Illustrate and Explain grazing optimization theory
There are many ways of grazing plants respond differently to different levels of grazing, and most do well at 50% Slide 55
Why do unpalatable plants replace palatable plants under heavy grazing?
Because they have the reproductive capacity to do so as well as the energy required to grow larger, and take up the space around them. When heavy grazing occurs the plant doesn’t have the ability to recover as quickly as plants that aren’t grazed.
First, define grazing avoidance and grazing tolerance. Next, describe 4 mechanisms that grasses and forbs have developed that increase their resistance to grazing
A) Avoidance: Try not to be consumed
B) Tolerance: Ability to recover from grazing
Actively growing leaves usually retain all of their photosynthate (carbohydrates) and import carbohydrates from older leaves. this has to do with “source-sink” relationships relative to carbohydrates
A) What is a carbohydrate “source”?
B) What is a carbohydrate “sink”?
A) source is where carbs are generated (leaves)
B) sink is where the carbs are used or stored (reproductive tissue, fruits)
Indicate the following grass tiller parts on the diagram:
If a grass plant is grazed, its ability to regrow is dependent on:
A) carbohydrate reserves
B) Amount of material left there (leaf area)
If a plant, regardless of the species, must be capable of physiologically responding to an aerially-applied herbicide or one might as well not sprayed.
Perennial herbaceous plants should be sprayed in the vegetative stage in order to be most effective
the rosette stage of herbaceous plants is the reproductive stage.
What two criteria must be met in order to successfully root-kill mesquite?
1) It must be 75 degrees Fahrenheit at 12-18 inches below the soil surface
2) It must be in the yellow bud, elongation stage
In what phenological stage can one root-kill broom snakeweed?
reproductive (post flower)
It is easy to root-kill regrowth mesquite when the plants are in the long-shoot stage.
The reproductive shoots in woody plants are (long-shoots, short shoots)
Droughts are the norm in the southern Great Plains rather than the exception to the rule.
Prolonged weather pattern characterized by what percentage of the precipitation constitutes a drought, by definition?
Two strategies that one must observe and follow to properly manage for a drought.
1)Manage according to how grasses grow.
2) have a grazing system where there are areas you don’t graze for a period of time or ability to relocate animals
What is range inventory and monitoring?
A) Inventory: taking into account what is on the range (everything on the land)
B) Monitoring: Watch how the grasses respond to grazing as well as how the animals do on the range land and keeping the balance
Long-shoots in grasses are (vegetative or reproductive)?
The most important period in the annual cycle of a plant, regardless of whether it is an annual or a perennial is what stage?
seed formation and reproductive
Illustrate and explain the major trends in plant food use and food storage during a year and indicate where heavy grazing would be most detrimental. Explain Why.
September/October would be detrimental because thats when the plant is preparing for next year’s growth.
Mention the three primary growth stages in grasses that you need to be able to recognize for grazing management.
Give 5 reasons why plants must store carbohydrates
1. Increase number of tillers
2. root regrowth
3. Increase seed production
5. Tolerance to drought
What is the main differnce between Structural and non-structural carbohydrates?
Non-structural carbs are used for energy
Name one herbicide that could be considered a “silver bullet” that is effective in controlling noxious plants regardless of physiological stage of growth of the noxious plant or prevailing environmental conditions at the time of herbicide application.
Effects of grazing on plants depends on grazing intensity. Write 4 positive effects of light grazing and 4 negative effects of heavy grazing on plants.
Positive of Light:
1. increase root growth
2. Increase number of tillers
3. decrease shade to new tillers
4. increase photosynthesis
Negative of Heavy:
1. decrease number of tillers
2. Harm root growth
3. decrease photosynthesis
4. decrease seed production/ reduce carbohydrates
The ability to tolerate or avoid grazing differs from species to species and from season to season. Why? What are the implications for grazing management (Plant-herbivore interactions)?
If the plant is a decreaser then they will not tolerate grazing very well.
If the plant is an increaser it will tolerate grazing in a moderate fashion and recover very well, but if overgrazed then it gives up its growing capacity to compete with invader species.
