|
2002 Annual Report Grassland Section |
Dickinson
Research Extension Center
1089 State Avenue Dickinson, ND 58601 |
PROGRESS
REPORT
Defoliation Effects
on the Structure and Dynamics of Grassland Ecosystems
Llewellyn
L. Manske PhD
Range Scientist
North Dakota
State University
Dickinson Research
Extension Center
Abstract
Grassland plants developed
defoliation resistance mechanisms to compensate for herbage removal by herbivores
and fire during the long period of evolution. Two types of defoliation resistance
mechanisms are of particular importance to grassland managers: changes in physiological
responses within grassland plants and changes in activity levels of symbiotic
soil organisms in the rhizosphere. Grassland managers can beneficially manipulate
these defoliation resistance mechanisms by timing grazing for a short period
(7-17 days) of partial defoliation of young leaf material between the third-leaf
stage and anthesis phenophase. Grass tiller numbers, aboveground herbage biomass,
and nutrient content of herbage increase as a result of plant defoliation at
early phenological growth stages. These increases allow for subsequent increases
in stocking rate and result in improved individual livestock weight performance
during a second grazing period after anthesis.
Introduction
The diverse and complex
nature of grassland ecosystems causes considerable difficulty in development
of sound management recommendations. However, increasing knowledge of ecological
principles and the intricacies of numerous mechanisms in the grassland ecosystem
has allowed for development of improved management strategies.
Recently several greenhouse
and laboratory studies have led to the initial understanding of defoliation
resistance mechanisms grassland plants developed as evolutionary responses to
defoliation by herbivores and fire. Defoliation resistance mechanisms are described
in two categories (Manske 1999). External mechanisms involve herbivore-induced
environmental modifications (Briske and Richards 1995). Internal mechanisms
are associated with herbivore-induced physiological processes (McNaughton 1979,
McNaughton 1983) and are divided into two subcategories: tolerance mechanisms
and avoidance mechanisms (Briske 1991). Defoliation tolerance mechanisms facilitate
growth following grazing and include both increased activity within the plant
meristem and compensatory physiological processes (Briske 1991). Defoliation
avoidance mechanisms reduce the probability and severity of grazing and include
the modification of anatomy and growth form. Grazing resistance in grass is
maximized when the cost of resistance approximates the benefits. Plants do not
become completely resistant to herbivores because the cost of resistance at
some point exceeds the benefits provided by the resistance mechanisms (Pimentel
1988).
Grassland management by
defoliation with herbivores has the greatest beneficial effect if planned to
stimulate two mechanisms: vegetative tillering from axillary buds and increased
activity of symbiotic soil organisms. The physiological responses to defoliation
do not occur at all times, and the intensity of the response is variable. The
physiological responses can be related to different phenological growth stages
of the grass plants. The key to ecological management by effective defoliation
is to apply defoliation during the phenological growth stage at which the desired
outcome will be triggered.
Understanding the defoliation
resistance mechanisms that work within grassland plants and that stimulate the
symbiotic organisms in the rhizosphere following defoliation is necessary to
accomplish beneficial manipulation of these mechanisms under field conditions
and to develop ecologically sound recommendations for management of our grassland
ecosystems. The goals of this research project were to study the ecological
effects of defoliation and to determine the season of use for domesticated grasses
and native range in the Northern Plains.
Methods and Materials
The long-term study site
is on the Dickinson Research Extension Center ranch, operated by North Dakota
State University and located 20 miles north of Dickinson in southwestern North
Dakota, U.S.A. (47o14'N.lat., 102o50'W.long.).
Soils are primarily Typic
Haploborolls. Mean annual temperature is 42.3oF (5.7oC).
January is the coldest month, with a mean temperature of 14.0oF (-10.0oC).
July and August are the warmest months, with mean temperatures of 68.9F (20.5oC)
and 68.9F (20.5oC), respectively. Long-term annual precipitation
is 16.31 inches (414.16 mm). The growing-season precipitation (April to October)
is 13.81 inches (350.63 mm), 85.0% of annual precipitation (Manske 2002). The
vegetation is the Wheatgrass-Needlegrass Type (Barker and Whitman 1988) of the
mixed grass prairie. The dominant native range species are western wheatgrass
(Agropyron smithii), needleandthread (Stipa comata), blue grama (Bouteloua gracilis),
and threadleaved sedge (Carex filifolia).
