FACTORS INFLUENCING INTERMUSCULAR FAT DEPOSITION IN THE BEEF CHUCK

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1 FACTORS INFLUENCING INTERMUSCULAR FAT DEPOSITION IN THE BEEF CHUCK K. L. Christensen, D. D. Johnson, D. D. Hargrove, R.L. West and T. T. Marshall SUMMARY Fifty-nine steers produced from the crossing of Braford, Simbrah, Senepol and Simmental bulls to Brahman-sired and Romana Red-sired cows, and from the mating of Brahman bulls to Angus cows were used in this study to determine effects of breed group and age-season of feeding on fat distribution in depots of the chuck. Half of the steers were placed in the feedlot immediately afterweaning, and the others were stockered during the winter and fed as yearlings during the summer. Steers were slaughtered at a compositional equivalent endpoint (.4 in external fat) as determined by visual appraisal and with realtime ultrasound monitoring. Percentage seam fat in the chuck did not differ ( P <.05) among breeds or age-season of feeding; however, percentage subcutaneous fat was greater (P<.05) in the Braford-sired steers than in steers of the other sire-breed groups. INTRODUCTION Recent studies indicate consumers have become increasingly conscious of dietary concerns related to the consumption of fat and have conveyed a desire for leanness through their purchasing decisions of fresh retail beef. These concerns have prompted approximately 85-90% of meat retailers to respond to the consumers' desires by implementing 1/4 in or less trim programs for beef retail cuts (Cross et al., 1986). More recently, Savell et al. (1988) reported on the National Beef Market Basket Survey that the average overall external fat thickness on cuts offered at retail was.11 in. The reduction in external fat trim, however, has posed an additional problem for the retailer. As the subcutaneous fat trim levels are reduced, the amount of fat between the muscles (seam or intermuscular fat) appears amplified and becomes even less appealing to the consumer. Seam fat comprises nearly 63% of all the separable fat in retail cuts (Savell et al., 1988) and is difficult to remove without destroying the typical shape and integrity of retail cuts, especially those from the chuck. Variability in carcass composition, especially fat deposition, can be attributed to variability in: 1) genetics 2) physiological maturity and 3) nutrition (Berg and Butterfield, 1968). Early research indicates that breeds can differ in composition; however, few data exist to explain how biological animal types differ in their tendency to deposit intermuscular fat in the chuck in relation to subcutaneous fat. Therefore, if differences in seam fat deposition can be attributed to various animal types or influenced by changing feeding regimen, it would be desirable to select for those breeds or management practices that can produce steers with less fat in this depot. The objective of this study was to identify the factors which contribute to intermuscular fat deposition in the chuck, particularly the influences of breed of sire and management practices (feeding weanling calves vs yearlings).

