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David Davis
21262 Genoa Road
Linneus, MO 64653
Phone: 660 895-5121
FAX: 660 895=5122
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July 1, 2002
Forage Systems Update
Vol 11, No. 3
The Meaning and Significance of Utilization Rate
Jim Gerrish
Research Assistant Professor
MU - Forage Systems Research Center
Pasture utilization rate is a term that gets thrown around a
lot at grazing conferences and pasture walks. There does seem to be
some confusion about what it actually means and what is its
significance. One of the reasons utilization confuses some people is
that there are two distinctly different types of utilization rate we
need to be concerned with and they don't always get properly
differentiated. The first concept is temporal utilization, which is
the harvest efficiency for a single grazing period. What this
relates to is how much of the forage that is standing in the pasture
during this grazing period will be harvested during this grazing
period. The second concept is seasonal utilization or how much of
the forage that grows during an entire growing season is actually
consumed by grazing animals. To use some other terms graziers are
familiar with, temporal utilization relates to grazing pressure
while seasonal utilization relates to carrying capacity.
Temporal utilization rate is largely determined by the target
residual to be left at the end of the grazing period relative to the
forage mass at the beginning of the grazing period. On of the
grazing management principles which I hold near and dear is the old
‘take half, leave half' adage. In utilization terms, this means
graze for a 50% temporal utilization. As long as regrowth does not
occur during the grazing period, this is fairly easy to monitor and
achieve. It is important to remember that the grazing animal is
taking half of the forage mass, not half of the height. Because the
top parts of plants are lighter than the lower parts, grazing 50% of
the biomass will actually remove more than 50% of the height. As
long as grazing periods are less than three days during good growing
conditions and less than a week in drier conditions, we generally
don't worry about factoring regrowth into the utilization equation.
There is both an animal and a plant management basis for the take
‘half-leave half' rule.
As the grazing animal is forced to utilize more of the forage
mass, forage quality declines and bite size becomes smaller. As a
general rule, higher temporal utilization rates result in lower
forage intake which subsequently leads to lowered animal performance
but may result in increased animal output per acre. How long it
takes to achieve the target residual also affects intake level
because of selective grazing. Intake depression increases with
longer grazing periods as we try to achieve some preset utilization
target (Figure 1). In this study, we compared daily and weekly
rotational grazing of big bluestem at the same stocking rate and
same target utilization rate. Lets look at intake in the week long
grazing period as an example. During the first couple days of the
period, grazing animals selected only the highest quality forage and
intake was quite high. By midweek they were consuming a higher
percentage of stems and fewer leaves and their overall diet quality
declined and intake followed. By the end of the week, the decline in
quality had continued and physical availability of forage was also
restricting bite size and intake was significantly lower. When the
same target residual had been achieved in one day through increased
stock density, or grazing pressure, selection was minimized and the
lower quality stem material entered the rumen at the same time as
the high quality leaf material and overall digestion and diet
quality was enhanced. The result was a higher level of intake for
the daily rotation cows even though beginning forage mass and target
residual were the same for both grazing periods. From an animal's
perspective, a higher degree of utilization can be achieved with
shorter grazing periods without adversely affecting intake.
The plant basis for ‘take half-leave half' is the effect of
residual leaf area on regrowth rate and root growth. For most plant
species, regrowth will be fairly rapid as long as at least 50% of
the forage mass is left after grazing. Removing more leaf area
results in lower photosynthetic output and both top and root growth
slow significantly. Removal of all leaf area forces the plant to
rely on stored carbohydrates for regrowth which results in a much
slower regrowth rate. Some plants have a greater regrowth response
to residual forage mass than do others (Figure 2). We compared
effect of residual on tall fescue and smooth bromegrass based
pastures. Beginning forage mass was similar for all treatments, so
different levels of residual reflect differences in temporal
utilization rate. Species like tall fescue that have a lot of leaf
area close to the ground have an optimal level of utilization.
Grazing too short leaves too little leaf area for rapid regrowth
while leaving too much residual leaves too much shading and aged
leaf material behind. With a typical pre-grazing target forage mass
of 2800 to 3200 lb/acre, 50% utilization leaves residual between
1400 and 1600 lb/acre, which optimizes regrowth potential. On the
other hand, species like smooth bromegrass or native tall grasses
respond to increasing levels of residual forage mass with higher
regrowth rates. Over utilization of these more erect growing species
seriously reduces annual production and regrowth potential.
