Low Residual Feed Intake Steers Research Articles

Association of glucose metabolism and insulin resistance with feed efficiency and production traits of finishing beef steers.

Increasing nutrient utilization efficiency is an important component of enhancing the sustainability of beef cattle production. The objective of this experiment was to determine the association of glucose metabolism and insulin resistance with dry matter intake (DMI), average daily gain (ADG), gain:feed ratio (G:F), and residual feed intake (RFI). Steers (n = 54; initial body weight = 518 ± 27.0kg) were subjected to an intravenous glucose tolerance test (IVGTT) where glucose was dosed through a jugular catheter and serial blood samples were collected. Three days after the last group's IVGTT, steers began a 63-d DMI and ADG test. Body weight was measured on days 0, 1, 21, 42, 62, and 63, and DMI was measured using an Insentec Roughage Intake Control system (Hokofarm Group, Emmeloord, the Netherlands). To examine relationships between DMI, ADG, G:F, and RFI with IVGTT measurements, Pearson correlations were calculated using Proc Corr of SAS 9.4 (SAS Inst. Inc., Cary, NC). Additionally, cattle were classified based on DMI, ADG, RFI, and G:F, where the medium classification was set as mean ± 0.5 SD, the low classification was < 0.5 SD from the mean, and the high classification was > 0.5 SD from the mean. No associations between DMI and IVGTT parameters were observed, and no differences were detected when classifying cattle as having low, medium, or high DMI. Peak insulin concentration in response to the IVGTT tended to be correlated with ADG (r = 0.28; P = 0.07), indicating cattle with greater ADG tend to have a greater insulin release in response to glucose. Glucose nadir concentrations tended to be positively correlated with ADG (r = 0.26; P = 0.10). Additionally, the glucose nadir was greater in high-ADG steers (P = 0.003). The association of greater glucose nadir with high-ADG could indicate that high-ADG steers do not clear glucose as efficiently as low-ADG steers, potentially indicating increased insulin resistance. Further, RFI was not correlated with IVGTT measurements, but low RFI steers had a greater peak glucose concentration (P = 0.040) and tended to have a greater glucose area under the curve (P = 0.09). G:F was correlated with glucose area under the curve (r = 0.33; P = 0.050), glucose nadir (r = 0.35; P = 0.011), and insulin time to peak (r = 0.39; P = 0.010). These results indicate that glucose metabolism and insulin signaling are associated with growth and efficiency, but the molecular mechanisms that drive these effects need to be elucidated.

Meat and sensory quality of major muscles from Angus, Charolais, and Angus crossbred steers with high and low residual feed intake

Effects of residual feed intake (RFI) and genetic group on growth, carcass, and meat quality characteristics of bovine longissimus lumborum (LL), triceps brachii (TB), semimembranosus (SM), and gluteus medius (GM) muscles were investigated using 72 purebred Angus, purebred Charolais, and Angus crossbred steers (n = 24 per genetic group) classified as either high (inefficient) or low (efficient) RFI (n = 12 high and low RFI steers within genetic group). There was no RFI effect (P &gt; 0.05) on growth, carcass, and meat quality measurements except high RFI steers had the highest dry matter intake (P &lt; 0.05), and low RFI TB was rated as having reduced beef flavour intensity and sustained juiciness (P &lt; 0.05). Purebred Angus and Charolais LL and GM had lower shear force values (P &lt; 0.05) than Angus crossbreds and ageing reduced mean shear force values except in TB. For TB, SM, and GM, Angus crossbred steers had the highest mean beef flavour intensity scores, and Charolais SM and TB were less tender than those of Angus crossbred (P &lt; 0.05). Overall, RFI did not influence most meat quality traits; therefore, low RFI animals may contribute to reducing feed costs or environmental impact without compromising meat quality and palatability.

