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An estimate of the incidence of dark cutting beef in the United Kingdom
Meat Science 27 (1990) 249-258
An Estimate of the Incidence of Dark Cutting Beef in the United Kingdom
S. N. Brown, E. A. Bevis & P. D. Warriss AFRC Institute of Food Research Bristol Laboratory, Langford, BS187DY, UK (Received 22 June 1989; revised version received 1 August 1989; accepted 8 August 1989)
ABSTRACT Eight slaughterplants with throughputs ranging from 20 to 300 animals per day were examined to estimate the incidence of dark cutting beef in the United Kingdom. Four thousand, eight hundred and sixteen animals were surveyed and information concerning animal category, source, season and preslaughter handling conditions recorded. Muscle samples were removed to estimate glycogen concentration and after incubation, ultimate pH. The overall incidence of dark cutting (pHu > 6.0) was 4"1%. Increased incidence was associated with short ( < 20 miles) and long (> 150 miles) transport distances. Slaughter on the day of arrival rather than overnight lairage also increased the incidence. Plants were classified into small (killing < 50 animals per day) or large (killing > 100 per day ). Eighty per cent of the animals slaughtered passed through the large plants, and a higher incidence was also associated with these plants. Bulls had the highest incidence and heifers the lowest. A seasonal effect was recorded with the highest incidence found between July and October. The results, however, indicate that factors in addition to those examined are also important.
INTRODUCTION D a r k cutting beef(DCB) is a meat quality problem caused by chronic stress preslaughter which depletes muscle glycogen stores (Fisher & Augustini, 1979; Warriss et al., 1984). The meat is dark is colour (MacDougall & Rhodes, 1972), more prone to bacterial spoilage (Newton & Gill, 1980), has 249 Meat Science 0309-1740/90/$03.50 © 1990 ElsevierSciencePublishers Ltd, England. Printed in Great Britain
250
S. N. Brown, E. A. Beois, P. D. Warriss
reduced flavour (Dransfield, 1981), and is therefore discriminated against by the retail trade and consumer (Tarrant, 1981). Even so there are no published estimates for its incidence in the UK. Surveys in several countries have shown incidences ranging from 3% (Ireland; Tarrant & Sherington, 1980) to 22% (Finland; Puolanne & Aalto, 1981). Some of this variation is attributable to the makeup of the population of animals surveyed. For example, the study by Puolanne and Aalto (1981) examined only bulls which are known to be more prone to dark cutting. The present survey investigated the incidence of dark cutting in the U K and some preslaughter stress factors which may be important in influencing it.
MATERIALS A N D METHODS The survey was carried out over a period of 1 year commencing in July 1986 and involved 4816 animals. Eight meat plants were studied, located in the southern part of England, which had throughputs ranging from 20 to 300 animals per day. Each plant was visited once every 2 months and all the animals killed on one day were sampled. Information was collected on animal category, distance travelled, whether killed on the day of arrival or lairaged overnight, the source of the animals (whether they came directly from producers or via dealers or markets) and the month of slaughter. Each plant could be categorised according to daily kill rate into large ( > 100 animals per day) or small ( < 50 per day). Animals having a pHu > 6.0 were classified as dark cutting.
Sampling Samples of M. longissimus dorsi (LD) weighing about 1 g were collected 30 min after death, from each side of the carcass at the level of approximately the 12th rib. The samples were removed using a procedure similar to that described by Petersen (1982), and were frozen in liquid nitrogen and stored at -20°C.
Ultimate pH (pHu) measurements One sample from each animal was allowed to thaw under liquid paraffin (29436 paraffin liquid light BDH Chemicals Ltd, Poole) for 48 h at 5°C. Excess paraffin was removed and the sample homogenised in ten volumes of 5 mM sodium iodoacetate, 150 mM potassium chloride (pH 7"0). The pH was measured using a Radiometer PHM 62 meter and combined glass electrode (GK2321C).
Incidence of dark cutting beef in the UK
251
Glycogen and lactate analysis Glycogen and lactate were determined in the second frozen sample. The sample was homogenised while still frozen in 10 ml of 1N hydrochloric acid. Lactate was measured using a commercial kit (No. 139084 Boehringer, Mannheim). The remaining homogenate was heated for 2 h at 95°C to hydrolyse the glycogen to glucose. This was measured using a commercial kit (No. 12044 Boehringer, Mannheim). The lactate concentrations were used to correct the muscle glycogen values for glycolytic loss before sampling.