Table 1. Effect of different degree of repeated leaf removal on root
growth three days after clipping.
|
Percent leaf removal | Rhodes grass
(single clipping) | Rhodes grass | Smooth bromegrass | Kentucky bluegrass |
|
| | Percent | Root | Stoppage |
|
10 | 0 | 0 | 0 | 0 |
|
20 | 0 | 0 | 0 | 0 |
|
30 | 0 | 0 | 0 | 0 |
|
40 | 0 | 0 | 0 | 0 |
|
50 | 2 | 8 | 13 | 38 |
|
60 | 50 | 80 | 36 | 54 |
|
70 | 78 | 97 | 76 | 77 |
|
80 | 100 | 100 | 81 | 91 |
|
90 | 100 | 100 | 100 | 100 |
Table 1 contains data from classic root growth studies
conducted by Dr. Fred Crider at Ohio State University in the 1950's.
It illustrates the drastic effect of utilization beyond 50% on root
growth of three grass species. The effect of over-utilization is
clear and significant. The additional effects of overutilization are
reduction of soil organic matter, decreased water infiltration and
increased surface runoff, less drought tolerance, and increased
pollution potential. The unfortunate part of all this is that 90% of
the farms and ranches I visit around the US are all over utilizing
their pastures in the temporal utilization rate concept. Now to
thoroughly confuse the issue, they are at the same time under
utilizing pastures in the seasonal sense.
So what affects seasonal utilization rate? Timeliness is a big
factor. While some graziers have the idea that they must harvest a
high percentage of pasture with every grazing, that is not the case
at all. Very high seasonal utilization rates can be achieved without
ever exceeding the ‘take half-leave half rule'. One of the first
lessons taught in grazing management is to understand the ‘S'-shaped
growth curve (Figure 3). Pasture plants are most photosynthetically
efficient during Phase 2. This corresponds to a large degree with
that optimal grazing window of pre-grazing mass of 3000 /b/acre +/-
and residual of 1500 lb/acre +/-. Keeping the pasture working in
this range and harvesting fairly frequently at the 50% level
produces surprisingly high seasonal utilization rates. Table 2 is a
typical paddock record sheet which shows pre-graze and residual
forage mass along with regrowth and temporal utilization rates.
Table 2. Paddock record for a single paddock in a MiG pasture system.
|
DATE
|
Pre-graze
Height
|
In Yield
|
Yield/
acre-inch
|
Residual
Height
|
Residual
Yield
|
Temporal
Utilization
|
Forage
Consumed
|
Re-
growth
|
Rest
Period
|
Daily
Growth
Rate
|
Total
Yield
|
Seasonal
Utilization
|
|
21-Apr-2000
|
6.0
|
2411
|
402
|
3.3
|
1436
|
40%
|
975
|
1611
|
20
|
81
|
2411
|
|
|
09-May-2000
|
8.0
|
3092
|
387
|
4.0
|
1546
|
50%
|
1546
|
1656
|
18
|
92
|
4067
|
|
|
01-Jun-2000
|
6.2
|
2600
|
423
|
3.3
|
1372
|
47%
|
1227
|
1054
|
23
|
46
|
5121
|
|
|
07-Jul-2000
|
7.7
|
2956
|
386
|
2.2
|
837
|
72%
|
2119
|
1584
|
36
|
44
|
6705
|
|
|
15-Aug-2000
|
8.7
|
2811
|
325
|
4.2
|
1827
|
35%
|
984
|
1974
|
39
|
51
|
8679
|
|
|
22-Sep-2000
|
7.7
|
2625
|
342
|
4.2
|
1042
|
60%
|
1583
|
798
|
38
|
21
|
9477
|
|
|
Summary
|
7.4
|
2749
|
377
|
3.5
|
1343
|
51%
|
8435
|
1446
|
29
|
56
|
9477
|
89%
|
Seasonal utilization rate equals total forage consumed divided by total seasonal
yield. Even though temporal utilization rate averaged only 51%, the seasonal
utilization was 89%. This level of forage utilization can be achieved in intensively
managed pastures in temperate climates and very short grazing periods. In this
example, grazing periods were one day. As grazing period length increases, temporal
utilization rate must be decreased to maintain a target level of animal performance,
and seasonal utilization rate will subsequently decline. For 3 to 5 day grazing
periods, seasonal utilization between 60 and 70% is a reasonable goal and has
been measured in several studies at FSRC. Traditionally, we have considered
continuous grazing to provide between 30 and 50% seasonal utilization with the
lower level of utilization supporting higher animal gains and the higher utilization
producing more output per acre but at a lower level of individual performance.
As a note for our readers in drier climates and on native
range, appropriate seasonal utilization rates are much lower than
those presented here. Very often pastures are only grazed one time
during the limited growing season. In this case, temporal and
seasonal utilization rate are very nearly one and the same. In some
environments, appropriate seasonal utilization rate may be as low as
20 to 30% during the growing season followed by some additional
usage during the dormant season. Utilization beyond 50% in both
growing and dormant season can be very detrimental in some rangeland
plant communities. Irrigated pastures in the US West can be managed
and utilized at nearly the same intensity as temperate pastures as
long as adequate water is available to ensure the pastures enter the
dormant season in vigorous condition.
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