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This review delves into the intricate realm of ruminal microbes, shedding light on their complexity beyond previously recognized dimensions. Existing publications have probed the essential facets of these microorganisms on a finer scale than previously acknowledged. Mathematical models have emerged to simulate the behavior of approximately 200 distinct bacterial species, 25 genera of ciliate protozoa, and five genera of anaerobic fungi, constituting about 10% of the total viable bacteria in the rumen. However, the expansive diversity and density of rumen microbes challenge the complete characterization of species or genera in relation to substrates or end products. Consequently, continuous revision and re-engineering of mathematical models become imperative. In the context of ruminants, prevalent models often categorize bacteria into fiber-fermenting and nonfiber-fermenting groups. An additional subgroup featuring hyper-ammoniaproducing bacteria has been proposed, but its explicit integration into nutrition models remains unexplored. Furthermore, incorporating a more mechanistic evaluation of protozoa and fungal growth may be warranted to improve nutrition models. The time horizon and intervals at which models are constructed present another critical consideration, with feed efficiency variations among ruminants believed to be closely tied to differences in the ruminal microbiota. Addressing immediate concerns, this review underscores the need for a comprehensive exploration of factors influencing microbial growth in the rumen. Several indices have been suggested and adopted to predict microbial efficiencies, but they may provide simplification for practical application. Thus, a mechanistic approach should be considered to remove dependencies between estimates and improve predictive accuracy and precision. While artificial intelligence holds promise in elucidating gaps within empirical and mechanistic models, the generation of accurate, vetted datasets is paramount. Furthermore, this review posits that factors such as recycled nitrogen, endotoxins, and microbial population dynamics (i.e., cross-feeding and competition) may be limiting our understanding of microbial growth, suggesting the necessity of modeling these intricacies concurrently to achieve a more nuanced comprehension. This multifaceted approach aims to propel the understanding of ruminal microbial dynamics into new frontiers, paving the way for more informed and effective modeling in the future. [ABSTRACT FROM AUTHOR]

Considering the growing accessibility of dried distillers grains with solubles (DDGS) in Brazil, coupled with the limited research examining its integration into conventional confinement diets in the southeast region, comprising ingredients such as flint corn, citrus pulp, and whole cottonseed, there is a clear imperative for conducting studies to deter- mine optimal inclusion levels of this product in such diets. The objective of the study was to compare the performance (experiment 1) and metabolism (experiment 2) of Nellore bulls finished with diets containing 0, 10, 20, 30, or 40% DDGS (% DM) in total replacement of whole cottonseed and partial replacement of ground flint corn and citrus pulp. In experiment 1, Nellore bulls [n = 368; body weight (BW) = 420 ± 30 kg] were used, allocated in 60 experimental pens for 125 d (n = 12 pens per treatment), evaluating variables such as dry matter intake, average daily gain (ADG), feed efficiency, observed net energy (NE) for maintenance and gain, and carcass characteristics. In experiment 2, rumen-cannulated Nellore bulls (n = 30; BW = 503 ± 46) were kept in individual pens for 21 d (n = 6 animals per treatment), evaluating ruminal parameters (proportion of short-chain fatty acids, ammonia concentration, ruminal pH) and apparent digestibility of nutrients in the total tract. The experimental design adopted was randomized blocks, and the data were analyzed using