Statistical analysis With unbalanced material the effects of transport distance, lairage time, plant size, source, category and season and the corresponding two way interactions were assessed by generalised linear models using a binomial error and logit link function. Approximate F statistics are presented. Chi-square tests were carried out to test specific comparisons.
RESULTS AND DISCUSSION
Dark cutting incidence A pHu > 6.0 was used to identify dark cutting carcasses. The pHu frequency distribution is shown in Fig. 1. The average pHu was 5-58. The incidence of animals with a pHu > 6.0 was 4.1% and those > 5.8, a value above which the meat presents a problem of rapid spoilage when vacuum packed (Anon, 1981) was 7.1%. This is in agreement with the value found by DezeureWallays et aL (1986) in Belgium, higher than that of Tarrant and Sherington 50'
40"
30
20 ¸
10i 0
r-5
, 6
> 6.5
Ultin~e pH
Fig. 1.
Frequency distribution o f ultimate pH.
252
S. N. Brown, E. A. Bevis, P. D. Warriss
(1980) in Ireland, but lower than that of Fabianssion et al. (1984) in Sweden, Munns and Burrell (1966) in America and Poulanne and Aalto (1981) in Finland. However, direct comparisons with other surveys are difficult, because of differences in the pHu value used for indicating dark cutting, the type of slaughter populations (e.g. bulls only), the number and type of plant and the time of year. Most of the factors recorded had significant influences on the incidence of dark cutting. Lairage time, size of plant and animal category had the largest effects (F values 48.0, 39.8 and 30.4, respectively, P < 0-001) but transport distance, source of animal and season were also important (Fvalues 17.6, 6.8 and 8-5, respectively, P < 0.001). Because of the confounding of the various factors it is impossible to separate the causes of the dark cutting precisely between these factors. The relatively low overall incidence of dark cutting precludes meaningful inclusion of all possible factors separately in one analysis. Nevertheless, it is unlikely that the reported overall incidences attributable to the major factors would be greatly affected by this confounding because of the large sample size involved.
Transport and lairage conditions Table 1 shows that very short (<20 miles) or long (_> 150 miles) transport distances were associated with a high incidence, as was killing animals on the day of arrival at the slaughter plant (5.5°,/o)compared with keeping them in lairage overnight (3.1%) (Z2 = 25.16, P < 0.001). Puolanne and Aalto (1981) found that with longer distances the incidence of dark cutting increased. Although this effect was small it is in agreement with the present findings. However, the increases found with short transport TABLE 1 The Distance Travelled from Farm to Slaughter Plant in Relation to the Incidence of Dark Cutting Miles
Number
Number DCB
%
0-20 21-40 41-60 61-80 81-100 101-150 151-200 > 200
1 201 1 026 765 420 334 480 407 183
70 32 19 11 12 8 27 18
5"8 3-1 2"5 2"6 3"6 1"7 6'6 9"8
253
Incidence of dark cutting beef in the UK
and short lairage are contrary to those of other workers (Augustini, 1981; Malmfors et al., 1983; Fabiansson et al., 1984). It is known that dark cutting arises from chronic stress (Hedrick, 1981) and that 24-48 h is required for recovery of the animal to give a normal muscle pHu although still with reduced glycogen content (Warriss et ai., 1984). Therefore, if stress occurred in the period before being sent to slaughter, for example, by mixing in holding pens or a period of fasting, animals could be potentially dark cutting before entering the slaughter chain and short transport and lairage would be of no benefit. It is important to bear in mind that the various studies have considered animals of different types which may, in part, explain the differences in findings. An additional consideration is that husbandry practices vary. For example, unlike the situation in the UK many cattle in continental Europe are reared intensively and often in individual stalls or tethered. This may influence considerably their reaction if mixed with unfamiliar animals during preslaughter handling. Plant size
Approximately 80% of the animals slaughtered in the UK pass through plants killing more than 100 animals per day. Large plants in this survey had on average a higher incidence of DCB (4.6%) compared with smaller plants (2.0%) (X2 = 10-89, P < 0-001) (Table 2). Some of these differences could be explained by the treatment that animals receive preslaughter to achieve the required rapid throughputs, such as long transport, short lairage, mixing and crowding. Also the proportions of the different categories varied, there being more bulls in the large plants and more heifers in the smaller plants. TABLE 2
Effects of Plant Size on Incidence of Dark Cutting Code
1 2 3 4 5 6 7 8
Size a
Large Large Large Large Small Small Small Small
Number
! 168 1080 1026 661 297 164 170 250
Number DCB
41 66 38 34 2 0 6 10
%
3"5] 6.1 3.7 5-1 0"7) 0~ 3"5[ 4 J
Average
Large = 4.6%
Small = 2"0%
a Large= slaughtering >_100 animals per day; Small= slaughtering < 50 animals per day.
