The Growth and Distribution of Carcass Fat in Fattening Steer of Different Breeds

The growth and distribution patterns of carcass fat were investigated in three breeds of beef cattle entering fattening phase. The study involved 23 grass-fed steer Brahman, 24 Hereford and 22 Brahmanx Hereford crosses with a live weight range from 300-600 kg. An allometric Huxley model was used to study the growth and distribution patterns of fat tissue within wholesale cut. In most cases, Brahmans had significantly higher growth coefficients than Herefords and/or BrahmanxHereford crosses while Herefords and BrahmanxHereford crosses had similar growth coefficients in wholesale cuts. At log natural of 75 kg side muscle+bone weight (4.313 kg), Herefords had significantly a higher fat weight within wholesale cuts than Brahmans and BrahmanxHereford crosses. Comparison at log natural of 114 kg side muscle+bone weight (4.733 kg), whilst Brahmans and Herefords were similar in fat weight distribution, the cross-bred steer had lower fat weights than the other two breed groups in almost all wholesale cuts.


�NTRODUCT�ON
During the fattening phase, the growth impetus of fat was predominantly high relative to that of muscle and bone (Priyanto et al., 2009). This made the former tissue having an important role in determining compositional changes of a carcass and consequently the yield of beef (Mukai et al., 2004;Vieira et al., 2006). The changes in carcass composition after the onset of fattening phase are largely attributable to the increasing proportion of fat in a carcass. Advancing in carcass fatness would be followed by increasing carcass fat but decreasing carcass muscle percentages due to the much higher growth impetus of carcass fat relative muscle and bone (Priyanto Priyanto et al., 2009). Consequently, increasing carcass fatness . Consequently, increasing carcass fatness during the fattening phase might have a deleterious effect on the yield percentage of saleable beef. It was reported that cattle with larger frame size had better yield percentages of saleable beef, if compared to that with smaller frame size (Bidner Bidner et al., 2009). The variation in beef yield were associated primarily with maturity type differences because of the differential growth patterns of their carcass tissues (Priyanto Priyanto et al., 2009)

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Fat-free carcass weight was as an effective adjustment factor in breed comparison for muscularity and muscle to bone ratio (Purchas Purchas et al., 2002). Previous finding showed significant but small between breed differences in the muscle growth and distribution relative to fat-free carcass weight (Priyanto & Johnson, 2011). In this study, the influence of beef cattle breed on carcass fat and its distribution throughout the wholesale cuts relative to fatfree carcass weight was examined in fattening steer.

MATER�ALS AND METHODS
The cattle used in the study included 69 grass-fed steers, consisted of 23 Brahmans, 24 Herefords, and 22 BrahmanxHereford Crosses which had enter, or were progressing along, their fat deposition phase. The steers were sequentially slaughtered at approximately 300, 400, 500, and 600 kg live-weight. All steers were fasted but access to water for 24 h prior to slaughter. Following dressing, the carcasses were divided into two sides, weighed and then chilled at 3 o C for 24 h. The right sides were broken down into 15 wholesale cuts, namely Thin Thin flank, Loin, Tenderloin, Rump, Thick Flank, Topside, Silverside, Shank, Point End (PE) Brisket, Navel End (NE) Brisket, Shin, Blade, Rib Set, Chuck, Neck+Sticking (AUS- (AUS-MEAT, 2003). The cuts were then dissected into muscle, fat, bone, and connective tissue. The weights of the carcasses, the hot and chilled sides, wholesale cuts were recorded. All dissection products, including muscle, fat, bone and connective tissue from each cut were weighed and recorded. The dissection of the carcass components were based on recovered weights of the right side.

Statistical Analysis
An allometric model, Y= αX β (Huxley, 1932), was used to study the growth patterns of the muscle tissue. In order to obtain a linear relationship, the equation was transformed into log natural form. Breed effects on the relationship between the weight of muscle in the wholesale cut and the weight of side muscle+bone were examined using the following model: = residual error of the measurement of Y ij , assumed to be normally distributed around a mean of zero with a variance of σ 2 This analysis allowed comparisons of breed regression coefficients as suggested by Kaps & Lamberson (2004), and estimates of dependent variables (Y's) at a particular independent variable (X). In order to obtain more accurate results, the dependent variables were estimated using breed regressions at a particular X value and compared between breeds.
The carcass weight range covered both traditional (light-weight) and specific (heavy-weight) markets, thus it was of particular interest to compare the Y values at two different values of the independent variable (X), one where the carcasses suitable for the traditional market and one where they were suitable for the specific (hotel, restaurant and institution) market. The traditional market prefer carcasses averaging 200 kg which correspond to 75 kg of side muscle+bone weight, while the specific market prefers carcasses averaging 300 kg or 114 kg side muscle+bone weight.

