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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
* University of California, Davis 95616;
and
Universidade Federal de Viçosa, Viçosa-MG, Brazil; and
and
Universidade Católica de Goiás, Goiânia-GO, Brazil
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: beef steer • maintenance • protein turnover • residual feed intake
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Residual feed intake (RFI), the difference between actual and expected intake, is an efficiency trait that is independent of BW and BW gain (Koch et al., 1963). Like growth, RFI is moderately heritable (Herd and Bishop, 2000; Arthur et al., 2001), but little is known about the biology underlying differences in growth potential or RFI.
Genetic selection for high growth rate produces cattle with faster muscle protein accretion and greater efficiency of gain (i.e., G:F), perhaps due to a lower maintenance cost or a greater efficiency of gain (Herd et al., 1991; Oddy et al., 1998). Because the rate of protein accretion depends on the relative rates of protein synthesis and degradation, rapidly growing animals must have a greater ratio of protein synthesis to protein degradation than slower growing animals.
The current study aimed to examine the relationships among growth potential, feed intake, G:F, carcass composition, energy metabolism, and myofibrillar protein turnover. The main hypotheses were as follows:
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Twenty-four crossbred beef steers, approximately 14 to 18 mo of age, with a mean weight of 403 ( ± 3) kg were housed and fed in individual pens for 112 to 135 d (mean = 122 d). The pens were approximately 10 x 2.5 m, and the steers had ad libitum access to feed and water. Twelve steers came from the University of California, Davis (UCD) herd, and the other 12 were purchased from a local producer. Six additional steers (3 from each group) were slaughtered at the beginning of the experiment to estimate initial body composition. Before the experiment, all steers underwent a growing program for about 120 d at the same (UCD) feedlot to minimize the effects of previous nutrition, so that the steers were grown and finished between July 2003 and March 2004. All animals were predominantly Angus x Hereford, with the purchased cattle having some Gelbvieh influence as well. The UCD herd has undergone genetic selection for postweaning growth for over 15 yr, including the purchase of semen only from Angus bulls, with an EPD for yearling weight of > 33.6 kg for the previous 5 yr. By contrast, the purchased cattle came from a herd with no selection or improvement program. These 2 groups of animals were designated the high growth (HG) and low growth (LG) groups, respectively.
All steers were fed the same diet on an ad libitum basis. The diet was composed (as-fed basis) of flaked corn (80%), alfalfa hay (5%), oat hay (5%), molasses (4%), fat (2.5%), sodium bicarbonate (1.25%), urea (1%), oyster shell (0.50%), trace mineralized salt (0.5%), ammonium chloride (0.25%), monensin sodium (0.002%; Rumensin 80, Elanco Animal Health, Greenfield, IN) and tetrachlorvinphos (0.006%; Rabon 97.3, KMG-Bernuth, Inc., Houston, TX). Estimated ME and CP contents on a DM basis were 3.06 Mcal/kg and 13.6%, respectively. Feed offered and refused were recorded daily, and the ration levels were adjusted every week on the basis of the average intake of the previous week to maintain approximately 10% refusals. The steers were weighed every 28 d, after being deprived of feed (but not water) overnight, and ADG was estimated as the slope of the regression of BW on time in days for each steer. At each weighing, the steers were scanned by real-time ultrasound using an Aloka 500V instrument (Corometrics Medical Systems, Wallingford, CT) with a 3.5-MHz, 17.2-cm linear array transducer. Measures of subcutaneous fat over the 12th to 13th ribs were used to determine the optimum slaughter end point, aiming for an average of 12 mm.
Two weeks before being slaughtered, 12 animals (6 from each group) were taken to metabolic crates, where they stayed for a period of 10 d (5 d of adaptation and 5 d of collection). Because of a limited number of metabolic crates, collections and slaughters were staggered over a 3-wk period. At the beginning of the collection period, 20 mL of H2SO4 was added to the urine collection container as a preservative. Every 6 h, the urine was weighed and a 50-mL sample was obtained. Specific gravity of a subsample was measured to calculate the total volume of urine excreted in a 24-h period. The remaining urine sample was frozen at –20° C until analysis.
