Review: Biological determinants of between-animal variation in feed efficiency of growing beef cattle.

Animal's feed efficiency in growing cattle (i.e. the animal ability to reach a market or adult BW with the least amount of feed intake), is a key factor in the beef cattle industry. Feeding systems have made huge progress to understand dietary factors influencing the average animal feed efficiency. However, there exists a considerable amount of animal-to-animal variation around the average feed efficiency observed in beef cattle reared in similar conditions, which is still far from being understood. This review aims to identify biological determinants and molecular pathways involved in the between-animal variation in feed efficiency with particular reference to growing beef cattle phenotyped for residual feed intake (RFI). Moreover, the review attempts to distinguish true potential determinants from those revealed through simple associations or indirectly linked to RFI through their association with feed intake. Most representative and studied biological processes which seem to be connected to feed efficiency were reviewed, such as feeding behaviour, digestion and methane production, rumen microbiome structure and functioning, energy metabolism at the whole body and cellular levels, protein turnover, hormone regulation and body composition. In addition, an overall molecular network analysis was conducted for unravelling networks and their linked functions involved in between-animal variation in feed efficiency. The results from this review suggest that feeding and digestive-related mechanisms could be associated with RFI mainly because they co-vary with feed intake. Although much more research is warranted, especially with high-forage diets, the role of feeding and digestive related mechanisms as true determinants of animal variability in feed efficiency could be minor. Concerning the metabolic-related mechanisms, despite the scarcity of studies using reference methods it seems that feed efficient animals have a significantly lower energy metabolic rate independent of the associated intake reduction. This lower heat production in feed efficient animals may result from a decreased protein turnover and a higher efficiency of ATP production in mitochondria, both mechanisms also identified in the molecular network analysis. In contrast, hormones and body composition could not be conclusively related to animal-to-animal variation in feed efficiency. The analysis of potential biological networks underlying RFI variations highlighted other significant pathways such as lipid metabolism and immunity and stress response. Finally, emerging knowledge suggests that metabolic functions underlying genetic variation in feed efficiency could be associated with other important traits in animal production. This emphasizes the relevance of understanding the biological basis of relevant animal traits to better define future balanced breeding programmes.

Consistency of feed efficiency ranking and mechanisms associated with inter-animal variation among growing calves.

This study investigated the possible mechanisms for explaining interanimal variation in efficiency of feed utilization in intact male Holstein calves. Additionally, we examined whether the feed efficiency (FE) ranking of calves (n = 26) changed due to age and/or diet quality. Calves were evaluated during three periods (P1, P2, and P3) while fed a high-quality diet (calculated mobilizable energy [ME] of 11.8 MJ/kg DM) during P1 and P3, and a low-quality diet (calculated ME of 7.7 MJ/kg DM) during P2. The study periods were 84, 119, and 127 d, respectively. Initial ages of the calves in P1, P2, and P3 were 7, 11, and 15 mo, respectively, and initial body weight (BW) were 245, 367, and 458 kg, respectively. Individual dry matter intake (DMI), average daily gain (ADG), diet digestibility, and heat production (HP) were measured in all periods. The measured FE indexes were: residual feed intake (RFI), the gain-to-feed ratio (G:F), residual gain (RG), residual gain and intake (RIG), the ratio of HP-to-ME intake (HP/MEI), and residual heat production (RHP). For statistical analysis, animals' performance data in each period, were ranked by RFI, and categorized into high-, medium-, and low-RFI groups (H-RFI, M-RFI, and L-RFI). RFI was not correlated with in vivo digestibility, age, BW, BCS, or ADG in all three periods. The L-RFI group had lowest DMI, MEI, HP, retained energy (RE), and RE/ADG. Chemical analysis of the longissimus dorsi muscle shows that the L-RFI group had a higher percentage of protein and a lower percentage of fat compared to the H-RFI group. We suggested that the main mechanism separating L- from H-RFI calves is the protein-to-fat ratio in the deposited tissues. When efficiency was related to kg/day (DMI and ADG) and not to daily retained energy, the selected efficient L-RFI calves deposited more protein and less fat per daily gain than less efficient H-RFI calves. However, when the significant greater heat increment and maintenance energy requirement of protein compared to fat deposition in tissue were considered, we could not exclude the hypothesis that variation in efficiency is partly explained by efficient energy utilization. The ranking classification of calves to groups according to their RFI efficiency was independent of diet quality and age.

Application of fecal near-infrared reflectance spectroscopy profiling for the prediction of diet nutritional characteristics and voluntary intake in beef cattle.