2 MATERIALS AND METHODS Fifty-nine steers calved in 1988 at the University of Florida Beef Research Unit (BRU), Gainesville, were selected for this study. The steers were produced from crossing Braford, Simbrah, Senenol and Simmental bulls to Brahman-sired and Romana Red sired cows, and Brahman bulls to Angus cows. Approximately one-half of the steers in each breed group were randomly assigned feed as weanling calves during the cool period of the year (November to May), while the others were assigned to a winter stocker program followed by feeding during the warm period of the year (June to October). The steers were fed a 75% dry-matter diet consisting of high moisture corn (79%), cottonseed hulls (12%), a commercial 50% protein pellet (4%), and molasses (5%). All steers were implanted with Ralgro just prior to being placed in the feedlot. Steers still on feed 90 days later were implanted with Synovex S. External fat (subcutaneous fat) of the feedlot steers was monitored visually and by ultrasound. Upon reaching.4 in subcutaneous fat measured at the 12th rib, each steer was weighed in the morning before feeding and shipped to the University of Florida Meats Laboratory. Steers were allowed overnight access to water and slaughtered the following morning. Standard slaughter procedures were followed. Carcasses were chilled 24 h at 32 to 34 F prior to being ribbed. The carcasses were ribbed between the 12th and 13th ribs and subsequently evaluated for USDA quality and yield grade factors by University of Florida personnel. The crosscut chuck was removed between the 5th and 6th ribs perpendicular to the dorsal line of the carcass. Further dissection included removal and separation of subcutaneous fat, intermuscular fat, major muscles, and bone. Individual muscle weights were recorded of those weighing greater than 5 lb. The Triceps brachii, Serratus ventralis, and Supraspirzatus muscles were ground separately, mixed, and reground through a 3/8 in plate. A random grab sample was taken and further ground for uniformity using a food processor. Chemical analyses were performed by oven drying and ether extraction procedures to determine intramuscular fat deposition in the three muscles. The results were converted to a physical separation basis and averaged to determine an estimate of average intramuscular fat percentage for the chuck. The same procedure was used on the lean trim (trim + muscles weighing less than.5 lb) to determine the chemical fat content of the lean. The chemical fat was subsequently used in determining the amount of fat-free lean within each chuck. Data were analyzed using the least squares, fixed model procedures of the Statistical Analysis System (SAS, 1982). Main effects of sire breed group and season of feeding, along with their interaction were included in the mathematical model. Adjusted fat over the ribeye was used in the model as a covariate. RESULTS AND DISCUSSION Table 1 reveals that Simmental-sired steers had a heavier (P <.05) mean hot carcass weight than all other breed groups except Simbrah-sired and Senepol-sired steers. Hot carcass weights from Simbrah-sired and Senepol-sired steers were intermediate among the breeds. Additionally, the yearling steers had greater hot carcass weights than did steers fed as calves (P<.07). Carcasses from Simmental-sired steers also had larger mean ribeye areas than did steers of the other breed groups, while Simbrah-sired, Senepolsired, and Braford-sired steers had ribeyes that were intermediate in size and Brahmansired, the lowest. When ribeye area was expressed per 100 lb of hot carcass weight, almost the exact opposite ranking of breed groups was observed. The Brahrnan-sired steers were superior (P<.05) to Braford-sired steers, while Simbrah-sired, Senepol-sired,

3 and Simmental-sired steers were intermediate. Differences in the results of the two ribeye measurements were due to variations in hot carcass weight across the different breed groups. Steers with average ribeye areas and lighter weight carcasses generally rank above steers with heavier carcasses and larger ribeye areas, when ribeye area is expressed per 100 lb of carcass. This study was designed to slaughter all steers at an equivalent composition (measured as fat over the ribeye) and the actual fat over the eye did not differ among breed of sire or age-season of feeding groups. By equalizing the variation in subcutaneous fat deposition, any differences in seam fat deposition should be more meaningful. Percentage of kidney, pelvic, and heart fat (KPH) was greater in carcasses of Brahman-sired, Senepol-sired, and Simbrah-sired steers than in Braford-sired steers (P<.05). The Simmental-sired steers were intermediate in percentage KPH. The Braford group had the lowest percentage of KPH, and the Simmental group was intermediate to the others. Steers fed as calves during the cool season had a higher percentage of KPH (P <.05) than did those fed as yearlings. This was expected since internal fat is an early developing depot and comprises a higher percentage of carcass composition at lower side weights. Simmental sired steers were superior (P<.05) to Braford-sired steers in curability (lower USDA yield grade), while the Senepol-sired, Simbrah-sired, and Brahman-sired steers were intermediate. There was no difference in USDA yield grade between steers fed as calves during the cool season and those fed as yearlings during the warm season of the year. The Simmental-sired group had a higher average marbling score (P<.05) than the Simbrah-sired group. The Senepol-sired, Brahman-sired, and Braford-sired steers were intermediate and not different from the other groups. The mean marbling scores for all breed groups fell within the USDA Select grade. Comparison of the characteristics of the chuck by breed of sire and age-season of feeding, indicate fewer differences were apparent (Table 2). The Simbrah-sired, Senepolsired, Simmental-sired, and Brahman-sired steers had chucks with significantly less (P<.05) subcutaneous fat on a percentage basis than did chucks from Braford-sired steers, suggesting a more even covering of fat over the chuck in the Braford-sired steers. Percentage seam fat was not different with respect to breed groups or age-season of feeding. Brahman-sired, and Senepol-sired steers exhibited a higher percentage of intramuscular fat in their chucks than did the Braford-sired or Simbrah-sired steers (P<.05), and Simmental-sired steers were intermediate. Percentages of bone and major muscles were not different among breed groups; however, the calves exhibited a higher percentage of major muscles and a lower percentage of trim than did the yearlings. The trim (% fat-free lean) was significantly greater in the chucks of Senepol-sired steers (P<.05) than in the chucks of Brahman-sired steers, and intermediate in the Braford-sired, Simbrah-sired, and Simmental-sired steers. Total percentage of fat-free lean (major muscles + fat-free lean) did not differ among breed groups or age-season of feeding. Muscle to bone ratio of the chuck also did not very among the breed of sires or ageseason of feeding. The results of this study indicate that seam fat deposition is not affected by breed of sire or to changes in management practices used in this study. Therefore, the selection of different biological animal types, by using these breeds and feeding regimens, would not result in the reduction of seam fat within the beef chuck.

