Agriculture Reference
In-Depth Information
the ability of ruminants to turn human-inedible
products into human-edible products is of large
significance in terms of global food security.
Increases in livestock production will
largely have to come from further advancement
in the efficiency of livestock systems in convert-
ing human inedible resources into human edi-
ble food whilst at the same time improving
animal performance. This review will focus
on the production efficiency of cattle, primarily
that of dairy cattle, at the animal level.
Improvements in production efficiency are dis-
cussed with respect to the amount of feed
required for production and associated losses
of N and CH
4
from cattle.
Nutrient in feed
Nutrients in faeces
Digested nutrients
Urine
Methane
Fermentation heat
Metabolizable
nutrients
Heat
Efficiency of Converting
Feed to Product
Nutrients in milk
Fig. 2.1.
Steps and losses in the conversion
of feed into milk in dairy cattle.
In general, efficiency of cattle production is con-
cerned with minimizing the amount of inputs
(including feed) and losses of nutrients or unde-
sired outputs (e.g. CH
4
and NH
3
) to produce a
given quantity of meat or milk. Improved effi-
ciency may come from maximizing production
(milk production by dairy cattle or growth rate
of beef cattle) or from minimizing waste output.
The efficiency of feed use for milk or meat
production has obvious economic impact, and is
the major determinant of feed conversion effi-
ciency (FCE, the amount of product produced
per kg of feed dry matter intake; Reynolds
et al
.,
2011). Producers have continued to improve the
efficiency with which cattle convert feed into
meat and milk through development and appli-
cation of new technologies and methodologies.
In the conversion of feed energy to milk energy,
several steps may be distinguished (see Fig. 2.1
for major steps in milk production). Each of
these steps in the conversion of feed to milk
occurs with varying efficiency, and represents a
potential opportunity for improvement.
Not all feed eaten by cattle is useful for
production because some of it is not digested
but rather lost in faeces. Feed digestibility
varies widely, in general with high digestibility
values for concentrate feed ingredients rich in
non-structural carbohydrates, and low values
for roughages high in structural carbohydrates.
Numerous factors affect feed digestibility,
including level of intake, retention time in the
various gastro-intestinal compartments, and
feed processing methods (Mertens, 2005; Van
der Poel
et al
., 2005). Although concentrate,
and in particular cereal grains, contains high
levels of non-structural carbohydrates and have
a high digestibility compared with most forages,
their rapid fermentation in the rumen may lower
pH and decrease digestibility of fibre (Zebeli
et al
.,
2008; Dijkstra
et al
., 2012).
Next, some of the digested feed is lost as
gaseous energy, primarily CH
4
produced during
ruminal and to a lesser extent hindgut fermenta-
tion, and as urinary energy, primarily urea pro-
duced during the catabolism of organic molecules
containing N. Production of CH
4
and urinary N
losses are discussed in subsequent sections.
Of the remaining digested feed available for
metabolism, some is used for body maintenance
purposes, some is lost as heat increment associ-
ated with the work of digesting and metaboliz-
ing nutrients, and some is converted into milk,
accreted body tissues and conceptus. In particu-
lar, liver and gut may account for a large and
variable part of nutrient metabolism (Lapierre
et al
., 2006; Reynolds
et al
., 2011) and contrib-
ute to maintenance requirement and nutrient
delivery to the various body tissues or organs. At
the tissue or organ level, nutrient utilization
depends upon the nature and quantity of the
absorbed nutrients as well as the physiological
