LCA Food 2012 in Saint Malo, France! - Manifestations et colloques ...

PARALLEL SESSION 3C: SHEEP AND DAIRY PRODUCTION SYSTEMS 8 th Int. Conference on LCA in the
306
n
i1
c
ij
X
i
0,
j 1...
p
Agri-Food Sector, 1-4 Oct 2012
where cij is the supply or demand of j by activity i, which describes the relationship between enterprises.
Demands are negative and supplies are positive and total supply must equal total demand. For example,
purebred lowland flocks produce rams, which are, in turn, demanded as terminal sires by lowland finishing
flocks. The total amount of material k flowing into the system is:
M
k
n
Eq. 2
mik
X i,
k 1...
q
i
1
Eq. 3
where mik is the flow of material k into activity i. The life cycle inventory (LCI) for the system is the total of
each burden l
B
l
p
k1
M
k
b
k l
, l 1...
r
where bkl is the amount of burden l produced by the use or disposal of material k and Mk is the total amount
of material. The LCI identifies the contribution of each material
Bkl M kbkl
Eq. 5
or activity
B
il
X
2.3 Feed demands
q
i
k
Eq. 4
mikbkl
1 Eq. 6
The main demand for feed energy is met by grazing, with relatively small contributions from conserved forage
or concentrates. The energy demand of a ewe was calculated from the AFRC feeding system (Alderman
and Cottrill, 1993), much of which is derived from ARC (1980). These include terms for maintenance (a
function of body weight, wool production and activity), pregnancy (a function of lamb weight) and lactation
until weaning, a function of lamb size and daily liveweight gain (DLWG). The metabolisable energy demand
is calculated per ewe each year and includes the energy needs for lambs until they leave the system.
Energy demand is converted to a demand for grazing dry matter intake (DMI) by allowing for the management
choice of the quantitative supply of concentrates and conserved forage to lambs and ewes and thus
obtaining the grazing energy demand by difference. This is converted to DMI from the energy density of
grazed pasture. The combination of grazing, conserved forage and concentrates normally meets sheep protein
requirements when energy demand is met. So, these are not explicitly calculated separately.
2.4 Grazing land requirements
A grass sub-model was developed to calculate the area of grass required together with the managerial inputs
needed. Grass yield was modelled using the grass site class system (Brockman and Gwynn, 1988). Site
classes range from 1 to 7, with 7 being least productive. The main determinants of site class are summer
rainfall (low rainfall restricts production) and soil texture (lighter soils drain more quickly and have lower
yield potentials). An allowance is also made for reduced productivity at higher altitudes. Effects of site class
and N application rate on grass yield are shown in Figure 1.
In a grazing system, N may be supplied applied fertiliser, fixation by clover, atmospheric deposition and by
the animals’ excreta. This causes the organic matter to build up and cycle round the system to become available
to both the pasture and loss processes to air and water. The resulting system can be described by a system
of equations which can be solved for a steady state.