Fertility Management
in Grazed Pastures
Jim Gerrish
Research Assistant Professor
We receive numerous questions concerning pasture fertility
at our grazing workshops and when tour groups visit FSRC.
In this issue of Forage Systems Update, we will address some
of the questions we commonly are asked.
Haying vs Grazing: One question is how much fertilizer should
pastures receive? Many fertilizer recommendations for pasture
are actually based on hay harvest concepts. When we harvest
hay and feed it elsewhere, we are physically carrying mineral
elements away from the field. Each ton of mixed grass- legume
hay will typically contain 10 - 12 lbs of phosphorus, 40 - 50
lbs of potash, and varying amounts of other elements.
Continually harvesting hay without replacing the P and K removed will
deplete the fertility of most Missouri soils over time. The
deeper loess derived soils of north Missouri can support
unfertilized hay production much longer than the more highly
weathered soils of south Missouri but even the best soils in
the state must ultimately have P and K replenished to sustain
economic hay yields.
If we contrast hay harvest to grazing, we must give
fertility credit to the manure that is being returned to the
soil and the plant litter that is trampled down but left on-site.
This simple concept suggests that pasture fertility
rates should be lower than hay field rates. The effectiveness
of returned manure in maintaining pasture fertility is highly
dependent upon a number of factors.
The first factor is the uniformity of manure distribution.
Redistribution is affected by size of pasture and
distance the animals must travel to water, stocking density,
and presence or absence of shade. As the distance to water
increases, particularly in hot weather, the less uniform
manure distribution becomes. Concentration near the water
source increases and decreases at the farthest reaches of the
pasture. We would like to see pastures set up with all
paddocks within 600 to 800' of water.
Manure distribution is more even at higher stock densities.
When the travel area of the animal is restricted,
grazing distribution and manure distribution are both enhanced.
Forage quality affects the rate at which manure is
degraded and the minerals contained within it are made
available. Grazing management which enhances overall forage
quality will encourage a more rapid nutrient cycle.
Shade in the pasture acts as a magnet for manure deposition,
even more so than water in many cases. If shade is
dispersed across the paddock in many locations the impact of
shade on manure distribution is not so pronounced. In the case
of the single shade tree in a paddock, a nutrient gradient
will develop across the pasture with the shade tree as the
focal point. The soil test values for P and K within 50' of a
lone shade tree are commonly 3 to 5 times greater than the
general pasture situation.
In summary, maintenance of pasture fertility based
strictly on manure is very site specific and management
dependent and may either be quite adequate or totally inadequate.
Legumes vs nitrogen fertilization: Many producers immediately
think of nitrogen when pasture fertilization is discussed. The
visual color and growth response to N is very impressive but
may or may not be cost effective. Since the early 1980's, our
pasture focus at FSRC has shifted more and more to managing
the legume component of pastures. Thus, our fertility focus
has been on soil pH and P and K levels.
Table 1 shows the soil test criteria that we use at FSRC
for fertilization decisions when we are interseeding legumes.
The values indicated in the table are the minimum soil test
values we would like to have to help ensure successful
interseeding and stand maintenance. The required soil test
levels will vary across the state, with higher levels of K
being required in south Missouri and slightly lower P levels
on some of the deep loess soils. Check specific recommendations
for your region at your area Extension office.
Table 1. Minimum soil test levels used at FSRC for legume
establishment and maintenance.
------------------------------------------
Legume pH P1 K
------------------------------------------
Alfalfa 6.5 30 300
Red Clover 6.0 25 250
Ladino Clover 5.5 25 250
Birdsfoot
Trefoil 5.5 20 225
Lespedeza 5.0 20 200
The cost effectiveness of legume based pastures compared
to N fertilization is illustrated by a study we did 1986 -
1989. In this study we compared several forage systems. Within
a twelve paddock grazing cell we had 4 paddocks each of
brome-alfalfa, brome-trefoil, and brome + 80 lb N/A applied
as 40 lb in April and 40 lb in August if moisture conditions
warranted the treatment. In the five years prior to this study,
the entire pasture area was handled as a brome - red clover
pasture with uniform management over the entire area. A
fertility build up program was initiated in 1981 to bring
three replications of this pasture system set to similar
fertility levels. In fall of 1985, additional fertilizer and
lime were applied to the 4 paddocks where alfalfa was to be
interseeded based on the criteria presented in Table 1.