The grazing treatments
and nongrazed control were organized as a paired-plot design. The nongrazed
control, 4.5-month twice-over rotation treatment, and 6.0-month seasonlong treatment
had two replications. The 4.5-month seasonlong treatment had three replications.
The long-term nongrazed treatments had not been grazed, mowed, or burned for
more than 30 years prior to the start of data collection.
The 4.5-month twice-over
rotation (4.5 TOR) management strategy began on a fertilized (50lbs N/acre on
1 April) crested wheatgrass pasture, with grazing starting as close as possible
to 1 May and continuing on that forage type for about 31 days. The livestock
then followed a rotation sequence through three native range pastures during
the next 135 days. Each pasture was grazed for two periods, one period of 15
days between 1 June and 15 July (third-leaf stage to anthesis phenophase), followed
by a second period of 30 days after 15 July and prior to mid October. The first
pasture grazed in the sequence was the last pasture grazed the previous year.
The livestock were moved to an Altai wildrye pasture on 15 October, where they
grazed for about 30 days, until as close as possible to 15 November, when the
calves were weaned at about 244 days of age.
The 4.5-month seasonlong (4.5 SL) management strategy began on an unfertilized crested wheatgrass pasture, with grazing starting as close as possible to 1 May and continuing on that forage type for about 31 days. The livestock were moved to one native range pasture on 1 June, where they grazed for 135 days, until 15 October. Cows and calves were then moved to crop aftermath, where they grazed for about 30 days, until as close as possible to 15 November, when the calves were weaned at about 244 days of age.
The 6.0-month seasonlong
(6.0 SL) management strategy began as close as possible to 16 May, with grazing
on one native range pasture. The livestock remained on this pasture for 183
days, until as close as possible to 15 November, when the calves were weaned
at about 244 days of age.
Each treatment was stratified
on the basis of three range sites (sandy, shallow, and silty sites). Samples
from the grazed treatments were collected on both grazed quadrats and quadrats
protected with cages (ungrazed). Aboveground plant biomass was collected on
11 sampling dates from April to November. The major components sampled were
cool- and warm-season grasses, sedges, forbs, standing dead, and litter. Plant
species composition was determined by the ten-pin-point frame method (Cook and
Stubbendieck 1986) between mid July and August. A standard paired-plot t-test
was used to analyze differences between means (Mosteller and Rourke 1973).
Commercial crossbred cattle
were used on all grazing treatments in this trial. Individual animals were weighed
on and off each treatment and on each rotation date. Cow and calf mean weights
were determined for each grazing period. Live-weight performance of average
daily gain and accumulated weight gain for cows and calves was used to evaluate
each treatment. Cow animal unit equivalent (AUE) was determined through calculation
of the metabolic weight of the average animal as a percentage of the metabolic
weight of a 1000-pound cow (Manske 1998).
Results and Discussion
Forage feed costs of pasture-forage
management strategies by range cow production periods are compared in table
1.
Forage feed for a range
cow during the 32 days of the dry gestation production period, from mid November
to mid December, cost $1.27 per day, or $40.64 per period, for native range
pasture; $0.42 per day, or $13.44 per period, for mature crested wheatgrass
hay; $0.44 per day, or $14.08 per period, for cropland aftermath pasture; and
$0.38 per day, or $12.16 per period, for forage barley hay.
Forage feed for a range
cow during the 90 days of the third trimester production period, from mid December
to mid March, cost $1.67 per day, or $150.30 per period, for native range pasture;
$0.62 per day, or $55.80 per period, for mature crested wheatgrass hay; and
$0.38 per day, or $34.20 per period, for forage barley hay.
Forage feed for a range
cow with a calf during the 45 days of the early lactation production period,
from mid March to late April, cost $1.97 per day, or $88.65 per period, for
native range pasture; $0.80 per day, or $36.00 per period, for crested wheatgrass-alfalfa-corn
silage feed mix; and $0.41 per day, or $18.45 per period, for forage barley
hay.
Forage feed for a range
cow with a calf during the 31 days of the spring lactation production period,
from early to late May, cost $1.35 per day, or $41.85 per period, for native
range pasture; $0.52 per day, or $16.12 per period, for crested wheatgrass pasture;
and $0.51 per day, or $15.81 per period, for fertilized crested wheatgrass pasture.
