Download or read book The Relationship Between Performance Residual Feed Intake and Temperament Assessed in Growing Heifers and Subsequently as Three year Old Suckled Beef Cows written by Tera Elizabeth Loyd and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Heifers were ranked by RFI and placed into Low (most efficient; 0.5 SD; n = 24), Med (0.5 SD; n = 24), and High (least efficient;0.5 SD; n = 26) RFI groups. Initial BW, final BW and ADG were similar among groups. However, daily DMI differed for all groups (P

Download or read book The Influence of Breed and Temperament on Circulating Concentrations of Insulin like Growth Factor I IGF I and Its Relationship to Feed Efficiency in Beef Cattle written by Lisa Caldwell and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Insulin-like growth factor-I (IGF-I) is a growth hormone that acts as a key modulator of the growth axis. Serum and plasma concentrations of IGF-I have been linked to economically important traits in beef cattle. In order to determine whether concentrations of IGF-I differed among breeds of beef cattle, plasma samples from purebred and crossbred animals were analyzed. Two calf crops were derived from three-breed diallel matings using temperate and tropically-adapted breeds of cattle. The breeds consisted of temperate Bos taurus (A; Angus), tropical Bos indicus (B; Brahman), and tropical Bos taurus (R; Romosinuano). Plasma samples were obtained from 10 heifers and 10 steers of each breed-type at weaning, and two dates post-weaning. Concentrations of IGF-I were determined by radioimmunoassay (RIA). Breed differences were observed (P 0.001). Relative to the temperate Bos taurus breed, IGF-I was greater in tropically-adapted breed-types. In an effort to select for the improvement of economically important traits, experiments were performed to explore the possible use of concentration of IGF-I and temperament assessment as tools for selection. Using a Calan gate system, 3 Brahman heifer crops were fed during70-day trials. Performance and feed intake data were collected to determine feed efficiency. Temperament, determined by exit velocity and pen score, was evaluated at weaning. Serum samples were taken at weaning and days 0 and 70 of each trial. Concentrations of IGF-I and cortisol were determined by RIA. Correlations including IGF-I were weak (P 0.05). Temperament had no significant effect on RFI but may affect ADG. In an attempt to examine the relationship between IGF-I and RFI, body weight and feed intake data were collected during individual finishing phase feeding trials, on steers at El Reno, OK. The breeds consisted of temperate Bos taurus (A; Angus), tropical Bos indicus (B; Brahman), and tropical Bos taurus (R; Romosinuano). Plasma samples were obtained from 10 steers of each breed-type at weaning and days 0 and 60 of each finishing phase. Concentrations of IGF-I were determined by RIA. Correlations between IGF-I and RFI were weak (P> 0.05). Breed and year significantly influenced RFI (P

Download or read book Examining Mechanisms Contributing to the Biological Variation of Residual Feed Intake in Growing Heifers and Bulls and in Mid gestation Females written by Aimee Hafla and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The objectives of this study were to characterize residual feed intake (RFI) in growing bulls and heifers and in mid-gestation females to examine relationships with performance, body composition, feeding behavior, digestibility (DMD) and heart rate (HR) and evaluate the impact of RFI on bull fertility and cow forage utilization. Additionally, use of the n-alkane method to predict individual animal variations in intake was investigated. To accomplish these objectives, multiple RFI studies were conducted. In all studies RFI was computed as the difference between actual and expected DMI from linear regression of DMI on mid-test metabolic BW and ADG. To evaluate phenotypic relationships between feed efficiency, scrotal circumference (SC) and semen-quality an experiment was conducted with yearling bulls (N=204). Residual feed intake was not correlated with BW and ADG, but was positively associated with 12th-rib back fat (BF) such that the more efficient bulls were leaner. Bulls with low RFI had similar SC and progressive motility of sperm compared to high-RFI bulls. However percent normal sperm were weakly associated with RFI in a negative manner. To examine phenotypic relationships between heifer postweaning RFI, and performance, efficiency, HR, and DMD of mid-gestation cows, RFI was measured in growing Bonsmara heifers (N=175). Forty-eight heifers with divergent RFI were retained for breeding. Subsequently, intake, performance and feeding behavior was measured on mid-gestation females. Pregnant females classified as having low postweaning RFI continued to consume 22% less feed, spent 25% less time eating, and had 7% lower HR while maintaining similar BW, ADG and body composition compared to high RFI females. A moderate association between RFI in growing heifers and subsequent efficiency of forage utilization in pregnant cows was found. Growing heifers identified as efficient had greater DMD, however DMD in mature cows was similar between RFI groups. The n-alkane method of predicting intake detected differences in intake between divergent RFI groups in mid-gestation females. Results from this study indicate that inclusion of RFI as a component of a multi-trait selection program will improve feed efficiency of growing animals and mid-gestation females with minimal impacts on growth, body composition, and fertility traits.

