Effect of diets containing linoleic acid- or oleic acid-rich oils on ruminal fermentation and nutrient digestibility, and performance and fatty acid composition of adipose and muscle tissues of finishing cattle 1 A. N. Hristov 2 , L. R. Kennington, M. A. McGuire, and C. W. Hunt Department of Animal and Veterinary Science, University of Idaho, Moscow 83844-2330 ABSTRACT: Two trials were conducted to determine the effect of linoleic acid- or oleic acid-rich safflower oil on ruminal fermentation, nutrient digestion, feedlot performance, carcass characteristics, and fatty acid composition of adipose and muscle tissues of beef cattle. In both trials, cattle were fed a finishing diet based on barley grain, wheat silage, and alfalfa hay. Oils were fed at 5% of dietary DM. In a metabolism trial, four ruminally and duodenally cannulated Angus crossbred steers were subjected to linoleic acid-rich oil or oleic acid-rich oil in a crossover design with covariate periods (no oil supplementation). In a finishing trial, 16 individ- ually fed Angus crossbred steers and heifers (eight per diet) received linoleic acid- or oleic acid-rich oils during the last 86 d of a 116-d feeding period. Ruminal pH, ammonia concentration, protozoal counts, major VFA concentrations, acetate-to-propionate ratio, polysac- Key Words: Cattle, Conjugated Linoleic Acid, Performance, Protozoa, Safflower Oil 2005 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2005. 83:1312–1321 Introduction Fat consumption continues to increase in the United States (Putnam et al., 2002), and techniques allowing modification of tissue and milk fat composition (De- meyer and Doreau, 1999) could improve the healthful characteristics of the fat produced by ruminants. Conju- gated linoleic acids, present in meat and milk from ruminant animals, have potential health benefits (Be- 1 This study was supported by a grant from the Idaho Beef Council and funds from the Idaho Agric. Exp. Stn. The authors gratefully acknowledge G. Pritchard, J. K. Ropp, J. Szasz, R. Manzo, R. Falen, and J. Parker for technical assistance, and thank R. Richard for performing the carcass analyses, W. Price for assistance with statisti- cal evaluation of the results, and the staff of the Dept. of Anim. and Vet. Sci. Beef Res. Center for their conscientious care of the experimental animals. 2 Correspondence: P.O. Box 442330 (phone: 208-885-7204; fax: 208- 885-6420; e-mail: ahristov@uidaho.edu). Received August 20, 2004. Accepted March 9, 2005. 1312 charide-degrading activities, microbial N flow to the duodenum, and the efficiency of microbial N synthesis in the rumen were not affected (P = 0.18 to 0.96) by type of oil. Type of oil had no effect on total-tract apparent digestion of nutrients (P = 0.46 to 0.98). Ruminal true nutrient digestibilities did not differ between oils (P = 0.15 to 0.99), except that the linoleic acid-rich oil de- creased (P = 0.05) NDF digestibility. Dry matter intake, ADG, G:F, and carcass characteristics did not differ (P = 0.11 to 0.84) between the two oils. Overall, the difference in dietary fatty acids provided to the cattle produced few changes in tissue fatty acids. Weight per- centages of c9t11 CLA were unaltered by the addition of linoleic acid to the diet compared with oleic acid, probably as a result of low vaccenic acid production in the rumen, as the pathway of biohydrogenation was apparently primarily through the t10 pathway. lury, 2002) and are formed through isomerization of linoleic acid by ruminal bacteria (Harfoot and Hazle- wood, 1997) and via desaturation by body tissues of another product of biohydrogenation, trans-vaccenic acid (t11 18:1, VA; Griinari et al., 2000). Thus, it may be possible to increase the content of CLA in fat and muscle from beef animals through increased dietary availability of the substrate, linoleic acid. Linoleic acid also may increase CLA concentrations by altering rumi- nal protozoa (Sutton et al., 1983; Hristov et al., 2004), either directly through increased lipolysis, or indirectly by affecting the bacterial population, including Group A bacteria that are thought to be primarily responsible for the biohydrogenation of dietary linoleic acid (Har- foot and Hazlewood, 1997). Oleic acid, on the other hand, produces no CLA and very little VA during its biohydrogenation (Mosley et al., 2002), and thus should not lead to enhanced CLA concentrations in body tis- sues. Therefore, oleic acid provides a good comparison with linoleic acid, as it also is an unsaturated fatty acid that undergoes biohydrogenation in the rumen, but it will lead to very minor CLA formation in tissues.