R v. 4.3.1 (R Core Team, 2023). Linear increases were observed in carcass adjusted final BW (P = 0.013), carcass adjusted ADG (P = 0.024), carcass adjusted feed efficiency (P = 0.0003), hot carcass weight (P = 0.0004), and dressing percentage (P < 0.0001) with the incremental inclusion of DDGS in the diets (Table 1). For every 10-percentage unit inclusion in these type of diets, hot carcass weight is increased by 2.56 kg. Additionally, the inclusion of DDGS resulted in linear increases in observed NE for maintenance (P = 0.001) and NE for gain (P = 0.001), calculated according to Zinn and Shen (1998). Digestibility out- comes, determined through the difference of nutrients present in the consumed diet and in the feces, exhibited linearly decreasing effects with the escalating inclusion of DDGS for dry matter (P = 0.004), organic matter (P = 0.004), neutral detergent fiber (P = 0.01), acid detergent fiber (P = 0.04), and total digestible nutrients (P = 0.004). We also observed linear increases for isobutyrate (P = 0.004) and isovalerate (P = 0.002) as the DDGS level in the diet increased. The 40% DDGS diet exhibited 0.6 mmol/100 mol and 0.7 mmol/100 mol more of isobutyrate and isovalerate, respectively, compared with the 0% DDGS diet. In summary, a high inclusion of DDGS emerges as a viable alternative in the nutrition of Bos indicus cattle fed diets containing ground flint corn and citrus pulp, manifesting significant improvements in performance and carcass quality. [ABSTRACT FROM AUTHOR]

The inclusion of the coproduct dry distil- lers grains plus soluble (DDGS) in the diet of finishing cattle can lead to changes in the nutritional profile of the diets, causing an increase in protein content and a reduction in starch. It is necessary to understand how changes in rumen substrates affect rumen metabolic activity, acids, and enteric methane (CH4) production. Five treatments were used, considering levels of DDGS inclusion in the dry matter (DM) of 1) 0 g/kg of DDGS, 2) 150 g/kg of DDGS, 3) 300 g/kg of DDGS, 4) 450 g/kg DDGS, or 5) 600 g/kg DDGS in finishing diets. The metabolism experiment was conducted with rumen-cannulated Nellore bulls (n = 20) with initial body weight (BW) of 401.8 ± 27.0 kg. The production of short-chain fatty acids (SCFA) and CH4 were determined using the ex-situ ruminal fermentation technique. Rumen content was manually collected from 0, 4, 8 and 12 h post-feeding. The liquid and solid fractions of rumen content were used in four 50 mL penicillin-type flasks per animal. Of the four flasks prepared, two are called “blank” and inactivated in boiling water to stop fermentation, and the other two (microrumen) are incubated in a thermostatic bath, simulating the conditions existing in the rumen of animals (presence of microorganisms, anaerobes, temperature of 39ºC, natural animal saliva, physiological pH) for 30 min. After 30 min, incubated flasks were also inactivated in boiling water to stop fermentation. Subsequently, measurements of the final products of rumen fermentation (SCFA and CH4) were carried out in each bottle. Total rumen emptying was performed to determine the rumen pool of DM. The data will be analyzed in the SAS software, the mean of the treatments will be compared using test of mean and orthogonal contrasts. The declared significance is P ≤ 0.05. The inclusion or levels of DDGS did not affect CH4 production of gּ·kg DM-1ּ·d-1ּ and gּ·kg DM-1ּ d-1ּ (P > 0.05). There was an effect of the treatments on the production of propionate (gּ·kg DM-1ּ d-1ּ; P = 0.0448), with a decreasing linear effect (P = 0.0087) with the increase in the inclusion of DDGS in diet (0; 150; 300; 450; 600 g/kg of DDGS; means 137.20; 160.32; 117.60; 87.09; 91.52 gּ·kg DM-1ּ d-1ּ propionate, respectively). There was no effect of DDGS levels on the production of Acetate, Butyrate, or total SCFAs (gּ·kg DM-1ּ d-1ּ). In summary, DDGS inclusion affects rumen metabolic activity, decreasing propionate production, but does not affect the production of other SCFAs nor CH4 production. [ABSTRACT FROM AUTHOR]

