THE MEASUREMENT OF GROWTH
At an intermediate stage of measuring the endomysium connective tissue framework of meat with an image analysis system.
Measuring the growth of meat animals is a lot more difficult than measuring
the growth of people. You can tell a people to stand still on the weigh-
scale, you can ask them not to come to the measuring session plastered with
several kilograms of mud, you can see their degree of subcutaneous fat cover
and muscle bulging, and people normally do not fill their stomachs with fodder
or their bedding material at every opportunity. Thus, it is difficult to
assess the growth of meat animals. Of critical importance for the butcher, the
live weight of an animal tells us little or nothing about the amount of fat it
has, and a fair amount of groping is required to make even a rough estimate of
the contribution of body fat to live weight.
METHODS, PROS & CONS
CAT and NMR
Medical methods for the non-destructive analysis of body composition, such
as computer-assisted tomography (CAT) and nuclear magnetic resonance (NMR)
imaging, can be used to measure the growth of meat animals, but their
extremely high cost and complexity restrict these methods to research. They
would be extremely difficult to apply routinely in commercial agriculture and
could not be cost effective.
Ultrasonic imaging is widely used for humans, and it is far, far less
expensive than CAT or NMR. Ultrasonic measurements of fat and muscle
thickness generally are quite reliable and are used routinely in comemrcial
- High frequncy sound waves are generated by a transducer on the animal's
back and, since the velocity of the waves though the tissue is known,
the time taken for echoes to return from tissue boundary layers may be
used to determine their depth.
- Strong echoes are returned from the muscle-fat boundary over the
longissimus dorsi muscle, but other echoes are returned from connective
tissue septa in the subcutaneous adipose tissue and from the skeleton.
- Depth measurements may be assembled into a map of the carcass section by
moving the transducer in an arc over the animal's back.
- An alternative ultrasonic method is to measure changes in the velocity
of the signal as it passes through the tissue, since the proportion of
adipose tissue is correlated with the reciprocal of velocity.
The relative development of commercially valuable carcass components such
as the loin and hindquarter is difficult to assess subjectively in the live
animal, but may be done by photogrammetry. This utilises the parallax effect
of a stereo photograph compounded from two photographs taken simultaneously by
two cameras separated by a known distance. The volume of the animal body or
anatomically defined parts of the body may be calculated from the degree of
the stereo effect. Although this method may be accurate, its cost and
complexity have so far limited its use.
The relative development of the carcass also may be studied using specific
gravity. Instead of being immersed in water by the classical method of
Archimedes, the air displacement technique uses negative air pressures around
an animal in an experimental chamber. The accumulation of fat (low density)
decreases the overall density of the body so that density then provides an
index of fatness. In practice, however, it is difficult to obtain accurate
volumetric measurements that allow for the offsetting effects of bones. In
cattle, bone density increases with age and varies between sexes and breeds.
An alternative method for adipose tissue volume is based on the fact that
adipose tissue has a very low water content. Chemicals such as antipyrin or
one of its derivates are injected into the vascular system from whence they
diffuse into the total volume of aqueous body fluids. Provided that the
injected substance is only slowly metabolized or excreted by the body, and
provided that it becomes adequately dispersed, the extent to which the
injected substance becomes diluted indicates the total diluting volume. In
general, fat animals have a proportionately smaller aqueous volume than lean
Another indirect method of estimating body composition is based on the fact
that nearly all the potassium in the body is intracellular. Since adipose
cells contain only a trace of potassium in their scanty amount of cytoplasm,
the total amount of potassium in the body is approximately proportional to the
lean body mass.
Radioactivity also may be employed in a similar manner using isotopes of
hydrogen (deuterium or tritium) to estimate the water volume and, indirectly,
the fat content of a live animal. The advantage of deuterium is that it is not
radioactive, while the advantage of tritium is that it may be measured rapidly
by liquid scintillation.
- A constant proportion (0.012%) of potassium atoms are radioactive (the
isotope 40K), and they emmit gamma rays that may be converted into small
flashes of light and counted electronically.
- If an animal is shielded from environmental gamma rays, its total gamma
emission gives an index of its total muscle mass. A background count of
the extrinsic radiation that penetrates the shield is subtracted from
the count made during the three minutes when the animal is being
- Animals are taken off their feed and water for 24 hours prior to being
measured to avoid complications from potassium in the rumen, but
different types of animals require different calibration factors from
which to predict their muscle mass. Individual muscles of the carcass
also differ in their concentration of potassium.
The volume of electrolytes in the animal body is estimated from the
disturbance that their electrical conductivity creates in an
externally-applied electromagnetic field. However, this system is adversely
affected by any variation of the animal's position and orientation within the
electromagnetic field and, consequently, it is not accurate enough for live
animals. However, it works very well for boxed beef and carcasses that can be
neatly lined up on a conveyer to pass through the machine - but that's another
Serial slaughter experiments
After an animal has been slaughtered its composition may be determined very
accurately. Thus, a group of animals can be slaughtered one at a time
throughout their growth to assemble a pseudo-growth curve. The major problem
is the degree of variation between individual animals, the high cost involved,
and the fact that all the animals are dead when the experiment is finished.
The method can be used with the off-spring of expensive breeding stock
(progeny testing), but all this takes a long time and is one step removed from
the animal of interest - usually a prize bull or boar.