254
S. N. Brown, E. A. Bevis, P. D. Warriss
Even so there was a wide range of incidence found between similar throughputs indicating the influence of factors which were in addition to those measured and possibly specific to individual plants. Examples are the type of animal produced in the region, or required to fill orders and the protocol laid down for delivery and slaughter of animals. Beef may come from prime beef breeds, including continental crosses, as a secondary product from the dairy industry, principally Friesians, or from culled dairy cows. Market requirements may demand carcasses of different weights, fatness and conformation. Transport and lairage schedules vary. All of these factors may influence the tendency of animals to dark cut.
Source Sixty per cent of the animals were supplied directly to the plant by producers, 23% came from dealers and 17% via markets. The incidences in these groups were 4.4%, 4.1% and 2.8%, respectively (Z2 = 1685, P < 0.001). As dark cutting is known to arise from chronic stress, this result is rather surprising. Animals subjected to the trauma of live auction market selling might have been expected to have the highest incidence. However, a large proportion of the animals passing through markets were heifers, which are known to be less prone to dark cutting and there were very few bulls; this led to a category × source interaction ( F = 14.4, P < 0.001). Further investigation of dark cutting in animals supplied by producers showed that the incidence could be related to a relatively small number of suppliers. They constantly produced above-average numbers of dark cutting animals, indicating a specific problem. This was likely to be related to stress which occurred on the farm before animals came under the control of the slaughter plant.
Animal category The animals were divided into five categories--bulls, cows, steers, heifers and veal calves. Twelve per cent of the animals slaughtered were bulls, 58 % steers and 12% heifers. The incidence of dark cutting in bulls was 8%, which was almost double that for steers and nearly four times that for heifers (Table 3). The incidence of dark cutting in groups of animals of the same category was wide, ranging from 0 to 80%. The effect of category on the incidence of dark cutting has been recorded by many workers (Tarrant & Sherington, 1980; Puolanne & Aalto, 1981; Fabianssion et al., 1984). The problem of the high incidence, although well known, may become more important following the recent EEC legislation which prevents the use of anabolic steroid implants in steers. To ensure
Incidence of dark cutting beef in the UK
255
TABLE 3
The Effect of Animal Category on Incidence of Dark Cutting
Animal
Number
Number DCB
%
Bull Steer Cow Heifer Calf
649 2 929 252 716 270
52 110 15 10 10
8"0 3"8 6.0 1'4 3-7
efficient production, the number of bulls used will probably increase, as could potentially dark cutting meat. Cows were also found to have a relatively high incidence, although this is in agreement with the study of Munns and Burrell (1966). These animals could potentially have been in oestrus making them less tolerant of preslaughter handling (Hedrick, 1981; Kenny & Tarrant, 1988), and a number of animals were in slightly poorer condition and again likely to be more prone to preslaughter stress. Season
Tarrant and Sherington (1980), Duchesne (1978), Munns and Burrell (1966) and Fabianssion et al. (1984) have all reported a seasonal variation in the incidence of dark cutting. Generally these showed a peak in late autumn and again in spring. The present work indicated a rise during July-October and then a gradual fall over the rest of the period under study (Fig. 2). Severe
@ o co "D
i
July/Aug 'SepVOct 'Nov/Dec ' Jan/Feb' MaffApr' May/Jun 86
86
86
87
87
87
Time of year Fig. 2.
Effect of time of year on dark cutting incidence.
256
S. N. Brown, E. A. Bevis, P. D. Warriss
climatic changes have been suggested as important in Canada (Munns & Burrell, 1966), but this seems unlikely in the mild conditions of the UK. Another possible reason for the autumn peak is that animals are still being fed exclusively on grass. Blaxter et al. (1971) showed that there is a progressive decrease in the quality, digestibility and soluble carbohydrate content of the grass as the growing season progresses. This may lead to lower muscle glycogen reserves and the possibility of animals being more prone to dark cutting. In the present work a slight reduction in muscle glycogen was found during July-October compared to the rest of the year (9.71 versus 10-22mg/g) although it was probably insufficient to cause the increased incidence recorded.
