Beef Cattle Ultrasound Measurements

3.7 Ultrasound measurements

Introduction

Real time ultrasound imaging equipment to record carcass characteristics in live animals for livestock improvement programs has been in use for more than two decades. Its usefulness in beef cattle has been well demonstrated e.g. Brethour (1994), Wilson et al. (1998).

Ultrasound scanning has been used since the late 1980’s in many beef cattle breeding programs to overcome the inherent difficulty of recording carcass data from progeny tests under extensive production systems and in performance test situations where access to carcass information is not possible. A number of genetic evaluation programs have now included scan data in their routine analysis.

Practical Application of ultrasound imaging

The application of ultrasound is highly technical and requires:

1. The use of sophisticated equipment.
2. Strict adherence to proper equipment calibration.
3. Proper animal preparation.
4. Adherence to a standard scanning protocol.
5. Adherence to a standard image interpretation protocol.
6. Suitable animal handling facilities.

Animals to be scanned

Scanning for genetic evaluation

It is important for genetic evaluation that animals are allowed to express their inherent genetic potential. As fat measurements are directly related to the nutritional state of the animals it is essential to record only groups of animals which are on a reasonable level of nutrition. Otherwise too many animals will be recorded with minimum fat levels and no intramuscular fat thus generating information of little value since the true genetic potential will not have been expressed. Such data is useless for genetic evaluation where the intention is to identify genetic differences.

As ultrasound measurements are used to provide an insight into a number of carcass characteristics and to a limited extent into meat quality, the most valuable records will come from young animals undergoing selection for breeding and on which no direct carcass information can be collected. Yearling bulls and yearling heifers are the most obvious animals to scan. In many commercial production systems a progeny test through steers or bulls is also possible.

In summary, scanning can provide useful information for the estimation of carcass EBVs or EPDs using records from

1. Yearling bulls.
2. Yearling heifers.
3. Groups of progeny fed for slaughter.

The most common age window for young breeding stock is between 320 to 500 days. It may vary depending on production system. The development of body composition EBVs or EPDs requires that scanned animals be associated with a well-defined contemporary group.

For animals scanned on the farm of birth a contemporary group is comprised of all animals of the same sex that are reared and managed together. A 60 days birth window is recommended. Where herd sizes are small and calving season extended the contemporary group may cover a longer birth season window. A typical contemporary group definition would include herd code, birth season, weaning group (date, location, and management), operator (if scanned by more than one operator) and scanning group (date, location, and management). For animals scanned at a central station test, the contemporary group should include animals from the same sex born within 60-90 days age

window and the same test end. The herd of origin and other birth and weaning group information may also be included. The practise of harvesting/slaughtering animals from groups when they reach market target weights reduces the management group size as records from animals slaughtered on different days and in particular in different abattoirs should not be directly compared. Scanning for carcass traits of all animals prior to the first selection of any animals to be slaughtered will provide a basis for direct comparison of all animals in the group.

Scanning of slaughter animals

Real time ultrasound scanning for subcutaneous fat can also be used to determine market suitability of commercial slaughter animals. However, scanning of animals that have reached target market specifications should not be compared with the use of the same technology for performance recording purposes.

Special care must be taken to avoid any bias in the mean of the observations. Such a bias could have severe financial implication if animals are slaughtered and found to be outside market specifications. For the purpose of genetic evaluation a consistent bias will be part of the management group effect and will not affect the accuracy of genetic evaluation.

Technical requirements

Recording device

A number of real time ultrasound recording devices are on the market. Most of them have been developed for human health or veterinary purposes (e.g. pregnancy testing). The small transducer used for medical purpose is of limited use for scanning of carcass characteristics and so special transducers are required when scanning for carcass traits.

For a list of scanning devices used in animal recording see Appendix 1 following. Ultrasound equipment is undergoing continuous improvement resulting in smaller and more sophisticated models being developed and marketed on an ongoing basis.

Facilities

Efficient ultrasound scanning of large groups of animals requires well designed yards, races and chutes to hold the animals in a stress free and secure manner and release them as soon as all necessary information has been recorded. The operator should insure that the cattle handling facilities for scanning are adequate in respect of health and safety considerations before he commences scanning. A squeeze chute with fold-down side panels is best for scanning beef cattle.