Mitochondrial abundance and function in muscle from beef steers with divergent residual feed intakes

The objective of this study was to evaluate the relationship between muscle mitochondrial function and residual feed intake (RFI) in growing beef cattle. A 56-day feeding trial was conducted with 81 Angus × Hereford steers (initial BW = 378 ± 43 kg) from the University of California Sierra Foothills Research Station (Browns Valley, CA, USA). All steers were individually fed the same finishing ration (metabolizable energy = 3.28 Mcal/kg DM). Average daily gain (ADG), DM intake (DMI) and RFI were 1.82 ± 0.27, 8.89 ± 1.06 and 0.00 ± 0.55 kg/day, respectively. After the feeding trial, the steers were categorized into high, medium and low RFI groups. Low RFI steers consumed 13.6% less DM (P < 0.05) and had a 14.1% higher G : F ratio (P < 0.05) than the high RFI group. No differences between RFI groups were found in age, ADG or BW (P > 0.10). The most extreme individuals from the low and high RFI groups were selected to assess mitochondrial function (n = 5 low RFI and n = 6 high RFI). Mitochondrial respiration was measured using an oxygraph (Hansatech Instruments Ltd., Norfolk, UK). State 3 and State 4 respiration rates were similar between both groups (P > 0.10). Respiratory control ratios (RCRs, i.e., State 3 : State 4 oxygen uptakes) declined with animal age and were greater in low RFI steers (4.90) as compared to high RFI steers (4.26) when adjusted for age by analysis of covariance (P = 0.003). Mitochondrial complex II activity levels per gram of muscle were 42% greater in low RFI steers than in high RFI steers (P = 0.004). These data suggest that skeletal muscle mitochondria have greater reserve respiratory capacity and show greater coupling between respiration and phosphorylation in low RFI than in high RFI steers.

Effects of Breed Type, Residual Feed Intake and Post-Mortem Aging on Physio-Chemical Properties of Triceps Brachii Muscle and Their Relationships With Beef Toughness

ObjectivesThe influence of breed type, residual feed intake (RFI) and post-mortem aging on meat and carcass quality attributes and intramuscular connective tissue characteristics were examined in the bovine Triceps brachii, a high connective tissue muscle from the chuck. The hypothesis that selection for low RFI in beef cattle increases beef toughness, increases collagen content and reduces collagen heat solubility of the Triceps brachii was tested.Materials and MethodsSeventy-one beef steers from Angus (n = 23), Charolais (n = 24) and Angus crossbred (n = 24)) genetics were used. Each breed had high RFI and low RFI steers (n = 12). Muscles collected were aged for 3- and 13-days post-mortem (dpm). Final animal live weight, grade marbling, intramuscular pH, water holding capacity (WHC), intramuscular fat, cooking loss, drip loss, protein, temperature, moisture, color, RFI, and Warner Bratzler shear force (WBSF) data were collected for carcass and meat quality measurements. Total collagen, collagen heat solubility, and collagen cross-link Ehrlich’s chromogen (EC) of the isolated perimysium were quantified. Data were analyzed using a split-plot experimental design procedure (R software 3.4.1) with breed and RFI as main effects in the whole plot and postmortem aging included at the subplot level.ResultsFinal weight was significantly greater for Charolais (683 ± 9.58 kg) than Angus (554 ± 9.65 kg) and Angus crossbred (568 kg ± 9.58 kg) steers (P = 0.017), and grade marbling score was higher for high RFI (421 ± 19.85) than for low RFI steer carcasses (385 ± 19.82) (P = 0.001). No significant effects of breed type and RFI (P &gt; 0.05) were observed on meat quality attributes. WBSF value at 3 dpm (3.72 kg) was significantly higher than at 13dpm (3.21 kg) (P &lt; 0.005). Collagen solubility was significantly higher at 13 dpm (25.88%) than at 3 dpm (18.03%) (P &lt; 0.005). Total collagen and wet endomysium were positively correlated (r = 0.44) as were total collagen and EC in raw muscle (r = 0.76), EC and wet perimysium (r = 0.42) and WBSF and EC at 13 dpm (r = 0.27) (P &lt; 0.005). Total collagen and collagen solubility at 3dpm (r = –0.36) and 13 dpm (r = –0.63) were negatively correlated, as were EC and solubility at 3 dpm (r = –0.38) (P &lt; 0.005).ConclusionIncreasing postmortem aging periods reduced WBSF and increased collagen heat solubility of the Triceps brachii muscle. With no effect of RFI on meat quality measurements, the production cost can be reduced by selecting for low RFI animals without sacrificing product quality.Table 7.Least square means (± standard error of the mean) for WBSF, soluble collagen, and collagen solubility of Triceps brachii muscles at 3 and 13 d post-mortem aging (dpm)