Download or read book Identification of Feed Intake Traits Related to Beef Reproductive Improvement written by Cameron Alexander Olson and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Reproductive efficiency in the Western Canadian beef cow herd has not improved over the past 3 decades, despite consistent and measurable improvement in several related areas of production. The general objective of this thesis was to evaluate the relationships between early-observation traits (feeding behaviour), weight change, and reproductive efficiency in beef cows. Estimates of total dry matter intake and the relationship between residual feed intake (adjusted for body composition; RFIFAT) observed in heifers and mature cows were also evaluated as traits of interest and use to the Canadian beef industry. Chapters 3 and 4 investigated the relationship between feeding behaviours and reproductive efficiency and provided phenotypic and genetic correlations that may be of use in selecting heifers with greater reproductive potential if those heifers have feeding behaviours reported. Feeding behaviour and reproductive efficiency were both correlated with dry matter intake; feeding behaviours that promote dry matter intake may be useful in the identification of heifers with greater reproductive potential. Chapter 5 evaluates the usefulness of tracking cow weights over time and the usefulness of comparing cow weights to an idealized growth curve estimation. Generally, cows that gained weight over time were more likely to be culled for reproductive failure, and the same was true for cows that were heavier than their estimated body weight. Producers may be able to use weight-monitoring technology currently available and in development to identify cows that abort their calves after a positive pregnancy evaluation and remove them from their herd at a time when feed resources are expensive. Chapter 6 was a comprehensive and unique estimation of cow dry matter intake over time, including energy estimates based on birth, weaning, residual feed intake tests as a heifer and as a cow, and subsequent calving events with associated energy expenditures for milk production. These estimates were compared to genomic retained heterozygosity, breed composition, and winter-feeding environment to evaluate the effects of those variables on dry matter intake predictions. Chapter 7 investigated the relationship between heifer residual feed intake and residual feed intake observed in the same animals as mature cows. Residual feed intake in heifers can be used to select cows that maintain a proportion of their efficiency observed as heifers, and ultimately provides evidence that the selection of feed-efficient heifer calves as replacement animals should result in a more efficient mature cow herd. This thesis provided evidence to support the selection of heifer replacements using feeding behaviours observed during a feed intake test, the use of weight monitoring technology to identify cows that may have had reproductive issues. This thesis also provided some of the first estimates of dry matter intake over the course of the production cycle in a large number of animals under normal production environments and provided estimates of the relationship between heifer and cow residual feed intake.

Download or read book Residual Feed Intake in Beef Cattle in Relation to Breed Selection and Behavior written by Marcus McGee and published by . This book was released on 2014 with total page 310 pages. Available in PDF, EPUB and Kindle. Book excerpt: Feed efficiency is a necessary factor in beef production systems due to rising feed ingredient costs, population growth, and beef supply demands. With this known, it is important for researchers to delve into mechanisms involved in efficiency to provide producers with a knowledge base for future selection and breeding programs. Though the use of residual feed intake (RFI) is fairly well know, traditional traits are often considered more desirable due to costs associated with testing. However, previous research has provided evidence of benefits using a less biased measurement (RFI) in comparison with its counterparts (e.g. ADG, FCR, F:G etc.) Though RFI does provide indication of efficiency status, there are underlying mechanisms that also play a role in these measurements that are not completely understood. One attribute of efficiency often overlooked is feeding behavior, in which this body of work attempts to interpret via relationships with RFI. Previous research has provided has examined relationships between these two parameters, however further studies are necessary to gain a more solid foundation. Research within this dissertation will add to this knowledge base using two breeds of cattle examined for behavior traits (bunk visit frequency, bunk visit duration, feed bout frequency, feed bout duration, meal frequency, meal duration, and average meal intake) during the course of RFI evaluation. Niche breeds of beef cattle (e.g. Japanese Black Cattle) are often excluded from feed efficiency examinations, due to focuses of output, carcass quality traits, and increased specialty market value. However, the recent rise of feed ingredient costs are universal, and producers of niche breeds may benefit as well. Within this body of work, 3 cohorts of Wagyu cattle were evaluated for feeding behavior parameters with some insight towards relationships with efficiency. Red Angus cattle were also evaluated over both growing and finishing phase RFI to clarify feeding behaviors' role in efficiency. Results from these studies provide researchers with additional tools for elucidating these relationships as well as identifying specific feeding behaviors that contribute to efficiency status in beef cattle.

