April 1, 2000
Forage Systems Update
Vol 9, No. 2
Net energy supply and demand in pasture rotationally grazed pastures
Introduction: Both forage availability and forage quality may limit animal
performance in grazing situations. In many research trials, only one of
these parameters may be measured. Failing to take both into account often
leads to over-prediction of expected animal performance or erroneous
conclusions about what factor is limiting animal performance. In other
cases both are measured but are not combined to provide a meaningful
relationship to animal requirements and performance. Knowing both forage
production and nutritive value of the forage allows the grazing manager to
determine the nutrient supply and demand balance.
Net energy demand of most classes of livestock under given production and
environmental scenarios has been determined. Net energy content of forage
can be calculated from forage analysis and forage availability or
accumulation rate can be measured. With this information, supply and demand
relationships can be determined and appropriate stock policy and stocking
rate decisions can be made. Our objective was to compare yearling steer net
energy demand per acre with forage net energy available per acre and
determine what factors most likely limit gain of grazing yearlings.
Materials and Methods: This project determined net energy availability for
cool-season grass-legume pastures rotationally stocked at four stocking
rates. Energy demand of yearling steers grazing these pastures was
determined based on net energy requirements for maintenance and growth.
Pastures consisted of endophyte-free tall fescue (Festuca arundinacea
Schreb.), orchardgrass (Dactylis glomerata L.), and Kentucky bluegrass
(Poa pratensis L.) overseeded with red clover (Trifolium pratense L.) and
birdsfoot trefoil (Lotus corniculatus L.) at the beginning of the study in
1995. Within each block, pastures were randomly assigned a stocking rate
and then spilt to either continuous or rotational stocking treatments. The
rotational grazing cells consist of 12 equal sized paddocks. Target
stocking rates were 300, 600, 900, or 1200 lb liveweight per acre at turn-
out as yearling steers weighing approximately 575 lb/head. Sixteen 10 acre
pastures were used in the study to provide two replications of each
treatment in a randomized complete block design with split plot assignment
of treatments.
The study was conducted from 1996 through 1999 with grazing beginning in
early to mid-April and ending around September 10. First grazing cycle
consisted of daily rotation through the 12 paddocks and subsequent cycles
usually consisted of 2-day grazing periods with 22 day rest periods. All
stocking rates were managed on the same rotation frequency.
In each rotationally grazed pasture, four paddocks were sampled in each
grazing cycle to determine forage availability. Nine 3.2 ft2 quadrats were
clipped both pre- and post-grazing in each paddocks. Bulk wet weight was
measured and a 150 g (+/-) subsample was oven dried to determine forage dry
matter. Oven dried samples were retained from each clipping for forage
analysis. All forage samples were analyzed by NIRS to determine crude
protein, ADF, and NDF levels. Each year approximately 100 samples were
analyzed through standard wet chemistry procedures to determine the same
parameters and calibrate the NIRS data. Net energy for maintenance (NEm)
was calculated from ADF value using the relationship:
Forage NEm = 1.04-(0.0104 X ADF)
Availability of NEm per acre was calculated as:
Forage NEm/acre = Herbage mass (lb/acre) X NEm (Mcal/lb)
Net energy demand for yearling steers was determined by the following
equations:
NEm=.077 X ((liveweight/2.205).75
NEg=.0493 X ((liveweight/2.205).75) X ((ADG/2.205)1.097)
Animal demand per acre was calculated as:
Required NE/acre = NE/head X Head/acre
Results and Discussion: Forage availability was significantly affected by
stocking rate with date of peak forage availability also being determined
by stocking rate (Figure 1). Increased animal demand of higher stocking
rates resulted in lower mean forage availability throughout the season.
While peak forage availability occurred on July 29 for 300 pound/acre
stocking rate, the same event occurred on June 8 for 1200 pound/acre
stocking rate. By the end of the grazing season, forage availability on
1200SR was approximately 35% of 300SR. The figures shown are forage
availability at the beginning of each grazing period. Both mean and
residual forage availability were below the levels shown.
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| Figure 1 |
At some level of forage availability, forage intake is limited by the
physical inability of the grazing animal to consume any more forage. From
the standpoint of grazing mechanics, intake is determined by time spent
grazing, biting rate, and bite size. Low forage availability limits bite
size to the extent that the animal cannot spend enough time grazing or take
enough extra bites per minute to compensate. A commonly cited figure for
the lower threshold for intake restriction of 1800 pounds/acre. Forage
availability at initiation of grazing in 1200 SR was below this level from
late July until grazing ended in September. Residual forage availability
was well below 1800 pounds for most of the season. Residual availability
did not fall below 1800 pounds all season for only 300 SR.
The NE content of forage samples was very good throughout the season with
mean levels remaining above .65 Mcal/pound for all treatments (Figure 2).
This data represents whole plant samples so animal selection would be
expected to result in even higher level of dietary energy intake. The idea
that cool season grasses are low quality during the summer months is not
born out by this data. In general, cool season grasses are low quality only
if they are allowed to become so. The highest stocking rate produced the
highest NE forage while lowest stocking rate produced the lowest NE forage.
Crude protein content , while not reported in this paper, responded to
stocking rate similarly.
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| Figure 2 |
These two pieces of data suggest that animal performance was more limited
by declining forage availability as compared to forage quality. The
observed NE levels in the forage should be adequate to produce average
daily gain in excess of 1.75 pounds/day if intake were not limited.
However, by mid-July all treatments were gaining less than 1.0 pound/day.
It may be that reduction in ADG at this time was due more to environmental
stress than forage conditions. Nighttime low temperatures were frequently
above 80oF with humidity in excess of 70%. These conditions are not
conducive to steers achieving high pasture intake.
Net energy available per acre was greatest throughout the season for the
lowest stocking rate and least for the highest stocking rate (Figure 3).
Available net energy declined rapidly after mid-June for 1200SR. During a
typical two-day grazing period, NE consumption by the steers would exceed
50% of the available NE. Forage utilization in excess of 50% will usually
result in depressed animal intake unless forage is of exceptional quality.
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| Figure 3 |
Daily net energy demand declined through the season for all treatments with
more rapid decline occurring as stocking rate increased (Figure 4). The
decline was due to decreased rate of gain as the season progressed. It is
very difficult to say whether gain decreased due to lack of energy intake
or other environmental factors. For all but the highest stocking rate, both
forage availability and net energy content were at levels which would not
normally be expected to restrict voluntary intake.
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| Figure 4 |
Steer average daily gain did not meet expectations based on available
forage and net energy content of the forage. It appears that factors other
than the measured forage parameters determine intake level and steer
performance. High nighttime temperatures and relative humidity may limit
grazing time and total daily intake. Maintaining high quality pastures with
adequate availability will help maintain summer rate of gain on yearling
steers but other factors such as proper environmental adaptation of
livestock, parasite management, and minimizing stresses may be equally
important.
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