Yield and quality grades were measured in two studies evaluating common breeds (Angus, Hereford, Hereford × Angus and Holstein) and nutrition programs for production of highly marbled beef for the Japanese market. Weaned heifer calves (n = 221, initial weight 265 kg) were slaughtered at 500, 590, and 680 kg in exp. 1. In exp. 2, yearling heifers (n = 216, initial weight 395 kg) were slaughtered at 590 and 680 kg. Cattle were assigned to either a high plane of nutrition (HP) until slaughter or to a moderate plane of nutrition until 500 kg (exp. 1) or 550 kg (exp. 2), followed by HP to slaughter (MHP). Carcasses were graded at the Japanese (6/7th rib) and Canadian/US (12/13th rib) grading sites. Extended feeding of heifers from the three beef breeds for 199 to 474 d and to slaughter weights up to 680 kg enhanced marbling at the cost of very poor USDA and Canada yield grades (extremely fat carcasses). Most carcasses had sufficient marbling to qualify for Canada AAA or USDA Choice quality grades, but only achieved average Japanese quality grades. Carcasses were frequently downgraded by the Japanese grading system because of coarse marbling texture, poor fat texture, and soft or coarse muscle texture, especially at the higher slaughter weights. Compared with the beef breeds, Holsteins produced leaner carcasses with at least as much marbling, and had superior Japanese quality grades, similar Japanese yield grades, and superior Canadian and US yield grades. However, carcasses with dairy conformation may be subjectively downgraded by grading systems in most countries, including Japan. Feeding early-maturing cattle high grain diets to excessive slaughter weights was not effective in producing beef with sufficient marbling to achieve the highest Japanese (5) or US (Prime) quality grades, but produced carcasses that were too fat for North American markets. Key words: Beef cattle, carcass quality, marbling, yield, grade
POSSIBILITIES OF USING MORPHOMETRIC MEASUREMENTS AS A TOOL TO ESTABLISH A LIVE BEEF CATTLE GRADING SYSTEM IN TUNISIA