The objectives of this study were to examine the relationship between residual feed intake (RFI) and DM and nutrient digestibility, in vitro methane production, and volatile fatty acid (VFA) concentrations in growing beef cattle. Residual feed intake was measured in growing Santa Gertrudis steers (Study 1; n = 57; initial BW = 291.1 ± 33.8 kg) and Brangus heifers (Study 2; n = 468; initial BW = 271.4 ± 26.1 kg) fed a high-roughage-based diet (ME = 2.1 Mcal/kg DM) for 70 d in a Calan-gate feeding barn. Animals were ranked by RFI based on performance and feed intake measured from day 0 to 70 (Study 1) or day 56 (Study 2) of the trial, and 20 animals with the lowest and highest RFI were identified for subsequent collections of fecal and feed refusal samples for DM and nutrient digestibility analysis. In Study 2, rumen fluid and feces were collected for in vitro methane-producing activity (MPA) and VFA analysis in trials 2, 3, and 4. Residual feed intake classification did not affect BW or BW gain (P > 0.05), but low-RFI steers and heifers both consumed 19% less (P < 0.01) DMI compared with high-RFI animals. Steers with low RFI tended (P < 0.1) to have higher DM digestibility (DMD) compared with high-RFI steers (70.3 vs. 66.5 ± 1.6% DM). Heifers with low RFI had 4% higher DMD (76.3 vs. 73.3 ± 1.0% DM) and 4 to 5% higher (P < 0.01) CP, NDF, and ADF digestibility compared with heifers with high RFI. Low-RFI heifers emitted 14% less (P < 0.01) methane (% GE intake; GEI) calculated according to Blaxter and Clapperton (1965) as modified by Wilkerson et al. (1995) , and tended (P = 0.09) to have a higher rumen acetate:propionate ratio than heifers with high RFI (GEI = 5.58 vs. 6.51 ± 0.08%; A:P ratio = 5.02 vs. 4.82 ± 0.14%). Stepwise regression analysis revealed that apparent nutrient digestibilities (DMD and NDF digestibility) for Study 1 and Study 2 accounted for an additional 8 and 6%, respectively, of the variation in intake unaccounted for by ADG and mid-test BW0.75. When DMD, NDF digestibility, and total ruminal VFA were added to the base model for Study 2, trials 2, 3, and 4, the R 2 increased from 0.33 to 0.47, explaining an additional 15% of the variation in DMI unrelated to growth and body size. On the basis of the results of these studies, differences in observed phenotypic RFI in growing beef animals may be a result of inter-animal variation in apparent nutrient digestibility and ruminal VFA concentrations. [ABSTRACT FROM AUTHOR]

The utilization of natural plant secondary compounds as feed additives in animal nutrition has been extensively studied because of their ability to modify digestive and metabolic functions. Condensed tannin (CT) supplementation can potentially alter ruminal fermentation, and mitigate methane (CH4) emissions. The objective of this study was to determine the effect of quebracho CT extract (QT; Schinopsis balansae) within a roughage-based diet on overall fermentability and CH4 production utilizing the in vitro gas production technique (IVGP). Twenty rumen cannulated steers (227 ± 19 kg) were randomly assigned to four dietary treatments (n=4): QT at 0, 1, 2, and 3% of DM (QT0, QT1, QT2, and QT3). A roughage-based diet containing 88% bermudagrass hay and 12% concentrate was fed daily at 2.1% of shrunk body weight. The animals were adapted to the basal diet for 24-d then introduced to predetermined treatments for 35d. Rumen inoculum was collected weekly from each steer to perform the incubations. Two hundred milligrams of air-dried base diet were incubated for 48-h with a composite rumen inoculum for each treatment over 5 wk. Kinetic analysis of cumulative 48h gas production was performed using Gasfit. Measurements of CH4 were performed via gas chromatography and digested residue was determined post-incubation. Data were analyzed using a random coefficients model. Total gas production was higher for QT0 compared to QT1 and QT3 (P = 0.001), but not different from QT2 (P = 0.554). The fractional rate of gas production was higher for QT2 compared to QT0 (P = 0.011). First and second pool gas production decreased linearly as QT inclusion increased (P = 0.042 and 0.010, respectively). There was no dietary effect in ivNDFD (P = 0.567). However, there was a linear tendency to decrease CH4 production with the addition of QT (P=0.071) likely due to changes in the microbial population. [ABSTRACT FROM AUTHOR]

