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ANIMAL NUTRITION |
* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, T6G 2P5 Canada; and
and
Alberta Agriculture, Food and Rural Development, Lacombe Research Centre, Lacombe, AB, T4L 1W1 Canada
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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Key Words: beef cattle • energy partitioning • feed efficiency • methane production
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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; Arthur et al., 2001). Thus, improvements in feed efficiency would lead to cost reduction and better overall production system efficiency. Residual feed intake is the difference between an animal’s actual feed intake and its expected intake based on BW and growth rate over a specified period.
Considerable research progress has been made in defining the variation in RFI using different biological types of cattle; however, the biological reasons underlying the observed variations are generally unknown, although several physiological mechanisms have been proposed. Johnson et al. (2003) and Richardson and Herd (2004) suggested that sources that may contribute to the variation in RFI are feed intake, digestion of feed, heat increment, protein turnover and overall tissue metabolism, feeding behavior and activity, body composition and rate of gain, BW, prolificacy, and several other presently unknown factors.
In a recent study, Richardson et al. (2004) reported significant metabolic differences in Angus steers divergently selected for RFI. Generally, there is considerable variation among cattle in energy use and partitioning. This variation is mainly related to differences in dietary energy losses (fecal, methane, and urinary), heat production (HP), and energy retention (Delfino and Mathison, 1991; Saama and Mao, 1995; Basarab et al., 2003). The current study examined the relationship of feedlot feed efficiency, performance, and feeding behavior with metabolic rate, digestion, and energy partitioning in beef cattle ranked by RFI.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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. At the end of each feedlot test, steers were ranked by their RFI and selected to be halter-trained for use in the metabolic and digestion trials at the University of Alberta Metabolic Research Center, Edmonton. Standard deviations above and below the mean RFI were used to group the selected steers into high RFI (RFI >0.5 SD above the mean; n = 11), medium RFI (RFI ± 0.5 SD above and below the mean; n = 8), and low RFI (RFI <–0.5 SD below the mean; n = 8). Respective BW ± SD for the high-RFI, medium-RFI, and low-RFI groups during the measurements were 495.6 ± 12.7, 529.1 ± 18.6, and 501.2 ± 15.5 kg.
The feedlot test diets for the 2 yr are shown in Table 1. In each year, the same feedlot test diet was used in subsequent metabolic and digestion trials. Corn was used in yr 1 instead of barley and oats because of a feed barley shortage that particular year; however, the diets used in both years were formulated to contain similar levels of ME. At the Metabolic Research Center in Edmonton, animals were housed individually in adjacent holding pens in a climate-controlled thermoneutral environment and adapted to individual metabolism crates and confinement-type respiration calorimetry stanchions. Each experimental period consisted of a 14-d adaptation period, during which steers were acclimated (or re-acclimated if previously used in a trial), gentled, and gradually brought to the specific feeding level. All steers in the study were cared for according to the guidelines of the Canadian Council on Animal Care (CCAC, 1993).
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maintenance requirement (0.077 Mcal NEm/BW0.75)]. Animals were weighed twice during the acclimation period, and the average BW was used to determine the 2.5 times the NEm feeding level. The metabolic crates permitted steers to lie down or stand during the trial. The digestibility trial consisted of a 5-d period during which a total collection of feces and urine was made. Aliquots of feed, orts, feces (10%), and pre-acidified urine (5%) were collected daily (after thorough mixing) and stored at –20°C for later processing and analyses. Feed, orts, and fecal samples were dried in a forced-air oven at 60°C for 72 h and ground in a Wiley mill (Arthur H. Thomas, Philadelphia, PA) to pass a 1-mm screen.
Dry matter contents of the feed, orts, and feces were determined by oven drying at 100°C to a constant weight. A standard macro-Kjeldahl procedure (AOAC, 1980) was used to determine N in feed, orts, feces, and urine samples. Gross energy contents of feed, feces, orts, and urine were determined in an automatically controlled Parr adiabatic oxygen bomb calorimeter (Parr Instrument Co., Inc., Moline, IL). Neutral detergent fiber was determined according to the procedure of Van Soest et al. (1991). Acid detergent fiber was determined according to AOAC (1997). These analyses were determined in the ANKOM 200 fiber analyzer (Ankom Technology Corp., Fairport, NY). The NDF and ADF analyses were carried out in triplicate; the intra-sample CV for fiber determination was <5% in 95% of samples.
