AGRICULTURAL | LANDBOUKUNDIG
AGRICULTURAL
There are many practical reasons for minimizing forage in rations of feedlot cattle.
some protein contamination.
NDF is not an ideal nutritive entity
because its digestibility varies with lignin
concentration and other factors in the feed.
Variation in dry matter digestibility (DMD) is
related primarily to the concentration and
digestibility of NDF in feeds. It is only NDF
that measures the differences within and
among feed types and has the potential
for developing a system of general feeding
recommendations across all feeds.
EFFECTIVENESS OF FIBER
The concept of effective fiber (EF) was
developed by nutritionists to formulate
rations that would maintain milk fat per-
centage in dairy cows and is not appro-
priate over the full range of feeds fed to
ruminants. A roughage value unit (RVU)
system was proposed for measuring the
effectiveness of feeds in stimulating chew-
ing activity that was based on a chemical
measure of fiber (NDF) and a physical
measure of particle size. A standard or
reference value with an RVU of 100 was
defined as a hypothetical feed containing
100% NDF with all particles large enough
to stimulate chewing (a long grass hay
containing 100% NDF). A direct rela-
tionship between the NDF content plus
physical length of fiber and stimulated
chewing activity were found. To clarify
these concepts definitions was proposed
for both effective NDF (eNDF) to main-
tain milk fat and physically effective NDF
(peNDF). The peNDF of a feed is related
to the physical properties of its fiber (pri-
marily particle size) that stimulates chew-
ing activity and establishes the biphasic
stratification of ruminal contents (floating
mat of large particles on a pool of liquid
and small particles). However, peNDF is
a feed attribute that is based on a fixed
scale (0 to 1) and reference value (long
grass hay). The peNDF of every feed can
be calculated taking the NDF value and
physical fiber length into account.
DETERMINING MINIMUM peNDF
REQUIREMENTS FOR RUMINANTS
Feedlot production is a terminal process
that rarely lasts beyond 180 days. Given
the short-term nature of feedlot production
and the need to maximize animal perfor-
mance, the minimum fiber requirements
for feedlot cattle may be substantially
lower than that for dairy cows. Because
fiber has lower productive energy density
than concentrates and is poorly digested
in high concentrate diets, lower fiber con-
centration in feedlot rations may improve
animal performance and reduce manure
excretion.
It appears that there are positive
relationships between ADG and peNDF
within trials when peNDF is less than 10%
of ration DM and negative relationships
within trials when peNDF is greater than
18%. The peNDF that maximizes ADG
was determined as 15.3%. However, there
is little difference in ADG when peNDF in
the ration is between 12 and 18%. The
optimum peNDF in the ration to minimize
liver abscesses was about 22%, and
the peNDF that maximized intake was
about 25%. The relatively broad range in
acceptable peNDF (12 to 18% of ration
DM) suggests that recommendations can
be modified to match multiple objectives
and account for other factors that may
influence minimum peNDF requirements
for feedlot cattle.
Another problem in ration consistency
is related to particle size, mixing effec-
tiveness and selection by animals. It is
difficult, if not impossible, to uniformly mix
and deliver rations containing concen-
trates and coarsely chopped roughage,
especially if the ingredients are dry.
In addition, animals will selectively eat
roughage and grain when the forage is
chopped coarsely even if a uniform ration
is delivered. Thus, there is a tendency to
reduce the particle size of the roughage
to improve handling, mixing and delivery.
When this occurs it is important that the
peNDF value of the roughage be adjusted
to reflect its effectiveness.
CONCLUSIONS AND
RECOMMENDATIONS
Both the chemical and physical charac-
teristics of rations are important in deter-
mining animal performance. Physically
effective NDF attempts to take into
account both the chemical and physical
nature of fiber that influences the chewing
activity and ruminal function of ruminants.
Although chewing activity is important in
providing salivary buffers for controlling
ruminal pH, it is also an indicator of the
physical environment of the rumen that
helps to establish optimal ruminal fermen-
tation and production.
Om die produktiwiteit van produksiestelsels
vir beeste te verbeter.
Adviesdiens vir herkouervoeding
Voerprosessering, voedingsbestuur en formulering
van voere vir volhoubare produksieprestasie
Formuleer en verskaf konsentrate om formulasies te
komplimenteer
Dr. Kobus Swart 083 262 0946 • [email protected]
SENWES SCENARIO | WINTER 2019
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