The objective of this study was to evaluate the use of fecal near-infrared reflectance spectroscopy (NIRS) profiling to predict diet nutritional characteristics and voluntary DMI in beef cattle. Fecal samples were collected for growing cattle across 11 experiments in which individual animal performance and DMI was measured. Dried and ground fecal composite samples collected from each animal were subjected to fecal NIRS analysis by a Foss NIRS 6500 scanning monochromator (Foss, Eden Prairie, MN) at the Grazingland Animal Nutrition Laboratory (Temple, TX). Fecal spectra were then used to develop equations to predict diet composition (trials 1 to 11; = 408), digestibility (trials 1 to 5; = 155), and DMI (trials 1 to 11; = 408). Coefficients of determination for calibration () and cross-validation () for prediction of diet nutritional characteristics were lower for NDF ( = 0.85; = 0.82) than for CP ( = 0.90; = 0.88). For the prediction of DMI, and ranged from 0.69 and 0.67 for the prediction of trial-average DMI to 0.76 and 0.73 for the prediction of fecal-collection-period DMI. While the and obtained for the prediction of DMI were lower than those obtained for the prediction of diet composition or digestibility, fecal NIRS prediction equations for DMI were successful in predicting the mean DMI of groups, as no differences were found for the prediction of fecal-collection-period DMI (Diff. = 1.10; = 0.72) or trial DMI (Diff. = -0.47; = 0.86).

Changes in feeding behavior patterns and dry matter intake before clinical symptoms associated with bovine respiratory disease in growing bulls.

Methods to improve accuracy of preclinical detection of bovine respiratory disease (BRD) are needed to reduce the economic impact of this disease, improve animal welfare, and promote more judicious use of antimicrobials in beef cattle. The objectives of this study were to retrospectively characterize time-series deviations in DMI and feeding behavior patterns preceding the display of observed clinical symptoms associated with BRD and to identify those feeding behavior traits that would be most predictive of BRD. The study was conducted with 231 seed stock bulls (391 ± 55 kg initial BW) representing 5 breeds that were housed in a facility equipped with GrowSafe feed bunks at a commercial bull-test facility. All bulls were vaccinated against standard viral and bacterial pathogens before and on arrival at the facility. Daily DMI and feeding behavior traits (frequency and duration of bunk visit events, head-down duration, variance of nonfeeding intervals, and time to approach feed bunk following feed-truck delivery) were measured for 70 d with a GrowSafe system. During a 10-d period from Day 28 to 37 of the trial, 30 bulls were administered antimicrobial therapy for clinical symptoms of BRD (rectal temperatures > 39.5°C). All remaining bulls ( = 201) were administered metaphylactic therapy on Day 38 of the trial in response to an acute decrease in feed intake. A retrospective analysis was conducted using a 2-slope broken-line regression model to identify inflection points in DMI and feeding behavior traits relative to onset of illness. The bulls were separated into 2 cohort groups based on observed clinical illness ( = 30) or those metaphylactically treated ( = 201), with the 2-slope broken-line regression model applied separately to each cohort. The model-detected inflection points for DMI were 6.8 and 3.8 d before observed clinical illness and metaphylactic treatment, respectively, and the reductions in DMI from detected inflection points to the day of observed clinical illness and day of metaphylactic treatment were 39.3 and 49.8%, respectively. Furthermore, the model-detected inflection points for individual feeding behavior traits ranged from 1.3 to 14.2 d before observed clinical illness and from 3.8 to 12.6 d before metaphylactic treatment. Results from this study demonstrate the potential value of electronic behavior-monitoring systems to improve the sensitivity and specificity of preclinical detection of BRD in feedlot cattle.

Relationships of feeding behaviors with average daily gain, dry matter intake, and residual feed intake in Red Angus-sired cattle.

Feeding behavior has the potential to enhance prediction of feed intake and to improve understanding of the relationships between behavior, DMI, ADG, and residual feed intake (RFI) in beef cattle. Two cohorts, born in 2009 and 2010, the progeny of Red Angus bulls (n = 58 heifers and n = 53 steers), were evaluated during the growing phase, and the latter group of steers was also evaluated during the finishing phase. All behavior analyses were based on 7 feeding behavior traits (bunk visit frequency, bunk visit duration [BVDUR], feed bout frequency, feed bout duration, meal frequency, meal duration, and average meal intake) and their relationships with ADG, DMI, and RFI. During the growing phase, feeding duration traits were most indicative of DMI with positive correlations between BVDUR and DMI for cohort 1 steers, growing phase (n = 28, r = 0.52, P = 0.00); cohort 2 steers, growing phase (n = 25, r = 0.44, P = 0.01); and cohort 2 heifers, growing phase (n = 29, r = 0.28 P = 0.05). There were similar trends toward correlation of BVDUR and RFI for both steer groups and cohort 1 heifers, growing phase (C1HG; n = 29; r = 0.27, P = 0.06; r = 0.30, P = 0.07; and r = 0.26, P = 0.08, respectively). Feed bout frequency was correlated with ADG in C1HG and in cohort 2 steers, finishing phase (r = -0.31, P = 0.04, and r = 0.43, P = 0.01, respectively). Feed bout duration was correlated with ADG in heifer groups (r = 0.29 and r = 0.28, P = 0.05 for both groups) and DMI for all growing phase animals (r = 0.29 to 0.55, P ≤ 0.05 for all groups). Evaluation of growing vs. finishing phase steer groups suggests that all behaviors, RFI, and DMI, but not ADG, are correlated through the growing and finishing phases (P ≤ 0.01 for all variables excluding ADG), implying that feeding behaviors determined during the growing phase are strong predictors of DMI in either life stage. Sire maintenance energy EPD effects (measured as high or low groups) on progeny feeding behaviors revealed a difference in meal duration with a tendency to differ in average meal intake (P = 0.01 and P = 0.07, respectively). Feeding behavior duration traits may be useful predictors of DMI in Red Angus cattle.

