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Grass Mechanics 101

Posted Mar. 1, 2001

Many ranchers use tractors, four-wheelers, grain drills, hay balers, and other assorted machinery to help them accomplish their everyday work. These tools of the trade help increase the efficiency of many agricultural operations. A good many producers know how to repair machinery such as this, and if they don't, they can refer to the instruction manual that usually comes with these devices. However, instructions don't come with the most important tool of the trade for most ranchers: GRASS. Ranchers spend many dollars every year for herbicides, fertilizer, and the like in an effort to fix grass. These are important tools, but one must first understand the mechanics of grasses before deciding what tools to use.

When you compare forage yields, you find that all grass is not created equal. However, when you look closely at grasses, you find that they all have the same parts. The primary unit of all grass plants is the phytomer, or shoot. All shoots comprise the following six parts: blade, sheath, axillary bud, node (joint), internode, and ligule (figure1). Some of these parts, such as the blade, node, and more often the ligule, are used to identify grasses when more readily identifiable parts such as the seed head are absent. Grasses also have several key growth points. The main growth point is in the crown of the plant (plant base) in the beginning of the growing season. When grasses begin developing stems, this growth point is elevated and eventually develops into a seed head. Therefore, the primary function of the main growth point is to manufacture shoots and eventually seed. All shoots originating from one main growth point are collectively called a tiller. Grasses also have what we'll call secondary growth points at each node (or joint) and at the base of each leaf blade. These growth points are responsible for leaf, sheath, and stem growth (figure 2).

Axillary buds (figure 1), which are located at the base of each shoot, are undeveloped growth points that can mature into main growth points and produce shoots of their own when the existing main growth point is grazed or cut off, or when it turns into a seed head. In some grasses, axillary buds can develop into main growth points regardless of the fate of the existing main growth point. However, in most native grasses, the main growth points remain toward the base of the plant where they can be protected until the change in day length triggers stem elongation, flowering, and seed production. Once the seed head develops, tiller growth is restricted to secondary growth points and axillary buds. One of the key things to remember in grass mechanics is that leaf growth stops when the ligule is formed, and sheath growth stops when the ligule is exposed (figure 3). A leaf that hasn't formed a ligule can be bitten by a cow and still regrow. However, once the ligule is there, further tiller growth will have to come from other leaves that haven't formed ligules or from previously developed axillary buds. Therefore, if you learn to identify grasses by the ligules, you will know that when you see one, you can write off any further leaf growth.

So why is all this important? Without knowledge of how grasses function, we often manage to mismanage them, which can lead to reduced production and lost opportunities for profit. The rate of regrowth after grazing is faster with secondary growth points because they expand previously divided plant cells of leaves and leaf sheaths. Although axillary buds are the slowest to respond after grazing, they have the highest potential for forage production because they can develop shoots of their own. For all of this to happen with any degree of success, most grasses need a break from being grazed so that they have enough leaf material to efficiently carry out photosynthesis. In other words, grasses need an opportunity for regrowth. The severity of grazing determines the length of this rest period. If a plant is grazed too short and the main growth point is removed before the axillary buds develop, the plant will regrow slower, extending the rest period. If livestock are rotated to another pasture before they can graze the plant too short, the rest period can be shorter because more leaf area is available to convert sunlight into plant tissue, and previously developed growth points are present.

The three plants in figure 4 illustrate the importance of understanding grass growth. On the left is a vigorous native grass plant that has yet to be grazed. The middle plant is a grass that has been properly grazed and is ready to be rested. The plant on the right has been overgrazed so that the small amount of leaf area can support only a much smaller root system, and most regrowth is restricted to slowly developing axillary buds.

As if the growth stage of this last plant were not tragic enough, matters can be compounded during dry weather by a root system shrinking farther away from available soil moisture and a weed population subsequently encroaching. Oftentimes, the tool used to fix the grass in this situation is herbicide, which remedies only the symptoms of the problem. In many cases such as this, knowledge of plant identification, grass growth mechanisms, and grazing management principles is often used to prevent the problem.

It's unrealistic to think that you won't have some overgrazed plants in your pastures. In fact, most profitable managers have pasture plants similar to all three shown in figure 4, but the occurrence of the last plant is generally isolated to high-use areas such as those around water points, shade, and mineral feeders.

One of the things that separate good managers from the rest is the ability to pick the right tool for the job. Learn more about grass and add another tool to your toolbox in the process.

Interested in grass identification? Take a look at our Plant Image Gallery.

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