Agriculture Reference
In-Depth Information
1944, but with 21% of the animals, 23% of the
feedstuffs and only 10% of the land. As a result,
the GHG intensity of milk production (cradle to
farm-gate) is estimated now to be 37% of what it
was back then (1.35 versus 3.66 kg CO
2
e kg
−1
milk). Decreased GHG intensity with increased
milk production is achieved mainly through
increased efficiency caused by a dilution of
maintenance energy requirements per kg of
milk (Place and Mitloehner, 2010). Thus,
increased productivity of cows offers considera-
ble mitigation potential for dairy production sys-
tems characterized by low productivity.
In addition to increased milk production,
improved feed utilization can be a highly
effective way of reducing CO
2
e emissions per
kg of milk (Vellinga
et al
., 2011). Bell
et al
.
(2011) showed that management and breed-
ing of dairy cows to improve feed use within a
system decreased the need for purchased feeds
and fertilizers. Similarly, Kristensen
et al
.
(2011) observed in an analysis of commercial
dairy farms in Denmark, that herd efficiency
explained most of the variation in the range
of GHG intensities of milk among farms, with
emission being 0.13 kg CO
2
e kg of ECM lower
for the most efficient group compared with
the least efficient group. This was mainly due
to a higher conversion of feed to milk of 1.32
versus 1.06 kg milk per kg DMI, which
reduced the CH
4
emission, and to some extent
due to the higher milk yield of 8488 versus
6964 kg ECM per cow.
lactation) increased the GHG intensity by 17%,
due to increased enteric CH
4
and manure
emissions. Additionally, harvesting forage
re quires more machinery and greater fuel con-
sumption compared with feed grain. The same
study showed that the type of forage was also
important. Increasing the use of maize silage
and decreasing the use of lucerne silage lowered
the GHG intensity of milk production by 13%.
Use of forages that contain high quantities of
starch lowers CH
4
emissions (Grainger and
Beauchemin, 2011). A number of other nutri-
tional strategies such as incorporating sources
of fat into the diet and supplementation with
various feed additives have been proposed as a
means of lowering methanogenesis in the rumen
(Johnson and Johnson, 1995; Beauchemin
et al
., 2009; Martin
et al
., 2010; Grainger and
Beauchemin, 2011). However, few of these strat-
egies have been explored in detailed for their net
effect on GHG intensity of milk production.
Pastoral versus Confined
Dairy Systems
Farm-based LCAs report that GHG intensity of
dairy production (kg CO
2
e kg
−1
ECM) in pastoral
systems is similar to (Flysjö
et al
., 2011b) or
higher than reported for confined systems
(Beukes
et al
., 2010; Bell
et al
., 2011). Differences
are partly attributed to higher enteric CH
4
pro-
duction of cows fed high forage diets. Diets with
high amounts of readily fermentable carbohy-
drates reduce enteric CH
4
production relative to
that in high-fibre pastoral diets, due to greater
propionic acid production and lower pH in the
rumen and the subsequent inhibitory effects
on methanogenesis (Beauchemin
et al
., 2009).
Additionally, the difference is due to level of ani-
mal performance; milk yield in intensive dairy
systems is often higher than that reported in
pastoral systems (e.g. Beukes
et al
., 2010; Flysjö
et al
., 2011b).
In pastoral systems, both milk production
and GHG emissions are driven by stocking rate,
through effects on feed intake. Thus, the chal-
lenge for pastoral systems is mitigating GHG
without causing a decline in milk production.
Beukes
et al
. (2010) investigated various poten-
tial GHG mitigation options for the pastoral
Nutrition
Diet manipulation is the most direct, and argua-
bly the most effective, means of lowering the
GHG intensity of milk for intensive dairy opera-
tions because the diet composition directly affects
CH
4
emissions. A number of dietary strategies
have been evaluated for their effects on GHG
intensity of milk and most studies agree that
increasing the proportion of forage in the diet
increases the GHG intensity of milk production
(Rotz
et al
., 2010; Bell
et al
., 2011). For example,
Rotz
et al
. (2010) reported for the US dairy sys-
tem that increasing the proportion of the forage
in the diet of lactating cows from 0.45 to 0.48-
0.70 of the dry matter (depending on stage of