Effects of Breed Type, Residual Feed Intake and Post-Mortem Aging on Physio-Chemical Properties of Triceps Brachii Muscle and Their Relationships With Beef Toughness

ObjectivesThe influence of breed type, residual feed intake (RFI) and post-mortem aging on meat and carcass quality attributes and intramuscular connective tissue characteristics were examined in the bovine Triceps brachii, a high connective tissue muscle from the chuck. The hypothesis that selection for low RFI in beef cattle increases beef toughness, increases collagen content and reduces collagen heat solubility of the Triceps brachii was tested.Materials and MethodsSeventy-one beef steers from Angus (n = 23), Charolais (n = 24) and Angus crossbred (n = 24)) genetics were used. Each breed had high RFI and low RFI steers (n = 12). Muscles collected were aged for 3- and 13-days post-mortem (dpm). Final animal live weight, grade marbling, intramuscular pH, water holding capacity (WHC), intramuscular fat, cooking loss, drip loss, protein, temperature, moisture, color, RFI, and Warner Bratzler shear force (WBSF) data were collected for carcass and meat quality measurements. Total collagen, collagen heat solubility, and collagen cross-link Ehrlich’s chromogen (EC) of the isolated perimysium were quantified. Data were analyzed using a split-plot experimental design procedure (R software 3.4.1) with breed and RFI as main effects in the whole plot and postmortem aging included at the subplot level.ResultsFinal weight was significantly greater for Charolais (683 ± 9.58 kg) than Angus (554 ± 9.65 kg) and Angus crossbred (568 kg ± 9.58 kg) steers (P = 0.017), and grade marbling score was higher for high RFI (421 ± 19.85) than for low RFI steer carcasses (385 ± 19.82) (P = 0.001). No significant effects of breed type and RFI (P > 0.05) were observed on meat quality attributes. WBSF value at 3 dpm (3.72 kg) was significantly higher than at 13dpm (3.21 kg) (P < 0.005). Collagen solubility was significantly higher at 13 dpm (25.88%) than at 3 dpm (18.03%) (P < 0.005). Total collagen and wet endomysium were positively correlated (r = 0.44) as were total collagen and EC in raw muscle (r = 0.76), EC and wet perimysium (r = 0.42) and WBSF and EC at 13 dpm (r = 0.27) (P < 0.005). Total collagen and collagen solubility at 3dpm (r = –0.36) and 13 dpm (r = –0.63) were negatively correlated, as were EC and solubility at 3 dpm (r = –0.38) (P < 0.005).ConclusionIncreasing postmortem aging periods reduced WBSF and increased collagen heat solubility of the Triceps brachii muscle. With no effect of RFI on meat quality measurements, the production cost can be reduced by selecting for low RFI animals without sacrificing product quality.Table 7.Least square means (± standard error of the mean) for WBSF, soluble collagen, and collagen solubility of Triceps brachii muscles at 3 and 13 d post-mortem aging (dpm)

Transcriptome analyses reveal reduced hepatic lipid synthesis and accumulation in more feed efficient beef cattle

