Proc. Aust. Soc. Anim. Prod. 1996 Vol. 21
THE REPEATABILITY OF MUSCLE SCORE IN BEEF ANIMALS BETWEEN BIRTH AND TWENTY MONTHS OF AGE
W.A. McKIERNANA and G.E. ROBARDSB
ANSW Agriculture, Scone, N.S.W. 2337
B University of NSW, Kensington, N.S.W. 2033
SUMMARY
One hundred and fifty calves were born at NSW Agriculture’s Elizabeth MacArthur Institute in 1992.
They were sired by high and low muscled Angus bulls out of a random selection of Hereford cows. The calves were assessed for muscling at 2 months of age and repeat scored on a number of occasions throughout their growth until 20 months of age. Correlations were derived between these scores. On a number of occasions muscle scoring was repeated on the same day and scored by 2 experienced, independent operators and simple correlations derived. Repeatability of scores on the same day with the same operator, and between operators, was shown to be quite high and correlations between scores over time to be reasonably consistent, but influenced by the variation in the population being assessed and possibly condition and/or fatness of the animal.
Keywords: muscle score, repeatability, correlations INTRODUCTION
Muscle scoring of live cattle has been shown (Perry et al. 1993 a,b) to be of use in predicting potential animal value because of its relationship with dressing percentage and yield of saleable and lean meat yield.
However, it is a subjective and visual assessment of animal shape and could present problems in implementation if it could not be consistently applied or was not repeatable. Kempster (1986) suggested that the weak link of a subjective system of evaluating muscularity may well be its poor repeatability. Also, the correlation of muscle score at a young age with later scores on the same animal at older ages or through various growth phases has not been tested. A strong relationship may aid in the prediction of market value of animals at an early age, and thus may assist in feeding management and marketing decisions.
MATERIALS AND METHODS
This experiment formed part of a cattle breeding project conducted at NSW Agriculture’s Elizabeth MacArthur Agricultural Institute (E.M.A.I.) at Camden, N.S.W., 70 kms south west of Sydney. It is relatively poor, natural, dryland country.
In 1991, 210 Hereford and Poll Hereford females were assembled on the property, varying in age (1 to 8 years old), and in place of origin (4 different properties). Cow age and origin were recorded. These cows were stratified by age and origin and randomly divided into 2 groups and mated to either 4 high or 4 low muscle scored Angus bulls. The high (H) bulls had an average muscle score of 11.25 pts and the low bulls an average muscle score of 6.00 pts on the basis of the visual-“muscle score” system: 15 point scale, 15 highest = A+, 1 lowest = E -.
The live “muscle score” used was based on visual assessment of the thickness and convexity of the body relative to the skeletal size of the animal, adjusting for subcutaneous fat depth (McKieman 1990). A 15 - point scale was used, from A+ to E-, where score A animals were well muscled, and score E animals poorly muscled.
Progeny were assessed for muscle score initially at an average age of 2 months and then at regular intervals until an averaging age of 20 months of age. The combined group of steer and heifer calves (n=l50) were scored a total of 8 times throughout this period by the same assessor. Steers (n=80) were scored an additional 4 times, the last 3 occasions on a feedlot. The steer data was analysed independently.
On 5 separate occasions, a number of calves (n = 25,49,64,15 1 and 8 1) were repeat scored twice on the same day by the same assessor. On 2 occasions 2 different assessors working independently scored calves (n = 153,80). Simple correlations were derived, as an indication of repeatability, using the GLM procedure of S.A.S. (1990) to determine: correlations of same day scores by 1 assessor, correlations between assessors, and correlations between scores over time.
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RESULTS
Over the 18 month period (8 scoring times) the calves grew from a mean birth weight of 35.4 kg (+ 4.9 kg) to a 20 months weight for heifers of 247 kg (2 22.4 kg) and steers of 444 kg (k 30.2 kg).
Repeatability of muscle score on the same day
The average correlation between 2 scores, given by the same assessor on the same day (usually about 6 hours between scores) on 5 separate occasions using animals selected at random from the base group at various times during their life was 0.81 (Table l), and on average the second score was either exactly the same or within 1 point of the first score 83% of the time.