A shaded area is required to allow the operator a good view of the monitor, as direct sunlight will make it difficult to see the images on the screen. Therefore, the chute should be located under a roof that can block direct sunlight and provide protection from rain or other inclement weather conditions. A clean and grounded power signal is required at the chute-side. It is best if the electrical circuit is a dedicated line to the chute, free from the interference of other electrical equipment such as motors etc.

Most ultrasound equipment does not operate efficiently and accurately when the ambient air temperature falls below 8 degrees Celsius or 45 degrees Fahrenheit. The breeder should make provisions to keep the facility heated in these situations. The operator should provide some portable supplemental heating systems to keep the ultrasound equipment warm if required.

Preparation of the animal

Animals should be cleaned and clipped particularly in winter or early spring when their hair is too long to get quality images. The requirement for clipping is even higher if scanning is used to determine intramuscular fat % (IMF%) as the lack of complete contact between the ultrasound transducer and the animal’s skin can have a direct effect on the predicted IMF%. In general the length of hair coat should be no more than 1,5 cm or 1/2 inches. Prior to scanning a liquid, commonly vegetable oil, should to be applied to the scan site to provide maximum contact between transducer and skin. The temperature of the oil applied to the skin should be above 20 degrees Celsius or 68 degrees Fahrenheit for best results. This might require a warm water bath for the bottle containing the oil during times of lower temperatures. Wet animals can be scanned successfully as the water can easily be removed from the scan area.

For the scanning of eye muscle area a curved guide or offset made from super-flap will help and will allow a curved image to be recorded without the need to apply excessive pressure to maintain good contact as this would result in distortions of the muscle or fat measurements resulting.

Recorded Traits

Real Time ultrasound imaging has so far been used for the measurement of subcutaneous fat cover as well as for Eye Muscle Area and Muscle Depth and the Intramuscular Fat Percent in the longissimus dorsi. The appropriate areas of interest are shown in Figure 3.3.

Figure 3.3. Areas of interest for ultrasound evaluation of carcase characteristics. where:

• A – Rump fat image
• B – Cross-sectional image for ribeye area/depth and 12th-13th rib fat thickness
• C – Longitudinal image for intramuscular fat

Rump fat thickness

Rump fat thickness or P8 scan is an indicator of fatness and can be used to improve the overall accuracy of external fat measurements. This measurement can be particularly beneficial when scanning leaner animals such as yearling bulls.

For measurements, the ultrasound transducer is aligned directly between the hook- and pin bones without a standoff guide to collect this image. Rump fat thickness is measured at the apex of the biceps femoris muscle. The site is located at the perpendicular intersection of the line from the high bone (third sacral vertebra) with a line from the inside of the pin bone (Tuber ischii ) (see Appendix 2, Figure 2 and 3). Rump fat thickness should be reported to the nearest millimeter or 1/25 of an inch. Operators may report to a higher degree of accuracy if desired.

Rib fat thickness

The selection of the site for rib fat and eye muscle depth or area may coincide with the traditional quartering site of beef carcasses in the country. In general the records on different sites are genetically highly correlated, however they might show different variation and are more or less easy to record as different muscles might interfere.

A common site assessed in a number of countries (e.g. Australia, Canada, New Zealand, US) is located ¾ of the distance from the medial to the dorsal end of the longissimus dorsi at a lateral point between the 12th and 13th rib. Rib fat thickness will be reported to the nearest millimeter or 1/25 of an inch. As with Rump fat thickness recordings may be reported to a higher degree of accuracy. Rib and rump fat thickness are well correlated (genetic correlation exceeding 0.70) with rib fat commonly having a lower mean. However, interactions between breed, management system and environment exist.

Eye Muscle Area (EMA)/ Eye Muscle Depth

Carcass ribeye usually is measured between the 12th -13th ribs of the ribbed carcass. The ultrasound ribeye measurement commonly is made from the same image used to measure 12th-13th rib fat thickness.

Eye Muscle Area/Eye Muscle Depth is measured as the cross sectional area of the longissimus dorsi muscle. Care should be taken not to include other muscles that occur at this site. Similarly, the image should be taken between the ribs not over a rib as the latter will cause distortion.

The presence of well-defined intercostal muscles under the Longissimus dorsi is an indication that the transducer is properly aligned directly between the 12th and 13th rib for this measurement.