Muscle pHu and initial glycogen The relationship between pHu and glycogen concentration is shown in Fig. 3. These results are in agreement with those of Warriss et al. (1984). Measurement of pHu has been used by most workers (Tarrant & Sherington, 1980; Augustini, 1981; Puolanne & Aalto, 1981; Fabianssion et al., 1984; Warriss et al., 1984) to indicate dark cutting carcasses. Because of the non-linear relationship, glycogen could be a more sensitive indicator of those animals, which although they still show normal pHu, have been subjected to stress. Muscle glycogen concentrations of about 8-9 mg/g result in elevation of pHu. Concentrations of less than 4-5 mg/g indicate a dark cutting carcass. It was seen that a large proportion of animals (30%) had lowered glycogen levels although these were not sufficient to raise pHu. If these animals had been subjected to a small additional stress which potentially could further reduce the glycogen stores, levels could be depleted sufficiently to elevate pHu. 6.6"
6.4'
==.
6.2'
=.
6.0 5.8 5.6 °
5.4 0
Fig. 3.
o
•
•
o
o•
1'0 Glycogen (mglg)
o
o
o•
2'0
Relationship between ultimate pH and initial glycogen concentration.
Incidence of dark cutting beef in the UK
257
CONCLUSIONS The overall incidence of dark cutting was 4.1%. Short ( < 20 miles) and long ( > 150 miles) transport caused an increase in dark cutting, as did killing on the day of arrival compared with after overnight lairage. The incidence in bulls was twice as high as in steers, indicating the need for improved handling of this group of animals. Large plants killing more than 100 animals per day had the highest incidence. This may have been caused by the high throughputs, but, as the incidence between plants of similar throughputs varied, other factors are likely to have been important. There was a seasonal effect with a large increase in the a u t u m n period. The results indicate that as well as plant size, transport and lairage conditions, season and category, other factors could be responsible for the large variations in the incidence of dark cutting beef between plants.
ACKNOWLEDGEMENTS The authors would like to thank Miss J. Ashby for invaluable technical assistance and Mr N. Bratchell for guidance on the statistical analyses.
REFERENCES Anon. (1981). Meat Research Letter, CSIRO Division of Food Research, Meat Research Laboratory, 81/3. Augustini, C. H. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 379. Blaxter, K. L., Wainman, F. W., Dewey, P. J. S., Davidson, J., Denerley, H. & Gunn, J. B. (1971). J. Agric. Sci. Camb., 76, 307. Dezeure-Wallays, B., Van Hoof, J. & Pensaert, R. (1986). Proceedings 32nd Europ. Meeting Meat Res. Workers, Ghent, 3, 2. Dransfield, E. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 344. Duchesne, H. E. (1978). The aetiology of dark cutting beef: A study on the management and behavioural factors that affect meat quality in entire male cattle. Thesis, University of Bristol. Fabianssion, S., Erichsen, I., Laser Reutersw~ird, A. L. & Malmfors, G. (1984). Meat Sci., 10, 21. Fisher, K. & Augustini, C. (1979). Fleischwirtschaft., 59, 1871. Hedrick, H. B. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 213. Kenny, F. J. & Tarrant, P. V. (1988). Meat Sci., 22, 21-31. MacDougall, D. B. & Rhodes, D. N. (1972). J. Sci. Food Agric., 23, 637 47.
258
S. N. Brown, E. A. Bevis, P. D. Warriss
Malmfors, G., Lundstrom, K. & Fabianssion, S. (1983). Proceedings 29th Europ. Meeting Meat Res. Workers, Palma, Vol. 1, pp. 70-75. Munns, W. O. & Burrell, D. E. (1966). Food Technol., 20, 1601. Newton, K. G. & Gill, C. O. (1980). Meat Sci., 5, 223. Petersen, G. V. (1982). Meat Sci., 7, 37. Puolanne, E. & Aalto, H. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 462. Tarrant, P. V. & Sherington, J. (1980). Meat Sci., 4, 287. Tarrant, P. V. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 3. Warriss, P. D., Kestin, S. C., Brown, S. N. & Wilkins, L. J. (1984). Meat Sci., 10, 53.