Relationships between residual feed intake and hepatic mitochondrial function in growing beef cattle.

The objective of this study was to evaluate the relationship between hepatic mitochondrial function and residual feed intake (RFI) in growing beef cattle. In Trial 1, RFI was measured in 29 Angus heifers (initial BW = 258.0 ± 24.9 kg) from divergent IGF-I selection lines created at the Eastern Agricultural Research Station (The Ohio State University) fed a grain-based diet (calculated ME = 2.85 Mcal/kg DM). In Trial 2, RFI was measured in 119 Santa Gertrudis steers (initial BW = 308.4 ± 28.1 kg) fed a roughage-based diet (calculated ME = 2.21 Mcal/kg DM). At the end of the RFI measurement period, cattle in Trial 1 (n = 7 low RFI and n = 7 high RFI) and in Trial 2 (n = 6 low RFI and n = 8 high RFI) with measures of RFI exceeding 0.5 (Trial 1) or 1.0 (Trial 2) SD from the mean RFI were selected to measure mitochondrial function. Overall ADG, DMI, and RFI were 1.19 ± 0.15, 9.31 ± 1.12, and 0.00 ± 0.63 kg/d and 0.83 ± 0.16, 9.48 ± 1.00, and 0.00 ± 0.86 kg/d in Trial 1 and 2, respectively. Cattle with low RFI consumed 13 and 24% less (P < 0.05) DM and had 14 and 56% greater (P < 0.05) G:F than cattle with high RFI in Trial 1 and 2, respectively, even though ADG and BW were similar (P > 0.10). In Trial 1, cattle with low RFI tended (P = 0.06) to have greater state 3 respiration rates than cattle with high RFI, but state 3 respiration rates were similar (P > 0.10) between cattle with low and high RFI in Trial 2. In both trials, cattle with low RFI had greater (P < 0.05) acceptor control ratios than their high RFI counterparts. The respiratory control ratio tended (P = 0.09) to be greater for cattle with low RFI compared with high RFI cattle in Trial 1, but no difference (P > 0.10) was observed in Trial 2. Proton-leak kinetics were similar (P > 0.05) between cattle with low and high RFI in both trials. These data suggest that ADP has greater control of oxidative phosphorylation in liver mitochondrial of cattle with low RFI compared to their high RFI counterparts.

Relationships of feeding behaviors with efficiency in RFI-divergent Japanese Black cattle.

New approaches to limit expenses associated with input, without compromising profit, are needed in the beef industry. Residual feed intake (RFI) is an efficiency trait that measures variation in feed intake beyond maintenance, growth, and body composition. The addition of feeding behavior analysis to standard RFI tests may provide an approach to more readily identify feed-efficient cattle. The current study analyzes 7 feeding behaviors (BVFREQ: bunk visit frequency, BVDUR: bunk visit duration, FBFREQ: feed bout frequency, FBDUR: feed bout duration, MFREQ: meal frequency, MDUR: meal duration, and AMINT: average meal intake) and their relationships with RFI, ADG, and DMI in Japanese Black (Wagyu) cattle. Three cohorts of yearling Wagyu animals were studied using a standard 70-d RFI test, and data from divergent ( ± 0.5 SD from population RFI mean) subsets of animals were analyzed for feeding behaviors [n = 58, bulls on high-concentrate diet (C1); n = 36, bulls on a lower-concentrate diet (C2); n = 34, heifers on a lower-concentrate diet (C3)]. The following behaviors were correlated with ADG: BVFREQ (r = 0.32, P = 0.01; C1 bulls), BVDUR (r = 0.42, P = 0.01, C2 bulls), FBFREQ (r = 0.37, P < 0.01; C1 bulls), FBDUR (r = 0.46, P < 0.01, C1 bulls), and MFREQ (r = 0.42, P < 0.01, C2 bulls). Behaviors were trending or significantly correlated with DMI for all cases except for MFREQ for C3 and AMINT for C2. Residual feed intake was positively correlated with MDUR across all cohorts (r = 0.31, P = 0.02; r = 0.38, P = 0.02; r = 0.54, P ≥ 0.01, respectively). For C2 bulls and C3 heifers, RFI was positively correlated with behavior frequency categories (BVFREQ; r = 0.44, P = 0.01; r = 0.60, P ≤ 0.01, respectively, and FBFREQ r = 0.46, P ≤ 0.01; r = 0.60, P ≤ 0.01, respectively). Bunk visit frequency and FBFREQ were highly correlated with RFI status (high or low) in C2 bulls and C3 heifers. Behavior duration categories (BVDUR, FBDUR, and MDUR) were most correlated with efficiency status in C1 bulls. However, behavior frequency categories (BVFREQ and FBFREQ), as well as MDUR, were most correlated with efficiency status in C2 bulls and C3 heifers. Inclusion of meal duration measurements when evaluating RFI provides an additional tool in understanding the drivers of variation in this important trait in Wagyu cattle. The present study provides new insights into feed intake patterns of a beef breed for which there are few reports of feeding behavior.

