Ministry of Agriculture, Food and Rural Affairs

A New Method for the Determination of Lean Meat Yield in Beef Cattle

Lean meat yield is an economically important trait in the beef industry, as producers are paid on a $/lb basis of carcass weight, with a possible differential for yield grade. The Canadian Beef Grading Agency (www.beefgradingagency.ca) sets out guidelines for the grading and quality assessment of beef carcasses. They assign yield grades based on the estimated percent of lean yield; Y1 greater than 59%, Y2 54-58%, Y3 53% or less. The grader uses the Canadian Yield Ruler (Figure 1) to determine the yield grade; they do so by combining muscle score (a combination of three categories of length and depth of the rib-eye (longissimus dorsi)), and fat class score (fat depth) in the chart. The Yield Grade is based on the yield equation:

Lean % = 63.65 + 1.05 X (muscle score) - 0.76 X (grade fat) (Canadian Beef Grading Agency)

From here they assign the yield grade based on the estimated percent lean.

Figure 1. The Canadian Yield Ruler (Canadian Beef Grading Agency).

In the current system, producers are paid based on carcass weight, with a potential differential for yield grade. However, the problem with the current system is that carcasses are placed into 1 of 3 broad categories, each of which contains a wide range of percent lean yield. It fails to distinguish within yield grade and award carcasses based on the proportion of lean. Therefore, it would be much more beneficial to producers if they were paid on a $/lb of predicted lean basis, (which is a much better reflection of the true value of a carcass) rather than lumping cattle into broad yield grade classes.

An example calculation is shown below in which two carcasses which weigh the same (800 lbs), and are both graded Yield 1 are compared for a $ value based on the amount of lean:

Average price for a carcass weighing 800 lbs = $1.68/lb* X 800 lbs = $1344

lbs of lean for a carcass with 59% lean yield = 0.59 X 800 lbs = 472 lbs of lean tissue

If the producer were paid on a $/lb lean tissue basis then

$/lb of lean tissue = $1344 / 472 lbs = $2.85/lb lean

lbs of lean for a carcass with 63% lean yield = 0.63 X 800 lbs = 504 lbs of lean tissue

Difference in lbs of lean = 504 lbs - 472 lbs = 32 lbs

Difference in value = 32 lbs X $2.85/lb of lean = $91.20

*The $/lb value comes from the most recent average rail grade price from the Ontario Cattlemen's Association (December, 2008). Based on the above calculations, the increased value of an 800 pound carcass with 63% lean yield compared to a carcass with 59% lean yield is $91.20. This is a significant difference for beef producers!

Therefore, an accurate estimate of % lean at the time of carcass assessment is critical to rewarding producers for differences in yield. Several studies have been conducted to come up with prediction equations based on carcass measurements to accurately estimate lean yield. Traditionally, the area of the rib-eye (l. dorsi) is taken at the 12/13th rib interface and is used in combination with fat depth and carcass weight in prediction equations. Other methods have been explored by Crews (2001) and Bergen et al. (2003) in which they examined alternative measures from the l. dorsi; including width (herein referred to the same measurement as length) and depth of the muscle taken at the 12/13th rib interface (Bergen et al. (2003) employed two methods for length and depth). The results from both studies indicate that length and depth measurements can be used as alternatives to tracing the rib-eye area (which can be a more time-consuming process). Bergen et al. (2003) also explored alternatives using ultrasound imaging to obtain measurements of bodywall depth, gluteus medius depth, rump depth, etc. It was apparent from this study that adding bodywall depth to the standard area (or length and depth combination), fat depth, and carcass weight in a prediction equation generated a more accurate estimate of the actual percent lean. In addition, when the ultrasound and carcass methods of evaluation were compared, it was determined that the ultrasound method was superior to that of the carcass method.

Measurements in This Study

The work done by Bergen et al. (2003) prompted the current University of Guelph study in which measurements were taken post-slaughter on carcasses and on accompanying photographs of the same sections. Cattle from the Elora Beef Research station feedlot that were processed at the University of Guelph meat laboratory were used in the current study. There were 90head, analyzed over a 23 week period. Measurements included 2 methods of measuring rib-eye length and depth to calculate rib-eye area; , fat depth; carcass weight; bodywall depth; and bodywall percent lean (Figures 2 to 9) (for more detail see below¹).

