Agriculture Reference
In-Depth Information
(i.e. containing all essential amino acids),
whereas vegetable sources generally lack one or
more of the essential amino acids. The nutri-
tional value of dietary proteins can be assessed
adequately by expressing the content of the first
limiting essential amino acid of a protein as a
percentage of the content of the same amino
acid in a reference pattern of essential amino
acid required by humans, corrected for true fae-
cal digestibility of the dietary protein. This scor-
ing method is the protein digestibility-corrected
amino acid score (PDCAAS) (Schaafsma, 2000).
The PDCAAS of milk is well above 100%
(although values higher than 100% are trun-
cated to 100%) and higher than many plant pro-
tein sources including peas, beans and wheat
(PDCAAS between 40% and 90%). The high
nutritive value of ruminant products needs to be
taken into account when evaluating feed
resources or excretion of waste material in vari-
ous production systems. For example, Smedman
et al
. (2010) calculated the composite nutrient
density of 21 essential nutrients in relation to
costs in greenhouse gas (GHG) emissions from a
life cycle assessment (LCA) methodology of vari-
ous beverages. The GHG emission of milk pro-
duction is relatively high, but the Nutrient
Density to Climate Impact index (NDCI) of milk
was 0.54, compared with 0.28 (orange juice)
and 0.25 (soy drink). Thus, the high nutritive
value of milk and beef positively impacts the effi-
ciency of ruminant production systems when
expressed on a human-edible basis and in this
respect livestock may add to the food supply
beyond what can be provided by crops.
enteric formation of CH
4
, being the major GHG
in cattle production systems. Complex interac-
tions occur between various GHG and their rela-
tion with other aspects of sustainability, such as
eutrophication and animal welfare. For exam-
ple, mitigation options aimed at reducing uri-
nary N excretion may result in elevated CH
4
emission levels (Dijkstra
et al
., 2011). The trade-
off between N excretion and enteric CH
4
produc-
tion needs to be understood at the animal level to
allow accurate data to be used at the whole farm
level. Detailed analyses using LCA methodolo-
gies (de Boer
et al
., 2011), which recognize these
interactions, are addressed in Chapter 14.
Nitrogen efficiency
Ruminants have a lower overall efficiency of N
utilization and a higher excretion of N in faeces
and urine per unit N intake compared with non-
ruminants (Kohn
et al
., 2005). Several factors
affect this N efficiency of cattle. In general, the
level of N intake itself is a major factor that deter-
mines efficiency. High dietary N levels may result
in low efficiencies in both beef (Yan
et al
., 2007)
and dairy cattle (Huhtanen and Hristov, 2009).
In particular, urinary N excretion increases rap-
idly when N intake increases (Kebreab
et al
.,
2002), which is of particular importance given
that N in urine is susceptible to rapid volatilization
as ammonia. Yan
et al
. (2007) observed a moder-
ate negative relationship in beef cattle between
level of feeding (as an indication of growth rate)
and N excretion as a fraction of N intake, whereas
a strong negative relationship between dietary N
content and N efficiency was present. Similarly, in
a review, Calsamiglia
et al
. (2010) concluded that
high N efficiency in dairy cattle is associated in
particular with low dietary N levels, but also to a
smaller extent with high production levels. In
dairy cattle, reduced dietary N levels may impair
feed intake and/or milk production (Law
et al
.,
2009; Brun-Lafleur
et al
., 2010). This lowered
productivity may actually reduce FCE, even
though N efficiency is improved. In early lactation
though, it may be hypothesized that low-protein
diets can improve energy status of dairy cattle by
reducing milk energy output, and this may con-
tribute to reducing the various health and fertility
problems associated with severe negative energy
balance and consequently improve FCE.
Production Efficiency and Emissions
In view of the expanding world population,
where prospects to increase significantly the
amount of arable agricultural land are small,
efficiency of ruminant production needs to
increase. Or, looking from another angle, there is
a need to reduce waste of natural resources in
ruminant production systems. An increase in
productivity (amount of product per animal)
not only offers a pathway to satisfy increasing
demands for milk and beef but also a possible
mitigation approach to reduce the emission of
various pollutants. In this chapter, we focus on
the excretion of N in faeces and urine and on