Relationships between postweaning residual feed intake in heifers and forage use, body composition, feeding behavior, physical activity, and heart rate of pregnant beef females.

The objectives of this study were to determine if residual feed intake (RFI) classification of beef heifers affected efficiency of forage utilization, body composition, feeding behavior, heart rate, and physical activity of pregnant females. Residual feed intake was measured in growing Bonsmara heifers for 2 yr (n=62 and 53/yr), and heifers with the lowest (n=12/yr) and highest (n=12/yr) RFI were retained for breeding. Of the 48 heifers identified as having divergent RFI, 19 second-parity and 23 first-parity females were used in the subsequent pregnant-female trial. Pregnant females were fed a chopped hay diet (ME=2.11 Mcal kg(-1) DM) in separate pens equipped with GrowSafe bunks to measure individual intake and feeding behavior. Body weights were measured at 7-d intervals and BCS and ultrasound measurements of 12th-rib fat depth, rump fat depth, and LM area obtained on d 0 and 77. Heart rate and physical activity were measured for 7 consecutive d. First-parity females had lower (P<0.05) initial BW, BW gain, and initial hip height and tended (P=0.07) to have lower DMI compared to second-parity females. Females with low RFI as heifers consumed 17% less (P<0.01) forage compared to females with high RFI as heifers but maintained the same BW, BW gain, and body composition. Likewise, RFI classification did not affect calving date. An interaction (P=0.04) between heifer RFI classification and parity was found for calf birth weight. Calves from first-parity low-RFI females were lighter at birth (P<0.01) than calves from high-RFI females, but RFI classification did not affect BW of calves born to second-parity females. Residual feed intake classification did not affect bunk visit frequency, but low-RFI females spent 26% less time (P<0.01) at the bunk compared to high-RFI females. First-parity females had more (P<0.05) daily step counts and greater lying-bout frequencies compared to second-parity females, but physical activity was not affected by RFI classification. Heart rates of females classified as low RFI were 7% lower (P=0.03) compared to high-RFI females. Heifer postweaning RFI but not G:F or residual gain were positively correlated with forage intake (r=0.38) and RFI (r=0.42) of pregnant females. Results indicate that heifers identified as having low postweaning RFI have greater efficiency of forage utilization as pregnant females, with minimal impacts on growth, body composition, calving date, and calf birth BW, compared to their high-RFI counterparts.

Plane of nutrition x tick burden interaction in cattle: effect on fecal composition.

Effective tick management on grazing animals is facilitated by accurate noninvasive detection methods. Fecal analysis provides information about animal health and nutrition. Diet affects fecal composition; stress may do likewise. The constituents in feces that may be affected by tick burdens and in turn affect near-infrared spectra have not been reported. Our objective was to examine the interaction between plane of nutrition and tick burden on fecal composition in cattle. Angus cross steers (n = 28; 194 ± 3.0 kg) were assigned to 1 of 4 treatments (n = 7 per group) in a 2 × 2 factorial arrangement: moderate (14.0 ± 1.0% CP and 60 ± 1.5% TDN) vs. low (9.0 ± 1.0% CP and 58 ± 1.5% TDN) plane of nutrition and control (no tick) vs. tick treatment [infestation of 300 pair of adult Lone Star ticks (Amblyomma americanum) per treated animal]. Fecal samples were collected at approximately 0700 h on d -7, 0, 7, 10, 14, 17, and 21 relative to tick infestation. Fecal constituents measured were DM, OM, pH, Lactobacillus spp., Escherchia coli, acetate, propionate, butyrate, isobutyrate, valerate, isovalerate, IgA, and cortisol. Experimental day affected (P < 0.05) all constituents measured. Plane of nutrition affected (P < 0.05) DM, OM, VFA, and IgA. Tick treatment numerically (P = 0.13) reduced cortisol. A multivariate stepwise selection model containing cortisol and E. coli values on d 10 and d 14 accounted for 33% of the variation in daily adult female tick feeding counts across both planes of nutrition (P < 0.07). Within the moderate plane of nutrition, a model containing only cortisol on d 10 and d 14 described 59% of the variation in the number of feeding ticks (P < 0.02). Similarly, a model including cortisol, propionate, isovalerate, and DM at d 10 and d 14 d described 95% of the variation in total feeding ticks in the low plane of nutrition. Of the constituents measured, fecal cortisol offers the best possibility of noninvasively assessing stress by way of a single assay but the presence of ticks would still need to be confirmed visually. Although several constituents measured in this study should exist in sufficient quantity to directly affect near-infrared spectra, none stood out as a clear descriptor of prior observed differences in fecal spectra between tick-treated versus non-tick-treated animals. There were, however, groups of fecal constituents related to daily adult female tick feeding numbers (as a visual estimation of tick stress).