The measurements were taken on the carcasses using rulers, placed directly on the surface of the meat. Four digital photographs were taken for each carcass: two of the rib-eye, and two of the bodywall. They were scaled by placing two rulers within the frame of the image, so that the measurements could be converted from pixels to cm or cm2. The measurements on the images were performed using Sigma Scan Pro 5.0 software. The rib-eye area was determined using the area function in the Sigma Scan Pro 5.0 software. For carcass measurements, the rib-eye and bodywall were both traced onto acetate paper, and the area was analyzed using a planimeter. The percent lean in the bodywall was determined by taking the area of the lean and dividing by the total area of the section. The University of Guelph meat lab staff determined the carcass weight, dissected the carcasss, and weighed the proportion of lean, from which the actual percent lean was derived.

A digital camera was used to capture the images, in order to mimic a method which could be easily employed by small packing plants and would not require costly sophisticated computer imaging systems that are feasible only in large packing plants.

Figure 2: Rib-eye area-Method 1 length

Figure 3: Rib-eye area - Method 2 length

Figure 4: Rib-eye area - Method 1 depth

Figure 5: Rib-eye area - Method 2 depth

Figure 6: Fat depth

Figure 7: Tracing of rib-eye area

Figure 8: Bodywall depth

Figure 9: Bodywall lean

The results from the current study suggest that bodywall measurements (for image and carcass based methods) significantly improve the prediction of lean yield (Table 1). This can be seen by the large increase in R2 values (correlation coefficients) when the bodywall measurements are added (especially in the case of the carcass evaluations). Therefore, we can conclude that adding bodywall depth and lean measures to the current methods improves the accuracy with which we can make predictions. These should be included in the assessment of carcasses for lean yield.

Table 1. Correlation of Predicted Lean Yield of Beef Carcasses (Various Methods) With Dissected Lean Yield

Evaluation of Carcass or Digital Image	Rib-eye Measurement Method	R2
		No Bodywall	Bodywall*
Carcass	Method 1 Length & Depth	0.22	0.39
	Method 2 Length & Depth	0.24	0.39
	Area	0.17	0.37
Digital Image	Method 1 Length & Depth	0.52	0.58
	Method 2 Length & Depth	0.49	0.56
	Area	0.49	0.57

*Includes Bodywall depth and Bodywall percent lean

Also apparent from these results is that length and depth measurements are slightly better than taking the rib-eye area, supporting the work done in previous studies. Additionally, equations developed from the image data were superior to those developed from the carcass data in their ability to predict lean yield. However, there are a couple of downsides to the image analysis method; for one, it is difficult to ensure each photograph is unskewed and clear (ie. the image is not blurred), and two, it is both labour intensive and time consuming in the follow up measuring stage. However, if technology (such as cameras and image analysis software) improves, and techniques become perfected, then these issues may be overcome in the future. There is promise in using this technique, and therefore further research should be conducted to evaluate the merit of such methods.

¹Measurement details

Method 1 length was measured as the greatest distance from the medial to lateral edges of the l. dorsi, and method 2 length was taken from the apex of the acorn fat to the extreme edge of the muscle. The depths were measured as the greatest distance from the dorsal to ventral edge, and the distance from apex of the acorn fat to the point of maximum depth of the muscle for methods 1 and 2, respectively. The fat depth was taken in the fourth quadrant of the rib-eye at the point of least fat coverage, and the bodywall depth was measured at a point 4.5 cm from the lateral edge of the l. dorsi

References

Bergen, R. D., Crews, D. H., Jr., Miller, S. P. and McKinnon, J. J. 2003. Predicting lean meat yield in beef cattle using ultrasonic muscle depth and width measurements. Can. J. Anim. Sci. 83: 429-434.

Crews, Jr., D. H. 2001. Alternative ultrasound predictors of beef carcass longissimus muscle area. Prof. Anim. Sci. 17: 303-308.

Ontario Cattlemen's Association

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