Indirect Calorimetry Procedure
Oxygen consumption and methane production were measured in a 4-chamber, open-circuit, indirect calorimetry system (Delfino et al., 1988). The calorimetry system is designed for individual animals to stand or lie down in stanchions with their heads in hoods. The hoods were located in a climate-controlled thermoneutral environment, and animals were allocated randomly to the calorimetry hoods after acclimation. The calorimetry system was designed such that air could be drawn from the hoods at a mean oxygen concentration of 20%. Respired air was passed through Drierite (W. A. Hammond Drierite Co., Ltd, Xenia, OH) to remove water vapor before passing through a paramagnetic oxygen analyzer (Servomex Inc., Sussex, UK) or methane analyzer (Model 880A Infrared Analyzer, Rosemount Analytical, Orville, OH). A Foxboro 823 IFO integral flow orifice with cell transmitter (Invensys Systems, Inc., Foxboro, MA) was used to measure airflow rate. Pressure was measured with a Foxboro 821AL absolute pressure transmitter (Invensys Systems, Inc.). Temperature and relative humidity also were measured by cellular temperature and relative humidity transmitters (General Eastern, Fairfield, CT).
The system also allowed measurement of the concentration of ambient oxygen. The calorimetry system was calibrated by the N injection method (by releasing a weighed amount of N2 gas into the system) as described by Young et al. (1984). Two 16-h measurements at 3-d intervals were obtained from each steer at the estimated (NRC, 1996) 2.5 times maintenance requirement feeding level after the digestion trial. To estimate heat increment of feeding (HIF), oxygen consumption also was measured at 1.2 times maintenance feeding level. To remove residual feed caused by the higher feeding level from the gut, steers were kept on the 1.2 times maintenance feeding level for 5 d before measurements were made.
Calculations and Statistical Analyses
Procedures for obtaining the measures of feedlot performance and feed efficiency have been described previously (Nkrumah et al., 2004). Each animal’s ADG during the feedlot test was computed as the coefficient of the linear regression of BW (kg) on time (d) using the regression procedure of SAS (SAS Inst., Inc., Cary, NC; Version 9.1). The metabolic BW of each animal over the feedlot test period was computed as the midpoint BW0.75 of a 70-d test. The total feed intake of each animal over a 70-d test period was used to compute the daily DMI. Residual feed intake was computed for each animal as the difference between actual DMI and predicted expected daily DMI based on the ADG and metabolic BW over the test period using procedures described by Arthur et al. (2001). The PEFFG (i.e., energetic efficiency for ADG) above maintenance of each animal was computed as the ratio of ADG to the difference between average daily DMI and expected DMI for maintenance (Arthur et al., 2001) in which DMI for maintenance was computed according to NRC (1996). Feed conversion ratio was computed as the ratio of DMI to ADG on test.
Feedlot behavior traits studied were daily feed bunk attendance and daily feeding duration (FD). Procedures for determining feeding behavior from the GrowSafe System have previously been described (Basarab et al., 2003). Daily feed bunk attendance in this study was defined as the number of independent visits or attendances in a day by a particular animal to a feed bunk, irrespective of the duration of the visit. Daily FD was computed as the sum of the difference between feeding event end times and start times per day for each animal. It was equal to the total number of minutes each day spent in feeding-related activities (prehension, chewing, backing away from the bunk and chewing, socializing, scratching, or licking) at the feed bunk.
All energy intake and partitioning values during the postfeedlot trial were expressed per unit of metabolic BW (i.e., BW0.75). With the exception of HIF, which by convention must be estimated using two different feeding levels, all energy partitioning and digestibility values reported in the study are measurements taken at the 2.5 times maintenance feeding level. Digestible energy was calculated by multiplying the daily intake energy by the energy digestibility of the diet from each animal. The energy lost as methane was calculated as the total methane produced in liters per day at standard temperature and pressure (STP) x 9.45 kcal/L (Brouwer, 1965). Metabolizable energy (kcal/kg of BW0.75) was calculated by subtracting energy losses (kcal/kg of BW0.75) in urine and methane from DE (kcal/ kg of BW0.75). Heat production (kcal/kg of BW0.75) was computed as (–4.90 kcal/L of O2) x (volume of expired air at STP) x (oxygen in exhaust air – oxygen in inlet air at STP). This approach has been shown to give accurate estimates of HP (±1.2%; McLean and Tobin, 1990). Heat increment of feeding was calculated as the change in HP per unit change in ME intake for the same animal (McDonald et al., 2002). Retained energy (RE; kcal/kg of BW0.75) was calculated as the difference between ME and HP (NRC, 1996) at 2.5 times maintenance requirement.
