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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
Department of Animal and Range Sciences, South Dakota State University, Brookings 57007
Abstract
Angus steers of known age (265 ± 17 d) and parentage were used in a 2-yr study (yr 1, n = 40; yr 2, n = 45) to evaluate the relationship between percentage of i.m. fat content of the longissimus dorsi at the 12th rib and carcass characteristics during growth of nonimplanted steers. Steers were sorted by age and EPD of paternal grandsire for marbling into high- and low-marbling groups so that steers with varying degrees of genetic potential for marbling were evenly distributed across slaughter groups. All steers were fed a 90% concentrate corn-based diet. Steers were allotted to five slaughter groups targeted to achieve hot carcass weights (HCW) of 204, 250, 295, 340, and 386 kg over the course of the feeding period. Data were analyzed as a completely random design with a factorial arrangement of treatments (year, marbling group, and slaughter group). Marbling group did not affect backfat, LM area, yield grade (YG), or marbling score. Regression equations were developed to quantify the change in carcass characteristics and composition over slaughter groups. Hot carcass weight increased in a linear fashion and differed (P < 0.01) among the slaughter groups as anticipated by design. Yield grade followed a quadratic upward pattern (P < 0.01) as HCW increased. Slaughter group affected the degree of marbling linearly (P < 0.01). There were no slaughter group x marbling group interactions, indicating that no differences occurred in the pattern of marbling attributable to paternal grandsire EPD. Carcasses expressed small degrees of marbling at 266 kg of HCW and obtained a YG of 3.0 at 291 kg of HCW. Fractional growth rates decelerated with increasing HCW. Greater advances in marbling relative to total carcass fatness occurred at HCW less than 300 kg. Management practices early in growth may influence final quality grade if compensatory i.m. fat content development does not occur.
Key Words: Beef • Marbling • Serial Slaughter
Introduction
Carcass composition is affected by sex, age, genetic background, plane of nutrition, and BW. Preston (1971)
stated that the variable with the greatest effect on composition is BW and that the growth and distribution of fat is important when quantifying growth. As an animal grows and approaches market weight, fattening occurs as a normal part of growth. Selecting for lean meat yield is presumed to be antagonistic to marbling.
The development of tissue depots was first characterized by Hammond (1932) and McMeekan (1940) to be in the sequence of skeleton, muscle, and fat. The authors reported that bone develops early, then muscle, and fat is the last tissue to develop. Berg and Butterfield (1968) conducted research in cattle that supported this principle. Andrews (1958) quantified the order of deposition of different fat depots to be in the following order: internal, intermuscular, s.c., and i.m. Duckett et al. (1993) reported that marbling increased in a quadratic fashion, increasing at a decreasing rate with increasing days on feed. Research has not clearly defined the accumulation of marbling relative to carcass composition and weight, nor has the deceleration concept (Brody, 1945) of maturing tissues been quantified for marbling in relation to growth.
This study was conducted to determine the rate and extent at which marbling develops in serially slaughtered steers. Additionally, changes in carcass composition and accretion rates of carcass fat and protein were quantified.
Materials and Methods
Animals
Purebred Angus steers of known parentage and age were used in a 2-yr study (yr 1, n = 40; yr 2, n = 46). Steers (approximately 8 mo old) were weaned at the ranch and transported 588 km to the South Dakota State University Nutrition Unit, where they were individually tagged and processed. Before initiating the study, steers were backgrounded (70 d) at a targeted gain of 1 kg/d. Each year, the experimental group was selected from 120 contemporaries of known parentage and age. A uniform set of steers with an initial BW of 268 ± 19.1 kg (yr 1) and 297 ± 21.7 kg (yr 2) were selected and sorted into high- and low-marbling groups by EPD of paternal grand-sire for marbling and assigned to one of five slaughter groups. The difference in marbling EPD was not great enough to merit a response, but ensured cattle of different marbling potentials were evenly distributed across treatment groups. In yr 1 of the study, four paternal grandsires were represented in the high-marbling group (+0.21), and two paternal grandsires were represented in the low-marbling group (–0.19). In yr 2, the high- and the low-marbling groups used five paternal grandsires each, with an average EPD for marbling of +0.28 for the high group and –0.02 for the low. Slaughter groups (n = 5) were targeted to produce hot carcass weights (HCW) of 204, 250, 295, 340, and 386 kg. Paved outdoor pens measuring 9.5 m in length with a 3.7-m fence-line feed bunk contained (DM basis) four or five steers of a marbling group x slaughter group designation. A typical finishing diet formulated to meet or exceed nutrient requirements (NRC, 1996) contained 10% grass hay, 77.2% whole shelled corn, 8.8% soybean meal, and 4.2% supplement with Rumensin (1.35 Mcal of NEg/kg; DM basis) and was delivered once daily in the afternoon. Steers had ad libitum access to feed and water. Steers were weighed approximately every 28 d to monitor BW for scheduling appropriate slaughter dates. There was no restriction of feed or water before weighing. When the mean BW of the slaughter group reached the desired target, feed was removed the afternoon before slaughter. On the designated day of slaughter, steers were individually weighed and transported (1.5 km) to the South Dakota State University Meat Laboratory on campus for slaughter using conventional procedures. Care, handling, and sampling of animals used in this study were approved by the South Dakota State University Animal Care and Use Committee.