Download or read book The Relationship Between Residual Feed Intake and Feeding Behavior in Growing Heifers written by Glenda Marie Bingham and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The objective of this study was to determine if feeding behavior traits are correlated with performance and feed efficiency traits in growing heifers. Individual dry matter intake (DMI) was measured in Brangus heifers (n = 115) fed a roughage-based diet (ME = 2.1 Mcal/kg) for 70 d using Calan gate feeders (6 heifers/pen). Residual feed intake (RFI) was computed as the residuals from linear regression of DMI on mid-test BW0.75 and average daily gain (ADG). Heifers with the highest (n = 18) and lowest (n = 18) RFI were identified for feeding behavior measurements. During days 28 through 56 of the 70-d feeding trial, continuous video recordings were obtained for all heifers. Video images of two sets of four 24-h periods, two weeks apart, were analyzed for the focal animals. All occurrences of feeding were timed and counted per day, and the eight 24-h periods averaged to derive the overall feeding event (FE) and meal duration and frequency for each focal heifer. Total feeding event duration was defined as the total min per day the animal0́9s head was down in the feed bunk. A meal included all visits an animal made to the feed bunk that were separated by less than 5 min. The mean RFI values for the low and high RFI heifers were (mean " SE) - 1.03 and 1.00 " 0.03 kg/d, respectively. Low RFI heifers consumed 21.9% less (P

Download or read book Relationships Between Residual Feed Intake and Performance of Heifers of Diverse Breedtypes and Brahman Cows written by Andrea N. Loyd and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: These studies were designed to evaluate the relationships between residual feed intake (RFI) and performance of growing heifers and Brahman cows. Residual feed intake was determined for 77 heifers of diverse breedtypes (Angus, Brahman, Hereford, Holstein, Jersey and F1 crosses) during both the pre- and post-pubertal periods. Heifers were individually fed and allowed ad libitum access to feed for 84 +/- 6 d during the prepubertal feeding trial and 90 +/- 4 d during the post-pubertal feeding trial. Brahmaninfluenced heifers had lower RFI than heifers without Brahman influence during both the pre-pubertal (P

Download or read book Effects of Residual Feed Intake Classification on Temperament Carcass Composition and Feeding Behavior Traits in Growing Santa Gertrudis Heifers written by Justin Anthony Ramirez and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Objectives of this study were to evaluate the associations of feed intake, feeding behavior, temperament, and carcass composition with performance and feed efficiency traits in growing heifers. Santa Gertrudis heifers (n = 369) with initial BW of (275.3 ± 37.4 kg) were used in this study. Intake and feeding behavior traits were collected for 70 d using a GrowSafe system while consuming a forage-based diet (ME = 2.19 Mcal/kg DM). Heifers were weighed at 14-d intervals and ultrasound traits measured on d 0 and 70. Residual feed intake was computed by regression of DMI on mid-test BW0·75 and ADG, and heifers classified into low, medium, and high RFI (± 0.50 SD) groups. A 2- population distribution model was fit to log10-transformed non-feeding interval lengths to estimate meal criterion (MC), which was used to compute meal traits (frequency and duration). As expected, RFI was positively correlated with DMI (r = 0.72; P