RESULTS AND D�SCUSS�ON
Unlike the growth of muscle within wholesale cuts (Priyanto & Johnson, 2011), the growth of fat showed more widespread breed variations, 11 of the 15 wholesale cuts differing significantly among breed groups (Table 1). In most cases, Brahmans had significantly higher growth coefficients than Herefords and/or BrahmanxHereford crosses while Herefords and BrahmanxHereford crosses had similar growth coefficients in wholesale cuts. The differential fat growth patterns which were clearly demonstrated among breeds led to some important implications for fat weight distribution when breeds were compared at two growth stages. Comparison of fat weight within wholesale cuts at log natural of 75 kg side muscle+bone weight (4.313 kg) showed that Herefords had consistently and significantly a higher fat content than Brahmans and BrahmanxHereford crosses (Table 2). This occurred in all wholesale cuts except topside, silverside and PE brisket. Brahmans and BrahmanxHereford crosses tended to have a similar fat weight distribution. The differences became more obvious when breeds were compared at the heavier fat-free carcass weight. At log natural of 114 kg side muscle+bone weight (4.733 kg), Brahmans and Herefords were similar in the weight estimates of fat in most wolesale cuts due mainly to the relatively higher rate of fat deposition in the Brahmans (Table 3). BrahmanxHereford crosses had a lower total fat weight than the other two breed groups in almost all wholesale cuts, attributable to their generally low fat growth coefficient. Belk et al. (1991) reported minor influence of breed on the growth and distribution of fat tissue when fat in the wholesale cuts was related to the total carcass fat. This suggests diminished effect of breed on fat growth and distribution when comparisons were made at equal fatness bases. Whilst such studies could provide useful information about patterns of fat growth and distribution, the genetic variation in fattening patterns could not be clearly identified by this approach. Priyanto & Johnson (2011) reported breed variation in muscle weight distribution whitin wholesale cuts when adjusted to the same carcass weight. The present study confirmed the previous findings that there were significant between breed differences in the growth and distribution of carcass fat whitin wholesale cuts relative to fat-free carcass weight.
There were important breed variations in both fat growth and weight distribution, which followed a sys-tematic pattern. In lightweight carcasses, Hereford steers deposited more fat in the wholesale cuts than Brahmans and BrahmanxHereford steers. Because fat deposition was relatively faster in Brahmans once they entered the Note: † All breed regression coefficients highly significant (P<0.01).
Means in the same row with different superscript differ significantly (P<0.05); the weights of fat within wholesale cut and side muscle+bone were measured in kg.  Table 1. Allometric relationship between fat weight of the wholesale cut (Y) and side muscle+bone weight (X), Ln Y= Ln a + b Ln X Note: The weights of fat within wholesale cut and side muscle+bone were measured in kg; Means in the same row with different superscript differ significantly (P<0.05). fattening phase, the fat weight distribution between Herefords and Brahmans was similar in heavyweight carcasses. BrahmanxHereford crosses showed the slowest development for fat and consequently had less fat weight particularly in the proximal fore-limb and lumbar regions when compared with the other two breeds. It was suggested that maturity type of cattle breed differences in the fattening pattern might be explained in terms of the onset of fattening and/or the rate of fattening (Bidner et al., 2002;Gotoh et al., 2009), and these were reflected in fat-free carcass weight differences among breed at which the fattening phase commenced.

CONCLUS�ON
In fattening steer, there were significant between breed differences in fat growth and distribution whitin wholesale cuts. At the lower side muscle+bone weight, Hereford generally had more fat within wholesale cuts relative to Brahman and BrahmanxHereford cross. At the heavier side muscle+bone weight, however, Brahman and Hereford had similar fat weight distribution in the carcass, indicating a faster growth of fat from Brahman steer.