The objective of this trial was to determine the influence of live yeast supplementation (LY) and temperature exposure (TEMP) on the ruminal fermentation characteristics of steers receiving a grower diet. The effects of LY and TEMP were investigated using a 2 x 2 crossover design that spanned five periods. Eight Angus crossbred steers (365 ± 32 kg) were randomly split into pairs and housed in four outdoor pens outfitted with an individualized feeding system. Animals were limit fed a grower diet (DIET) at 1.2% SBW with no live yeast supplementation (NOY) or a grower diet top-dressed with 10 g live yeast/d for 14 days (1.2 × 1012 CFU/d). On days 13 and 14, animals were subjected to one of two TEMP conditions, thermoneutral (TN; 18.4 ± 1.1°C, 57.6 ± 2.8 % RH) or heat stress (HS; 33.8 ± 0.6°C, 55.7 ± 2.7 % RH), in two side-by-side, single-stall open-circuit, indirect respiration calorimetry chambers. Data were analyzed using a random coefficients model. Carryover effects were examined and removed from the model if not significant (P > 0.05). There was no effect of DIET, TEMP, or DIET×TEMP (P > 0.05) on ruminal pH, redox, ciliated protozoa count, acetate, butyrate, total VFA, and ruminal ammonia concentrations. Similarly, the acetate to propionate ratio was not influenced by DIET, TEMP, or DIET × TEMP (P = 0.190). Propionate concentration was the greatest in animals in TN conditions receiving LY (P = 0.008). Compared to HS+NOY, HS+LY (P = 0.003) and TN+LY (P = 0.043) had greater ruminal enumerations of Fusobacterium necrophorum. This suggests LY (P = 0.010) provided a favorable environment for F. necrophorum during heat stress. Live yeast supplementation did not improve overall ruminal fermentation during heat stress. Additional research is required to better understand the dynamic relationship between live yeast and temperature exposure. [ABSTRACT FROM AUTHOR]

Rumen acidosis is a common metabolic disorder occurring when organic acid production exceeds clearance capacity, reducing ruminal pH. Acidosis occurrence has been directly correlated to the ratio of concentrate to forage in the diet. However, the rates of substrate fermentation and acid absorption vary at different locations in the rumen. The objective of this study was to determine the pH in different locations of the rumen using 16 rumenally-cannulated steers (309 ± 43 kg) receiving quebracho extract (QT; Schinopsis balansae) within a grower-type diet [25:75 forage-toconcentrate, dry matter (DM) % basis]. Animals were randomly assigned to one of four dietary treatments (n = 4): QT at 0, 1, 2, and 3% of DM (QT0, QT1, QT2, and QT3). Animals were adapted to the basal diet (QT0) for 12-d before being introduced to predetermined treatments for four weeks, with feed provided twice daily to allow ad libitum intake. Weekly measurements of ruminal fluid pH and redox potential (Eh) were taken four h post-feeding using a portable pH and redox meter probe in four locations of the rumen (cranial sac, ventral sac, dorsal sac, and reticulum). Data were analyzed using a random coefficients model with the pen as a random effect and week as repeated measures. The DM intake was included as a covariate. There was no interaction among diet, location, and week (P ≥ 0.925) on pH. Overall, ruminal pH was lower for QT0 and QT1 compared to QT3 (P < 0.001). Ruminal pH in the cranial sac and reticulum was greater than in the dorsal sac (5.98, 6.03, and 5.87, respectively; P = 0.001). Redox potential was lower for QT0 in week 1 than all other treatments (P = 0.042). This study indicated that pH differs among locations of the rumen regardless of QT supplementation level and days on feed. [ABSTRACT FROM AUTHOR]

