high-RFI cattle, but no differences were observed for ADG, final BW, ultrasound fat thickness and Longissimus area. In the finishing phase, no interactions were observed between RFI and slaughter BW. No differences between more and less efficient cattle were observed for hot carcass weight, dressing percentage, kidney, pelvic and inguinal fat, visceral mass, Longissimus area, backfat thickness, marbling score, trimmings and retail product yield. Low-RFI steers presented less fat on the gastrointestinal tract (GIT) than high-RFI cattle. No differences were observed for myofibrillar fragmentation index, Warner-Bratzler shear force and calpain system activities. Fractional rates of protein degradation, synthesis and accretion were similar between high- and low-RFI cattle. Low-RFI Nellore steers may store less subcutaneous carcass fat at body weights ranging from 340 and 460 kg. At higher body weights (460-550 kg), carcass traits and body composition are not affected by RFI, but least efficient cattle present greater visceral fat mass. Breeding zebu cattle for improved RFI may decrease feed intake and improve feed efficiency without compromising meat quality.

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