Influence of acetogenic versus propiogenic supplements on adipose tissue accretion in stocker steers grazing ryegrass pasture

Abstract

Fifty-eight high grade Bonsmara steers were used to evaluate effects of high-fiber versus high-starch pasture supplements on subcutaneous (s.c.) and intramuscular (i.m.) adipose tissue accretion during growing and finishing phases. Cattle were stratified by body weight (BW), randomly assigned to one of three treatments, and placed on irrigated ryegrass pastures. Treatments were 1) no supplement (NC); 2) commercially available, pelleted high roughage (HR) supplement, designed to promote higher acetate fermentation, fed at 1.36 kg/hd (as-fed) 6 d/wk; or 3) corn-based high starch (HS) supplement, designed to promote higher propionate fermentation, fed at the same rate and frequency as HR. Throughout growing (140 d) and finishing (119 d) phases, full BW was measured every 28 d. Ultrasound ribeye area (REA), percent i.m. fat (IMF), and 12th rib fat thickness (BF) were measured on d -15, 56, 112, 182, and 231. Mixed model repeated measures analysis was performed on growth performance and ultrasound data. All responses increased over time (P < 0.01). Treatment by time interaction (P = 0.05) for BW was due to treatment rank changes among days; within day separations were minimal. Neither treatment nor interaction affected (P > 0.20) IMF, but treatment influenced ultrasound REA (P = 0.05); HS-fed steers had larger REA than HR-fed steers; NC steers were intermediate. Treatment effects on REA with similar IMF suggest that HS-fed steers accreted a greater total amount of i.m. fat. One-way structure analysis of carcass data was performed. Treatment did not affect hot carcass weight (HCW), carcass REA, or carcass fat thickness (FAT) (P > 0.48), but tended (P = 0.15) to affect marbling score (MARB). Supplemented cattle tended to have greater MARB than non-supplemented steers, and MARB was greater for HS-fed steers than that of HR-fed steers. The relationship between carcass REA and MARB is consistent with the relationship observed between ultrasound REA and IMF. These observations suggest that source of energy supplementation partitioned nutrients during the growing phase to favor i.m. fat accretion.