Anaerobic metabolism in a biological system, such as the rumen, depends largely on the reducing power of the bacterial population present. For that reason, it is crucial to understand the relationship between redox potential (Eh) and fermentative activity. The diverse microbial profile is characterized by enzyme systems capable of maintaining a constant, yet ever-evolving rumen environment. Variation in Eh is a result of intrinsic and extrinsic factors, including measurement procedure, diet, temperature, and animal. Because Eh is sensitive to oxygen contamination, the objective of this study was to assess different procedures for post-collection rumen inoculum to determine Eh. Four hours after feeding, ten rumen fistulated steers (267.9 ± 8.4 kg), receiving a grower diet, had rumen inoculum (600 mL per animal) extracted using a vacuum suction device and Eh was measured in vivo simultaneously. One subset of samples was incubated in a 39°C water bath (Procedure 1), while the second subset of samples was placed in an ice bath (Procedure 2). Redox potential and temperature were measured at 5, 15, 30, 60, 120, and 240 minutes for both Procedures. For Procedures 1 and 2, Eh values ranged from -235.4 to -313.1 mV and -198.0 to -293.4 mV, respectively. Data were analyzed using orthogonal contrasts with animal within-day and day as random effects in the statistical model. There was no effect for Procedure × Time (P = 0.273) or Time (P = 0.396), but there were differences between Procedures (P < 0.001). Polynomial contrasts indicated quadratic tendencies for Procedure 1 and the interaction of Procedure x Time (P = 0.051 and P = 0.0776, respectively). This study concluded that Procedure 2 more accurately represents in vivo Eh values, likely due to the reduced rate of fermentation associated with samples preserved on ice. [ABSTRACT FROM AUTHOR]

Methods have been developed to measure the effectiveness of many roughages, but few evaluations have been conducted with tropical feeds. The objectives of this research were to determine the effectiveness of roughage sources based on bioassay and laboratory methods and identify the biological attributes of the diets that correlate with these methods. Six ruminally cannulated Nellore steers (408 ± 12 kg of BW) were randomly assigned to a 6 × 6 Latin square design within six diets: negative control diet (NC) with aNDF as 10% from corn silage (CS); positive control diet (PC) with aNDF as 20% from CS; and four diets containing 10% aNDF from CS and 10% aNDF from each of the following sources: sugarcane (SC), sugarcane bagasse (SCB), soybean hulls (SH), or low oil cottonseed hulls (LOCH). Physical effectiveness factor (pef , related to the physical characteristics of aNDF) and effectiveness factor (ef , related to the ruminal pH) were determined based on a linear model approach that uses a bioassay method in which CS aNDF was assumed to be the standard fiber source. Laboratory methods to estimate pef of roughage sources were based on the proportion of DM of roughage retained on a 1.18-mm sieve pef(>1.18 mm) or retained on the 8.0-mm Penn State Particle Separator screen pef(>8.0 mm). The pef calculated by the bioassay method (total chewing time and ruminal mat resistance) for CS, SCB, and SC were higher values (P < 0.05) compared with SH and LOCH. The pef(rumen mat) of SC and SCB were higher (P < 0.05) than that of CS, SH, and LOCH. The pef(rumen mat) of LOCH was 61% higher than SH. The ef(rumen pH) of SC and LOCH was higher (P < 0.05) than CS and SH. The pef(chewing, min/d), pef(chewing, min/kg of DM), pef(rumen mat), and ef(rumen pH) positively correlated with rumination time, total chewing time, and ruminal mat resistance (values from transit time in seconds). No correlation was observed (P > 0.05) between pef(>8.0 mm) and rumination time, chewing time, and ruminal pH. The pef calculated using the bioassay method as well as pef (>8.0 mm) were negatively correlated with rumen pH (P > 0.05). The values of the effectiveness of fiber sources obtained in this research can be used as a guideline for nutritionists aiming to replace roughage sources from tropical regions in beef cattle finishing diets. Under our conditions, the pef using the bioassay method or laboratory methods were not adequate in predicting ruminal pH. [ABSTRACT FROM AUTHOR]

