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It is unclear how age, physiological maturity, and genetics affect intramuscular fat (IM) desposition in cattle. The study used beef cattle of known age and parentage to study the development of primal cuts, total carcass fat and IM fat depots as part of the growth process. Selecting cattle for marbling with the use of paternal grandsire's EPD for marbling was not indicative of differences in the onset or the rate, of development of marbling. Greater differences in EPD for marbling may be needed to observe phenotypic differences. Harvest group affected the level and extent of marbling (P<.10), however there was no harvest group x marbling group interactions indicating no differences occurred in the pattern of marbling development due to marbling EPD. Carcasses expressed a Small degree of marbling between the hot carcass weights of 550 and 650 Ibs. and at a back fat depth of approximately .30 in. In this study utilizing non-implanted steers of the same breed, we found that as days on feed increased, hot carcass weights, back fat depth, and percent carcass fat increased along with marbling score as well as percent 12th rib lipid content. No differences were observed in the weight of the primal cuts when expressed as a percentage of the chilled carcass between marbling groups at each of the five end points. As HCW increased across harvest groups, primal weight increased without a change in the percentage of the carcass represented by the middle meats (sirloin, shortloin, rib).

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