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Digitized by Kansas Correctional Industries

Typescript (photocopy). / Digitized by Kansas Correctional Industries

Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 52-64). Also available on the World Wide Web.

Objectives of this study were to determine if variation in energy expenditures contributed to differences in feed efficiency between low and high RFI steers. Nine steers with the lowest and highest residual feed intakes (RFI) were selected from 169 Braunvieh-sired crossbred steers that were individually fed a pelleted roughage-based diet for 77 d. Following the RFI measurement period, heat production (HP) measurements were obtained using indirect calorimetry while steers were fed the same roughage diet (RD) and on a high-concentrate diet (CD). Linear regression analyses of log HP or retained energy on ME intake were used to determine energy partitioning. Motion and lying activity were measured concurrently with HP on the RD and CD. During the RFI measurement period, low RFI steers had lower (P < 0.01) RFI (-1.7 vs. 1.6 ± 0.17 kg/d), DMI (7.7 vs. 10.2 ± 0.42 kg/d) and feed:gain ratio (F:G; 7.2 vs. 10.6 ± 0.60), but similar final BW and ADG compared to high RFI steers. However, there were smaller differences in DMI (8.4 vs. 9.7 ± 0.38 kg/d; P < 0.05; 7.56 vs. 8.16 ± 0.31; P = 0.19) and F:G (10.0 vs. 10.9 ± 0.40; P = 0.36; 6.5 vs. 7.5 ± 0.30; P < 0.05) between low and high RFI steers, on the RD and CD, respectively. ME for maintenance (MEm; kg .75 d–1) and the partial efficiencies of ME used for maintenance and gain were similar for low and high RFI steers. Likewise, no differences were found in fasting HP or fed HP. Motion activity was lower (P < 0.05) for low RFI steers compared to high RFI steers during fasting HP. Covariate analysis of HP at the same activity level yielded similar results. At slaughter, weights of lung and trachea (P < 0.05), spleen (P < 0.05) and adrenal gland (P = 0.07) were higher for low RFI cattle. The lack of differences in energy partitioning between divergent RFI steers may have been the result of alterations in feeding behavior or stress imposed by adapting steers to calorimetry chambers.

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Monensin was fed at levels of 0 and 33 ppm in a series of experiments to determine its effect on nitrogen (N) utilization by steers fed diets high in sorghum grain (76%, steam-processed, flaked) and corn (90%, whole shelled). Total ruminal volatile fatty acid (VFA) concentrations measured 3 hr postprandially were not affected by monensin addition to either diet. Molar proportion of acetic acid decreased (P < .05) was observed with monensin addition to the sorghum grain-based diet. Monensin had no effect on apparent total tract digestibility of dry matter or energy in either diet, but consistently improved apparent total tract digestibility of crude protein (CP) in both diets. Improvement was greater (P < .05). Retention of N tended (P > .05) to improve in response to monensin with the sorghum grain-based diet (24 vs 20% of N intake; 41 vs 36% of N absorbed). Ruminal ammonia concentrations measured 3 hr post-prandially were decreased (P < .10) ruminal digestion of feed N (44 vs 51%) for steers fed monensin. Monensin decreased (P < .05) the contibution of bacterial N (42 vs 50%) and increased (P < .05) that of ruminally undegraded feed (bypass) N (58 vs 50%) to total N digested postruminally, resulting in less (P < .10) bacterial N (23 vs 28 g/day) and a tendency (P > .10) for more feed bypass N (32 vs 27 g/day) to be digested in the intestines of steers fed monensin with the corn-based diet. That monensin caused a greater proportion of feed OM and N to be digested and absorbed in the intestines than in the rumen (with possibly greater resultant metabolic efficiency) may account for some of the benefit of feeding this compound with high grain diets, as losses incurred in the ruminal transformation of feed nitrogen to bacterial nitrogen appear to be partially eliminated.

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With feed costs representing on average 65% of the total cost of production improvements in feed efficiency could have a major impact on the beef industry. Postweaning performance and feed efficiency data were collected on 114 heifers born on the University of Arizona's V bar V Ranch. Residual feed intake (RFI) was significantly correlated with dry matter intake (DMI) (r = 0.46) and G:F (r = -0.43), while G:F was moderately correlated with initial age (r = -0.54), initial body weight (r = -0.40), and average daily gain (ADG) (r = 0.68). Residual gain (RG) was moderately correlated with RFI (r = -0.41) and highly correlated with ADG (0.62) and G:F (0.73). In addition, RG was significantly correlated (r = -0.21) with initial weight suggesting that selection for RG could lead to increased cow mature size. Heifers were sorted into three breed categories: Hereford, Hereford cross, and Wagyu cross and breed effect on performance and feed efficiency traits were analyzed. Significant differences in DMI, FCR, and RG were noted, but likely due to significant differences in age at the time of testing. RFI was not significantly different between breed groups, suggesting that RFI is less influenced by animal age than other measures of feed efficiency. In addition, sires of study heifers with 5 or more progeny were selected and progeny performance and feed efficiency traits were compared between sires. No significant differences in ADG, RFI, FCR, or RG were noted between sire progeny. This was likely due to an insufficient number of progeny used in the trial. Further research and an increased number of animals used may uncover significant impacts of sire on RFI.