Just before slaughter, the animals were weighed after an overnight feed deprivation. Carcass composition was estimated by carcass specific gravity according to Garrett and Hinman (1969). Briefly, the right side of the carcass was chilled for 48 h, and each quarter was weighed under water using a digital balance suspended over a cylindrical tank (diameter, 152 cm; depth, 168 cm). The carcasses were weighed as rapidly as possible, and the water temperature was recorded so that the carcass density could be corrected to 4° C. Cold carcass weight was used in the equation to calculate carcass density. Identical procedures were used for the 6 animals at the initial slaughter.
Energy Calculations
The procedures used to compute the energy retained and the maintenance energy requirement were similar to those of Lofgreen and Garrett (1968). Mean values of compositions of the initial slaughter groups were used to determine the initial empty body composition of each animal, using different values for each group. Gains of body components were calculated as the difference between initial and final weights of the respective components. Empty body compositions were estimated from carcass density, and heats of combustion of retained fat and protein were assumed to be 9.385 and 5.539 Mcal/kg, respectively (Garrett and Hinman, 1969). Whole body energy gains (retained energy; RE) were expressed as Mcal · kg of BW0.75 · d–1 to account for differences in BW. Metabolizable energy intakes (MEI) were estimated by multiplying the average DMI during the experimental period by the formulated ME content (NRC, 2000) of the diet, and were also expressed as Mcal · kg BW0.75 · d–1. The individual MEm was taken to be the x-intercept of the linear regression between RE and MEI, after calculation of the slope of this equation (i.e., the net energetic efficiency of gain, kg) from each steer’s gains of protein and fat (Williams and Jenkins, 2003).
Residual Feed Intake
For calculation of RFI, DMI was regressed against the average metabolic BW and ADG (Archer et al., 1997) as follows:
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where 
represents the RFI (actual DMI minus the expected DMI). Because there was no significant difference between equations for the HG and LG groups (data not shown), a single regression equation was fitted to all of the data.
The animals were classified into 2 RFI classes according to their individual RFI. Animals with individual RFI 
0.5 SD above or below the mean were classified as high or low RFI (n = 8/class), respectively. However, only 4 steers per RFI class were used for the metabolism studies.
Myofibrillar Protein Metabolism
The myofibrillar protein breakdown rate was estimated from the urinary excretion of 3-methylhistidine (3MH; Harris and Milne, 1981). The breakdown rate of all myofibrillar proteins was assumed to be similar, or at least similarly affected by the phenotype groups. The fractional degradation rate (FDR) of myofibrillar protein was calculated from total 3MH excretion and estimates of the 3MH pool size based on carcass composition, as follows:
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where [3MH]urine represents the concentration of 3MH in the urine, and 3MHmuscle is the total muscle 3MH pool.
The size of the 3MH pool in myofibrillar protein was calculated by multiplying the estimated skeletal muscle protein mass by the 3MH content of mixed muscle protein (3.5106 µmol of 3MH/g of muscle protein; Nishizawa et al., 1979). Initial and final skeletal muscle protein contents (ISMP and FSMP, respectively) were estimated by using the following equations derived using these cattle (Sainz, unpublished observations):
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where pISM is the proportion of skeletal muscle in the body of the initial groups (0.3675 and 0.3891 for HG and LG steers, respectively), and pFSM is the proportion of skeletal muscle in the carcass of the final slaughter groups, given by
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In both equations, the skeletal muscle was assumed to be 18% protein. This assumption was based on previous experience as well as published muscle protein contents that ranged from 15 to 23% (Atwater and Woods, 1896; Gopinath and Kitts, 1984; Brackebusch et al., 1991). The rate of skeletal muscle protein gain was calculated as the difference between the final and initial skeletal muscle protein contents divided by the number of days the animals were on trial. The fractional accretion rate (FAR) of protein was calculated as the rate of skeletal muscle protein gain divided by the average total skeletal muscle myofibrillar protein pool. The fractional synthesis rate (FSR) of the myofibrillar protein pool was calculated as the sum of FDR and FAR.
