22-Agric-A1 Applied Plant, Animal or Human Physiology · May 2017
Question 5 of 6: Metabolizable-Energy Partition in a Growing Pig
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
Paper format. 04-Agric-A1 Animal or Human Physiology, National
Exams May 2017 — a three-hour closed-book examination; one of two
approved calculator models (Casio or Sharp) is permitted. The rubric states that five
(5) questions constitute a complete exam paper and that the first five questions
appearing in the answer book are marked (worth 20 marks each, 100 marks total); all six (6)
printed questions are worked here as a complete study resource.
Reference texts. M.K. Yousef (ed.), Stress Physiology in
Livestock, Vol. I — Basic Principles, CRC Press (thermoregulation, thermoneutral
zone, endotherm/ectotherm physiology, external stressors); J.A. DeShazer (ed.) and ASABE
Standards (American Society of Agricultural and Biological Engineers), Livestock Energetics
and Thermal Environmental Management (sensible heat production, metabolic body-size
scaling, animal housing design); P. McDonald et al., Animal Nutrition, 7th ed.
(gross/digestible/metabolizable/net energy, feed-energy partition); K. Schmidt-Nielsen,
Animal Physiology: Adaptation and Environment, 5th ed. (Bergmann's rule, comparative
thermal biology, calorimetry); R.L. Curtis, Environmental Management in Animal
Agriculture, Iowa State University Press (animal housing microclimate).
Question 5: Metabolizable-Energy Partition in a Growing Pig (20 marks)
Energetic efficiency, protein / lipid / NFVC deposition
0.5 / 0.9 / 0.5
Find. The metabolizable energy used for growth, MEg, and the
metabolizable energy used for maintenance, MEm.
Approach. For each of the three growth components, convert the fresh-tissue
growth rate to a dry-matter mass, multiply by that tissue's energy content to get the energy
actually deposited, then divide by the component's energetic efficiency to get the
metabolizable energy the pig had to spend to deposit it; sum the three components to
get MEg, then take the remainder of the ME intake as MEm.
Energy deposited in fat. Convert the fresh growth rate to dry matter, then
to deposited energy at the lipid energy content:
$$m_{DM,fat} = 0.087\ \text{kg/day} \times 0.90 = 0.0783\ \text{kg DM/day}$$
$$E_{dep,fat} = 0.0783 \times 39.6 = \boxed{3.1007\ \text{MJ/day}}$$
Energy deposited in lean tissue. Same two-step conversion, using the lean
tissue's DM fraction and the protein energy content:
$$m_{DM,lean} = 0.303 \times 0.22 = 0.06666\ \text{kg DM/day}$$
$$E_{dep,lean} = 0.06666 \times 23.7 = \boxed{1.5798\ \text{MJ/day}}$$
Energy deposited in NFVC. NFVC uses the same energy content as protein
(23.7 MJ/kg DM) but its own DM fraction:
$$m_{DM,nfvc} = 0.107 \times 0.22 = 0.02354\ \text{kg DM/day}$$
$$E_{dep,nfvc} = 0.02354 \times 23.7 = \boxed{0.5579\ \text{MJ/day}}$$
Substituting the numbers, the fat component deposits by far the most energy per day even
though its fresh-mass growth rate is the smallest of the three, because it is both the most
energy-dense tissue (39.6 vs 23.7 MJ/kg DM) and the driest (90% DM, so almost all of the fresh
mass counts).
ME spent on each growth component. Deposited energy is not the same as ME
spent — each pathway converts ME to deposited tissue energy at its own efficiency, so
divide back out:
$$\text{ME}_{fat} = \frac{3.1007}{0.9} = 3.4452\ \text{MJ/day}, \quad
\text{ME}_{lean} = \frac{1.5798}{0.5} = 3.1597\ \text{MJ/day}, \quad
\text{ME}_{nfvc} = \frac{0.5579}{0.5} = 1.1158\ \text{MJ/day}$$
Notice fat deposition is the most efficient use of ME (0.9) even though it is not the
most energy-dense pathway in absolute deposited terms once efficiency is folded in relative to
lean tissue's lower (0.5) efficiency.
Total ME for growth and for maintenance. Sum the three growth components,
then take maintenance as whatever is left of the 18 MJ/day intake:
$$\text{ME}_g = 3.4452 + 3.1597 + 1.1158 = \boxed{7.72\ \text{MJ/day}}$$
$$\text{ME}_m = \text{ME} - \text{ME}_g = 18 - 7.72 = \boxed{10.28\ \text{MJ/day}}$$
Maintenance therefore claims the larger share (about 57%) of this pig's daily energy intake,
which is the usual pattern for a growing pig well short of its maximum growth rate.