An Estimate of the Incidence of Dark Cutting Beef in the United Kingdom
S. N. Brown, E. A. Bevis & P. D. Warriss AFRC Institute of Food Research Bristol Laboratory, Langford, BS187DY, UK (Received 22 June 1989; revised version received 1 August 1989; accepted 8 August 1989)
ABSTRACT Eight slaughterplants with throughputs ranging from 20 to 300 animals per day were examined to estimate the incidence of dark cutting beef in the United Kingdom. Four thousand, eight hundred and sixteen animals were surveyed and information concerning animal category, source, season and preslaughter handling conditions recorded. Muscle samples were removed to estimate glycogen concentration and after incubation, ultimate pH. The overall incidence of dark cutting (pHu > 6.0) was 4"1%. Increased incidence was associated with short ( < 20 miles) and long (> 150 miles) transport distances. Slaughter on the day of arrival rather than overnight lairage also increased the incidence. Plants were classified into small (killing < 50 animals per day) or large (killing > 100 per day ). Eighty per cent of the animals slaughtered passed through the large plants, and a higher incidence was also associated with these plants. Bulls had the highest incidence and heifers the lowest. A seasonal effect was recorded with the highest incidence found between July and October. The results, however, indicate that factors in addition to those examined are also important.
INTRODUCTION D a r k cutting beef(DCB) is a meat quality problem caused by chronic stress preslaughter which depletes muscle glycogen stores (Fisher & Augustini, 1979; Warriss et al., 1984). The meat is dark is colour (MacDougall & Rhodes, 1972), more prone to bacterial spoilage (Newton & Gill, 1980), has 249 Meat Science 0309-1740/90/$03.50 © 1990 ElsevierSciencePublishers Ltd, England. Printed in Great Britain
250
S. N. Brown, E. A. Beois, P. D. Warriss
reduced flavour (Dransfield, 1981), and is therefore discriminated against by the retail trade and consumer (Tarrant, 1981). Even so there are no published estimates for its incidence in the UK. Surveys in several countries have shown incidences ranging from 3% (Ireland; Tarrant & Sherington, 1980) to 22% (Finland; Puolanne & Aalto, 1981). Some of this variation is attributable to the makeup of the population of animals surveyed. For example, the study by Puolanne and Aalto (1981) examined only bulls which are known to be more prone to dark cutting. The present survey investigated the incidence of dark cutting in the U K and some preslaughter stress factors which may be important in influencing it.
MATERIALS A N D METHODS The survey was carried out over a period of 1 year commencing in July 1986 and involved 4816 animals. Eight meat plants were studied, located in the southern part of England, which had throughputs ranging from 20 to 300 animals per day. Each plant was visited once every 2 months and all the animals killed on one day were sampled. Information was collected on animal category, distance travelled, whether killed on the day of arrival or lairaged overnight, the source of the animals (whether they came directly from producers or via dealers or markets) and the month of slaughter. Each plant could be categorised according to daily kill rate into large ( > 100 animals per day) or small ( < 50 per day). Animals having a pHu > 6.0 were classified as dark cutting.
Sampling Samples of M. longissimus dorsi (LD) weighing about 1 g were collected 30 min after death, from each side of the carcass at the level of approximately the 12th rib. The samples were removed using a procedure similar to that described by Petersen (1982), and were frozen in liquid nitrogen and stored at -20°C.
Ultimate pH (pHu) measurements One sample from each animal was allowed to thaw under liquid paraffin (29436 paraffin liquid light BDH Chemicals Ltd, Poole) for 48 h at 5°C. Excess paraffin was removed and the sample homogenised in ten volumes of 5 mM sodium iodoacetate, 150 mM potassium chloride (pH 7"0). The pH was measured using a Radiometer PHM 62 meter and combined glass electrode (GK2321C).
Incidence of dark cutting beef in the UK
251
Glycogen and lactate analysis Glycogen and lactate were determined in the second frozen sample. The sample was homogenised while still frozen in 10 ml of 1N hydrochloric acid. Lactate was measured using a commercial kit (No. 139084 Boehringer, Mannheim). The remaining homogenate was heated for 2 h at 95°C to hydrolyse the glycogen to glucose. This was measured using a commercial kit (No. 12044 Boehringer, Mannheim). The lactate concentrations were used to correct the muscle glycogen values for glycolytic loss before sampling.
