Proc. Assoc. Advmt. Anim. Breed. Genet. Voll2 EFFECTS OF STRAIN AND SELECTION FOR GROWTH ON TENDERNESS IN SHEEP
AND BEEF MEATS
P.A. Speck, J.N. Clarke and C.A. Morris
AgResearch, Ruakura Agricultural Research Centre, Private Bag 3 123, Hamilton, New Zealand SUMMARY
Tenderness is the most important consumer attribute of meat. Data are presented which demonstrate that selection for growth in both sheep and cattle can greatly enhance the rate at which meat tenderizes. The implication for this relates to the impact of the contribution of genetics to the final processing of meat as a consumer product.
Keywords: Selection, growth, meat tenderness.
INTRODUCTION
Tenderness is an important attribute of meat which has a significant impact on consumer acceptability of the product. Considerable variation in meat tenderness exists even under modem production and handling systems (Morgan et al., 1991). Final tenderness of meat is determined by the rate and extent ofpost mortem proteolysis of key myofibrillar proteins. Also results of a number of experiments have indicated that the calpain proteolytic system plays a major role in the tenderization of meat post mortem (Koohmaraie, 1988, 1992a,b).
Few studies have examined the impact of genetic improvement for growth on meat quality and in particular meat tenderness. Mrode (1988) has summarised direct and correlated responses to genetic selection for liveweight at a fixed age in beef cattle experiments. Burrow et al. (1991) demonstrated that shear values for meat tenderness did not differ in lines of cattle selected for
increased weight at a given age. Pharmacological improvement of growth performance using p- adrenergic agonists undoubtedly reduces proteolysis, tenderization and meat tenderness, and is also associated with an increase in calpastatin activity (Koohmaraie et al., 1994 and Speck et al., 1993) given the possible role of the calpain system in normal growth and metabolism (Go11 et al., 1989) it is likely that differences exist between animals selected for improved growth performance, and these may impact on meat tenderness post mortem. The paper highlights recent data on the impact of selection for growth on meat tenderness in both sheep and cattle lines.
Slaughter and meat sampling procedures.
Cattle study. Male progeny from lines selected for high yearling weight (ASl) and Control line (ACO) were fust weighted off pasture at Tokanui one day before slaughter. They were then transported to a commercial abattoir and tailgate slaughtered. Animals were handled together at all times, and pre-slaughter handling (which could have affected ultimate pH) was the same for all groups. Immediately after slaughter the carcass was split into left and right sides, and the hot carcass weight was obtained from the two side weights.
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Proc. Assoc. Advmt. Anim. Breed Genet. Vol12 The M. longissimus dorsi was obtained from the left side of each carcass. The muscles were removed during the hot boning procedure within 30 minutes of slaughter and taken back to a temperature controlled room (10°C) for pH and temperature monitoring. Twenty four hours after slaughter, muscles were portioned and vacuum packed before being placed at 1°C for ageing.
Sheep study. Lambs, which represent the progeny from 12 sire breeds and strains, at 11 months of age were weighed off pasture transported to experimental abattoir and slaughtered after an overnight fast. Animals were handled together at all times. Four hours after slaughter, the M.
Longissimus dorsi was removed and placed in a 15°C waterbath for up to 6 days of ageing, sub samples were removed essentially at 24 hour intervals for tenderometer measurements. Twenty four hours alter slaughter ultimate pH was recorded.
Tenderometer studies - sheep and cattle. The M. longissimus and M.psoas portions (approximately 6 cm in length) were placed inside a plastic cooking bag and heated in a waterbath at WC), until they reached an internal temperature of 75°C. They were then removed, cooled rapidly in ice, to an internal temperature of 2°C (Graathuis et al., 1991). The sample was trimmed of nay external dried tissue, and cores of 10 mm cross-section and 50 mm length were cut with the long axis running parallel to the fibres. A minimum of 10 samples per animal was sheared through, using a MIIUNZ tenderometer (Frazerhurst and MacFarlane, 1983). Cooking loss was calculated as the difference between the weight of the uncooked and cooked samples.
pH and temperature determination. pH was recorded using a spear tip electrode and an Orion meter with temperature compensation. The temperature of each muscle was recorded using Orion temperature loggers.
Statistical methods.
Cattle studies. Results were analysed by least squares (Genstat, 1994) and in cases where measurements were repeated over a time series, a restricted maximum likelihood (REML) programme was used to account for repeated records (Genstat, 1994). Effects were fitted for herd, sampling day, year of birth (or year of kill) and their two-way interactions. Preliminary statistical models were fitted to test for the significance of two possible fixed factors in addition: age of dam and date of birth covariate. Where any of these latter two effects or the two-way interactions were not significant they were removed and the model was re-run.
