Agriculture Reference
In-Depth Information
animal at a given point in time as the sum of
the requirements for maintenance and pro-
duction. When applied to pig populations,
however, the requirements for a nutrient
should rather be defined as the amount
needed for specified production purposes
such as optimal growth rate, protein depos-
ition, feed efficiency, etc. (Hauschild
et al
.,
2010). That is, the concept of nutrient re-
quirements when applied to populations
should be considered in the context of nutri-
ents provided to heterogeneous populations
over long periods of time (Ferguson
et al
.,
1997; Knap, 2000; Leclercq and Beaumont,
2000; Pomar
et al
., 2003; Vautier
et al
.,
2013). An individual animal's response to
dietary nutrient levels may differ in magni-
tude and pattern from the response of a
population (Pomar
et al
., 2003) and, as indi-
cated before, population nutrient require-
ments should be seen as the desired balance
between the proportion of pigs that are going
to be overfed and underfed acknowledging
that this proportion will change over time.
In practice, there are two methods used
to estimate the nutrient requirements of do-
mestic growing animals: the empirical and
the factorial methods (Patience
et al
., 1995).
In the empirical method, nutrient require-
ments are estimated by feeding groups of pigs
with increasing levels of the nutrient under
evaluation and measuring one or several sets
of performance parameters (e.g. growth rate)
at given time intervals. In this empirical
method, the nutrient level at which the opti-
mal population response is observed within
a given growing period is identified as the
population requirement for this nutrient and
for this growing interval. This population re-
sponse may be biological, technical, economic
and/or environmental in nature (Jean dit
Bailleul
et al
., 2000), but different response
criteria may also suggest different nutrient
requirement estimations (Baker, 1986). For
example, Hauschild
et al
. (2010) found that
by simulating the growth of a population of
growing-finishing pigs optimal lysine (Lys)
to net energy (NE) ratio (Lys/NE ratio) for
average daily gain (ADG) was 9%, 6% and
3% higher than the optimal Lys/NE ratio for
feed conversion ratio (FCR), respectively, in
the three feeding phases simulated between
25 kg and 105 kg of live body weight (BW).
In fact, feed intake and daily gain (DG)
evolved differently in response to changes
in Lys/NE, thus explaining why FCR and
ADG do not necessarily reach the same Lys/
NE optimal value. The results of Hauschild
et al
. (2010) indicate that the amount of Lys
required for optimal FCR of a given popula-
tion can be lower than the amount of Lys re-
quired for maximal ADG, in agreement with
other studies (O'Connell
et al
., 2005; Main
et al
., 2008). Besides the diversity of the re-
sponses of the pigs raised in groups, the stat-
istical model used to establish this popula-
tion optimal response with the empirical
method should be considered. The linear-
plateau model is frequently the preferred
model for representing the responses of ani-
mals to graded levels of limiting nutrients
(Baker, 1986; Hauschild
et al
., 2010). Although
this model may provide adequate statistical
it, it tends to underestimate optimal nutri-
ent requirement levels since it does not take
into account the physiological differences
that exist between the individuals in a popu-
lation (Remmenga
et al
., 1997). In this re-
spect, the model may not be suitable because
it does not consider the curvilinear nature of
the response of a population to graded levels
of a limiting nutrient (Pomar
et al
., 2003;
Wellock
et al
., 2004). A curvilinear-plateau
model has been recommended for describ-
ing the curvilinear nature of the responses
of heterogeneous populations (Baker
et al
.,
2002; Pomar
et al
., 2003; Simongiovanni
et al
., 2011). In this type of model, the opti-
mum nutrient level is attributed to the inter-
section between the curvilinear function and
the plateau. From that point onward, in-
creases in the ingestion of the limiting nutrient
are assumed not to have any effect on popu-
lation responses. Furthermore, maximal ADG
or minimal FCR may not necessarily result
in maximum economic return. This is due
to the fact that population responses to in-
creasing levels of limiting nutrients (i.e. Lys)
progressively decline as the limiting nutri-
ent approaches the plateau level. Because Lys-
or protein-rich diets are more expensive
than low-Lys or low-protein diets, marginal
economic returns can be expected to de-
crease faster than Lys marginal efficiency
