Agriculture Reference
In-Depth Information
Production Level, Nitrogen Efficiency
and Methane Production
(fibre-rich forages and by-products) into human
edible food. Cattle production systems based on
fibrous feeds may be associated with high levels
of GHG emissions per unit product, prompting
general calls to reduce the consumption of
beef and milk products, but in many situations
where the output of human-edible food is
larger than its input, cattle production systems
still have a major role in food security.
There are several other dietary options to
reduce enteric CH
4
formation that have been
discussed in various reviews (Ellis
et al
., 2008;
Iqbal
et al
., 2008; Beauchemin
et al
., 2009;
Martin
et al
., 2010; Cottle
et al
., 2011; Grainger
and Beauchemin, 2011). The addition of die-
tary fat or oil reduces CH
4
production by about
5% per 1% added lipid to the diet (reviewed by
Grainger and Beauchemin, 2011). Dietary
lipid supplementation decreases CH
4
produc-
tion mainly by reducing the activity of metha-
nogens and protozoal numbers, and reducing
fermentation of substrate in the rumen. This
CH
4
reduction appears not to be influenced by
the type of fatty acid (saturated versus unsat-
urated; carbon chain length) as long as the
addition of fat does not negatively affect feed
intake or nutrient digestibility (Van Zijderveld
et al
., 2011a). Rumen modifiers such as mon-
ensin may efficiently reduce CH
4
production,
but the persistency of the effect has been ques-
tioned (Martin
et al
., 2010). Yeast cultures
based on
Saccharomyces cerevisiae
are widely
used on commercial dairy farms in North
America and Europe to improve milk yield and
production efficiency, and thus may decrease
CH
4
per unit product, although other effects
including a shift in partitioning of nutrients to
microbial biomass and various VFA are not
documented well (Grainger and Beauchemin,
2011). Alternative electron acceptors, in par-
ticular nitrate, may be highly effective in
reducing CH
4
, but require careful introduction
and adaptation of cattle in view of the risk of
nitrite poisoning (Nolan
et al
., 2010). In dairy
cattle, nitrate has been shown to have a persis-
tent CH
4
reducing effect for at least 4 months
(Van Zijderveld
et al
., 2011b), corresponding
to its mode of action. In contrast, a number of
dietary additives that have shown promising
results
in vitro
have not been effective
in vivo
(Reynolds
et al
., 2011; Van Zijderveld
et al
.,
2011a).
As discussed previously, increased production is
a powerful means of reducing waste excretion
per unit product. Potential improvements in N
and CH
4
efficiency are most pronounced at low
levels of production. At higher production levels,
according to the law of diminishing returns, effi-
ciency gains are smaller, but still worthwhile to
pursue. An example of efficiency gains for N and
for CH
4
production per unit product is presented
in Fig. 2.5. Annual milk production (fat and pro-
tein corrected milk, FPCM) in the Netherlands
increased from 6270 (1990) to 8350 (2009) kg
per cow, with a rise in feed intake of about 18%.
The FCE (kg feed DM per kg FPCM) decreased
from 0.88 to 0.78. The N efficiency (g N in milk g
−1
N
intake with feed) increased from 0.18 to 0.27,
and CH
4
declined from 17.6 to 15.4 g kg
−1
FPCM,
when comparing 1990 and 2009 (Bannink
et al
.,
2011). Similar improvements in efficiency and
reduction of waste per unit product have been
shown in other dairy and beef cattle production
systems (e.g. Capper
et al
., 2009; Capper, 2011).
The rise in N efficiency was due in particular to a
decline in dietary N-content, whereas the reduced
CH
4
production per unit milk was related particu-
larly to the increased feed intake level. Such
improvements were achieved without increasing
the proportion of concentrates and wet by-
products in the diet (which actually decreased
very slightly, from 0.30 to 0.29 of diet DM) whilst
reducing N fertilizer input. Thus, major gains in
reduction of excreta and emissions from cattle
production systems are possible.
It is crucial to note that results based on
comparisons between systems are not necessarily
a good reflection of the results achieved when
improving productivity within a system. Gerber
et al
. (2011) explored the relationship between
productivity of dairy production and GHG emis-
sions on a global scale. A LCA methodology was
used that included the emissions of CO
2
, CH
4
and
N
2
O, for 155 countries. Gerber
et al
. (2011) pre-
sented two equations to predict GHG emissions
per kg FPCM (Fig. 2.6). These equations, based on
the same dataset, have different implications. In
the non-linear equation they derived, GHG emis-
sions per kg FPCM stabilize at a milk production
level of about 6000 kg year
−1
and the asymptotic
value of emissions is 1.4 kg CO
2
-e year
−1
. In the
