Agriculture Reference
In-Depth Information
100
485 days birth-slaughter
113 MJ ME kg
-1
beef produced
90
80
70
609 days birth-slaughter
128 MJ ME kg
-1
beef produced
55%
60
50
47%
40
30
20
45%
53%
10
0
1977
2007
Growth
Maintenance
Fig. 11.3.
The comparative daily energy requirements for maintenance and growth of beef cattle
produced under management systems characteristic of US beef systems in 1977 and 2007.
discussion exists within the beef industry as to
whether selection for growth rate and slaughter
weight has indirectly reduced the efficiency of the
mature cow. Within an efficient cow-calf system,
calves should be weaned at approximately half
the dam's mature weight, yet this is difficult to
achieve when mature cows approach or exceed
635 kg bodyweight given the nutritional limita-
tions of pasture-based diets. Life cycle assess-
ments (LCA) of beef production also indicate that
the cow-calf sector contributes the greatest pro-
portion of carbon emissions per unit of beef
(Beauchemin
et al
., 2011) and is less susceptible
to dietary mitigation of methane emissions due to
the extensive nature of the production system.
Although Notter
et al
. (1979) reported some
advantages of increased mature weight upon
cow-calf system efficiency, this author predicts
that if calf growth rates and weaning weights can
be maintained from cattle with a mature weight
of less than 580 kg, environmental sustainability
will be improved. The question therefore becomes
whether future environmental sustainability may
be achieved through further dilutions of mainte-
nance, or whether a reduction in the mainte-
nance requirement of the supporting population
through changes in mature weight may play a
significant role.
The US dairy industry has made a whole-
scale shift away from smaller breeds to larger-
framed cows over the past century - within the
previously discussed historical comparison, the
1944 dairy population contained 54% smaller
breeds (Jersey and Guernsey) compared with
90% Holstein cows in 2007 (Capper
et al
., 2009).
Genetic selection for increased milk yield has
increased the proportion of Holstein cattle within
the national dairy herd, yet one criticism often
levelled at intensive dairy production systems is
that high-producing cows tend to have an
increased bodyweight, and thus consume more
feed and emit greater quantities of GHG on a daily
basis. If daily GHG emissions per animal are the
correct metric by which to evaluate environmen-
tal impact, cows with a greater mature weight
will have increased GHG output compared with
their smaller counterparts - for example, the daily
GHG output per lactating cow was 13.5 kg CO
2
-eq
in 1944 compared with 27.8 kg CO
2
-eq in 2007
(Capper
et al
., 2009). None the less, expressing
results on a 'per head' basis fails to consider milk
yield, milk composition and other productivity
indices that may have a significant effect upon
population size and thus environmental impact.
As the population maintenance nutrient require-
ment has a sizeable impact upon resource use and
GHG emissions per unit of dairy product, the
impact of reducing population maintenance
through changes in mature cow weight warrants
further investigation.
Jersey cattle confer two potential breed-
specific advantages over the Holstein in terms of