Because the total pasture area had been treated similarly
from 1981 through 1985 and because the paddocks of each forage
type were managed similarly from 1986 through 1989, we can
make a valid comparison of the costs associated with each base
forage (Table 2).
Table 2. Grazing days and fertility costs per acre and
animal unit day for three pasture types at FSRC
from 1986 through 1989.
-----------------------------------------------------
Forage Grazing Fertilizer
system days/A cost/acre $/AUD
-----------------------------------------------------
Brome + N 101 $18.90 .187
Brome + alf 109 $18.92 .174
Brome + BFT 101 $ 8.35 .083
----------------------------------------------------
These costs reflect both the annual costs of the buildup
program as well as the actual fertilizer applied in the 1986 -
1989 time period. The higher costs associ-aited with alfalfa
compared to trefoil reflect the higher fertility requirements
specified in Table 1. At the end of the study, the soil test
values were similar for all three forage types.
These numbers illustrate two points. One, cost per
grazing day is typically lower for grass-legume pastures than
for N-fertilized pastures. Coupling this with the fact that
animal performance is generally 5 to 20% higher on grass-legume
pastures compared to N-fertilized grass means that cost
per lb of gain will be proportionally even lower for grass-legume
pastures as long as carrying capacity is similar. The
second point is that a legume that is adapted to the existing
fertility conditions may be more cost effective to raise than
a more productive legume that may have higher fertility
requirements.
What is the fertilizer value of hay: If we take the figures
mentioned earlier concerning the P and K content of a ton of
forage harvested as hay (12 lb P/T and 50 lb K/T) and apply
commercial fertilizer prices to these nutrients we can arrive
at a fertilizer value of hay.
P) 12 lb P/2000 lb hay = 6 lb /1000 lb bale
6 lb P/bale X $0.22/lb P = $1.32/bale
K) 50 lb K/2000 lb hay = 25 lb/1000 lb bale
25 lb K/bale X $0.14/lb K = $3.50/bale
The animals consuming this hay will retain about 10% of
the minerals and excrete the remaining 90%. Thus, the value of
P and K returned to the soil will be approximately $4 per 1000
lb bale. Organic matter and other minerals will also be added
but it is not as easy to place a dollar value on these inputs.
As with the grazing situation, this fertility input is only
useful if it is deposited in a useful location. Continually
feeding in the same area will lead to very high soil test
values in the immediate feeding area but will do little to
impact fertility away from the feeding area. Feeding along
brushy ditches to provide shelter for the animals while eating
will concentrate the nutrients along the ditches and result in
high nutrient losses via surface runoff. To make the most
effective use of brought-in hay to enhance pasture fertility,
the bales must be fed in such a manner as to allow good manure
distribution over a large part of the field and minimize
runoff losses. Unrolling large round bales results in high
feeding losses but is probably the most effective means of
ensuring uniform manure distribution.
What can be done to shift manure distribution: Placement of
waterers, fencing shade in or out of paddocks, location of
mineral feeders, and hay feeding system can all be used to
produce a more desirable manure distribution pattern. In
smaller scale grazing operations, the use of a portable water
system allows the manager to shift the focus of manure
distribution with every grazing cycle. A similar effort can be
made by changing location of mineral feeders regularly.
However, mineral feeders do not provide nearly the concentration
focus that water and shade will provide. If you feel that
the animals are spending too much time under the shade tree
and concentrating too many nutrients there, do not hesitate to
fence the shade out of the grazing area.
Practical conclusion: In summary, good manure distribution
management can go a long ways toward reducing the need for
purchased fertilizer but in most cases in Missouri, some
off-farm fertilizer input will likely be needed.
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