When modelling the effect of a changing nutrient supply to growing animals,
it is important to distinguish the individual response curve of an animal
from the change in this response that may occur during growth. A data analy
sis model is proposed where, for an individual animal, the relation between
protein deposition (PD) and metabolizable energy (ME) intake above mainten
ance (MEp) is curvilinear, so that PD intersects the origin and reaches ifs
maximum at the maximum protein deposition rate (PDmax). An increase of MEp
beyond that required to attain PDmax would not change PD. The MEp not used
for protein synthesis can be used for lipid deposition (LD). The relation
between PD and LD on the one hand and ME on the other hand can then be desc
ribed as a function of the maintenance energy requirement (MEm), PDmax, the
level of ME required to attain PDmax (F; as a multiple of MEm) and the ene
rgetic efficiencies of PD (k(p)) and LD (k(f)). Of these statistics, only k
(p) and k(f) were assumed to be independent of body weight (BW), age or gen
otype. Variation in PDmax was described as a Gompertz function (of age) whe
reas variation in F was assumed a linear function of BW. Maintenance energy
requirement was expressed as a power function of BW. To evaluate the model
, 145 nitrogen and energy (indirect calorimetry) balances were obtained fro
m three types of pigs (Large White castrated males (cLW) and Pietrain X Lar
ge White castrated males (cPPX) and males (bPPX)) ranging in BW between 45
and 100 kg and housed under thermoneutral conditions. Animals were allotted
to one of four energy levels ranging from 0.70 to 1.00 of ad libitum intak
e. The MEm was not different between genotypes (849 kJ/kg BW0.60) whereas t
he k(p) and k(f) were 0.56 and 0.75, respectively For castrated animals on
ad libitum intake, PDmax started limiting PD at approximately 130 days of a
ge (78 and 86 kg BW for cLW and cPPX, respectively). Before this age and fo
r bPPX, PD was limited by MEp. In bPPX, the difference between PD and PDmax
was small (less than proportionately 0.05). The F did not change with BW f
or bPPX (2.85 X MEm) whereas for the other genotypes, it decreased linearly
from 4.47 at 45 kg to 2.00 at 100 kg of BW. Due to ifs nature, the model a
llows estimation of PDmax even when energy is restricting PD.