Chapter 5: Range Plant Physiology

1. Basic concepts of plant growth2. Importance of carbohydrate reserves3. Grazing effect on forage plants 4. Grazing resistance in grasses, forbs and shrubs5. Grazing theory a. Why palatable plants dominate rangelands with

good grazing management?b. Why unpalatable plants dominate rangelands

under sustained heavy grazing (over grazing)?

A few basic principles concerning the influence of grazing on plants

1. Plants must have leaves for photosynthesis.2. Grazing has the least effect on plants during the

dormant season when they are photosynthetically inactive.

3. Grazing has the most severe effect on plants towards the end of the growing season ( seed formation to seed hardening) because the plant’s demands for carbohydrates are higher and little time remains of optimal temperature and moisture conditions for regrowth.

4. Grazing early in the growing season has less effect on plants than late in the growing season because considerable time remains when temperature and moisture are optimal for regrowth.

Why plants must store carbohydrates

1. Root replacement and growth2. Regeneration of leaves and stems after

dormancy 3. Respiration during dormancy 4. Bud formation 5. Regrowth after top removal by grazing.

Grazing Influences on Plants.*Effect of grazing on plants depends on grazing intensity.

Heavy grazing

1. Decreased total plant photosynthesis

2. Reduced carbohydrate production and storage

3. Reduced root growth

Light –to –moderate grazing

1. Increased photosynthesis

2. Increased tillering

3. Reduced shading from other tillers and plants

Grazing Influences on Plants cont.

Heavy grazing

4. Reduced seed production

5. Reduced ability to compete with ungrazed or lighter grazed plants

Light –to –moderate grazing

4. Reduced transpiration losses

5. Inoculation of plant parts with growth promoting substances.

Grazing Influences on Plants cont.

*Positive effects of grazing become less likely the drier or more arid the climate becomes.

Characteristics associated with high grazing resistance in grasses

1. Higher proportion of culmless (stemless) shoots than species with low resistance.

2. Greater delay in elevation of the apical bud than species with low resistance.

3. Sprout more freely from basal buds after defoliation than species with low resistance.

4. Have a higher ratio of vegetative to reproductive stems than species with low resistance.

Grazing resistance factors in shrubs

1. Spines and thorns 2. volatile oils and tannins that reduce

palatability3. Branches make removal of inner leaves

difficult4. Only current year’s growth is palatable

and nutritious for most species.

Grazing resistance factors in forbs.

* 1. Produce a large number of viable seeds 2. Delayed elevation of growing points3. Poisons and chemical compounds that

reduce palatability.

Grazing effects are influenced by:

1. Intensity 2. Timing3. Frequency4. Grazing of surrounding plants

*Grazing resistance of forage classes from most –to –least resistant.

1. Grasses 2. Shrubs 3. Forbs

*Many exceptions do occur.