Carcass Data
Carcasses were chilled with an air temperature of 2°C for 48 h. Carcasses were ribbed by the same individual, with data collected by the same two trained South Dakota State University personnel. Collected data included HCW, LM area, s.c. rib fat thickness (RF), and percentage of kidney, pelvic, and heart fat (KPH) depots (USDA, 1996). The KPH depot was removed by physical separation from each side of the chilled carcass and weighed to determine the percentage of carcass weight represented. Estimates of bone maturity and marbling score were made by the same two personnel throughout the trial.
Carcass Composition
Following carcass data collection, the 9-10-11 rib section was removed from the right side of each carcass as outlined by Hankins and Howe (1946). Soft tissue was separated from bone and weights were obtained on each. The soft tissue was mixed and homogenized by emulsification. Samples were stored in polyethylene bags at –20°C. Chemical analysis of the soft tissue was conducted to determine water, ether extract (fat), and nitrogen content of the 9-10-11 rib section samples. Two 50-g samples were lyophilized to a constant weight (48 h) to determine DM. The lyophilized samples were then combined and immersed in liquid nitrogen and subsequently powdered with a Waring commercial blender (Waring Products Division, New Hartford, CT). Replicate 2-g samples were wrapped in ashless N-free filter paper and extracted with petroleum ether in a side arm soxhlet (AOAC, 1990) to a constant weight (60 h) for ether extraction of lipid followed by drying at 60°C for 12 h. Crude fat was calculated as the difference between lyophilized and extracted sample weight. Crude protein was measured on extracted samples (1 to 1.5 g) by macro-Kjeldahl method (AOAC, 1990). For ash determination, 1 g of lyophilized sample was placed in a preweighed crucible and held at 650°C for 12 h. Hankins and Howe (1946) equations for steers were used to predict composition of the carcass soft tissue from chemical composition of soft tissue from the 9-10-11 rib section. Equations are as follows: carcass fat = 3.49 + 0.74 (9-10-11 rib fat content), carcass protein = 61.9 + 0.65 (9-10-11 rib protein content; Hankins and Howe, 1946). Whole carcass values were calculated by equations outlined by Hankins and Howe (1946) and used for determination of fractional growth rates.
Longissimus Samples
A 10-cm-thick portion of the longissimus muscle was removed from the posterior portion of the 12th rib from the right side of the carcasses. All exterior fat and epimysial connective tissue was removed as outlined by Blummer et al. (1962). A 1-cm-thick, evenly cut slice was removed and cut into 1-cm3 cubes and stored in plastic bags (Whirlpack, Nasco, Fort Atkinson, WI) at –20°C. Samples were homogenized in liquid nitrogen as outlined previously. Ether extraction of the longissimus samples was performed in triplicate to quantify i.m. fat (IMF) content of the longissimus dorsi muscle as outlined previously with the 9-10-11 rib sample.
Statistical Analyses
One steer was removed from trial in year two due to structural unsoundness. Statistical analyses of data were performed using Fisher’s least squares ANOVA generated using the GLM procedure of the SAS (SAS Inst., Inc., Cary, NC). Individual steer was the experimental unit and the model sums of squares were partitioned into year (1 or 2), marbling group (high marbling or low marbling), slaughter group (1, 2, 3, 4, or 5), and the year x slaughter group x marbling group interactions (Dammon and Harvey, 1987). Where there were no year x slaughter group x marbling group interactions, marbling groups were combined in the model. The means for slaughter group are presented here to demonstrate the magnitude, as well as direction of change, in the measured traits. When main effects were significant, (P < 0.05) means were separated using Fisher’s LSD test.
