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FORAGE AND NUTRITION Guide 2017
FORA
GE AND NUTRITION
Guide 20
1
7
35
Duration of finishing period
Live weight gain over the finishing period is not constant
and, generally, is higher at the beginning and declines as
the duration of the finishing period increases. As slaughter
weight increases, the proportions of non-carcass parts and
total muscle decrease, while the proportions of non-carcass,
carcass and marbling fats all increase. As fat deposition
requires more energy than protein (muscle) deposition, more
feed is required to produce a kilogram of fat. The practical
implications of this are that feed e ciency deteriorates,
and the feed cost per kilogram live weight and carcass gain
increases as the length of the finishing period increases. For
example, Teagasc research has shown that in steers o ered a
high-concentrate diet over 160 days, the kilogram dry matter
(DM) intake/kg carcass was 37 per cent better during the first
80 days compared to the final 80 days of the finishing period.
Therefore, avoiding overly-long finishing periods and ensuring
that animals achieve minimum carcass fat score without
impairing carcass value are ways to reduce feed requirements
and costs.
Compensatory growth
This is the ability of an animal to undergo accelerated
growth when o ered unrestricted access to high-quality feed
after a period of restricted feeding or under-nutrition. This
phenomenon can be readily exploited by producers through
`storing' (target growth rate, 0.50-0.70kg live weight daily)
weanling cattle during the expensive indoor winter period and
subsequently availing of compensatory (or `catch-up') growth
when o ered cheaper produced grass during the following
grazing season. Compensatory growth may also be exhibited
during the indoor finishing phase by cattle that experienced
sub-optimal growth earlier.
Gender
Bulls are inherently more feed-e cient than steers, which,
in turn, are generally more e cient than heifers. Research in
Ireland and elsewhere comparing steers and bulls of similar
breed, reared under similar management on the same diet and
slaughtered at the same age, showed that growth and feed
e ciency traits were 10-20 per cent better for bulls than steers.
Di erences in favour of bulls were generally more pronounced
at higher feed energy levels and with increasing slaughter
weight. The enhanced performance of bulls over steers is due
to naturally occurring male steroid hormones.
Breed type
In general, beef breeds and beef crossbreds are more feed
e cient than beef x dairy breeds, which, in turn, are more
e cient than Friesian and Holstein. For example, a study at
Grange showed that suckler-bred beef cattle gained about
23 per cent more live-weight during the finishing period per
unit of energy consumed than Holstein/Friesian breeds when
slaughtered as either bulls at 15 months of age or as steers
at 24 months of age. However, because of the higher kill-out
proportion and the greater proportion of meat in the carcass
of beef compared to dairy breeds, the percentage of meat
produced per unit of energy consumed was, on average, 51
per cent greater for the beef than the dairy breed. Within the
beef breeds, late-maturing breeds are more feed e cient than
early-maturing breeds, especially in terms of carcass weight
and muscle production. It is important to bear in mind that
comparison of intake and e ciency data for cattle breed types
must be interpreted in the context of the production system
operated and slaughter end point of the comparisons, as the
ranking could vary with changes in these factors.
Variation in feed efficiency within cattle populations
The concept of RFI, rather than FCR, is becoming the preferred
measure of feed e ciency across many livestock production
enterprises, and in particular for beef cattle. Cattle with low
RFI (e cient) consume less feed than expected based on their
weight and growth. The advantage of using RFI as a means of
selecting for improved feed e ciency is that it is independent
of growth and carcass traits in growing beef cattle. This has
positive implications for maintenance requirements of both
growing and especially mature cattle (suckler cows). Teagasc
research has shown that in any group of growing cattle or
suckler cows there can be up to 20 per cent di erence
and greater in the feed consumed by the most e cient
compared to the least e cient animals for the same level of
growth and performance. The challenge now is to reliably
and cost-e ectively identify these feed e cient animals and
proliferate their genetics through structured animal breeding
programmes.
Current research on feed efficiency in beef cattle
Worldwide, breeding values of bulls for feed intake or feed
e ciency are typically derived from progeny performance
based on
ad libitum access to energy-dense rations whereas,
in many countries including Ireland, the lifetime gain of most
commercial beef cattle is achieved from diets consisting, to
a significant extent, of lower energy density feeds such as
grazed grass and/or ensiled forages.
There is evidence from Teagasc research that, although
relatively repeatable, ranking of beef cattle for feed e ciency
o ered the same diet is not necessarily consistent over
di erent phases of their lifetime, and this may be further
exacerbated when di erent diets are fed successively (ie.
forage versus concentrate-based diets), as per commercial
practice. This strongly indicates the presence of what is termed
a `genotype x environment' interaction for the trait, in other
words that the relative feed e ciency of a particular animal
depends on the type of feed it is o ered or management
system within which it is reared.
However, the existence of such a phenomenon has not been
adequately tested to date. Research at Teagasc Grange,
funded by the Department of Agriculture, Food and the Marine,
is currently examining this in order to aid the identification of
animals that are most profitable to produce under our grass-
based production systems.