Download or read book Phenotypic Relationships Between Residual Feed Intake Feeding Behaviour and Temperament in Western Canadian Beef Cattle written by Lindsey Lippens and published by . This book was released on 2018 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Feed costs have led the cattle industry to examine the merit of selecting cattle for inputs (feed efficiency) rather than for outputs exclusively (growth). Residual feed intake (RFI) has become a preferred measure of biological efficiency. Feeding behaviour traits (feeding event duration, frequency, head-down time, length and eating rate) and temperament may provide insight into the variation in feed efficiency. Feeding behaviour, RFI (n = 868) and temperament (n = 58) were examined using five classes of beef cattle. Cattle were sorted into low, medium and high RFI groups. Feeding behaviour traits were moderately (-0.21 to 0.56; P 0.05) related to RFI. Bulls fed a grain-based diet spent longer periods lying down than bulls fed a forage-based diet. No relationships (P 0.1) were observed between RFI and bull temperament, indicating that temperament is not correlated with RFI and selecting for low RFI cattle will not negatively impact temperament.

Download or read book Relationships of Residual Feed Intake with Performance Profitability Digestibility and Carcass Traits in Individual and Group Pens written by Gustavo Durante Cruz and published by . This book was released on 2007 with total page 152 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Determining Biological Sources of Variation in Residual Feed Intake in Brahman Heifers During Confinement Feeding and on Pasture written by Robert O. Dittmar and published by . This book was released on 2008 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Objectives were to characterize residual feed intake (RFI) and determine the phenotypic correlation between performance, feed efficiency, and other biological measurements in Brahman heifers, as well as the relationship between RFI determined in confinement and measurements of grazing activity on pasture. Three separate 70 d feeding trials were performed, and RFI was determined as the residual between actual and predicted dry matter intake (DMI) for a given level of production. Brahman heifers (n = 103; 5-to-9 mo of age) were individually limit-fed a pelleted 12% CP complete ration daily in Calan gates. Weekly body weight (BW) and DMI data were collected, and predicted DMI was determined by linear regression of actual DMI on mid-test metabolic BW. Ytterbium chloride was used to evaluate digestive kinetics, and fecal samples were collected to determine fecal volatile fatty acids (VFA) concentration and determine apparent dry matter digestibility (DMD) utilizing acid insoluble ash as an internal marker. Measurements of temperament were evaluated on all heifers at weaning. High (n = 6) and low (n = 6) RFI heifers (Exp. I) grazed fescue and ryegrass to determine variation in grazing behavior, DMI, and apparent DMD. Data from all three experiments were pooled, and RFI was not correlated with average daily gain (ADG), DMI, BW, partial efficiency of gain, feed conversion ration, fecal VFA concentration, or any measures of temperament. There were no significant differences in digestive kinetics between the RFI efficiency groups. Fecal samples taken for acid detergent insoluble ash (ADIA) determination were not collected at frequent enough intervals to account for weekly variation in fecal ADIA concentration. Pasture measurements were not different between the efficiency groups for heifers evaluated for grazing behavior, as well as estimated intake as a proportion of BW, or apparent DMD. Results of this study suggest that Bos indicus cattle appear to have similar efficiency traits as Bos taurus and Bos indicus influenced cattle, making this measure of efficiency equally as valid for use in both types of cattle. This indicates that selection based on RFI can be made to increase feed efficiency without affecting ADG or BW in Brahman cattle.

Download or read book Energy Expenditure in Growing Heifers with Divergent Residual Feed Intake Phenotypes Effects and Interaction of Metaphylactic Treatment and Temperament on Receiving Steers written by Zachary D. Paddock and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Cattle classified as having low residual feed intake (RFI) phenotypes are those that consume less feed than expected based on body weight and growth performance. Mechanisms contributing to the variation in RFI are not fully understood. Previous studies have shown that cattle of divergent RFI phenotypes have different levels of energy expenditures, which are associated with heat increment, basal metabolism, thermoregulation responses, and physical activity. The objectives of this experiment were to characterize residual feed intake (RFI) in growing heifers and to determine if variation in whole-animal energy expenditure contributes to differences in RFI. Brangus heifers (n =120) were individually fed a roughage-based diet (1.93 Mcal ME/kg DM) diet twice daily and feed refusals measured weekly. Heifers were weighed once weekly for 70 d and RFI calculated as the difference between actual and expected DMI from linear regression of DMI on ADG and mid-test BW0.75. Immediately following the 70 d study, oxygen pulse rate (mL O2/heart beat) and 48-h heart rates were measured on 8 high and 8 low RFI heifers to estimate energy expenditure. Daily heart rates and oxygen pulse rates were higher (P