Data were partitioned into comparisons for linear and quadratic relationships based on contrasts for the model (Steel and Torrie, 1960). Regression equations were developed to quantify the change in carcass characteristics and composition throughout the growth phase. The equation to calculate fractional growth rates was:
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where P1 is the later measure of carcass tissue, P0 is the earlier measure of carcass tissue, and T is the number of days between the two measurements (McCarthy et al., 1983). Data needed to calculate fractional growth rate were derived from developed regression equations, which described carcass protein, carcass fat, and IMF over the five slaughter groups. Using the experimental population regression equations for days on feed, kilograms of carcass fat, and kilograms of carcass protein were developed for use in the fractional growth rate. Interval fractional growth rate data were then calculated to quantify the change in fractional growth rate with increasing carcass weight.
Results
Performance and Carcass Traits
The slaughter schedule and production traits are reported (Table 1) by year. Body weight and carcass traits within slaughter group were not different between high- or low-marbling groups. No interaction was found between year and slaughter group x marbling group; thus, marbling groups were combined for further analysis. As BW increased with increasing slaughter group, cumulative ADG decreased linearly (P < 0.01) with progression of slaughter group.
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). Weight and marbling scores were different between years (P < 0.05; Table 3). Increases (P < 0.10) in rib fat, KPH, and yield grade between slaughter groups followed the anticipated upward trend. Regression equations developed for each year were not different. Due to the nature of the trial, which was to develop regression equations to explain the development of carcass components with increasing carcass weight, years were combined. Hot carcass weight, LM area, marbling score, and IMF content of the 12th rib increased (P < 0.0001) linearly with increasing HCW. Marbling score and backfat were regressed against the explanatory variable HCW (Figure 2). From these regression equations, carcasses expressed a small degree of marbling at a HCW of 266 kg and reached a yield grade of 3.0 at a HCW of 291 kg. The greatest advances in marbling relative to total carcass fatness occurred at HCW less than 300 kg (Figure 2).
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). As anticipated, (Haecker, 1920; Simpfendorfer, 1974) the percentage of whole carcass protein and the percentage of whole carcass moisture were inversely related to percentage of whole carcass fat and decreased (P < 0.05) in a quadratic fashion with advancing slaughter group. Bone as a percentage of carcass weight was highest (P < 0.10) in the first slaughter group, remained constant in slaughter groups 2, 3, and 4, and then decreased (P < 0.10) at slaughter for group 5. The percentage of IMF content increased (P < 0.10) with increasing slaughter group in a linear fashion (P < 0.05) (Table 2
). Lines were developed to represent the change in weight of carcass protein, fat, moisture, and bone by regression analysis of weight of tissue against HCW (Table 5).
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) were obtained by regressing the dependent variables of kilograms of carcass protein, carcass fat, and days against the explanatory variable HCW. Values for the dependent variables at the targeted slaughter group weights were then entered into the fractional growth rate equation of McCarthy et al. (1983). Fractional growth rate for carcass protein, carcass fat, and IMF decreased with increasing HCW (Table 6).
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The research presented here was conducted to answer the basic question of the rate of development of IMF relative to the growth of various carcass tissues.
Due to the design of this trial, tabular data are presented for only ADG (Table 1). The DMI and feed gain efficiency were not evaluated. Live BW basis ADG decreased with increasing slaughter weights, which is similar to Hicks et al. (1987), who reported a linear decrease in ADG with increasing days on feed. The decrease reported agrees with the deceleration of growth first proposed by Brody (1945). The results in this study follow a similar deceleration pattern of ADG to NRC requirements for beef cattle (1996).
Dressing percent increased linearly with increasing slaughter group, which is similar to research reported by May et al. (1992), who serially slaughtered cattle from 0 to 196 d on feed. In the present study, s.c. fat thickness increased in a quadratic fashion, which is similar to results reported by Brethour (2000), who reported an exponential increase. Similar results to the present study were reported by others (Dolezal et al., 1982; Hicks et al., 1987; Miller et al., 1987; Schroeder, 1990). May et al. (1992) and Van Koevering et al. (1995) both reported linear increases in s.c. fat depth. However, in those studies, the duration of growth may have been too short to identify the inflection point of s.c. fat growth. The percentage of KPH also increased in a quadratic fashion, similar to results reported by Hicks et al. (1987) and Schroeder (1990). Van Koevering et al. (1995) and May et al. (1992) reported an increase in KPH development before reaching a plateau. The percentage of KPH reported in these trials (Hicks et al., 1987; May et al., 1992; and Van Koevering et al., 1995) was visually estimated, whereas in the present study, KPH was determined by physical separation. Longissimus muscle area increased linearly with increasing HCW, which is similar results found by May et al. (1992). No difference was found for LM area between slaughter groups 4 or 5, which may signify the onset of maturity and the end of true growth.