Plane of nutrition by tick-burden interaction in cattle: effect on growth and metabolism.

Ticks are external parasites, which pose a significant economic burden to domestic animal agriculture. The effects of ticks on grazing animals may be exacerbated during periods of low nutrition, such as those encountered during drought. It is not completely understood how plane of nutrition and tick burden interact to affect metabolism in cattle. The objective of the current research was to examine the plane of nutrition by tick-burden interaction in cattle and determine the effects of this interaction on physiological indicators of growth and metabolism. Eight-month-old Angus cross steers (n = 28, 194 ± 3.0 kg) were stratified by pretrial BW and DMI into 1 of 4 groups (n = 7/group) in a 2 × 2 factorial arrangement. Categories were: moderate (14.0 ± 1.0% CP, 60 ± 1.5% TDN) vs. low (9.0 ± 1.0% CP, 58 ± 1.5% TDN) plane of nutrition and control (no tick) vs. tick treatment (300 pair of adult Amblyomma americanum per treated animal). Steers were individually fed their respective experimental diets ad libitum and feed intake was monitored for 35 d before and 21 d after the start of tick infestation (d 0). Blood samples were harvested via coccygeal venipuncture on d -7, 0, 7, 8, 9, 10, 11, 13, 17, and 21. Plasma cortisol and IGF-I were determined by RIA. Metabolic indicators were determined by colorimetric assay. Steers weighed 195 ± 6 kg on d -35, but on d -7 and d 21, the moderate steers weighed more than the low steers (244.1 ± 8.7 vs. 227.7 ± 8.4 kg, P < 0.07; and 283.4 ± 8.0 vs. 244.0 ± 7.9 kg, P < 0.001, respectively). Cortisol was affected by plane of nutrition and treatment (P < 0.08). Insulin-like growth factor-I was greater (P < 0.01) in moderate than in low and control animals (P < 0.02), compared with tick-treated animals. Tick treatment had no effect (P > 0.05) on any of the metabolites measured in this study. Plane of nutrition affected (P < 0.02) albumin, blood urea nitrogen, and glucose in that values from the moderate group animals were greater than those from the low group. Although cortisol was related to both tick treatment and nutritional status in the current study, with respect to the combination of parasitism and suboptimal nutrition, IGF-I was the most highly indicative constituent measured. Tick burden affected various characteristics of growth and metabolism in these growing cattle and the effects were exacerbated by a low plane of nutrition.

Relationships between feed efficiency, scrotal circumference, and semen quality traits in yearling bulls.

A meta-analysis was conducted to examine phenotypic relationships between feed efficiency, scrotal circumference, and semen quality traits in yearling bulls. Data evaluated were obtained from 5 postweaning trials involving Angus (n = 92), Bonsmara (n = 62), and Santa Gertrudis (n = 50) bulls fed diets that ranged from 1.70 to 2.85 Mcal ME/kg DM. After an adaptation period of 24 to 28 d, feed intake was measured daily, and BW was measured at 7- or 14-d intervals during the 70- to 77-d trials. Ultrasound carcass traits (12th-rib back fat thickness, BF; LM area, LMA) and scrotal circumference (SC) were measured at the start and end of each trial. Semen samples were collected by electroejaculation within 51 d of the end of the trials when the age of bulls averaged from 365 to 444 d and were evaluated for progressive sperm motility and morphology. Residual feed intake (RFI) was calculated as the difference between actual DMI and expected DMI from linear regression of DMI on ADG and midtest BW(0.75), with trial, trial by ADG, and trial by midtest BW(0.75) as random effects. Across all studies, bulls with low RFI phenotypes (<0.5 SD below the mean RFI of 0) consumed 20% less DM and had 10% less BF but had similar ADG, SC, and semen quality traits compared with high-RFI bulls (>0.5 SD above the mean RFI of 0). Gain to feed ratio was strongly correlated with ADG (0.60) and weakly correlated with initial BW (-0.17) and DMI (-0.26). Residual feed intake was not correlated with ADG, initial age, or BW but was correlated with DMI (0.71), G:F (-0.70), and BF (0.20). Initial SC (-0.20), gain in SC (-0.28), and percent normal sperm (-0.17) were correlated with G:F, but only sperm morphology was found to be weakly associated with RFI (0.13). These data suggest that RFI is not phenotypically associated with SC or sperm motility but is weakly associated with sperm morphology.

Technical note: Evaluation of bimodal distribution models to determine meal criterion in heifers fed a high-grain diet.

