To understand how defoliation impacts grass growth we must first understand how grasses grow. The basic unit of a grass plant is a phytomer, which consists of a blade, sheath, node, internode, and axillary bud (Figure 1). A series of phytomers make up a tiller and an assemblage of tillers make up a grass plant. Grass plants also have different types of growth forms which is a result of how the tillers are arranged within a plant. Bunchgrasses have the majority of the tillers growing vertically and remaining close together. Sodgrasses have tillers that grow out at angles, allowing the plant to spread out more.

Figure 1. Grass plant organization (Heitschmidt and Stuth 1991)

It is also important to understand the stage of growth that a plant is in in order to know how it will respond to defoliation. When a grass is in the vegetative state there is no true stem, it is instead a collection of leaf blades and sheathes rolled together. At the base of each tiller is a growing point referred to as the apical meristem (or dome) and new leaf initiation occurs on opposite sides of this point. At the base of each leaf is also a bud (axillary bud) that may grow into a new tiller. When a grass becomes reproductive the internodes elongate to form a true stem. At this time the apical meristem converts from forming new leaves to forming floral structures so that once this change occurs no new leaves can be formed on that tiller. Most grasses can also reproduce vegetatively (through tillering, rhizomes and stolons) and so do not have to set seed for continued survival.

Another factor that impacts how grass plants respond to grazing is how they store and use carbohydrates. Plants are able to photosynthesize and convert energy from the sunlight to store in the form of carbohydrates, which they can then access for continued growth. If a plant is defoliated it can draw on these carbohydrate reserves to help support new growth. However, in order to photosynthesize plants need green leaf material and continued removal of leaf material without allowing the plant an opportunity to regrow and store more carbohydrates can stress the plant. You can think of this as a battery, if the battery is continually drained without being given an opportunity to recharge it will eventually go dead.

Now that we have covered how grasses grow and how they store energy we can tie this back into our original question of how grasses respond to defoliation. In a nutshell, grass plants are well designed to deal with defoliation because the location of their growing points close to the ground means that they can continued to grow leaves if upper leaf material is removed and their ability to store carbohydrates also helps to give the plant energy to grow new leaves if defoliation occurs. Grass plants may also have different avoidance and tolerance mechanisms that help them to deal with defoliation. Avoidance mechanisms help to reduce the probability of grazing and examples would include awns, spikes, and biochemical compounds that could make the grazing animal sick. Tolerance mechanisms help to increase growth following grazing such as carbon allocation and leaf replacement potential. Some plants may exhibit both avoidance and tolerance mechanisms and this can give them a competitive advantage in the plant community when it comes to defoliation.

So what does all of this mean in terms of range management? Understanding how a grass responds to grazing can help us to minimize any negative impacts by adjusting the intensity, frequency, timing and duration of grazing. Intensity relates to how much of the plant material is removed and this relates back to both the location of the growing points and the carbohydrate storage. Frequency is how often the plant is defoliated and this defoliation can occur at several different times through out the grazing season. Timing refers to when in the season the plant is defoliated, for example spring versus fall. Finally, duration relates to how long the animals are left in an area. One other factor to keep in mind is the type of animal that is being managed as plant selection and grazing impacts vary by species.

In summary, grasses are well designed to deal with defoliation as their growing points are located close to the ground. Two key points to keep in mind are: 1) that if a grass plant is in a vegetative state and defoliated it can continue to produce more leaf material but if they are in a reproductive state that tiller will no longer produce more leaves and 2) if a grass plant is continually defoliated without a chance for recovery lack of carbohydrates may also impact plant growth. It is important to tie these concepts into range management to help keep our grasslands both healthy and productive.

References
Bittman, S.,Schmidt, O. and Cramer, T.N. 1999. Advanced Forage Management.
Available online at http://www.farmwest.com/library/bookindex.cfm?bookid=1
(See Chapter 1 – How Grasses Grow)

Heitschmidt, R.K. and Stuth, J.W. 1991. Grazing Management: an Ecological
Perspective. Available online at http://cnrit.tamu.edu/rlem/textbook/textbook-fr.html
(See Chapter 4 – Developmental Morphology and Physiology of Grasses)

Langer, R.H.M. 1972. How Grasses Grow. Edward Arnold (Publishers) Ltd. London. 60pp