Data were analyzed using the Mixed procedure of SAS; the model included fixed effects of RFI group (high, medium, and low), year (1 and 2), test group within year (2 groups per year), and random effects of metabolic crate or calorimetry chamber and sire of animal. All interaction terms that were not significant for a trait (P > 0.10) were dropped from the final model. There was no RFI group x year interaction on any of the traits considered. With the exception of the model for feedlot DMI and post-feedlot GE intake (GEI), the model for all other traits included feedlot DMI as a linear covariate within treatments. Mean separation among RFI groups for different test traits was carried out by least squares using the PDIFF option in SAS. The PROC CORR of SAS was used to obtain Pearson partial phenotypic correlations adjusted for the linear effects of DMI and the fixed effect of year.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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). These differences provided adequate variation among the animals for determining the relationships of the different measures of feed efficiency, performance, and behavior with the measures of metabolic rate, digestion, and energy partitioning considered in the study. Daily fecal DM, methane, orts, urine, urinary N excretion, and apparent digestibility of dietary components are presented in Table 3. Table 4 shows the associations between RFI and measures of daily energy partitioning. The phenotypic correlations between the feedlot measures of performance, efficiency, and feeding behavior with daily energy partitioning are presented in Table 5.
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Daily HP and energy retention were highly significantly associated with feedlot RFI (P < 0.001). Heat production was 21 and 10% less by low-RFI steers than by steers with high or medium RFI, respectively. Consistent with this result was the lower RE (44 and 23%) in high- and medium-RFI steers, respectively, compared with low-RFI steers. Simple regression analyses showed that differences in feedlot DMI, post-feedlot HP, and post-feedlot ME (kcal/d), respectively, accounted for 20, 48, and 16% of the variation in feedlot RFI among the animals. The regression equation was feedlot RFI = –6.52 + 0.320 x DMI (kg/d; feedlot) + 0.031 x HP (kcal/ kg of BW0.75; post-feedlot) – 0.005 x ME (kcal/kg of BW0.75; post-feedlot). The differences in metabolizability were mainly attributable to the observed differences in DE (kcal/d) and methane production.
No significant differences were observed among the RFI groups in HIF, measured at 2 different feeding levels above maintenance (P > 0.20), although low-RFI steers had 32.6% lower HIF. There was a tendency for a negative association between RFI and digestibility of dietary CP (r = –0.34; P < 0.10) and DM (r = –0.33; P < 0.10). Daily fecal DM production was 15.5 and 8.1% greater in high- and medium-RFI steers, respectively, compared with low-RFI steers, although these differences were not significant (P > 0.10). The results for NDF and ADF indicated that NDF digestibility was generally less in high-RFI compared with low-RFI steers, although differences were not significant. Correlations of NDF and ADF digestibility with RFI did not differ from zero (P > 0.10). Urinary N excretion was 17% greater in the urine of high-RFI steers compared with low-RFI steers, although these differences, as well as that of daily urine production, were not significant (P > 0.10).
The relationship of PEFFG with the various test traits consistently followed those with RFI. There were significant correlations (P < 0.01) between PEFFG with DE, ME, HP, RE, and methane production. Similarly, the PEFFG of the steers tended to be related to fecal output and digestibility of dietary components (P < 0.10). With the exception of daily HP (P < 0.05), feedlot FCR of the steers was generally unrelated to any of the metabolic rate and energy partitioning traits considered in the study. Feedlot DMI showed positive associations with methane production (P < 0.05) and energy lost through urine (P < 0.01), but it was negatively correlated with daily DE (P < 0.05), ME (P < 0.01), and RE (P < 0.01). There also was a tendency for feedlot DMI to affect fecal DM production (positive association), HP, and DM digestibility (negative association; P < 0.10).