Meals are clusters of feedbunk visit (BV) events that are differentiated from the next meal by a nonfeeding interval that is longer compared with the nonfeeding intervals within a meal. The longest nonfeeding interval considered to be part of a meal is defined as the meal criterion. The objective of this study was to determine which combination of 2 probability density functions [(PDF): Gaussian normal (G), Weibull (W), Log-Normal, Gamma, and Gumbel] used in a bimodal distribution model had the best fit of nonfeeding interval data collected in beef heifers. Feeding behavior traits (572,627 total BV events) were measured in 119 heifers fed a high-grain diet (3.08 Mcal ME/kg DM), using a GrowSafe system for 66 d. The frequency and duration of BV events averaged 75 ± 15 events/d and 73.0 ± 22.3 min/d, respectively. The bimodal PDF combinations were fitted to the log(10)-transformed interval lengths between BV events for each animal, using R mixdist package (2.13). The Akaike Information Criterion (AIC) was used to assess goodness of fit of the 25 bimodal PDF combinations. The PDF model with the least AIC value was selected as the best fit for each individual. A χ(2) analysis of the selected best PDF distribution across individuals revealed that 78.2% of the heifers best fit were G-W or W-W PDF models. The likelihood probability estimates were calculated from the average AIC deviation of each model from the standard G-G model. The G-W likelihood probability estimate was greater (P = 0.001) than the W-W combination (0.997 vs. 0.727). Our analysis indicated the G-W model had a statistically better fit and is most likely the best approach to define meal criterion in beef heifers fed high-grain diets.

Validation of a system for monitoring feeding behavior in beef cattle.

The objective of this study was to evaluate the use of an electronic radio-frequency-identification-based system (GrowSafe System Ltd., Airdrie, Alberta, Canada) to measure feeding behavior traits in beef cattle fed a high-grain diet. Feeding behavior data were recorded by the GrowSafe system and time-lapse video using 10 heifers over a 6-d period. Observed bunk visit (BV) and meal event data (frequency and duration) were compared with electronic feeding behavior data generated by the GrowSafe system at 5 parameter settings (MPS; 30, 60, 100, 150, and 300 s), which are used to define the maximum duration between consecutive electronic identification recordings to initiate a subsequent BV event. A random coefficient model was used to compare video and electronic data using orthogonal contrasts. Video data were regressed on the electronic feeding behavior data to obtain an estimate of precision (r(2)) and other statistical estimates, including mean square error of prediction and concordance correlation coefficient, to access the adequacy of the electronic system predictions. The variation in MPS values affected BV data, but not meal event data. Electronic meal frequency and duration data were not different (P > 0.50) from observed values, and were not affected by electronic MPS values. The optimal MPS value for prediction of BV and meal event frequency and duration traits was 100 s. Our evaluation indicated the GrowSafe system 4000E was able to predict BV and meal event data when the 100-s MPS was used to analyze the feeding behavior data.

Effects of a lone star tick (Amblyomma americanum) burden on performance and metabolic indicators in growing beef steers.

We examined the response of selected animal performance, endocrine, immune, and metabolic factors from 13 steers (254+/-6.1 kg) with and without a lone star (Amblyomma americanum) tick burden during progressive days of the tick feeding cycle. Steers were randomly assigned to either non-treated controls or treated with 300 adult pair of A. americanum per animal. Animals were weighed and blood sampled on days -7, 0, 10, 14, 21, 28, and 35 relative to tick treatment. Tick treatment did not affect (P<0.1) animal performance. Experimental day did (P<0.05) affect body weight gain and dry matter intake. Tick treatment did not affect (P<0.1) metabolic indicators. Experimental day affected (P<0.05) IGF1 and lactate, tended to affect cortisol (P<0.07), but did not affect (P<0.1) glucose concentrations. Tick treatment did not significantly (P<0.1) affect growth hormone receptor (GHR) mRNA in liver, but liver tissue from treated animals had numerically lower GHR mRNA than did tissue from control animals. Day had a significant (P<0.05) effect on liver GHR mRNA. There was a significant treatment by day interaction (P<0.05) for liver IGF1 gene expression, as IGF1 mRNA was reduced in tick-treated cattle versus control cattle on day 35. Overall, liver IGF1 gene expression was lower (P<0.05) in tick than in control animals while there was no effect (P>0.1) due to day. Within the tick-treated group, correlations were found between quantitative female tick feeding characteristics and host metabolic indicators. Feeding by adult female lone star ticks did cause acute stress in growing beef steers on a moderate plane of nutrition as indicated by some physiologic indicators. In particular there may be longer term effects on the somatotrophic axis in the liver which could affect subsequent (i.e. feedlot) performance. It is not known how these observed effects would be manifest under a lower plane of nutrition, as is common and may become more so within the current native range of A. americanum. Other acute effects due to tick feeding may have been masked by the effects of handling and invasive sampling. Non-invasive experimental procedures are called for in order to study the effects of a stressor such as arthropod infestation on grazing animals. Future research efforts will be aimed at non-invasively elucidating the effects of tick stress on grazing animals under various nutritional environments.

Phenotypic and genetic relationships of residual feed intake with performance and ultrasound carcass traits in Brangus heifers.