The genetic mechanisms controlling residual feed intake (RFI) in beef cattle are still largely unknown. Here we performed whole transcriptome analyses to identify differentially expressed (DE) genes and their functional roles in liver tissues between six extreme high and six extreme low RFI steers from three beef breed populations including Angus, Charolais, and Kinsella Composite (KC). On average, the next generation sequencing yielded 34 million single-end reads per sample, of which 87% were uniquely mapped to the bovine reference genome. At false discovery rate (FDR) < 0.05 and fold change (FC) > 2, 72, 41, and 175 DE genes were identified in Angus, Charolais, and KC, respectively. Most of the DE genes were breed-specific, while five genes including TP53INP1, LURAP1L, SCD, LPIN1, and ENSBTAG00000047029 were common across the three breeds, with TP53INP1, LURAP1L, SCD, and LPIN1 being downregulated in low RFI steers of all three breeds. The DE genes are mainly involved in lipid, amino acid and carbohydrate metabolism, energy production, molecular transport, small molecule biochemistry, cellular development, and cell death and survival. Furthermore, our differential gene expression results suggest reduced hepatic lipid synthesis and accumulation processes in more feed efficient beef cattle of all three studied breeds.

The Effects of Ractopamine and Hormonal Growth Promotants on Growth and Meat Quality of Crossbred Angus Steers

ObjectivesMeat tenderness is an important quality parameter that influences consumer preference. The cattle industry has over the years seen the emergence of feed additives and hormonal growth promotants in the form of β-adrenergic agonists (β-AA) and steroids, respectively. The objective of this study was to analyze the effect of hormonal growth implants and ractopamine on slaughter weight and meat quality parameters of steers selected for high (inefficient) and low (efficient) residual feed intake (RFI) performance.Materials and MethodsForty-eight crossbred Angus steers identified from individual GrowSafe data as high (n = 21) or low (n = 27) RFI cattle were randomly assigned to pens according to treatment (n = 12). Treatments included control (no ractopamine hydrochloride (RAC)/no steroids), RAC and steroids, steroids only, and RAC only in a 2x2x2 factorial design. Steers on steroid treatment received a first implant (200mg progesterone, 20mg estradiol benzoate and 29mg tylosin tartrate) at about 350 d of age and 450kg live weight and a terminal implant (120 mg trenbolone acetate and 24mg estradiol) at about 100 d before slaughter. RAC was fed to the appropriate group 28 d before slaughter at a rate of 200mg head–1d–1. Cattle were slaughtered at about 16 mo of age over 6 consecutive weeks by weight and back fat, with 1 animal per treatment represented in each kill for a total of 8 animals slaughtered per week. Hot carcass weights (HCW) were recorded. Gluteus medius (GM) muscles were obtained from the carcasses 3 d post mortem and halved for ageing, with one half aged a further 12 d under vacuum. After ageing at 4°C, muscle halves were assessed for pH, color, drip loss and Warner-Bratzler shear force (WBSF). For all data the experimental unit was the steer as the effect of ageing was not considered. Data was analyzed using the General Linear Model procedure in SAS (SAS Inst. Inc., Cary, NC) with RFI, steroids, RAC and their interactions as fixed factors with slaughter day used as a covariate. Mean differences were determined using Least Square Means and Tukey’s multiple comparisons.ResultsResults revealed no effect (P > 0.05) of RFI and RAC on slaughter weight (SW) but steroids increased (P < 0.0001) SW of steers. An interaction effect (P = 0.0381) was seen between RFI, steroids and RAC on HCW, where high RFI steers that were implanted with steroids and fed RAC had a higher HCW at 389.64 ± 8.70 kg than low RFI steers that were neither implanted nor fed RAC (325.68 ± 7.05kg). An interaction between RFI, steroids and RAC (P = 0.045) for drip loss was observed on muscles aged for 12 d, where high RFI steers that were implanted but not fed RAC had a higher drip loss (1.93 ± 0.23 g) than low RFI steers that were not implanted but feed RAC (0.63 ± 0.19g). Muscles from implanted steers had a higher mean WBSF value than muscles from non-implanted steers on d 12 post-mortem (P = 0.039), while high RFI steers that received RAC had the lowest mean WBSF (P = 0.015).ConclusionResults indicated that steroids compromised the development of tenderness during post mortem ageing in the GM. This suggests that the benefit of steroid use on slaughter and hot carcass weights will compromise tenderness of this muscle. Additional post mortem ageing beyond 12 d may be required. Conversely, the use of the β-AA RAC showed potential for decreasing cooked GM toughness in high RFI steers regardless of the ageing period.