4 LITERATURE CITED Berg, R. T. and R. M. Butterfield Growth patterns of bovine muscle, fat and bone. J. Anim. Sci. 27:611. Cross, H. R., J. W. Savell and J. J. Francis National consumer retail beef study. Proc. Recip. Meat Conf. 39:112. SAS SAS User s Guide Statistics. SAS Inst. Inc. Gary, N.C. Savell J. W., H. R. Cross, D. S. Hale, and L. Beasley National Beef Market Basket Survey. Meat Res. Brief, Texas A&M Univ., College Station. TABLE 1. LEAST SQUARES MEANS FOR CARCASS CHARACTERISTICS BY BREED OF SIRE AND AGE-SEASON OF FEEDING Hot Ribeye Ribeye Actual fat USDA Lean Marbling a No. of Carcass area area/100lb over the KPH yield Bone Maturity Trait steers wt. Ibs. in 2 carcass wt. eye. in. % grade maturity score Breed of Sire Brahman c 11.5 cd 1.8 b b 2.64 bc A 45 A 40 S154 bc Braford c 10.8 d 1.6 c c 2.89 b A 46 A 44 S157 bc Simbrah bc 12.5 bc l.7 bc b 2.66 bc A 44 A 45 S146 c Senepol bc 12.2 c 1.7 bc b 2.65 bc A 48 A 46 S163 b Simmental b 13.3 b 1.7 bc bc 2.55 c A 53 A 44 S180 b Age-Season Calves-cool l b 2.71 A 46 A 41c S151 Yearlings c 2.65 A 49 A 47b Sl69 Warm a SL = slight b,c,d Means in same column (within main effect) with different superscripts differ (P <.05).

5 TABLE 2. LEAST SQUARES MEANS FOR CHUCK CHARACTERISTICS BY BREED OF SIRE AND AGE-SEASON OF FEEDING Sub- Intramus- Major Trim Total Muscle/bone No. of cutaneous Seam cular fat, Bone muscle (fat-free) (fat-free) ratio of fat Trait steers fat,% fat,% fat,% a % % b lean (%) lean (%) free lean Breed of Sire Brahman d c d Braford 11 9.l c e cd Simbrah d de cd Senepol d c c Simmental d cd cd Season-age Calves-cool c 15.2 d Yearlings d 17.2 c warm a Calculated from the average Intramuscular fat of the Triceps brachii, Serratus ventralis and Supraspinatus. b Muscles weighing greater than.5 Ib. c,d,e Means In same column( within main effect) with different superscripts differ (P <.05)