Statistical analysis With unbalanced material the effects of transport distance, lairage time, plant size, source, category and season and the corresponding two way interactions were assessed by generalised linear models using a binomial error and logit link function. Approximate F statistics are presented. Chi-square tests were carried out to test specific comparisons.
RESULTS AND DISCUSSION
Dark cutting incidence A pHu > 6.0 was used to identify dark cutting carcasses. The pHu frequency distribution is shown in Fig. 1. The average pHu was 5-58. The incidence of animals with a pHu > 6.0 was 4.1% and those > 5.8, a value above which the meat presents a problem of rapid spoilage when vacuum packed (Anon, 1981) was 7.1%. This is in agreement with the value found by DezeureWallays et aL (1986) in Belgium, higher than that of Tarrant and Sherington 50'
40"
30
20 ¸
10i 0
r-5
, 6
> 6.5
Ultin~e pH
Fig. 1.
Frequency distribution o f ultimate pH.
252
S. N. Brown, E. A. Bevis, P. D. Warriss
(1980) in Ireland, but lower than that of Fabianssion et al. (1984) in Sweden, Munns and Burrell (1966) in America and Poulanne and Aalto (1981) in Finland. However, direct comparisons with other surveys are difficult, because of differences in the pHu value used for indicating dark cutting, the type of slaughter populations (e.g. bulls only), the number and type of plant and the time of year. Most of the factors recorded had significant influences on the incidence of dark cutting. Lairage time, size of plant and animal category had the largest effects (F values 48.0, 39.8 and 30.4, respectively, P < 0-001) but transport distance, source of animal and season were also important (Fvalues 17.6, 6.8 and 8-5, respectively, P < 0.001). Because of the confounding of the various factors it is impossible to separate the causes of the dark cutting precisely between these factors. The relatively low overall incidence of dark cutting precludes meaningful inclusion of all possible factors separately in one analysis. Nevertheless, it is unlikely that the reported overall incidences attributable to the major factors would be greatly affected by this confounding because of the large sample size involved.
Transport and lairage conditions Table 1 shows that very short (<20 miles) or long (_> 150 miles) transport distances were associated with a high incidence, as was killing animals on the day of arrival at the slaughter plant (5.5°,/o)compared with keeping them in lairage overnight (3.1%) (Z2 = 25.16, P < 0.001). Puolanne and Aalto (1981) found that with longer distances the incidence of dark cutting increased. Although this effect was small it is in agreement with the present findings. However, the increases found with short transport TABLE 1 The Distance Travelled from Farm to Slaughter Plant in Relation to the Incidence of Dark Cutting Miles
Number
Number DCB
%
0-20 21-40 41-60 61-80 81-100 101-150 151-200 > 200
1 201 1 026 765 420 334 480 407 183
70 32 19 11 12 8 27 18
5"8 3-1 2"5 2"6 3"6 1"7 6'6 9"8
253
Incidence of dark cutting beef in the UK
and short lairage are contrary to those of other workers (Augustini, 1981; Malmfors et al., 1983; Fabiansson et al., 1984). It is known that dark cutting arises from chronic stress (Hedrick, 1981) and that 24-48 h is required for recovery of the animal to give a normal muscle pHu although still with reduced glycogen content (Warriss et ai., 1984). Therefore, if stress occurred in the period before being sent to slaughter, for example, by mixing in holding pens or a period of fasting, animals could be potentially dark cutting before entering the slaughter chain and short transport and lairage would be of no benefit. It is important to bear in mind that the various studies have considered animals of different types which may, in part, explain the differences in findings. An additional consideration is that husbandry practices vary. For example, unlike the situation in the UK many cattle in continental Europe are reared intensively and often in individual stalls or tethered. This may influence considerably their reaction if mixed with unfamiliar animals during preslaughter handling. Plant size
Approximately 80% of the animals slaughtered in the UK pass through plants killing more than 100 animals per day. Large plants in this survey had on average a higher incidence of DCB (4.6%) compared with smaller plants (2.0%) (X2 = 10-89, P < 0-001) (Table 2). Some of these differences could be explained by the treatment that animals receive preslaughter to achieve the required rapid throughputs, such as long transport, short lairage, mixing and crowding. Also the proportions of the different categories varied, there being more bulls in the large plants and more heifers in the smaller plants. TABLE 2
Effects of Plant Size on Incidence of Dark Cutting Code
1 2 3 4 5 6 7 8
Size a
Large Large Large Large Small Small Small Small
Number
! 168 1080 1026 661 297 164 170 250
Number DCB
41 66 38 34 2 0 6 10
%
3"5] 6.1 3.7 5-1 0"7) 0~ 3"5[ 4 J
Average
Large = 4.6%
Small = 2"0%
a Large= slaughtering >_100 animals per day; Small= slaughtering < 50 animals per day.