Relationship between Liver Mitochondrial Respiration and Proton Leak in Low and High RFI Steers from Two Lineages of RFI Angus Bulls.

The objective of this research is to evaluate liver mitochondrial oxygen consumption and proton leak kinetics in progeny from two lineages of Angus bulls with high and low residual feed intake (RFI). Two Angus bulls were selected based on results from a genetic test for RFI and were used as sires. Eight offspring at 10-11 months of age from each sire were housed in individual pens for 70–105 days following a diet adaptation period of 14 days. Progeny of the low RFI sire had 0.57 kg/d (P = 0.05) lower average RFI than progeny of the high RFI sire. There was no difference in dry matter intake between low and high RFI steers, but low RFI steers gained more body weight (P = 0.02) and tended to have higher average daily gains (P = 0.07). State 3 and State 4 respiration, RCR, and proton leak did not differ between high and low RFI steers (P = 0.96, P = 0.81, P = 0.93, and P = 0.88, resp.). Therefore, the increase in bodyweight gain which distinguished the low RFI steers from the high RFI steers may be associated with other metabolic mechanisms that are not associated with liver mitochondrial respiration and proton leak kinetics.

Effect of residual feed intake on hypothalamic gene expression and meat quality in Angus-sired cattle grown during the hot season1,2

The relationship between heat stress, meat quality, and residual feed intake (RFI) is unknown in growing steers. To address this issue, high RFI (HRFI) and low RFI (LRFI) individuals were compared by assessing RFI in 48 Angus-sired steers during a 70-d feeding trial conducted during July through September to identify steers with calculated RFI at least 2 SD apart. The association of RFI with indices of meat quality and expression of genes within hypothalamic and adipose tissue was then determined in LRFI and HRFI steers. While on test, feed intake was recorded daily with BW and hip heights recorded every 14 d. Ultrasound measurements of rib eye area (REA) and backfat (BF) were recorded initially and before harvest. Carcass and growth data were analyzed using a mixed model with RFI level (LRFI and HRFI) as the independent variable. The least square means for RFI were -1.2 and 0.99 kg DMI/d, respectively, for the LRFI and HRFI cohorts (P < 0.0001). Dry matter intake was higher for the HRFI individuals versus the LRFI steers (P < 0.0001) while on-test gain was not different (P < 0.95). Marbling score was greater in LRFI than HRFI steers (P < 0.05). However, there were no differences in REA (P < 0.53), BF (P < 0.65), yield grade (P < 0.24), or objective Hunter color measures between LRFI and HRFI steers indicating there was no consistent relationship between RFI and indices of meat quality. Hypothalamic neuropeptide Y (NPY), agouti related protein (AGRP), relaxin-3 (RLN3), melanocortin 3 receptor, and relaxin/insulin-like family peptide receptor 1 (RXFP1) mRNA were expressed 280, 185, 202, 183, and 163% greater, respectively (P < 0.01), while proopiomelanocortin (POMC) mRNA was expressed 42% lower in LRFI than HRFI animals (P < 0.05). Hypothalamic GnRH mRNA expression was 67% lower while gonadotropin inhibiting hormone (GnIH) mRNA was 209% higher in LRFI than HRFI animals (P < 0.01). Pituitary expression of FSHβ and LHβ correlated to hypothalamic GnRH levels (P < 0.05) indicating changes in gene expression within the hypothalamus had functional consequences. Leptin mRNA expression levels were not different between adipose tissue of LRFI or HRFI steers (P < 0.84). These data indicate that animals with superior RFI evaluated during warm conditions have higher expression of orexigenic neuropeptide genes independent of the expression of adipose-derived leptin. Furthermore, the gonadotropin axis may also influence feed efficiency under these conditions.