The objective of this study was to characterize residual feed intake (RFI) and to estimate phenotypic and genetic correlations with performance and ultrasound carcass traits in growing heifers. Four postweaning feed efficiency trials were conducted using 468 Brangus heifers. The complete Brangus pedigree file from Camp Cooley Ranch (Franklin, TX), which included 31,215 animals, was used to generate genetic parameter estimates. The heifer progeny from 223 dams were sired by 36 bulls, whereas the complete pedigree file contained 1,710 sires and 8,191 dams. Heifers were individually fed a roughage-based diet (ME = 1.98 Mcal/kg of DM) using Calan gate feeders for 70 d. Heifer BW was recorded weekly and ultrasound measures of 12th- to 13th-rib fat thickness (BF) and LM area (LMA) obtained at d 0 and 70. Residual feed intake (RFIp) was computed as actual minus predicted DMI, with predicted DMI determined by linear regression of DMI on mid-test BW(0.75) (MBW) and ADG with trial, trial x MBW, and trial x ADG as random effects. Overall means for ADG, DMI, and RFI were 1.01 (SD = 0.15), 9.51 (SD = 1.02), and 0.00 (SD = 0.71) kg/d, respectively. Stepwise regression analysis revealed that inclusion of gain in BF and final LMA into the base model increased the R(2) (0.578 vs. 0.534) and accounted for 9% of the variation in DMI not explained by MBW and ADG (RFIp). Residual feed intake and carcass-adjusted RFI (RFIc) were strongly correlated phenotypically and genetically with DMI and FCR, but not with ADG or MBW. Gain in BF was phenotypically correlated (P < 0.05) with RFIp (0.22), but not with FCR or RFIc; however, final BF was genetically correlated (P < 0.05) with RFIp (0.36) and RFIc (0.39). Gain in LMA was weakly phenotypically correlated with FCR, but not with RFIp or RFIc; however, gain in LMA was strongly genetically correlated with RFIp (0.55) and RFIc (0.77). The Spearman rank correlation between RFIp and RFIc was high (0.96). These results suggest that adjusting RFI for ultrasound carcass composition traits will facilitate selection phenotypically independent of growth, body size, and carcass composition; however, genetic relationships may still exist between RFI and carcass composition.

Relationship between feeding behavior and residual feed intake in growing Brangus heifers.

Residual feed intake (RFI) is a measure of feed efficiency defined as the difference between actual feed intake and expected feed intake required for maintenance and production. The objective of this study was to determine the relationship between RFI, feeding behavior, and other performance traits in growing heifers. Individual DMI was measured in Brangus heifers (n = 115) fed a roughage-based diet (ME = 2.0 Mcal/kg) for 70 d using Calan-gate feeders. Residual feed intake was computed as the residuals from linear regression of DMI on mid-test BW(0.75) and ADG. Heifers with the greatest (least efficient, n = 18) and least (most efficient, n = 18) RFI were identified for quantification of feeding behavior traits. Continuous video recordings were obtained for all heifers during d 28 through d 56 of the 70-d feeding trial. Video data of 2 replications of four 24-h periods, 2 wk apart, were analyzed for the focal heifers. A head-down feeding event was defined as a heifer positioned in the feeder with her head lowered. A meal included all head-down feeding events that were separated by less than 300 s. The mean RFI for the high- and low-RFI heifers were 1.00 and -1.03 +/- 0.03 kg/d, respectively. High-RFI heifers consumed 21.9% more (P < 0.0001) DM but had similar BW and ADG compared with low-RFI heifers. The high-RFI heifers spent less time in head-down feeding events per day (P < 0.0001; 124 vs. 152 +/- 4.3 min/d), consumed DM at a faster rate (99.6 vs. 62.8 +/- 3.3 g/min), and ate more often per day (119.1 vs. 90.5 +/- 3.9 head-down feeding events/d) compared with the low-RFI heifers; however, meal duration and frequency were not related to RFI. We conclude that feeding behavior related to head-down feeding events may be more useful as an indicator of RFI than the number of meal events.

Effects of the methane-inhibitors nitrate, nitroethane, lauric acid, Lauricidin and the Hawaiian marine algae Chaetoceros on ruminal fermentation in vitro.

The effects of several methane-inhibitors on rumen fermentation were compared during three 24h consecutive batch cultures of ruminal microbes in the presence of nonlimiting amounts of hydrogen. After the initial incubation series, methane production was reduced greater than 92% from that of non-treated controls (25.8+/-8.1 micromol ml(-1) incubation fluid) in cultures treated with nitroethane, sodium laurate, Lauricidin or a finely-ground product of the marine algae, Chaetoceros (added at 1, 5, 5 and 10 mg ml(-1), respectively) but not in cultures treated with sodium nitrate (1 mg m1(-1)). Methane production during two successive incubations was reduced greater than 98% from controls (22.5+/-3.2 and 23.5+/-7.9 micromol ml(-1), respectively) by all treatments. Reductions in amounts of volatile fatty acids and ammonia produced and amounts of hexose fermented, when observed, were most severe in sodium laurate-treated cultures. These results demonstrate that all tested compounds inhibited ruminal methane production in our in vitro system but their effects on fermentation differed.

