FORAGERS/BROWSERS - NOT GRAZERS
Goats as a species are herbivores, i.e. plant-eating animals. As ruminants, goats grasp and swallow partially chewed food into one of their four stomachs, then regurgitate it for further processing through a process known as chewing their cud.
With the exception of deer, goats have the fastest metabolism of all ruminants; they must eat continually (frequently) and require a higher quality of plant material than other ruminants in order to thrive. Goats are very selective eaters, using their prehensile mouth (lips and tongue) to choose the tastiest plants. As a percentage of body weight, goats consume a larger volume of plant materials than cattle eat.
Goats get their nutrition through the use of their highly-adapted teeth, mouths, and digestive systems to break down the cell walls of plant leaves. Leaves are primarily comprised of cellulose, a complex carbohydrate that supports the plant's cell walls, and lignin, which is the chief non-carbohydrate part of the plant that binds with cellulose to harden and strengthen cell walls. Lignin is in the vein structure of plant leaves as well as in the stem to give the plant enough rigidity to stand upright and compete for sunlight. The cell contents within the cell walls are the most digestible parts of the plant.
There are two types of vein structures in plant leaves: net and parallel venation. Broadleaf plants, forbs, and browse have net venation. The only forbs that have parallel venation are those that are "grass-like," which includes members of the lily and sedge families. All grasses have parallel venation.
Goats do best by eating forbs (weeds) and browse (leaves of trees, shrubs, and vines that have woody stems) rather than eating grasses.
Here is why:
The micro-organisms in the rumen can break down the material between the net venation of broadleaf plants more quickly than the material in parallel veined grasses. Since a goat has a rapid passage rate of material through its rumen, it needs to eat a plant that its rumen micro-organisms can break down quickly. Because the parallel venation in very young grasses is lower in non-digestible lignin, the micro-organisms in the goat's rumen can break them down rapidly too.The more mature grasses -- the grasses with the higher lignin content -- are harder for the goat's rumen micro-organisms to break down and process into nutrition. This does not mean that goats cannot eat grasses, but it does mean that goats need to eat young immature tender grass leaves -- leaves far less mature than cattle readily eat.
Broadleaf plants (forbs and browse) have their growing points at the top of the stems (apical dominance), which is why goats tend to eat the young tender growing parts of these plants at the tip of their stems. Goats eat the seeds of such plants because they are high in energy. Grasses, however, grow from the base of the plant (basal dominance), so the most digestible part of the grass leaf is the newer growth that comes from the ground level of the grass plant. And producers should remember that is also where the parasitic worms are that cause so much trouble with goat health.
Pasture Management and Re-Growth: Browsing of woody plants usually occurs at the end of the stems where the new growth is taking place. When these growth points are eaten, it takes more time and more nutrients to start new growth than grasses require to do the same thing. Continued feeding on lower new growth of browse plants is why browse lines on woody plants take so long to recover. When browse plants are continually eaten at a certain height, the plants tend to quit trying to grow new stems there and instead shift to other growth points higher up the plants which are less reachable for browsing. The is a survival mechanism for browse plants.
Because grasses grow from the base of the plant, they rebound and produce re-growth more quickly from grazing pressure than do forbs and browse. Continued re-grazing or grazing too close to the ground (where the crown point of the plant contains the buds needed for new growth) can cause trauma to the growth points and delay re-growth or deplete the nutrient reserves needed for re-growth. Over-grazing can cause the preferred plants in the pasture to die out and be replaced by less desirable and usually less nutritious plants. Noxious or toxic weeds will then begin to dominate the pasture because goats do not eat these plants.
To sum up: Stocking rates for goats are not based upon plant materials per acre available for consumption by goats. The producer must take into account the types of plant materials available that will provide the nutritional levels that goats need while giving them access to sufficient acreage to forage/browse over so that they can largely eat "from the top down" to avoid the internal parasites that wait for them on grasses near ground level.
This writer thanks Kent Mills, livestock nutritionist with HiPro Feeds, Friona, Texas, for his invaluable help in the preparation and editing of this article.
Suzanne W. Gasparotto
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