Using genes differentially expressed in bulls to classify steers divergently selected for high and low residual feed intake

Feed efficiency is an economically important trait in livestock and residual feed intake (RFI) is a commonly used measure of the trait in beef cattle. Residual feed intake is the difference between the actual feed intake recorded over a test period and the expected feed intake of an animal based on its size and growth rate. It is a heritable trait, and efficient animals have lower RFI values. Several genes have been identified as being differentially expressed in the liver of Angus bulls that have been divergently selected for RFI. The objective of this study was to use genes that are differentially expressed in bulls to classify Angus steers from the same divergent RFI selection lines. Liver samples were collected at slaughter from 40 high RFI and 40 low RFI steers that were ~23 months old, and had just completed a 251-day feedlot feeding period. RNA samples from the livers were assayed by quantitative real-time PCR for 14 genes, which have been identified previously in bulls. Steers were not measured for RFI, hence the estimated breeding values (EBV) for RFI of their parents were used to calculate their mid-parent (average of the two parents) RFI-EBV. Correlation and discriminant analyses were conducted on the normalised quantitative real-time PCR data from the steers. Discriminant analysis was also conducted on the bull data for comparison. In the steers, 8 out of the 14 genes were significantly (P &lt; 0.05) correlated with RFI-EBV. Two genes from the glutathione S-transferase mu family (GSTM1 and GSTM2) and the S100 calcium-binding protein A10 (S100A10) had the highest correlations with RFI-EBV, with correlation coefficients of 0.59, 0.44 and 0.36, respectively. Based on the 14 expressed genes, 84% of the steers and 98% of the bulls were correctly classified into their respective RFI selection lines. The results of this study indicate that a high proportion of the genes that were differentially expressed in the original study with bulls were also differentially expressed in this study with steers. The high accuracy in classification obtained in this study shows that the transcriptional approach to the study of the biological processes involved in variation in RFI has great potential for identification of candidate genes.

Performance, residual feed intake, digestibility, carcass traits, and profitability of Angus-Hereford steers housed in individual or group pens1

Even though the concept of residual feed intake (RFI) is well accepted, several questions remain regarding other traits that may be associated with selection for decreased RFI. These include DM digestibility, carcass composition, profitability, and performance. The objective of this study was to investigate the difference in those traits between low- and high-RFI cattle. Sixty Angus x Hereford crossbred steers (296 kg of initial BW) were fed a corn-based finishing ration (1.68 Mcal of NE(m)/kg, 13% CP on a DM basis) during 2 periods of 60 d each. For both phases, the regression equation fitted without the intercept (not statistically significant) was DMI (kg/d) = 0.0701 x BW(0.75) + 2.714 x ADG, r(2) = 0.42. The 15 greatest and least RFI steers were classed as high and low RFI groups. There were no differences between low and high RFI groups for days on feed (162 vs. 168 d), slaughter weight (503 vs. 511 kg), HCW (317 vs. 315 kg), LM area (76.5 vs. 77.1 cm(2)), backfat (1.23 vs. 1.27 cm), KPH (3.1 vs. 3.7%), quality grade (average Choice for both groups), or carcass fat (32.4 vs. 33.1%). Visceral organ masses and abdominal fat were similar for low and high RFI groups (32.25 vs. 31.24 kg and 37.48 vs. 36.95 kg, respectively). These results do not support the existence of major differences in composition and organ mass between low and high RFI steers at slaughter. The RFI grouping had a significant effect on DMI, G:F, and RFI values. Stepwise regression showed that G:F alone or DMI and ADG together explained 98.5% of the variance in cost of BW gain, whereas RFI alone explained only 18%. We conclude that RFI is less useful than G:F as an indicator of feedlot efficiency and profitability.