254
S. N. Brown, E. A. Bevis, P. D. Warriss
Even so there was a wide range of incidence found between similar throughputs indicating the influence of factors which were in addition to those measured and possibly specific to individual plants. Examples are the type of animal produced in the region, or required to fill orders and the protocol laid down for delivery and slaughter of animals. Beef may come from prime beef breeds, including continental crosses, as a secondary product from the dairy industry, principally Friesians, or from culled dairy cows. Market requirements may demand carcasses of different weights, fatness and conformation. Transport and lairage schedules vary. All of these factors may influence the tendency of animals to dark cut.
Source Sixty per cent of the animals were supplied directly to the plant by producers, 23% came from dealers and 17% via markets. The incidences in these groups were 4.4%, 4.1% and 2.8%, respectively (Z2 = 1685, P < 0.001). As dark cutting is known to arise from chronic stress, this result is rather surprising. Animals subjected to the trauma of live auction market selling might have been expected to have the highest incidence. However, a large proportion of the animals passing through markets were heifers, which are known to be less prone to dark cutting and there were very few bulls; this led to a category × source interaction ( F = 14.4, P < 0.001). Further investigation of dark cutting in animals supplied by producers showed that the incidence could be related to a relatively small number of suppliers. They constantly produced above-average numbers of dark cutting animals, indicating a specific problem. This was likely to be related to stress which occurred on the farm before animals came under the control of the slaughter plant.
Animal category The animals were divided into five categories--bulls, cows, steers, heifers and veal calves. Twelve per cent of the animals slaughtered were bulls, 58 % steers and 12% heifers. The incidence of dark cutting in bulls was 8%, which was almost double that for steers and nearly four times that for heifers (Table 3). The incidence of dark cutting in groups of animals of the same category was wide, ranging from 0 to 80%. The effect of category on the incidence of dark cutting has been recorded by many workers (Tarrant & Sherington, 1980; Puolanne & Aalto, 1981; Fabianssion et al., 1984). The problem of the high incidence, although well known, may become more important following the recent EEC legislation which prevents the use of anabolic steroid implants in steers. To ensure
Incidence of dark cutting beef in the UK
255
TABLE 3
The Effect of Animal Category on Incidence of Dark Cutting
Animal
Number
Number DCB
%
Bull Steer Cow Heifer Calf
649 2 929 252 716 270
52 110 15 10 10
8"0 3"8 6.0 1'4 3-7
efficient production, the number of bulls used will probably increase, as could potentially dark cutting meat. Cows were also found to have a relatively high incidence, although this is in agreement with the study of Munns and Burrell (1966). These animals could potentially have been in oestrus making them less tolerant of preslaughter handling (Hedrick, 1981; Kenny & Tarrant, 1988), and a number of animals were in slightly poorer condition and again likely to be more prone to preslaughter stress. Season
Tarrant and Sherington (1980), Duchesne (1978), Munns and Burrell (1966) and Fabianssion et al. (1984) have all reported a seasonal variation in the incidence of dark cutting. Generally these showed a peak in late autumn and again in spring. The present work indicated a rise during July-October and then a gradual fall over the rest of the period under study (Fig. 2). Severe
@ o co "D
i
July/Aug 'SepVOct 'Nov/Dec ' Jan/Feb' MaffApr' May/Jun 86
86
86
87
87
87
Time of year Fig. 2.
Effect of time of year on dark cutting incidence.
256
S. N. Brown, E. A. Bevis, P. D. Warriss
climatic changes have been suggested as important in Canada (Munns & Burrell, 1966), but this seems unlikely in the mild conditions of the UK. Another possible reason for the autumn peak is that animals are still being fed exclusively on grass. Blaxter et al. (1971) showed that there is a progressive decrease in the quality, digestibility and soluble carbohydrate content of the grass as the growing season progresses. This may lead to lower muscle glycogen reserves and the possibility of animals being more prone to dark cutting. In the present work a slight reduction in muscle glycogen was found during July-October compared to the rest of the year (9.71 versus 10-22mg/g) although it was probably insufficient to cause the increased incidence recorded.