If the plant species is an invader then it avoids being grazed and can take over a property easily with out the proper grazing management plan in place.
Removal of the apical meristem from a tiller by grazing will:
generally stimulates new tiller growth by auxillary buds
Which of the following merisematic tissues yields the slowest rate of regrowth after a plant is defoliated?
basal axillary or lateral buds
Typical Cycle of
Why can’t all the material be removed from forage plants by grazing during dormancy?
You can’t remove all the material because then the plant will die from the inability to use carbohydrates stored in the leaves to reproduce and grow new tillers for the following year.
= to protect fall regrowth
= would take a long time to recover (protect soil)
Plants must have Leaves for _________.
Grazing has the least affect on plants during _________, because they are ________ ________.
Grazing has the most severe effect on plants towards the __________ (_____) because the plant’s demands for _______ are higher and little time remains of optimal temperature and moisture conditions for regrowth.
End of the growing season (seed formation to seed hardening)
Grazing _____ in the growing season has less effect on plants than ______ in the growing season because…
considerable time remains when temperature and moisture are optimal for regrowth
The key factor in meeting grass plant biological requirements is… Why?
proper timing of the defoliation
The effect of defoliation is not simply the removal of herbage from grass plants
Defoliation also changes physiological processes in all parts of the plants
Alters the plant community microclimate by changing light transmission, moisture relations, and temperature
Factors that Affect Response of a Plant Defoliation
Range plant survival depends primarily on…
1. Synthesis and storage of food.
2. Formation of vegetative structure for renewal of top growth.
3. Maintenance of a healthy root system.
4. Production of reproductive organs.
1.plants live only one growing season
2.two growing seasons. Normally these plants form a basal cluster of leaves the first year and send up a seed stalk the second year
3.live from one year to the next, production leaves and stems for more than two years from the same crown. Most range plants are perennial.
Cool season species:
1. Make most growth in cool weather of spring and fall
2. Flower mostly in early summer
3. Have C3 photosynthetic pathway – adapted to cool wet conditions
4. Provide spring/fall forage at lower elevations and summer forage at high elevations.
Warm season species:
1. Make most growth in warm summer periods
2. Flower from mid-summer to early fall
3. Have C4 photosynthetic pathway – adapted to hotter drier conditions.
4. Provide forage in summer months
5. Most plants in Texas are C4 plants
1. Woody plants that retain leaves throughout the year
2. Cool-season plants (remember they photosynthesize in winter)
3. Important forage in drought and winter
Forage Value –
Plants can also be classified by how well it provides nutrients to grazing animals.
The forage value of a plant varies depending on which animal is grazing it because nutritional needs and dietary preferences differ by species of grazing animals.
For example, a plant could have excellent forage value for cattle and poor forage value for deer
High forage value
designates plants that are nutritious, palatable, and produce abundant forage.
Medium forage value
denotes a plant that will provide adequate nutrients if eaten, however, it is not preferred by animals or does not produce abundant forage.
Low or poor forage value
describes plants that simply do not provide adequate nutrients to the grazing animal. Some plants with poor forage value would be described as poisonous or toxic plants. These plants contain natural chemicals that are harmful to grazing animals and can even cause death if eaten in sufficient quantities`
Plant Response to Grazing – Range managers classify plants based on level of grazing resistance or response to grazing:
are the first plant to die out under continued heavy grazing. These native plants decrease because they are either quite palatable and sought out by grazing animals or they lack physiological attributes that help them recover from grazing.
are native plants that generally increase their numbers as decreaser plants are reduced. Many increaser plants avoid grazing damage because they grow close to the ground or are less palatable than decreasers
Increasers often also possess physiological mechanisms that help them recover from grazing. These plants should be watched with caution because they are a sign of high grazing pressure and can increase in number and abundance beyond what is desirable. If overuse continues, even increaser plant may not survive.
are commonly weedy plants that become established because more desirable plants have been diminished by excessive grazing. Invaders are usually signs of overgrazing. However, many noxious rangeland weeds are able to invade into even healthy range communities.