Spectral analysis of feeding patterns of steers divergent in residual feed intake

Spectral analysis was used to quantify differences in feeding patterns of Angus steers genetically divergent in residual feed intake (RFI). Forty-two low RFI (more efficient) and 28 high RFI (less efficient) selection line steers were fed a feedlot diet through electronic feeders for 72 days. Mean (±s.e.) daily feed intake was 11.9 ± 0.076 and 12.7 ± 0.095 kg dry matter/day over the 72-day feeding period for low and high RFI steers, respectively. The high RFI steers exhibited different temporal cycles to the low RFI steers in daily feed intake and time spent feeding. However, patterns of number of eating sessions, feeding rate, the time spent eating and the number of eating sessions were similar. Further analysis of daily (24 h) feeding behaviour is required to help improve our understanding of the association between feeding behaviour and feed efficiency.

Growth, carcass quality, and protein and energy metabolism in beef cattle with different growth potentials and residual feed intakes1,2

Twenty-four beef steers (predominantly Angus x Hereford, 14 to 18 mo of age, 403 +/- 3 kg of BW), were housed and fed in individual pens for about 122 d. Twelve steers came from a herd that had been selected for growth (high growth; HG) and the other 12 from a herd with no selection program (low growth; LG). Another 6 steers (3 from each group) were slaughtered at the beginning to obtain the initial composition. All steers were fed the same corn-based diet (3.06 Mcal of ME/kg of DM, 13.6% CP) on an ad libitum basis. Two weeks before slaughter, total urine was collected for 5 d for estimation of 3-methylhistidine excretion and myofibrillar protein breakdown rates. Compared with LG steers, HG steers had less initial BW but greater final BW, DMI (7.52 vs. 6.37 kg/d), ADG (1.33 vs. 0.853 kg/d), G:F (0.176 vs. 0.133 kg/kg), ME intake (0.233 vs. 0.201 Mcal x kg of BW(0.75) x d(-1)), and retained energy (RE; 0.0711 vs. 0.0558 Mcal x kg of BW(0.75) x d(-1)); gained more fat (676 vs. 475 g/d); and tended to gain more whole body protein (100 vs. 72 g/d), with no difference in residual feed intake (RFI). Estimated net energetic efficiency of gain (k(g)) and ME for maintenance (ME(m)) did not differ between the 2 groups, averaging 0.62 and 0.114, respectively. The HG steers had greater HCW (350 vs. 329 kg), backfat (16.1 vs. 11.6 mm), and yield grades (3.53 vs. 2.80), with a similar dressing percent, KPH fat, LM area, and marbling score. Skeletal muscle protein gain (70.2 vs. 57.6 g/d) and fractional protein accretion rate (0.242 vs. 0.197%/d) tended to be greater in HG than in LG steers. Steers were classified into low (-0.367 kg/d) and high (0.380 kg/d) RFI classes. Compared with the high RFI steers, low RFI steers consumed less DM (6.61 vs. 7.52 kg/d) and ME (0.206 vs. 0.234 Mcal x kg of BW(0.75) x d(-1)) and tended to gain less fat (494 vs. 719 g/d), but were similar for initial and final BW, ADG, G:F, protein gain, HCW, dressing percent, backfat, KPH fat, LM area, marbling score, and yield grade, as well as for all observations related to myofibrillar protein metabolism. Residual feed intake may be positively [corrected] correlated with ME for maintenance. The maintenance energy requirement increased by 0.0166 Mcal x kg(-0.75) x d(-1) for each percentage increase in fractional protein degradation rate, confirming the importance of this process in the energy economy of the animal.