Muscle pHu and initial glycogen The relationship between pHu and glycogen concentration is shown in Fig. 3. These results are in agreement with those of Warriss et al. (1984). Measurement of pHu has been used by most workers (Tarrant & Sherington, 1980; Augustini, 1981; Puolanne & Aalto, 1981; Fabianssion et al., 1984; Warriss et al., 1984) to indicate dark cutting carcasses. Because of the non-linear relationship, glycogen could be a more sensitive indicator of those animals, which although they still show normal pHu, have been subjected to stress. Muscle glycogen concentrations of about 8-9 mg/g result in elevation of pHu. Concentrations of less than 4-5 mg/g indicate a dark cutting carcass. It was seen that a large proportion of animals (30%) had lowered glycogen levels although these were not sufficient to raise pHu. If these animals had been subjected to a small additional stress which potentially could further reduce the glycogen stores, levels could be depleted sufficiently to elevate pHu. 6.6"
6.4'
==.
6.2'
=.
6.0 5.8 5.6 °
5.4 0
Fig. 3.
o
•
•
o
o•
1'0 Glycogen (mglg)
o
o
o•
2'0
Relationship between ultimate pH and initial glycogen concentration.
Incidence of dark cutting beef in the UK
257
CONCLUSIONS The overall incidence of dark cutting was 4.1%. Short ( < 20 miles) and long ( > 150 miles) transport caused an increase in dark cutting, as did killing on the day of arrival compared with after overnight lairage. The incidence in bulls was twice as high as in steers, indicating the need for improved handling of this group of animals. Large plants killing more than 100 animals per day had the highest incidence. This may have been caused by the high throughputs, but, as the incidence between plants of similar throughputs varied, other factors are likely to have been important. There was a seasonal effect with a large increase in the a u t u m n period. The results indicate that as well as plant size, transport and lairage conditions, season and category, other factors could be responsible for the large variations in the incidence of dark cutting beef between plants.
ACKNOWLEDGEMENTS The authors would like to thank Miss J. Ashby for invaluable technical assistance and Mr N. Bratchell for guidance on the statistical analyses.
REFERENCES Anon. (1981). Meat Research Letter, CSIRO Division of Food Research, Meat Research Laboratory, 81/3. Augustini, C. H. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 379. Blaxter, K. L., Wainman, F. W., Dewey, P. J. S., Davidson, J., Denerley, H. & Gunn, J. B. (1971). J. Agric. Sci. Camb., 76, 307. Dezeure-Wallays, B., Van Hoof, J. & Pensaert, R. (1986). Proceedings 32nd Europ. Meeting Meat Res. Workers, Ghent, 3, 2. Dransfield, E. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 344. Duchesne, H. E. (1978). The aetiology of dark cutting beef: A study on the management and behavioural factors that affect meat quality in entire male cattle. Thesis, University of Bristol. Fabianssion, S., Erichsen, I., Laser Reutersw~ird, A. L. & Malmfors, G. (1984). Meat Sci., 10, 21. Fisher, K. & Augustini, C. (1979). Fleischwirtschaft., 59, 1871. Hedrick, H. B. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 213. Kenny, F. J. & Tarrant, P. V. (1988). Meat Sci., 22, 21-31. MacDougall, D. B. & Rhodes, D. N. (1972). J. Sci. Food Agric., 23, 637 47.
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Malmfors, G., Lundstrom, K. & Fabianssion, S. (1983). Proceedings 29th Europ. Meeting Meat Res. Workers, Palma, Vol. 1, pp. 70-75. Munns, W. O. & Burrell, D. E. (1966). Food Technol., 20, 1601. Newton, K. G. & Gill, C. O. (1980). Meat Sci., 5, 223. Petersen, G. V. (1982). Meat Sci., 7, 37. Puolanne, E. & Aalto, H. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 462. Tarrant, P. V. & Sherington, J. (1980). Meat Sci., 4, 287. Tarrant, P. V. (1981). In The Problem of Dark-cutting in Beef, ed. D. E. Hood & P. V. Tarrant. Martinus Nijhoff Publishers, The Hague, p. 3. Warriss, P. D., Kestin, S. C., Brown, S. N. & Wilkins, L. J. (1984). Meat Sci., 10, 53.