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somewhat. This can also be seen from the
difference in improvement of energetic effi-
ciency in the dairy cattle example described pre-
viously (Fig. 2.3), where the improvement
calculated with the Dutch system (correction
1.8% per multiple of maintenance) was smaller
(namely 16%) than calculated with the UK Feed
into Milk system (namely 19%).
The dilution of maintenance requirements
in terms of metabolizable protein is less pro-
nounced than that for energy (Fig. 2.4) for two
main reasons. First, the required amount of pro-
tein for maintenance purposes relative to require-
ment per unit milk is generally relatively small,
although this depends on what metabolic pro-
cesses are actually included in 'maintenance'.
In the Dutch metabolizable protein system for
example, at 2.5 kg FPCM day −1 , the protein main-
tenance requirement is half that of the total
metabolizable protein requirement, whereas in
the net energy system the maintenance require-
ment for energy is half that of the total require-
ment at some 11 kg FPCM day −1 . Thus, dilution
of maintenance in this protein evaluation sys-
tem provides far less opportunity to improve the
efficiency of conversion of metabolizable pro-
tein into milk protein compared with metaboliz-
able energy. Second, the marginal efficiency of
conversion of metabolizable protein into milk
protein rapidly decreases at high production
levels. The major reason for this decline in mar-
ginal efficiency is the significant intermediary
metabolism of amino acids between the duode-
num and the mammary gland, in particular
metabolism by gut and liver tissues (Lapierre
et al ., 2006).
Although increased production levels
coincide with improved FCE, high production
levels may negatively affect animal health or
fertility, which in themselves may reduce FCE.
The relationship between productivity and ani-
mal health or fertility is subject of much debate.
For example, in their review, Ingvartsen et al .
(2003) showed unfavourable genetic correlation
between milk yield and incidence of mastitis,
lameness and ketosis. However, there was no
phenotypical relationship between milk yield
and the risk of ketosis and lameness. Ouweltjes
et al . (2007) performed a detailed experiment
with heifers of low or high genetic merit, milked
two or three times a day, and fed diets low or high
in energy. They concluded that satisfactory
udder health is possible with high milk produc-
tion and that high production is not a major risk
factor for udder health in the first lactation.
From the same experiment, Beerda et al . (2007)
concluded that signs for severe negative energy
and protein balances and low body condition
scores, all of which may be indicative of health
risks, were not concentrated in the highest pro-
ducing cows. Low-energy feed and extra milking
did have an unfavourable effect on energy and
protein balances, which emphasizes the possible
effect of mismanagement on animal health
risks. It is important to stress that it is difficult
clearly to establish relationships between pro-
duction and health, since there are substantial
problems with confounding effects and unac-
counted for biological correlations (Ingvartsen
et al ., 2003). Overall, high production levels may
not necessarily result in increased health and
fertility problems, and consequently less than
expected improvements in FCE, provided high-
producing animals are adequately managed.
Efficiency on a Human-edible
Food Basis
Increased animal productivity reduces the
amount of feed and the amount of waste material
(manure, CH 4 ) produced per unit of animal
product. Much of the increase in livestock pro-
duction is occurring in intensive systems (Gerber
et al ., 2010), in part using feed produced on
arable lands that could be growing food crops.
Some 30-40% of cereals grown globally are
used as feed for livestock (Godfray et al ., 2010).
On a human-edible food basis, this may be con-
sidered inefficient. In the previous section, it was
shown that much of the food energy in plant
biomass is lost when it passes through animals.
The number of people fed per hectare of crop-
land declines when grain is diverted through
livestock into human edible food (Gill et al .,
2010; Godfray et al ., 2010). Unfortunately,
cereal grains saved by reducing consumption of
milk or beef does not necessarily all become
available to people who do not have sufficient
food (Stokstad, 2010), and neither would this
reduction in grain used for livestock automati-
cally result in more plant protein being grown
(FAO, 2009). Yet given the growing world
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