Forage feed for a range
cow with a calf during the 137 days of the summer lactation production period,
from early June to mid October, cost $1.16 per day, or $158.92 per period, for
native range pasture managed by the 6.0-month seasonlong treatment; $0.81 per
day, or $110.97 per period, for native range pasture managed by the 4.5-month
seasonlong treatment; and $0.58 per day, or $79.46 per period, for native range
pastures managed by the 4.5-month twice-over rotation management system.
Forage feed for a range
cow with a calf during the 30 days of the fall lactation production period,
from mid October to mid November, cost $1.59 per day, or $47.70 per period,
during the early portion of the fall and $1.82 per day, or $54.60 per period,
during the late portion of the fall for native range pasture grazed at the proper
fall stocking rate; $1.18 per day, or $35.40 per period, for native range pasture
grazed at the summer stocking rate (This high stocking rate during the fall
is not sustainable.); $0.44 per day, or $13.20 per period, for cropland aftermath
pasture; and $0.40 per day, or $12.00 per period, for Altai wildrye pasture.
The lowest cost forage
feed for range cows is forage barley hay during the dry gestation period, the
third trimester period, and the early lactation period; fertilized crested wheatgrass
pasture during the spring lactation period; native range pastures managed by
the twice-over rotation system during the summer lactation period; and Altai
wildrye pasture during the fall lactation period.
The twice-over rotation
system, used to manage native range during the 137 days of the summer lactation
production period, applies defoliation treatments to grass plants at the appropriate
phenological growth stages to stimulate the defoliation resistance mechanisms
within the plants and the activity of the symbiotic microorganisms in the rhizosphere.
This stimulation increases both secondary tiller development of grasses and
nutrient flow in the rhizosphere, resulting in increased plant basal cover and
aboveground herbage biomass and improved nutritional quality of forage. The
increase in quantity and quality of herbage permits an increase in stocking
rate levels, improves individual animal performance, increases total accumulated
weight gain, reduces acreage required to carry a cow-calf pair for the season,
improves net return per cow-calf pair, and improves net return per acre (Manske
et al. 1988, Manske 1994, 1996).
After the defoliation resistance
mechanisms have been
stimulated by partial defoliation at the proper phenological growth stages,
the quantity of herbage biomass produced is related to the amounts of sunlight
and soil water available to the plants and by the amount of remaining leaf surface
area that is photosynthetically active. When cows remove a greater amount of
leaf material than is required to promote high levels of herbage production,
the quantity of standing herbage biomass is reduced.
Conclusion
Additional research is
needed to quantify exudation material; soil organism activity and biomass; nitrogen,
carbon, and phosphorus cyclic flows; and axillary bud development into tillers.
Such research would lead to a more complete understanding of the defoliation
resistance mechanisms of grassland plants and enable grassland managers to manipulate
defoliation for the increased benefit of the grassland ecosystems.
Data have shown that defoliation
of grass plants between the third-leaf stage and anthesis has beneficial effects
on the physiological responses within the plant, which allow for greater tiller
development, and beneficial effects on the symbiotic rhizosphere activity, which
increase the amount of available nitrogen for plant growth. Deliberate and intelligent
manipulation of these defoliation resistance mechanisms can increase secondary
tiller development and total herbage biomass. The secondary tillers increase
the nutrient content of the herbage
and thereby improve individual animal weight performance during the later portion
of the grazing season. The additional herbage biomass allows for a higher stocking
rate and a greater amount of herbage remaining standing after grazing. Increased
secondary tiller growth results in increases in plant density, canopy cover,
and litter cover. These increases reduce the impact of raindrops, reduce and
slow runoff, reduce erosion, and improve water infiltration.
The twice-over rotation
strategy systematically grazes each of three to six native range pastures for
two grazing periods. The first rotation period occurs between the third-leaf
stage and anthesis phenophase (1 June to 15 July), with grazing for seven- to
seventeen-days in each pasture, and the second rotation period occurs between
mid July and mid October, with grazing in each pasture for a period double the
length of the first. The combination of these two grazing periods maximizes
the beneficial effects of the defoliation resistance mechanisms of grassland
plants when adequate quantities of photosynthetically active leaf surface area
remain standing after each grazing period.
Acknowledgment
I am grateful to Sheri
Schneider for assistance in research data processing and production of this
manuscript. I am grateful to Amy M. Kraus for assistance in preparation of this
manuscript.
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