Agriculture Reference
In-Depth Information
90
80
70
60
50
3,000
5,000 7,000 9,000 11,000
Milk production (kg FPCM year
-1
)
13,000
Dutch MP system
UK MP system
Fig. 2.4.
Efficiency of conversion of metabolizable protein (MP; g) into milk (kg fat and protein corrected
milk (FPCM) per cow per year) according to the Dutch protein evaluation system (van Duinkerken
et al
., 2011)
and the UK Feed into Milk system (Thomas, 2004).
largely constant across a wide range of ME
intakes, and there is no statistical reason to sug-
gest that feeding level affects this efficiency
(Kebreab
et al
., 2003).
Although the amount of feed required per
unit of milk or meat is reduced when milk pro-
duction levels increase, the associated higher
feed intake levels generally coincide with a
reduction in digestibility. The level of reduction
in digestibility is usually more pronounced for
structural carbohydrates compared with non-
structural carbohydrates. For example, Robinson
et al
. (1987) evaluated the effect of feed intake
level of a one-third hay, two-thirds concentrate
diet on faecal digestibility of dairy cattle. Based
on the intake and digestibility results reported,
the decline in digestion of organic matter (OM),
neutral detergent fibre (NDF) and starch was
2.9%, 6.9% and 0.1% per multiple of energy
maintenance requirement, respectively. This
may be related to the ratio between ruminal frac-
tional passage rate (
k
p
) (which is usually assumed
to increase when feed intake level is higher) and
fractional degradation rate (
k
d
). Usually, the
k
d
of
fibre is lower than that of non-structural carbo-
hydrates, and fibre requires longer retention
times to reach a certain degradability in the
rumen. When
k
p
is raised in relation to elevated
intake levels, the relative extent of degradation
(which may in simple terms be calculated as
k
d
divided by the sum of
k
d
and
k
p
) of fibre is more
affected than that of non-structural carbohy-
drates. Usually, tabulated feed values are based
on the digestibility coefficients measured with
sheep fed at maintenance level, and these coeffi-
cients are applied to cattle fed at higher intakes
of diets necessitating a correction for the level of
feed intake to be made. Feed evaluation systems
differ significantly in calculations of digestion
depression with increasing feed intake. The UK
Feed into Milk system (Thomas, 2004) does not
correct for the effect of feed intake level on
digestibility. The NRC (2001) system for dairy
cattle assumes a 4% reduction in digestibility per
multiple of maintenance. In the Dutch NE sys-
tem, a 3% reduction in digestibility per multiple
of maintenance is assumed. However, in this
system, an improvement of 2% in metabolizabi-
lity and conversion of ME to NE is assumed at the
same time, related to reduced CH
4
losses and
increased propionic acid molar proportions at
higher intake levels. At an average metaboliza-
bility of 0.57, the Dutch system thus adopts a
net correction of 1.8% per multiple of mainte-
nance. Although firm data on the actual correc-
tion for intake level are scarce and may depend
on diet composition, the digestibility is usually
adjusted for intake level. This correction reduces
the theoretical advantage of dilution of mainte-
nance requirement at increased intake levels