Daily feed bunk attendance of steers was positively related to HP and negatively related to NDF and ADF digestibility (P < 0.05), but it was unrelated to other traits considered in the study (P > 0.10). Daily FD showed significant correlations with fecal DM and methane production (P < 0.01; positive associations) and with daily DE, ME, RE, and apparent digestibility of CP and DM (P < 0.01; negative associations). Feedlot ADG was generally unrelated to the traits considered in the study, except for a weak trend toward an association with daily DE and ME (P = 0.12). With the exception of daily urinary energy (P < 0.05), HP, NDF, and ADF digestibility (P < 0.10), feedlot metabolic BW was generally not related to the traits considered in the study. The use of either corn or barley and oats in yr 1 or 2 did not result in any interactions between RFI and year in the analyses (P > 0.10); however, energy lost as methane (percentage of GEI) was less (P < 0.01) for the diet in yr 1 (corn-based diet; 3.25 ± 0.23%) than for yr 2 (barley-based diet; 4.59 ± 0.30%). Additionally, dietary and fecal NDF and ADF levels were lower (P < 0.05) for the corn-based diet compared with the barley-based diet.
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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; Basarab et al., 2003).
The current study identified significant differences in methane emission among animals differing in RFI, which represents the first experimental evidence demonstrating associations between RFI and methane production. Previous evidence (Herd et al., 2002; Okine et al., 2003) relating RFI to methane production was based only on estimates derived from the relationship between RFI and DMI and resulted in approximately 5% difference between low-RFI and high-RFI animals in methane production. Data from the current study indicated that methane production was 28 and 24% less in low-RFI animals than in high- or medium-RFI animals, respectively. Although feedlot DMI was also correlated with daily methane production from steers, the differences observed among RFI groups were still evident following covariate adjustment for feedlot DMI. These differences correspond to approximately 16,100 L/yr less methane in low-RFI animals compared with high-RFI animals.
The mechanisms behind the observed differences among animals in methane emission, independent of intake, are unknown, but they may be related to differences in metabolizability as well as possible individual animal differences in methane production. Methane production has been shown to be heritable (h2 = 0.42) in humans (Flatz et al., 1985). According to Hackstein et al. (1996), there is a genetic link between methanogens and their hosts such that the presence of methanogenic bacteria in an animal requires a quality of the host that is under phylogenetic rather than dietary constraint. Whether this link has any effects on the type of methanogens that are dominant in the rumen of individual animals is unknown.
Any inherent differences in animals that may lead to ecological changes in the ruminal microbial ecosystem may translate into differences in methane production. Increased methane production by high-RFI animals not only represents significant decreases in energetic efficiency, but it also has implications for the environmental sustainability of beef cattle production because of the significant contributions to atmospheric methane emissions. Agriculture in Canada contributes approximately 10% of the total Canadian green house gas emissions (Environment Canada, 2004), of which 2.6% is methane. The current study also identified a tendency toward differences in daily fecal DM production per unit of DMI, an observation consistent with previous estimates by Okine et al. (2003).
The current study also indicated a trend toward associations between RFI and apparent digestibilities of dietary DM and CP. These differences in apparent digestibilities between RFI groups reported in the current study were consistent with the results of Richardson et al. (1996), who reported greater DM digestibility by low-RFI steers than by high-RFI steers and concluded that small differences in digestibility can result in large differences in feed efficiency. The differences in apparent digestibility observed in the current study between high-RFI and low-RFI animals were, however, weak (approximately 5%). A recent study by Channon et al. (2004) demonstrated significant genetic and phenotypic associations between RFI and traits that are indicative of the extent of starch digestion in the gastrointestinal tract of cattle. Russell and Gahr (2000) indicated that individual animal variation in factors such as the mechanism of digestion and absorption, ruminal retention time, and feeding behavior might contribute to variation between individual animals in diet digestibility.