Download or read book Evaluation of Modeled Feed Efficiency in Beef Cattle written by Jason R. Bowey and published by . This book was released on 2014 with total page 73 pages. Available in PDF, EPUB and Kindle. Book excerpt: The objectives of this study were to evaluate animal performance and actual feed efficiency and how they relate to model predictions through feed allocation models. In addition, ultrasound measures, scrotal circumference, and molecular value predictions (MVP) were compiled to correlate for associations with efficiency. Six hundred forty-eight growing bulls comprised of Angus, Hereford, and percentage Simmental were placed on intake trials. Data was collected for intake, weight change, and animal performance at either 14-28 d intervals. Feed intake was also predicted with 2 biological simulation models: a) Cornell Value Discovery System and b) NRC update 2000 level 2. Observed residual feed intake (oRFI) was estimated using observed feed intake in a linear statistical model. Observed feed conversion ratio (oFCR) was estimated as observed feed intake divided by average daily gain. Observed partial efficiency of growth (oPEG) was calculated as ADG divided by the difference between dry matter intake and intake for maintenance from published standards. Observed relative growth rate (oRGR) and observed kleiber ratio (oKR) was also determined as the difference between the log of final BW and initial BW over the number of days on test and as the ratio of ADG to metabolic body weight, respectively. Each model also held a prediction for the various feed efficiency measures using feed allocation estimates and projected weight gains in their determinations. Data on feed efficiency measures on all 648 bulls, ultrasound measures and scrotal circumferences on 346 bulls, and hair samples on sixty-six half-blood Simmental bulls for a DNA analysis for MVP were collected and compiled. All data was correlated to determine relationships between feed efficiency measures, both observed and model predicted, as well as ultrasound, scrotal measures, and MVP data as they link to efficiency. Observed feed conversion ratio showed a high correlation both the CVDS FCR (cFCR, 0.769; P0.10) and the NRC FCR (nFCR, 0.847; P

Download or read book Relationship Between Feed Efficiency and Reproductive Measurements in Beef Cattle written by Erika Elaine Blair and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: It is important for animals to be feed efficient and reproductively sound to optimize profits for cow- calf producers. The objective of this study was to determine the relationship between feed efficiency and reproductive performance. Feed efficiency measures included residual feed intake (RFI), feed to gain ratio (F:G) and daily dry matter intake (DMI). Reproductive measurements were pregnancy rate, first service conception rate, pregnancy type (AI, natural. open), calving percentage, calving day (CD) and age at first calving. Two data sets which included 136 crossbred Angus females sired by bulls with high or low RFI estimated breeding values (EBV) with multiple parity information and 56 purebred Hereford heifers with their first parity calving information were analyzed. Initially, the crossbred Angus females were analyzed based on their phenotypic RFI values. There was no difference in pregnancy rate between the feed efficiency measures. Second parity pregnant females had lower (F:G) with first service conception (P=0.053), and pregnancy types (P=0.014) than the open (less efficient) females. In parity 5, phenotypically efficient RFI Angus females were pregnant to first service conception versus inefficient RFI Angus females (P=0.052) and those with lower DMI were diagnosed pregnant (P=0.0002). When evaluated as a repeated trait, RFI was not a significant indicator of CD (P=0.514). Crossbred Angus females were analyzed based on their sire's RFI EBV and grouped accordingly into high (inefficient) or low (efficient) RFI sires. Females sired by high or low RFI (EBV) bulls showed no difference for the reproductive traits, except for a tendency for inefficient sired heifers to have a lower calving percentage in parity 2 (P=0.048). When CD was analyzed as a repeated measure, no difference between sire groups was found (efficient CD of 35.64 d, inefficient CD of 34.23, p = 0.789). In the Hereford heifers, RFI was not an indicator of CD (P = 0.774). There was also no difference in RFI between pregnant and open Hereford females with pregnant females having an LSMean of 0.11 kg/d and the open females having an LSMean of 0.14 kg/d (P = 0.860). Further research needs to be done to confirm any relationships between feed efficiency and reproduction.