Table 1. Means and correlations between repeated muscle scores on the same day at differing ages of cattle
Repeatability of muscle score between assessors
The average correlation between the scores given by 2 experienced assessors independently scoring the calves was 0.87 (Table 2).
Table 2. Means and correlations between assessors independently scoring cattle for muscle score on the same day
Repeatability of muscle score over time
The average correlation of muscle scores from 2 months of age through to 20 months of age, scoring on 8 different occasions, combining both steer and heifer progeny scores (Table 3), was 0.7 (range 0.57 to 0.79).
Table 3. Simple correlations between muscle scores over time and muscle score means and range of scores at those times for all progeny (150 animals)
For the steer progeny there were 12 scores given over a period of 20 months which included periods of weight loss (from 8 to 14 months of age) and rapid weight gain (17b to 20 months of age) on a feedlot.
During this period the steers were scanned (real time ultrasound scan) for P8 (rump site) fat depth at various ages. *At weaning (8 months) the steers’ mean P8 fat depth was 2.3mm f 0.18, at 14 months 2.7mm f 1.24
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and at 17(b) months (feedlot start) 3.8mm + 1.39. At the end of the feedlot phase (20 months) the steers were slaughtered at an average of 444 kg and 13mm fat depth at the P8 fat site. The average correlation over time within the steers only was 0.58, with a range from 0.35 to 0.83 (Table 5).
Table 4. Simple correlations between times for steers only (80 animals)
scores over time and score means range scores at
DISCUSSION
The calves studied in this experiment were of common background, the same breed combination and the variation in muscle score was not as great as would be expected in other mixed breed populations containing dairy and/or continental breed combinations (Kempster 1986).
The repeatability (as measured via simple correlations) of muscle scores on the same day, by the same experienced assessor, was high with an average correlation of 0.8. This is consistent with earlier findings of McKieman (1990). On that occasion only a small sample was used and only on 1 occasion at 1 age of cattle, whereas these results indicate consistency with a large number of animals at various ages, weights and condition scores.
The correlation between operators was also high and consistent with earlier findings of McKieman (1990) and Sundstrom (personal communication) indicating that muscle score can be successfully applied consistently between experienced operators.
The repeatability of muscle score over time, assessed over the whole population of steer and heifer progeny studied, was reasonably good - an indication of the consistency of this subjective score during substantial growth and condition changes (average correlation of 0.7 over 8 scores during an 18 month period). However, within the more uniform steer data set the correlations were not as good.
The decrease in the correlations, derived using only the steer portion (80) as opposed to those using the total population of 150 animals, may be explained in part by the smaller variation in muscle score in the steer population than in the total population (Table 3). This limitation of correlation coefficients was identified by Houghton and Turlington (1992) where they reported that larger than normal variation will produce high correlation coefficients, whereas a uniform population will result in much lower correlation coefficients.
Within the steer data there appears to be a distinction around row 17(b). At this age steers were assessed at feedlot entry. Following this assessment rapid weight and fatness changes occurred. It would appear that the correlations of this score (17b) with earlier scores are reasonably good, but following rapid live weight and fattening changes, the correlations decrease (row 19 and 20). A possible explanation for this may be that either assessors’ perceptions of shape were influenced by differences in fatness, or inherent variation in fatness was coincidental in muscularity, implying that animals were more able to display their inherent muscling as they improved in condition. This later hypothesis is supported by Tatum et al. (1986) who, following a canonical variate analysis of their data, indicated that muscle thickness (assessed as muscle score) was not influenced materially by variation in fatness and by Perry et aZ. (1993a,b) who reported a negative correlation between subcutaneous fat and muscle score. Later carcass evaluation of these steers
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hoc. Aust. Sot. Anim. Prod. 1996 Vol. 2 I (unpublished) showed there was no significant difference due to muscle score in measured 10th rib fat depth, P8 fat depth or fat waste after retail yield.
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