Intramuscular fat percent (IMF%)

Intramuscular fat percent or marbling is an important meat quality characteristic in certain high-priced markets, because consumer equate it with outstanding eating quality. The carcase benchmark for intramuscular fat is the chemical extraction of all fat from a meat sample taken as a slice off the longissimus dorsi. Most analytical software for IMF% use a longitudinal image in the region of the 11th, 12th and 13th ribs approximately 2/3rds of the distance from the medial to the dorsal end of the longissimus dorsi.

In experiments, it has been demonstrated that the correlation between a longitudinal sample and a cross sectional sample is very high. Research has shown that variation between images on the same side is larger than variation within an image selecting different but overlapping areas for the analysis.

The IMF% trait is the most difficult of all ultrasound traits to measure accurately. Equipment calibration, animal preparation, electrical power signal noise, existence of atmospheric radio waves, and transducer-animal contact are just some of the factors that can influence the measurement accuracy. Therefore, it is strongly recommended that the IMF% result be reported as the average of at least three images and even better, the average of five images to increase the accuracy.

Most machines do not provide a direct measure of IMF% and thus there is a requirement for specialised PC software. An image frame from the ultrasound scan is digitised and analysed on a computer. Such analysis software is normally designed specifically for a particular ultrasound machine (Hassen et al., 2001).

Scanning weight

The scanning weight of each animal should be measured within +/- 7 days of the scanning date.

Recorded data

Recorded data should comprise as a minimum:

1. Identification of the operator.
2. Type of scanner used.
3. Scanning date.
4. Farm/Herd identification.
5. Animal number.
6. Trait definition.
7. Actual recorded measurement.
8. Unit of measurement.

Qualification of the operator

Image Interpretation

The accurate interpretation of real-time ultrasound images for fat thickness, eye muscle area and IMF% requires a high degree of skill. A number of training programs are currently recognised within the beef cattle industry. Ultrasound scanning operators should participate in and satisfactorily complete such a course in ultrasound methodology before undertaking scanning activities.

Certification of commercially operating operators

To guarantee high quality data for genetic evaluation and research purposes Real Time Ultrasound Scanners should be regularly tested for their proficiency (e.g. annually). Successful completion of such proficiency tests can be made a prerequisite for the acceptance of data into national genetic evaluation systems by those organisations, which control access and input to beef cattle data bases (e.g. recording organisations or breed societies).

Training and Testing Protocol

Test design

Attempts should be made to select a group of about 30 animals with a range of values for the traits of interest, namely fat depth, eye muscle area, muscle depth and intramuscular fat. All animals should be clipped with some oil applied to the measurement site prior to the test.

As each operator will measure the animals twice, all animals should be tagged with numbers (best on their backs) and these numbers have to be changed between runs.

All operators should have a scanning station to themselves and will be allowed fixed time (e.g. 6 minutes per animal) to complete all measurements on the animals. All crushes should be sequentially aligned so that any time delay by one operator will delay the whole team. Note no two machines should take power from the same power plug to avoid interference between machines, which can particularly influence the prediction of intramuscular fat.

Testing protocol

Official sheets should be provided to record measurements. Sheets should be customised for operators with different machines. No other recording is to be permitted. These sheets are to be collected at the end of each run with at least fat depths recorded. They will be photocopied and returned to those who need them to submit eye muscle area, muscle depth or intramuscular fat. Other measurement, e.g. eye muscle area should be submitted within 48 hours of completing the test. Operators recording images for eye muscle areas will be required to submit tapes when submitting the EMA records. Operators who wish to submit EMA measurements on the spot can do so.

Intramuscular fat measurements will be submitted within 48 hours of completing the test.

Test animals should be killed between 24 and 48 hours after the completion of the test and after allowing for a settling down period to overcome any stress related downgrading of the carcasses.

Carcass data should be recorded by at least two experienced staff independently to allow for measurement error correction. It has to be remembered that recording of carcass data in the chiller is not error free and also requires skills. Care must be taken to identify carcasses whose physical attributes have been changed through the slaughter process e.g. commonly used hide pullers can remove some of the subcutaneous fat on rump or rib. Tightly packed carcasses can deform and reduce muscle area. Left or right-handed quartering of carcasses can affect the surface area and may bias the results for the eye muscle measurement.