Download or read book Characterization of Residual Feed Intake and Relationships with Performance Carcass and Temperament Traits in Growing Calves written by James Trent Fox and published by . This book was released on 2004 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The objectives of this study were accomplished with two experiments in growing Bonsmara bulls (N = 68) (experiment 1), and Simmental crossbred calves (N = 132) (experiment 2). Specific objectives for experiment 1 were to characterize residual feed intake (RFI) in growing bulls, and examine relationships between RFI and performance, fertility, temperament and body composition traits. In experiment 2, the objectives were to examine stocker-phase supplementation effects on feedlot feed conversion ratio (FCR) and RFI and to characterize relationships between these feed efficiency traits, and performance and carcass traits in finishing calves. In both experiments, individual feed intakes and BW were measured. Ultrasound technology was used to measure body composition in experiment 1, while actual carcass measurements taken at harvest were used for experiment 2. Experiment 1 demonstrated that temperament affected ADG and DMI, but not FCR or RFI. Residual feed intake was not phenotypically correlated to scrotal circumference or bull fertility traits. Experiments 1 and 2 demonstrated that RFI was independent of ADG and BW, but that there was a tendency (P 0.5 SD above mean RFI). Results from experiment 2 suggest that RFI measured while calves are consuming high-grain diets may be less influenced by previous level of stocker supplementation compared to FCR or residual gain efficiency traits. In summary, RFI was found to be phenotypically independent of growth rate and BW, had no effect on bull fertility or temperament traits, and was less impacted by previous plane of nutrition compared to FCR.

Download or read book Effects of Maturity and Production Stage on Residual Feed Intake Classification of Beef Cows written by R. L. Barnett and published by . This book was released on 2013 with total page 50 pages. Available in PDF, EPUB and Kindle. Book excerpt: The objective of this research was to determine if the postweaning residual Feed intake (RFI) classification of beef heifers remain consistent as the animals matured, went through different stages of production and were placed in different feeding systems. Three feed intake trials, using the GrowSafe feed intake system, (postweaning, dry and lactating) and a two-year pasture trial were conducted on the same 33 Simmental X Angus females from a single herd. Data from all three GrowSafe trials were pooled and stepwise regression (SAS PROC REG) was used to calculate expected feed intake (EFI) across all trials. Individual RFIs were calculated as the difference between actual dry matter intake (ADMI) and EFI. Cows were then categorically grouped as Low (RFI 0.5 SD below the mean), Average (RFI ± 0.5 SD above and below the mean) and High (RFI 0.5 SD above the mean) based upon individual RFI classifications for each trial. No correlations (P > 0.1) were found between postweaning RFI classifications and either trial as mature animals (dry or lactating). The overall average daily intakes in the pasture study were lower (P

Download or read book Sources of Biological Variation in Residual Feed Intake in Beef Cattle written by Phillip Allan Lancaster and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Objectives of this study were to characterize residual feed intake (RFI) in growing calves and to examine relationships with growth, carcass composition, physiological indicators, energy metabolism and reproduction. To accomplish these objectives, multiple experiments were conducted. In all experiments, RFI was calculated as the difference between actual DMI and expected DMI from linear regression of DMI on mid-test metabolic BW and ADG. To examine the relationships between RFI and rate and composition of growth, and reproduction an experiment was conducted with postweaning Brangus heifers (N = 348). Measures of carcass composition (longissimus muscle area, LMA; and 12th rib fat thickness, BF) were obtained by ultrasound at the start and end of each experiment. To determine if serum IGF-I concentration is associated with RFI, two experiments were conducted with Angus bulls and heifers (N = 95) divergently selected for serum IGF-I concentration. To evaluate relationships with energy metabolism, calves with low and high RFI were selected for determination of heart rate as an indicator trait for energy expenditure (4 experiments) and liver mitochondrial function (3 experiments). Residual feed intake was not correlated with ADG, but was positively correlated with gain in BF such that the more efficient calves were leaner. Calves from the low IGF-I selection line had lower RFI suggesting that RFI and IGF-I are related and that IGF-I could be used as an indicator trait to aid in selection for improved RFI in Angus cattle. Calves with low RFI had lower energy expenditure and greater mitochondrial acceptor control ratios than calves with high RFI suggesting improved regulation of energy metabolism. A similar percentage of heifers with low RFI attained puberty as those with high RFI by the end of the experiment. Results from this study indicate that producers can utilize RFI to select for improved feed efficiency with minimal impact on growth, carcass composition and reproduction.