Variation in ruminal retention time among animals has been reported in cattle (Ørskov et al., 1988), and it might be associated with differences in DMI or feeding behavior. Significant differences in feedlot DMI among animals differing in RFI have been demonstrated in several studies (Arthur et al., 2001; Basarab et al., 2003; Nkrumah et al., 2004). In addition, considerable differences in FD were observed among the animals in the different RFI groups in the current study. The differences in FD in the current study were associated positively with differences in fecal and methane production and associated negatively with DM and CP digestibility. These associations also translated into differences in daily DE and ME among the steers differing in RFI.
Significant differences in DE, ME, and RE among the animals of the different RFI groups also were demonstrated in the current study. Part of the variation in efficiency of energy retention has been attributed, among other factors, to a decrease in metabolizability of the diet and to an increase in the HIF at high levels of intake above maintenance (Ferrell and Jenkins, 1998). The results of the current study are generally in agreement with these suggestions; the differences in feedlot DMI also were associated with significant decreases in DE, ME, and RE. Nonetheless, to evaluate whether differences in daily DE, ME, or RE were due to inherent differences in the different RFI steers, independent of the level of intake-associated effects proposed by Ferrell and Jenkins (1998), we included feedlot DMI as a linear covariate in the statistical models of analyses. This did not eliminate the relationships of the given traits with RFI, which demonstrated that part of the variation in the different RFI steers in DE, ME, and RE might be independent of the level of intake.
The results of the current study indicate that the greater DMI by animals with high RFI might be partly related to the low metabolizability of consumed feed and the accompanying increased need to attain the levels of energy intake required for maintaining BW and supporting body protein and fat accretion. According to the present results, the lowered metabolizability of feed in high-RFI steers in itself might be attributable, at least in part, to the decreased digestibility and increased fecal DM production and methane production but is less related to energy lost through urine. The current study, however, failed to demonstrate significant differences among RFI groups in HIF above maintenance, partly because of high within-RFI group variation in HIF.
The mean of ME:DE observed in the current study (0.91) is indeed greater than the 0.82 suggested by the beef cattle NRC (1996). Values similar to those reported in the present study have been reported for other studies (Rikhardsson et al., 1991). For feedlot steers on high-grain diets that also may contain vegetable fats or ionophores such as monensin, the proportion of intake energy lost through urine and methane is considerably less than for high-roughage diets (Van der Honing and Steg, 1990). The practice of adding vegetable oils and ionophores to high-grain feedlot diets to decrease extreme cases of bloat has been shown to decrease ruminal methanogenesis considerably (Mathison, 1997; McGinn et al., 2004), and this might have contributed to the high ME:DE observed in the current study.
The considerably greater HP in high-RFI steers compared with low-RFI steers observed in the current study, despite the lack of differences in GEI, might be attributable to variation in metabolic efficiency. Variation in energy expenditure related to differences in the size of visceral organs, for instance, has been proposed as contributing significantly to the differences in HP between animals with different RFI (Basarab et al., 2003). Residual feed intake is positively correlated with DMI, and it has been demonstrated that increased DMI in cattle is generally accompanied by significant increases in the size of visceral organs (Ferrell and Jenkins, 1998). Indeed, a study by Basarab et al. (2003), which indicated a greater HP from high-RFI animals compared with low-RFI animals, also indicated significantly greater visceral organ weights in high-RFI steers than in low-RFI steers.
According to Reynolds (2002), differences in visceral organ size contribute significantly to the variation in total oxygen consumption, and thereby HP, accounting for 40 to 50% of daily HP. In addition, Webster (1980) indicated that there is a strong linear relationship between protein synthesis and HP and that marked differences in metabolic rate could be explained almost entirely by differences in protein synthesis. Not surprisingly, the greatest proportion of the protein synthesis and associated HP takes place in visceral tissues such as the gastrointestinal tract and the liver, which are not normally associated directly with growth and meat production (Webster, 1980; Reynolds, 2002).
An evaluation of the differences in expression of genes involved in protein turnover and associated HP in certain metabolically active tissues, such as the liver and gastrointestinal tract, may help to explain part of the molecular mechanisms leading to variations in energy expenditure in cattle with similar BW and ADG. This may be even more important in ruminants because of the comparatively large size of the viscera in relation to the whole body.
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IMPLICATIONS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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Footnotes |
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2 Corresponding author: stephen.moore{at}ualberta.ca
Received for publication February 9, 2005. Accepted for publication August 18, 2005.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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