Criteria for certification

The criteria for a pass in the proficiency test has to be established. The standards established by the Performance Beef Breeders Association (PBBA) in Australia, Table 1 and the Beef Improvement Federation in the USA, Table 2 are presented as examples. These criteria may be adjusted if the mean and standard deviation of the carcass traits are found to be different to the values in the test that were used to establish these criteria. There does not need to be a requirement to achieve a minimum bias. As bias affects all animals in a similar way it is an effect confounded with the management group of the animal. However, note that a comparison of scanned records and real carcass records which reveal large biases will undermine the confidence of breeders in the technique.

Mean and standard deviations between animals and between carcass graders have to be recorded to monitor quality recording of carcass data and a consistent variation between the test animals.

A number of different statistics should be calculated to show the proficiency of the scanner.

1. Standard deviation of the difference between first and second scans of the same animals together with the correlation. For this, animals don’t have to be slaughtered and this statistic can be used to evaluate scanners during a training phase. Only scanners reaching minimum standards here, that means those that are consistent in what they are measuring will be allowed to attempt the expensive accreditation involving carcase data.
2. Standard deviation of difference between scan results and mean carcase value and the correlation between scan and carcase results.
3. The bias between scan and carcase measurement.

Table 3.3. Recommended standards for Proficiency Testing of Real Time Ultrasound Assessment of Live Cattle used in Australia.

Rib Fat Thickness (12/13th rib)
Maximum Standard error of repeatability	1.0 mm	0.04 inches
Maximum Standard error of measurement (prediction)	1.0 mm	0.04 inches
Correlation with carcase measurement	0.9	0.9
Rump Fat Thickness (12/13th rib)
Maximum Standard error of repeatability	1.5 mm	0.06 inches
Maximum Standard error of measurement (prediction)	1.5 mm	0.06 inches
Correlation with carcase measurement	0.9	0.9
Eye Muscle Area (EMA
Maximum Standard error of repeatability	6.0 cm2	0.90 inches2
Maximum Standard error of measurement (prediction)	5.5 cm2	0.80 inches2
Correlation with carcase measurement	0.8	0.8
Intramuscular fat percent (IMF%)
Maximum Standard error of repeatability	1.0 %	1.0 %
Maximum Standard error of measurement (prediction)	0.9 %	0.9 %
Correlation with carcase measurement	0.75	0.75

Table 3.4. Guidelines on the minimum requirements for operators as set by the Beef Improvement Federation of the United States of America.

Trait	Standard error of prediction		Bias
Fat thickness	< 0.10	< 0.10	< 0.10
Ribeye area	< 1.20	< 1.20	< 1.20
% IMF	< 1.20	< 1.10	< 0.70

Alternative statistical methods, like goodness to fit, can also be considered when the proficiency of operators (scanners) are evaluated.

Supervision of the operator

The responsible breeding organisation should establish a routine supervision procedure for the operator. The competency of all operators should be monitored and training should be provided at regular intervals.

Ultrasound scanner

Table 3.5. Ultrasound scanners used in Beef cattle performance recording

Model	Manufacturer		Comments
SSD 210 DX II	Aloka	Kansas State	Requires Software for IMF
SSD 500V	Aloka	Iowa State	Requires software (Iowa State)
Pie 200 Vet	Pie	Australia, US	Includes software for IMF
Scanner 200 SLC	Tequesta	US	Requires Software for IMF

Location of P8 Site

Figure 3.4. Location of P8 Site (Rump fat thickness).

Figure 3.5. Ultrasound rump fat image with typical landmarks identified. Notice how the point of biceps femoris is near the 2/3 position of the image, and the fat lines are very defined and not blurred. Additionally, the pelvic bone is absorbing the ultrasound waves on the lower right portion of the image. The transducer is placed above a straight line between the hooks and the pins. The animal’s head is to the right side of the image, and the tail is to the left of the image.

Figure 3.6. Cross-sectional ultrasound image and outline of important landmarks @ 12-13 rib, where a carcass would be broken into quarters.

• 1 – Spinalis Dorsi
• 2 – Acorn Fat or the “Hook” of the ribeye
• 3 – Longissimus Costarum
• 4 – “Break” in the intercostals
• 5 – Intercostal muscle boundaries or “Railroad Tracks”

Figure 3.7. Longitudinal ultrasound image taken over the 13th, 12th, and 11th ribs. The first uniform layer of is the hide of the animal. The second layer is the subcutaneous fat layer. Notice also the triangular shaped section of spinalis dorsi under the fat layer above the 11th rib, and the added brightness of the image under the spinalis dorsi.