Skeletal maturity increased linearly, which is similar to results reported by May et al. (1992). Van Koevering et al. (1995) reported maturity score increased at a decreasing rate, resulting in linear and quadratic responses. In contrast, Dolezal et al. (1982) detected a general increase in maturity score up to 130 d on feed. From 130 to 230 d, increases were not always directionally consistent. Cattle in this study were of similar age and genetics, which may have resulted in a more predictable response.
Marbling scores increased in a linear fashion with increasing slaughter group. Others (Dolezal et al., 1982; Miller et al., 1987; Schroeder, 1990; Alderson, 1994) have reported increases in marbling when the feeding time was extended. May et al. (1992) and Van Koevering et al. (1995) regressed marbling score against days on feed and reported that marbling developed quadratically, increasing to 112 (May et al., 1992) and 119 d (Van Koevering et al., 1995) before reaching a plateau. Hicks et al. (1987) reported a linear response for marbling development; however, no change in marbling score was reported between the authors’ last two slaughter groups. Results of other studies (Moody et al., 1970; Butts et al., 1980; Greene et al., 1989) have also shown a plateau in the development of marbling as time on feed increased. In the present study, IMF content of the LM increased linearly, similar to the increase found for marbling scores when regressed as a component of growth over HCW. Duckett et al. (1993), using LM samples from cattle of May et al. (1992), reported IMF doubled between 84 and 112 d on feed but did not increase from 112 to 196 d.
This plateau response may be attributable to how data are expressed. The experiments of others described here compared marbling development to days on feed or age, not as a component of growth. When data here were reported against time, a quadratic response for marbling was found. As days on feed increases, growth slows; however, our research indicates that marbling as a component of growth (not time) increased through the duration of the trial.
Composition data reported here are on a carcass basis, with serial slaughter being conducted during the finishing period only. The percentage of whole carcass fat increased linearly, whereas whole carcass protein and whole carcass moisture decreased linearly with increasing slaughter group, which is similar to previous research results (Guenther et al., 1965; Waldman et al., 1971; Fox and Black, 1972; Jesse et al., 1976; Schroeder, 1990; Johnson et al., 1996). Jesse et al. (1976) reported results that appeared to be linear in nature, with an increase in carcass fat of 15.5% from cattle slaughtered between the live weights of 341 to 541 kg. The pattern of development of whole carcass protein and whole carcass moisture in the present study was similar to that reported by Jesse et al. (1976).
Fractional growth rate for protein and fat decreased with increasing HCW. Alderson (1994) reported decreases in fractional growth rate of protein and fat from 385 to 500 kg of live weight in heifers. Fractional growth rate of the IMF depot of the longissimus dorsi muscle at the 12th rib had a greater numerical decline than did fractional growth rate of whole carcass fat or whole carcass protein. The decrease indicates that IMF of the LM is not necessarily a late-developing tissue, but it is a tissue that has the opportunity to develop early in growth if nutritional management permits. The fractional growth rate data presented here are consistent with the deceleration concept proposed by Brody (1945). A limiting factor to the growth of a tissue is the maturity of the animal. As an animal grows, reaching mature body size, the rate at which tissues develop slows, but the proportionality of tissue growth changes.
Implications
Results of this study provide a better understanding of the development of marbling in the longissimus dorsi muscle at the 12th rib and its relation to whole carcass composition. These data indicate that marbling is not a late-developing tissue, but rather one that develops at a consistent rate throughout the normal growth of cattle under typical high-energy feeding programs.
Footnotes
1 Partial funding by the South Dakota Beef Industry Council is acknowledged. Published as South Dakota State Univ. Agric. Exp. Stn., Brookings, SD, Journal Paper No. 3383. 
2 Correspondence: ASC 217 Box 2170 (phone: 605-688-5452; fax: 605-688-6170; e-mail: kelly_bruns{at}sdstate.edu).
Received for publication September 2, 2003. Accepted for publication December 19, 2003.
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