The objectives of this research were to evaluate the effects of source and concentration of α-amylase-treated neutral detergent fiber (aNDF) from roughage on feed intake, ingestive behavior, and ruminal kinetics in beef cattle receiving high-concentrate diets. Six ruminally cannulated Nellore steers (408 ± 12 kg of body weight) were randomly assigned to a 6 × 6 Latin square design with six diets: 10% aNDF from corn silage (10CS); 20% aNDF from corn silage (20CS); or four diets containing 10% aNDF from corn silage and 10% aNDF from one of the following sources: sugarcane (SC), sugarcane bagasse (SCB), soybean hulls (SH), or low oil cottonseed hulls (LOCH). The parameters of passage and degradation kinetics were estimated based on a two-compartmental model with gamma- and exponential-distributed residence times. The nonlinear models were fitted by nonlinear least squares, and a linear mixed-effects model was fitted to all variables measured from the Latin square design that were related to intake, digestibility, digestion kinetic parameters, and residence times. Mean particle size (MPS) between roughage sources (CS, SCB, and SC) and coproducts (SH and LOCH) was affected (P < 0.05). Dry matter intake (DMI) was not affected (P > 0.05) by 20CS, SC, SH, or LOCH. Steers fed 20CS or LOCH diets had 16% and 20% greater DMI, respectively, (P < 0.05) than steers fed 10CS diet. Steers fed SCB consumed the least dry matter (DM). The SH and LOCH diets had lower MPS values (about 8.77 mm) in comparison to 20CS, SCB, and SC diets (about 13.08 mm) and, consequently, affected (P < 0.05) rumen content, ruminal in situ disappearance, nutrient digestibility, and solid fractional passage rate. Chewing time was affected (P < 0.05) by roughage sources and concentration. Lower values of distance travel inside the rumen (min/cm) were observed (P < 0.05) for the SCB and SC diets in comparison with any other diet. Except for SCB, there was no difference (P > 0.05) in rumen fill, among other treatments. Mean daily ruminal pH was not affected (P > 0.05) by 20CS, SCB, SC, and LOCH diets, and it ranged from 6.1 to 6.23. Total short-chain fatty acids concentration was affected (P < 0.05) by roughage source and concentration. Based on our results, we recommend that under Brazilian finishing diets, replacing roughage sources, except for SCB, based on aNDF concentration of the roughage in high-concentrate diets containing finely ground flint corn does not affect DMI. [ABSTRACT FROM AUTHOR]

Condensed tannins (CT) might improve animal and system-level efficiency due to enhanced protein efficiency and reduced CH4. This study evaluated the impact of quebracho tannin (QT) extract fed at 0%, 1.5%, 3%, and 4.5% of dry matter (DM), within a roughage-based diet on apparent digestibility of DM, organic matter (OM), fibrous fractions, and N retention and energy partitioning of growing steers (236 ± 16 kg BW). A Latin rectangle design with eight animals and four periods was used to determine the whole-animal exchange of CO2, O2, and CH4 as well as the collection of total feces and urine over a 48-h period, using two open-circuit, indirect calorimetry respiration chambers. Following the removal of steers from respiration chambers, rumen inoculum was collected to determine ruminal parameter, including volatile fatty acids (VFA) and ammonia. Animals were fed a 56.5% roughage diet at 1.7% BW (dry matter basis). Dry matter and gross energy intakes were influenced by the level of QT