Residual feed intake of purebred Angus steers: Effects on meat quality and palatability

Relationships between residual feed intake (RFI) and other performance variables were determined using 54 purebred Angus steers. Individual feed intake and BW gain were recorded during a 70-d post-weaning period to calculate RFI. After the 70-d post-weaning test, steers were fed a finishing ration to a similar fat thickness (FT), transported to a commercial facility, and slaughtered. A subsample of carcasses (n = 32) was selected to examine the relationships among RFI, meat quality, and palatability. Steers were categorized into high (> 0.5 SD above the mean; n = 16), medium (mid; +/- 0.5 SD from the mean; n = 21), and low (< 0.5 SD below the mean; n = 17) RFI groups. No differences were detected in ADG, initial BW, and d 71 BW among the high, mid, and low RFI steers. Steers from the high RFI group had a greater DMI (P = 0.004) and feed conversion ratio (FCR; DMI:ADG; P = 0.002) compared with the low RFI steers. Residual feed intake was positively correlated with DMI (r = 0.54; P = 0.003) and FCR (r = 0.42; P = 0.002), but not with initial BW, d 71 BW, d 71 ultrasound FT, initial ultrasound LM area, d 71 ultrasound LM area, or ADG. The FCR was positively correlated with initial BW (r = 0.46; P = 0.0005), d 71 BW (r = 0.34; P = 0.01), and DMI (r = 0.40; P = 0.003) and was negatively correlated with ADG (r = -0.65; P = 0.001). There were no differences among RFI groups for HCW, LM area, FT, KPH, USDA yield grade, marbling score, or quality grade. Reflectance color b* scores of steaks from high RFI steers were greater (P = 0.02) than those from low RFI steers. There was no difference between high and low RFI groups for LM calpastatin activity. Warner-Bratzler shear force and sensory panel tenderness and flavor scores of steaks were similar across RFI groups. Steaks from high RFI steers had lower (P = 0.04) off-flavor scores than those from low RFI steers. Cook loss percentages were greater (P = 0.005) for steaks from low RFI steers than for those from mid RFI steers. These data support current views that RFI is independent of ADG, but is correlated with DMI and FCR. Importantly, the data also support the hypothesis that there is no relationship between RFI and beef quality in purebred Angus steers.

The relationship between mitochondrial function and residual feed intake in Angus steers1

The objective of this study was to examine the relationship between mitochondrial function and residual feed intake in Angus steers. Individual feed intakes were recorded for a contemporary group of 40 steers via the GrowSafe feed intake system. Intakes were then used to calculate residual feed intake (RFI), a measure of efficiency. Based on these calculations, 9 low (RFI = -0.83) and 8 high (RFI = 0.78) RFI animals were selected for further study. Blood samples were collected via jugular venipuncture 1 wk before slaughter for the determination of plasma glucose and insulin concentrations. Tissue samples were taken from the LM from both the high and low RFI animals and mitochondria were isolated for measurement of oxygen consumption and hydrogen peroxide production. Average daily gain and carcass composition were not different between the high and low RFI steers; however, ADFI by the high RFI animals was 1.54 kg/d greater (P < 0.001) than for the low RFI animals. Low RFI steers exhibited a greater (P < 0.05) rate of state 2 and 3 respiration, respiratory control ratio, and hydrogen peroxide production than high RFI steers when provided with glutamate or succinate as a respiratory substrate. The acceptor control and adenosine diphosphate:oxygen ratios were not different between the 2 groups for either substrate. When hydrogen peroxide production was expressed as a ratio to respiration rate there was no difference between groups, signifying that electron leak was similar for both groups. Plasma glucose concentration was greater (P < 0.05) in the high RFI steers than in the low RFI steers; however, plasma insulin concentration was not different (P = 0.22) between the 2 groups. The ratio between plasma glucose and insulin concentration was similar (P = 0.88) between the 2 groups indicating no difference in glucose metabolism. The increased plasma glucose concentration observed in the high RFI steers was presumed to be the result of a greater feed intake by these animals. It seems that mitochondrial function is not different between the high and low RFI groups but rather the rate of mitochondrial respiration is increased in low RFI steers compared with high RFI steers.