Analysis of Urine
Urinary 3MH analysis was performed with a Bio-Chrom 30 amino acid analyzer (Biochrom Ltd., Cambridge, United Kingdom). Urine samples were deproteinized by mixing 400 µL of urine with 400 µL of 10% sulfosalicylic acid solution. The mixture was centrifuged for 25 min at 24,000 x g, the pH was adjusted to 2 with 0.4 N lithium hydroxide, and 50 µL of the deproteinized supernatant was loaded on the cation-exchange column. A standard elution with 1.65 M lithium citrate buffer (pH = 3.55) was conducted using an 8-µm cation-exchange resin (Blom and Huijmans, 1992). The AA were detected by absorbance at 540 nm after postcolumn derivatization with ninhydrin reagent using a reaction coil set at 135° C. For calibration, norleucine (Sigma, St. Louis, MO) was used as an external standard. Data analysis was performed using the EZChrome software (Scientific Software, San Ramon, CA). The creatinine concentration in urine was measured using the Metra Creatinine assay kit (Quidel Corp., San Diego, CA).
Statistical Analyses
All calculations, except carcass composition and skeletal muscle protein degradation, were made from the first to the last day of the experimental period. The growth, composition, and protein degradation data were analyzed using ANOVA, with phenotype (i.e., growth potential group or RFI class) as the main effect. Simple Pearson correlation analyses were conducted to study the associations among and between ADG, DMI, protein and fat gains, G:F, MEm, RFI, and FDR. All statistical analyses were conducted using Minitab Release 13 (Minitab Inc., State College, PA). The results were interpreted as statistically significant with a 5% probability level, but actual P-values are also given.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. Animals in the HG group had lower initial BW (P = 0.003) but greater final BW (P = 0.037) than animals in the LG group. Steers in the HG group showed greater (P = 0.001) DMI (7.52 vs. 6.37 kg/d), ADG (1.33 vs. 0.853 kg/d), and G:F (0.176 vs. 0.133 kg/kg) compared with LG animals. Steers in the HG group tended to gain more whole body protein (100 vs. 72 g/d; P = 0.068) and gained more fat (676 vs. 475 g/d; P = 0.037) than LG steers. There was no difference between the groups in RFI. As seen for DMI, HG steers had greater MEI (0.233 vs. 0.201 Mcal · kg BW0.75 · d–1; P < 0.001) and RE (0.0711 vs. 0.0558 Mcal · kg BW0.75 · d–1; P = 0.026) compared with LG animals. Estimated kg and MEm (Mcal · kg BW0.75 · d–1) did not differ between these 2 groups, averaging 0.62 and 0.114, respectively.
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). This was a function of increased BW, because there was no difference in dressing percent. Steers in the HG group had greater backfat (16.1 vs. 11.6 mm; P = 0.008) and higher yield grades (3.53 vs. 2.80; P = 0.004) compared with the LG group. There were no differences between groups in KPH fat, LM area, or marbling score.
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). Values for final skeletal muscle protein, 3MH, and creatinine excretion in urine, FDR, and FSR were similar between the HG and LG steers. The ratio of 3MH to creatinine excretion can be regarded as an alternative indicator of the rate of fractional muscle protein degradation. These values all agree, showing that these groups did not differ in their muscle masses or in the rates of myofibrillar protein degradation.
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, there was no difference between HG and LG steers in RFI. This was expected, because RFI is meant to be independent of the size and rate of gain (Kennedy et al., 1993). To examine hypothesis 3, steers were classified according to their RFI. Compared with the high RFI steers, low RFI steers consumed less DM (6.61 vs. 7.52 kg/d; P = 0.029) and ME (0.206 vs. 0.234 Mcal · kg BW0.75 · d–1; P = 0.009), with no differences between RFI classes in initial or final BW, ADG, or G:F (Table 4). Rates of whole body protein gain were similar between RFI classes, but low RFI steers tended (P = 0.078) to gain less fat (494 vs. 719 g/d) than high RFI steers. The low RFI steers had a mean RFI of 0.747 kg/d less than the animals classified as high RFI (–0.367 vs. 0.380 kg/d, P < 0.001). However, estimated kg and MEm did not differ (P > 0.10) between the RFI classes. These observations are in agreement with those of Nkrumah et al. (2006), who reported positive correlations between RFI and the production of methane and heat.
