Suckler cow management systems generally require cows to
be dependent on their body reserves of energy and protein at some
stage of the production cycle. In nutritional studies with suckler
cows it is therefore important that the rate of use or replenishment
of body reserves be considered with contemporary nutrition. This
requires the quantification of body reserves and hence a means of
measuring body composition in the live animal.
The methods available for the estimation of in vivo body
composition in animals are reviewed and a number of techniques
(live weight, skeletal size, total body water as estimated by
deuterium oxide dilution, blood and red cell volumes as estimated
by Evans Blue dilution, ultrasonic measurement of subcutaneous
fat depth and eye-muscle area, and body condition scoring) were
examined using 73 non-pregnant, non-lactating cows of five genotypes
(Hereford x Friesian, Blue-Grey, Galloway, Luing and British
Friesian) ranging in body condition score from 0.75-4.5. Direct
measurement of body composition in terms of water, fat, protein
and ash were made following slaughter.
Live weight, deuterium oxide dilution, ultrasonic measurement
of subcutaneous fat depth and eye-muscle area, and body condition
scoring were all considered to be potentially useful predictors
of body composition, but a combination of techniques offered a
better prediction than did any single index. Using a combination
of techniques it was possible to predict body fat and protein with
residual standard deviations of 13.1 kg and 3.15 kg respectively.
Body composition changes were also examined, and it was
calculated that the composition of empty body-weight change was
dependent upon empty body weight, containing more fat and less
water, protein and ash at higher empty body weights.
Important breed differences were found in the partition of fat
among the main adipose tissue depots, with the Friesian cows having
a greater proportion of fat in the internal depots and a lower
proportion in the subcutaneous depot. The implications of breed
differences in fat partition are discussed in relation to in vivo
body composition measurement.
An ancillary study was carried out into the effects of body
condition on maintenance requirements and on the use of blood
metabolites to measure energy status in suckler cows. This
indicated that body condition affected maintenance requirements to
the extent that at 500 kg live weight, maintenance requirements
were 8 MJ ME/day less for each unit increase in condition score.
Plasma free fatty acids were shown to be particularly useful in
assessing energy status in cows, but 3-hydroxybutyrate concentrations
were of little value in non-pregnant, non-lactating animals.
Finally, the conclusions of the two studies are discussed in
relation to areas of study likely to prove useful in the development
of efficient systems of suckled calf production.