Factors Affecting Photosynthesis And Respiration
1. leaf area
1. C3 plants have an optimum temperature range of 65-75 degrees F. Growth begins when the soil temperature is 40-45 degrees F.
2. C4 plants grow best at 90-95 degrees F. They begin to grow when the soil temperature is 60-65 degrees F.
5. soil nutrient levels
6. carbon dioxide
How does grazing alter plant physiological activities?
1. Remove plant tissue
2. May remove meristematic tissue
3. Change Live:Dead ratio of tissue
4. Carbon gain = leaf area + photosynthetic rate. After grazing photosynthetic rate increases (15-50%)in some plants. This is called compensatory photosynthesis
Why plants must store carbohydrates?
1. Root replacement and growth
2. Regeneration of leaves and stems after dormancy
3. Respiration during dormancy
4. Bud formation
5. Regrowth after top removal by grazing.
Three kinds of Meristems
Apical – at tip of shoot or tiller. Responsible for growth and differentiation of tiller.
Axillary – located at the base of a phytomer, just above the node. Responsible for growth of the phytomer
Intercalary – base of leaf, sheath, and internode. Responsible for growth of leaf, sheath, and internode.
Regions of permanent tissue
A) base of internodes
B) leaf blades
C) leaf sheaths
Vegetative stage the stem, with its growing point, remains compact near the soil line.
is the period during internodes elongate and is often referred to as jointing
Tiller elongates —->response changes daylength
-uppermost internodes elongate.
-The lowermost internodes do not elongate and remain at the base of the plant.
-The elongation period is sometimes referred to as transition because it represents the transition between vegetative and reproductive growth.
reproductive shoot—–> inflorescence is still enveloped in the enclosing sheath or sheaths.
is the period during which the developing inflorescence emerges and pollination occurs
Grazing Optimization Theories
I. Removal of foliage must, to some degree, reduce the potential to compete and retain their status in the community.
II. However, removal of apical dominance by grazing or browsing has long been known to increase productivity (wheat).
III. Grazing optimization theory – since vegetation and herbivores evolved together, some degree of use is optimum.
An increase in plant growth results from increased utilization up to a point; and then any further increase in utilization will result in plant damage until death.
Mechanisms that reduce the probability of defoliation (anatomical and architectural attributes + secondary compounds)
Mechanisms that facilitate regrowth following defoliation
1. Higher proportion of culmless shoots than species with low resistance (basal leaves)
2. Greater delay in elevation of apical buds than species with low resistance (switchgrass)
3.Sprout more freely from basal buds after defoliation (Bermuda grass)
4. High ratio of vegetative to repro stems (WW B Dahl)
1. Produce large number of viable seeds
2. Delayed elevation of growing points
3. Poisons and chemical compounds that reduce palatability
1. Spines or thorns that discourage browsing
2. Volatile oils and tannins that reduce palatability
3. Branches that make removal of inner leaves difficult
4. Only current year’s growth of shrubs is palatable and nutritious
d. these are not reused in other processes
= available carbohydrates
e. these are translocated within the plant and used for growth, respiration, etc.
Role of Carbohydrates in Plant Growth and Development
(Moderate to high levels are necessary for/to)
1. Maintain plant vigor
2. Support plant function during dormancy (respiration)
3. Rapid regrowth after dormancy and defoliation
4. Root, rhizome and vegetative growth and seed production
5. Tolerance to drought, frost and heat
6. Resistance to insect damage and disease
7. Formation and maintenance of nodules in legumes
wherever carbohydrates are used and/or actively stored we have a “sink”
A. Meristematic activity
B. Underground parts******
C. Nongreen aerial parts – buds, flowers, fruits
D. Cells in stems and petioles
E. Immature leaves
wherever carbohydrates originate we have a “source”
B. Storage tissues
Note – many plant tissues act as both source and sink – depending on timing, plant development, etc
Typical Cycle of TNC Concentration
Maximum concentration immediately before dormancy
– Slow reduction during dormancy
– Rapid depletion during spring growth
Minimum levels reached during floral initiation
– Increase after seed development
– “Dip” during fall regrowth