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). Likewise, all observations related to myofibrillar protein metabolism were similar between the low and high RFI groups (P > 0.10, Table 6).
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shows the simple Pearson correlations among ADG, DMI, protein and fat gains, G:F, MEm, RFI, and FDR. For all animals in the experiment, DMI, ADG, and G:F were all positively and significantly correlated with each other. Fat gain was correlated (P < 0.05) positively with DMI, ADG, G:F and FDR, and negatively with MEm, and tended (P < 0.10) to be correlated positively with RFI. Dry matter intake was also correlated (P < 0.05) with RFI. Metabolizable energy for maintenance was positively correlated with FDR (P < 0.05) and tended to be positively correlated with RFI (P < 0.10).
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). The equation for this line indicates that MEm would increase by 0.0166 Mcal · kg BW0.75 · d–1 for each percentage unit increase in the fractional rate of myofibrillar protein turnover.
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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discussed the relationship between growth potential and intake, and concluded that the greater intake of faster growing animals was caused by their faster growth. The data from this experiment cannot confirm or refute that concept, but they do support the implicit association among these variables. Because of greater fat accretion, steers in the HG group had heavier and fatter carcasses than their LG counterparts, as shown by the greater back fat and yield grade. Results from the growth experiment showed a clear tendency for greater protein deposition in HG steers, which could occur only through increased protein synthesis, decreased protein degradation, or both. Animals selected for growth tended to have greater absolute and fractional accretion rates of muscle protein than the LG steers; however FDR and FSR did not differ between these 2 groups. In this experiment, animals chosen for the protein metabolism measurements were halter-broken and trained to the collection equipment months in advance of the RFI measurement. Therefore, only 4 steers from the high and low RFI classes were used for protein metabolism measurements, and the range of RFI between the high and low RFI groups was quite narrow (0.728 kg/d). Because of the small number of replicates in the protein metabolism study, these results must be viewed with caution.
The results of the current study can only partly confirm hypothesis 3: As expected, there were no differences between RFI classes in initial and final BW or ADG, and DM and ME intakes were associated with RFI. On the other hand, G:F, estimated kg, and MEm were similar between RFI classes, as were all of the muscle protein metabolism measurements. Therefore, hypothesis 4 could not be confirmed. Although not statistically significant, the numerical differences between low and high RFI steers in MEm (0.108 vs. 0.119 Mcal · kg BW0.75 · d–1), estimated kg (0.60 vs. 0.64), and fat:protein in the gain (5.26 vs. 8.67) may have biological relevance, because in combination they were able to explain the differences in RFI between groups.
Although the fat:protein ratio in the gain was less in low RFI steers than in high RFI steers, none of the measurements of carcass traits differed between RFI classes. This is in agreement with Arthur et al. (2001) and Basarab et al. (2003), who reported a low or nonexistent phenotypic correlation between RFI and ultrasound backfat thickness. However, the relationship between RFI and carcass characteristics, mainly fat traits, has been the most contentious since the introduction of the RFI concept (Basarab et al., 2003). Although some researchers have reported some phenotypic or genetic correlations or both between RFI and carcass and body fat (Szasz et al., 2004), usually those correlations were low and the SE relatively high (Basarab et al., 2003), suggesting that the data should be interpreted cautiously.
Three factors should be considered when evaluating these conclusions. First, at the age at which these studies were conducted (14 to 18 mo), both HG and LG steers were not in the steep part of their growth curve, where the phenotypic differences would have been more apparent. The metabolic rate tends to slow down as the animal ages; adult animals usually show lower levels of skeletal muscle protein synthesis, degradation, and accretion. Thus, it should be borne in mind that the results observed at the age of the animals in the current study may not be representative of those of other ages or stages of maturity. Second, different from the animal model used by Oddy et al. (1998), in which the selection for HG and LG rate involved only pure Angus cattle from a common herd, animals in this study came from different herds and the selection for HG and LG was done within 2 types of crossbred animals (Angus x Hereford vs. Angus x Hereford x Gelbvieh). Although the comparison between these 2 crossbred cattle might have confounded the difference that would have been found between the HG and LG lines if both were from the same pure breed, the groups studied reflect a very realistic situation, because crossbred cattle are more commonly raised than purebreds. Finally, it is important to consider the problems involved with estimation of the skeletal muscle protein pool size. Overestimation of this pool would lead to underestimation of FDR in the animal. Some researchers have used an approximation (Munro, 1969), considering that most mammals have between 40 and 50% muscle in their bodies, irrespective of the species or sex of the animals. This may be true in nonruminants; however, in cattle most reports would indicate that approximately 31 to 35% of the BW is muscle (Callow, 1961; Henrickson et al., 1965; Nishizawa et al., 1979) or that approximately 52 to 57% of empty body protein is associated with skeletal muscle. The use of an incorrect estimate of muscle in the animal body could lead to as much as a 25% difference in estimated FDR. In this study, skeletal muscle was estimated to comprise between 29 and 39% of the whole body.
