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Department of Animal Science, Texas A&M University, College Station, 77843
2 Correspondence: 2471 TAMU (phone: 979-845-3939; fax: 979-458-2702; E-mail: sbsmith{at}tamu.edu).
We predicted that providing rumen-protected starch to the small intestine would increase adiposity of intramuscular adipose tissue, and hence marbling scores. Eighteen 15-mo-old Brangus steers were assigned randomly to one of three dietary treatment groups: 1) cracked corn (Corn); 2) casein-formaldehyde-protected lipid (Canola Lipid); or 3) casein-formaldehyde-protected starch (Marble Plus). All diets were equally balanced for ME (2.91 Mcal/kg), CP (12.5%), and DM (89%). Ether extract was 3.7, 6.9, and 6.9% for the Corn, Canola Lipid, and Marble Plus diets, respectively, and the Marble Plus also contained 3.7% protected starch. Steers were fed the diets for 126 d before slaughter. Average daily feed intake (as-fed basis), ADG, and feed:gain ratio (P 
0.23) did not differ among treatments. Carcasses across treatments did not differ (P = 0.26) in adjusted fat thickness, longissimus muscle area, hot carcass weight, dressing percentage, marbling scores, or USDA quality grade. Percentage of kidney, pelvic, and heart fat was higher (P < 0.01) and USDA yield grade tended (P = 0.08) to be higher, for carcasses from Canola Lipid- and Marble Plus-fed steers than for carcasses from Corn-fed steers. Of the descriptive meat sensory attributes, connective tissue amount (P = 0.06) and painty flavor (P = 0.12) tended to be greater in meat from Marble Plus steers than from Canola Lipid steers. Percentages of 18:2n-6 and 18:3n-3 were higher (P < 0.01), and 15:0, 16:0, and 17:0 were lower (P 
0.07) in tissues from Canola Lipid- and Marble Plus-fed steers than in Corn-fed steers. Mean adipocyte volume was greater (P = 0.02) in i.m. adipose tissue and tended (P = 0.11) to be greater in s.c. adipose tissue of Canola Lipid steers (848 pL) vs. Corn steers (536 pL). Glucose incorporation into total lipids, glyceride-glycerol, and fatty acid fractions was highest (P < 0.01) in s.c. adipose tissue from steers fed Marble Plus but was unaffected (P 0.33) by diet in i.m. adipose tissue. Fatty acid synthetase activity tended (P = 0.08) to be higher in s.c. adipose tissue of Marble Plus steers, and NADP-malic dehydrogenase activity was higher (P = 0.03) in i.m. adipose tissue of Canola Lipid steers. We conclude that Marble Plus did not improve carcass quality, but also did not reduce beef sensory attributes. Any differences we observed in carcass characteristics, adipose tissue cellularity, or lipogenesis apparently were caused by the protected lipid rather than the protected starch.
Key Words: Adipose Tissue • Beef Cattle • Carcass Quality • Protected Fat • Starch
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