Relationship of temperament, growth, carcass characteristics and tenderness in beef steers.

Relationships of temperament evaluated at different production stages with growth, carcass characteristics and beef tenderness were determined in Bonsmara crossbred steers managed under commercial managent. Temperament was evaluated at weaning and at initiation of the finishing phase. Steers from a Roswell, NM ranch (n=156) and a Cline, TX ranch (n=21) were stratified at fall weaning by weight and source and randomly allotted to winter ryegrass at Uvalde or Overton, TX followed by feeding in a commercial feedlot near Batesville, TX. Cattle were observed for temperament (escape velocity, EV, m/s; pen and chute temperament score, PTS and CTS) at weaning and upon entry to the feedlot. Cattle were harvested at approximately 7 mm 12th rib fat. Carcass data was taken approximately 36 hrs post-mortem and 2.5cm thick steaks were removed from the 13th rib for Warner-Bratzler shear force (WBS) determination. The only measures of temperament significantly related to performance were EV and PTS. Weaning EV appeared to be more related to feedlot ADG (r=-0.26, P<0.003), ribeye area (r=-0.37, P<0.0008), yield grade (r=0.29, P<0.01) and WBS, r=0.27, P<0.005) than did the later measures of temperament. However, in-feedlot EV was associated with feedlot weights (r=-0.28, P<0.0004). Results of this research suggest temperament, particularly at weaning, is related to feedlot performance, carcass merit, and beef tenderness at a low to moderate level and evaluation of this trait may be a helpful management tool.

Characterization of feed efficiency traits and relationships with feeding behavior and ultrasound carcass traits in growing bulls.

The objectives of this study were to characterize feed efficiency traits and to examine phenotypic correlations between performance and feeding behavior traits, and ultrasound measurements of carcass composition in growing bulls. Individual DMI and feeding behavior traits were measured in Angus bulls (n=341; initial BW=371.1+/-50.8 kg) fed a corn silage-based diet (ME=2.77 Mcal/kg of DM) for 84 d in trials 1 and 2 and for 70 d in trials 3 and 4 by using a GrowSafe feeding system. Meal duration (min/d) and meal frequency (events/d) were calculated for each bull from feeding behavior recorded by the GrowSafe system. Ultrasound measures of carcass 12th-rib fat thickness (BF) and LM area (LMA) were obtained at the start and end of each trial. Residual feed intake (RFIp) was computed from the linear regression of DMI on ADG and midtest BW(0.75) (metabolic BW, MBW), with trial, trial by ADG, and trial by midtest BW(0.75) as random effects (base model). Overall ADG, DMI, and RFIp were 1.44 (SD=0.29), 9.46 (SD=1.31), and 0.00 (SD=0.78) kg/d, respectively. Stepwise regression analysis revealed that inclusion of BW gain in BF and LMA in the base model increased R(2) (0.76 vs. 0.78) and accounted for 9% of the variation in DMI not explained by MBW and ADG (RFIp). Residual feed intake and carcass-adjusted residual feed intake (RFIc) were moderately correlated with DMI (0.60 and 0.55, respectively) and feed conversion ratio (FCR; 0.49 and 0.45, respectively), and strongly correlated with partial efficiency of growth (PEG; -0.84 and -0.78, respectively), but not with ADG or MBW. Gain in BF was weakly correlated with RFIp (0.30), FCR (-0.15), and PEG (-0.11), but not with RFIc. Gain in LMA was weakly correlated with RFIp (0.17) and FCR (-0.19), but not with PEG or RFIc. The Spearman rank correlation between RFIp and RFIc was high (0.91). Meal duration (0.41), head-down duration (0.38), and meal frequency (0.26) were correlated with RFIp and accounted for 35% of the variation in DMI not explained by MBW, ADG, and ultrasound traits (RFIc). These results suggest that adjusting residual feed intake for carcass composition will facilitate selection to reduce feed intake in cattle without affecting rate or composition of gain.

Association of mitochondrial function and feed efficiency in poultry and livestock species.

As grain prices have increased dramatically in the past year, understanding the fundamental genetic, cellular, and biochemical mechanisms responsible for feed efficiency (FE; g of gain/g of feed) or residual feed intake (RFI; an alternative feed efficiency trait that quantifies interanimal variation in DMI that is unexplained by differences in BW and growth rate) in livestock and poultry is extremely important with respect to maintaining viable meat production practices in the United States. Although breed and diet have long been known to affect mitochondrial function, few studies have investigated differences in mitochondrial function and biochemistry due to interanimal phenotypic differences in FE or RFI (i.e., variation among animals of the same breed and fed the same diet). This paper reviews existing literature on relationships of mitochondrial function and biochemistry with FE and RFI in poultry and livestock. The overall goal of all of this paper is to assist the development of tools (e.g., genetic markers or biomarkers) to aid commercial breeding companies in genetic selection that, in turn, will help maintain viable livestock and poultry industries in the United States and around the world.