In balancing energy inputs and outputs, heat production represents a substantial component of the energy budget of ruminant livestock. The fraction of total maintenance energy lost as heat generally ranges from 100% at maintenance (by definition) to about 70% in growing heifers (Reynolds et al., 1991). The metabolic sources of this expenditure at both the cellular and organ level have been reviewed (McBride and Kelly, 1990; Baldwin and Sainz, 1995) and include the maintenance of cell structure and functional integrity and processes such as ion transport and protein turnover. Although variations in maintenance and G:F are sometimes more highly associated with weight and metabolic activity of the visceral organs, such as the gut and liver, than with body proteins, body fat, or composition of gain (Ferrell and Jenkins, 1985), our results showed a Pearson correlation coefficient between FDR and MEm of 0.76 and a positive linear association between MEm and FDR. Reduced rates of protein degradation allow an increase in lean body mass without a proportionate increase in maintenance energy needs. Tomas et al. (1991, 1998) and Oddy et al. (1998) reported that efficiency of feed utilization in selected lines of rats, broilers, and Angus cattle, respectively, was inversely related to rates of skeletal muscle protein breakdown. This study extends those observations to show a clear positive relationship between the rate of muscle protein breakdown and the maintenance energy requirement.
Frequently, observed differences in efficiency of growth have been attributed to differences in body composition. More specifically, differences in rates of water and protein accretion relative to the rate of fat accretion are thought to have a major influence on the rate and efficiency of BW gain, primarily because of the lower energy content of water and protein than of fat. Conversely, greater maintenance costs have been associated more closely with body protein than with body fat (Pullar and Webster, 1977; Ferrell et al., 1979). Our results show a positive association between MEm and FDR of myofibrillar protein. Therefore, it is reasonable to conclude at this point that the animal with a greater skeletal muscle protein turnover rate, as reflected by its degradation rate, will have a greater energy requirement for maintenance and consequently less gross G:F.
Selection for G:F has been shown to increase the final body size (Basarab et al., 2003) because of the high correlation between these variables. This could have a negative impact on the beef production system, especially when considering the high cost of animal feed. On the other hand, improved feed efficiency can be achieved by selecting animals using RFI without a correlated response in mature size. The animals classified as low RFI had BW gains similar to those classified as high RFI but ate 12% less feed, making them more efficient in terms of feed utilization. In other words, animals with lower RFI used less energy in the physiological processes involved in maintenance, resulting in more net energy available for tissue accretion.
This study demonstrated that RFI may be negatively correlated with the maintenance energy requirement. The maintenance energy requirement was positively related to the rate of myofibrillar protein turnover, confirming the importance of this physiological process in the energy economy of the animal.
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Footnotes |
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2 Corresponding author: rdsainz{at}ucdavis.edu
Received for publication June 8, 2006. Accepted for publication December 11, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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-methylhistidine by cattle: Validation as an index of muscle protein breakdown. Br. J. Nutr. 45:411–429.[CrossRef][Medline]This article has been cited by other articles:
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  W. G. Bergen Measuring in vivo intracellular protein degradation rates in animal systems J Anim Sci, April 1, 2008; 86(14_suppl): E3 - E12. [Abstract] [Full Text] [PDF]
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
) high residual feed intake (RFI) steers; (
), low RFI steers; (
), steers with an RFI within 0.5 SD of the mean (zero).