inclusion (P ≤ 0.036). Digestibility of DM, OM, and N was reduced with QT inclusion (P < 0.001), and fiber digestibility was slightly impacted (P > 0.123). QTs altered the N excretion route, average fecal N-to-total N ratio excreted increased 14%, and fecal N-to-urinary N ratio increased 38% (P < 0.001) without altering the retained N. Increased fecal energy with QT provision resulted in reduced dietary digestible energy (DE) concentration (Mcal/kg DM; P = 0.024). There were no differences in urinary energy (P = 0.491), but CH4 energy decreased drastically (P = 0.007) as QT inclusion increased. Total ruminal VFA concentration did not differ across treatments, but VFA concentration increased linearly with QT inclusion (P = 0.049). Metabolizable energy (ME) was not affected by the QT rate, and the conversion efficiency of DE-to-ME did not differ. Heat energy decreased (P = 0.013) with increased QT provision likely due to changes in the DE intake, but there was no difference in retained energy. There were no differences for retained energy or N per CO2 equivalent emission produced (P = 0.774 and 0.962, respectively), but improved efficiency for energy retention occurred for 3% QT. We concluded that QT provided up to 4.5% of dry matter intake (about 3.51% of CT, dry matter basis) does not affect N and energy retention within the current setting. Feeding QT reduced energy losses in the form of CH4 and heat, but the route of energy loss appears to be influenced by the rate of QT inclusion. [ABSTRACT FROM AUTHOR]

The objectives of this study were to examine the relationship between residual feed intake (RFI) and DM and nutrient digestibility, in vitro methane production, and volatile fatty acid (VFA) concentrations in growing beef cattle. Residual feed intake was measured in growing Santa Gertrudis steers (Study 1; n = 57; initial BW = 291.1 ± 33.8 kg) and Brangus heifers (Study 2; n = 468; initial BW = 271.4 ± 26.1 kg) fed a high-roughage-based diet (ME = 2.1 Mcal/kg DM) for 70 d in a Calan-gate feeding barn. Animals were ranked by RFI based on performance and feed intake measured from day 0 to 70 (Study 1) or day 56 (Study 2) of the trial, and 20 animals with the lowest and highest RFI were identified for subsequent collections of fecal and feed refusal samples for DM and nutrient digestibility analysis. In Study 2, rumen fluid and feces were collected for in vitro methane-producing activity (MPA) and VFA analysis in trials 2, 3, and 4. Residual feed intake classification did not affect BW or BW gain (P > 0.05), but low-RFI steers and heifers both consumed 19% less (P < 0.01) DMI compared with high-RFI animals. Steers with low RFI tended (P < 0.1) to have higher DM digestibility (DMD) compared with high-RFI steers (70.3 vs. 66.5 ± 1.6% DM). Heifers with low RFI had 4% higher DMD (76.3 vs. 73.3 ± 1.0% DM) and 4 to 5% higher (P < 0.01) CP, NDF, and ADF digestibility compared with heifers with high RFI. Low-RFI heifers emitted 14% less (P < 0.01) methane (% GE intake; GEI) calculated according to Blaxter and Clapperton (1965) as modified by Wilkerson et al. (1995) , and tended (P = 0.09) to have a higher rumen acetate:propionate ratio than heifers with high RFI (GEI = 5.58 vs. 6.51 ± 0.08%; A:P ratio = 5.02 vs. 4.82 ± 0.14%). Stepwise regression analysis revealed that apparent nutrient digestibilities (DMD and NDF digestibility) for Study 1 and Study 2 accounted for an additional 8 and 6%, respectively, of the variation in intake unaccounted for by ADG and mid-test BW0.75. When DMD, NDF digestibility, and total ruminal VFA were added to the base model for Study 2, trials 2, 3, and 4, the R 2 increased from 0.33 to 0.47, explaining an additional 15% of the variation in DMI unrelated to growth and body size. On the basis of the results of these studies, differences in observed phenotypic RFI in growing beef animals may be a result of inter-animal variation in apparent nutrient digestibility and ruminal VFA concentrations. [ABSTRACT FROM AUTHOR]