Sheep studies. Results were analysed by least squares (IMP ~3.1, SAS Inst. Inc. 1995). Sire breed means were obtained from a model which included, sire, dam breed, slaughter group, nutrition group and sex by slaughter group were evaluated for age trends in tenderness on days 1,2,4 and 7 days post-slaughter (natural log scale).
RESULTS
Cattle stu&. Live weight and carcass weight differed significantly between the AS1 and AC0 lines by 20% (data not shown). Tenderness as measured by shear force improved in the striploin form 13.2 kg on day 1 to 6.0 kg on day 28 (Figure 1). Most of the improvement in tenderness was
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Proc. Assoc. Advmt. Anim. Breed Genet. Voll2 observed within 7 days of slaughter. Significant differences in tenderness between herds were found on days 1, 3 and 7 days post mortem for the striploin (with the selected herd being on average 22% more tender than the control herd on those days. Ultimate pH did not differ between the two herds and averaged 5.32, twenty four hours post slaughter.
Figure 1. Effect of genotype of ageing rate in M. longissimus dorsi (* denotes P < 0.5).
Table 1. Initial shear force (cz), Ageing coefficients @), and time (A, days) to reach a shear force (F) of 6 (A6), from the relation F = CL As) [from F = de-s* in square brackets].
Breed Strain
Romney HBW
CL P A,
5.5 [5.5] 0.77 [-0.0751 0.7 r-1.21
CBW 8.7 [8.8] 0.65 [-0.0131 2.3 [3.0]
WWT 9.3 [9.6] 0.65 [-0.1331 2.7 [3.5]
Texel Tex 9.6 [9.6] 0.64 [-0.1281 2.9 [3.7]
Poll Dorset Dor 10.4 [ll.O] 0.60 [-0.1641 2.9 [3.7]
Southdown Sd+ 9.6 [9.6] 0.65 [-0.1231 3.0 [3.8]
Dorset Down DDn 10.4 [ 10.91 0.60 [-0.1571 3.0 [3.8]
Composite (Rom) Wir 9.6 [lO.O] 0.68 [-0.1251 3.3 [4.1]
Southdown Sd- 11.8 [11.6] 0.57 [-0.1601 3.3 [4.1]
Oxford Oxf 12.1 [12.5] 0.59 [-0.1601 3.8 [4.6]
Suffolk USf 12.7 [12.6] 0.58 [-0.1561 3.9 [4.7]
Romney cc0 12.1 [12.6 0.63 [-0.1421 4.7 [5.2]
13.8 [14.1] 0.62 [-0.1441 5.7 [5.9]
Sheep study. Tenderness as measured by shear force declined exponentially with ageing (day 1 average 11.7 to 5.2 kgF on day 8 conditioning). Variations among different breeds and selected and control lines was greatest on day 1 post slaughter and was substantially reduced by day 7 of conditioning. The C and H growth selected Romney strains had lower shear forces at times post- slaughter than their corresponding unselected controls (PcO.05). Crossbred lambs derived form a.
range of sire breeds mated to Romney ewes showed less variation (only 50% of the breed and strain 569
Proc. Assoc. Advmt. Anim. Breed. Genet. Voll2 are expressed in fust cross lambs). Generally rates of ageing over the first 3 days were higher for those strains showing high shear force on Day 1, most notably US suffolks and lean selected Southdowns and unselected Romney lines. Ignoring the extremely low value for the HBW Romney selection line, there was approximately a 2 fold range in the time to attain a shear force of 6 KgF.
Most crossbred strains were similar apart from the Oxford and Suffolk breeds. Between strain correlations indicated that the days to attain a shear force of 6 KgF was more closely related to shear force on day 1 (r-0.94) than it was with the ageing coefficient (r=O.61).
Conclusions. Both the sheep and cattle studies have demonstrated a significant effect for enhanced growth performance on the tenderness of the M. longissimus dorsi, which in itself has important implications with respect to processing from lines of animals even of the same breed. Fortunately, the negative genetic association between growth and toughness if not antagonistic, that is, the propensity exists for the simultaneous commercial improvement of both traits. Koohmaraie (1992) demonstrated that temperature and pH post mortem have profound effects on meat tenderness. In the present studies pH of muscles was monitored and found not to be affected by genotype in either the sheep or cattle studies (data not shown). Numerous studies have been reported in the literature highlighting the role of the calpain system in the conversion of muscle to meat (see Koohmaraie, 1993 for a review). However in our laboratory we fail to obtain consistency between animal relationships of meat tenderness with the calpain system (data not shown), this does not however detract from the fact that the components calpain system are important in the tenderising process of meat, although its usefulness as a selection indicator for meat tenderness may be limited. The data presented in this paper highlights the contribution of genetics to the final processing of the product, certainly an aspect which is generally ignored by meat processing researchers and the processors themselves. The ability to be able to predict the ageing rate of meat from known genetic sources has major implications on the cost and ultimately the competitiveness of red meat.
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