Kim Williams
Approximately three-fourths of the cultivated forage cropland and a major part of the rangeland in the United States includes grass species. In the North America alone there are 150 genera and 1500 species grown (Miller, 1984). Pastures have been a large part of the ecological history of the U.S.. Selection processes, by animals, environment, and man, have developed the dominant grazing species that are available today.
Although grazing can have a few detrimental effects, the advantages outweigh the drawbacks, when proper management procedures are applied. Valentine (1990) lists some of the positive effects of grazing as the following: 1) vegetative plant growth maintenance and delayed maturation, 2) stimulation of regrowth by defoliation, 3) maintenance of optimal leaf area index within a sward, 4) herbage nutritive value increases by ratio of new growth: old growth, 5) removal or prevention of thatch buildup, 6) reduced build up of dead vegetation and mulch that may stunt new growth and delay soil warming, and 7) selective grazing for biological control of plant composition. Implemented correctly, grazing can effectively enhance a farming and ranching system by increasing the yield and quality of the forage available from pastures.
Growth Habits
Management practices can directly influence the physiological and morphological characteristics of a grass. Grasses belong to the subclass monocotyledon which are characterized by monocot seeds. Grasses account for much of the pasture, turf, and hay fields that are used in production situations (Miller, 1984). Grasses can be differentiated by their adaptation to different climates. Warm-season grasses require higher and warmer temperatures like those in the tropics, whereas cool-season grasses prefer temperatures that are similar to the cooler, temperate climates. Examples of warm-season grasses are indiangrass (Sorghum nutans (L.) Nash), bermuda grass (Cyadon dactylon (L.) Pers.), big bluestem (Andropogon gerardi Vitman) and switchgrass (Panicum virgatum L.). The cool-season grasses include Kentucky bluegrass (Poa pratensis L.), orchardgrass (Dactylis glomerata L.), and smooth bromegrass (Bromus inermis Leyss.)
Grasses are also divided by seasonal growth habit, namely annual, winter annual, and perennial grasses. These seasonal differences can be further divided into the two growth types of sod forming and bunch type grasses. Sod forming grasses have a prostrate growth habit and have tillers that may grow horizontally through the leaf sheath to insure spreading. In contrast, bunch type grass growth is primarily between the leaf sheath and the culm to provide upright erect growth (Miller, 1984). Examples of sod forming grasses are bermuda grass and smooth bromegrass, while orchardgrass and timothy (Phleum pratense L.) are examples of bunch type grasses.
Grass Composition
The smallest unit of a grass is the phytomer. The phytomer consists of a blade, internode, node, axiliary bud , and root initials. Each young phytomer is organized into a tiller. The tiller is the basis from which most morphological classification occurs. These classifications are divided into vegetative, reproductive, and seed ripening growth stages. The seedling is composed of a primary tiller. The expansion of the young seedling is dependent on the original tiller producing buds from which replacement tillers are produced, ie. secondary tillers, tertiary tillers, etc.. The dormant or inactive buds can produce new shoots with their own growing point. These buds are the origin of new growing points when the old tiller dies. This process explains why a perennial plant can live for many years. As long as the tiller stays in the vegetative stage the potential for leaf production is increased (Valentine, 1990).
Physiology
Photosynthesis is the basic conversion of solar energy into chemical energy (Waller et al., 1985). The chemical energy produces and serves as an available source of nutrients to sustain plant life. Carbon, from the carbon dioxide (CO2) in the atmosphere, is combined with water and converted to carbohydrates with the utilization of solar energy. According to Gardner et. al (1985), photosynthesis occurs in all green tissues, especially in most crop leaves, that have the following: (1) a large, flat external surface (2) upper and lower protective surfaces (3) many stomata per unit area (4) extensive internal surface and interconnecting air spaces (5) an abundance of chloroplast in each cell and (6) a close relationship between the vascular and photosynthetic cells.
Fig 2. Process of carbohydrate production adapted from Understanding Grass Growth:
The Key To Profitable Production (Waller et al. 1985).
The leaves in grasses serve as the large, flat surface that intercepts light to maximize the solar energy being received. The epidermis or protective layer, limits the gas exchange due to the cuticle, a waxy substance creating a thin surface layer that lies between the plant and the atmosphere. The cuticle also prevents water loss by the plant. The guard cells, located within the stomata, serve as the transport mechanism for CO2 exchange when the stomata is open. Stomatal closure is important in prevention of water loss when water is a limiting factor.
The photosynthetic process is different for C3 and C4 plants. Within the leaf, anatomical differences in the structure of the mesophyll cells, bundle sheath cells, and the intercellular spaces affect the efficiency of the plant. The arrangement of mesophyll cells allows for an increase in the internal surface area, and therefore aids in the diffusion of CO2 from the stomata to the cell surface. These mesophyll cells contain large numbers of chloroplasts and are located where the actual light reaction for photosynthesis takes place. The C4 plant, usually considered as a tropical origin plant, seems to be more efficient in the utilization of photosynthetic substrate and has less respiration than C3 grasses when both are exposed to warm climates.
Carbohydrate
Waller et al., (1985) equated carbohydrate maintenance, storage, and utilization with a factory, warehouse, and consumer outlet. These are simulated for leaf area, storage, and growth demand, respectfully . The raw materials (sunlight, CO2, and H2O) are taken in at the factory. The factory can also be thought of as a source and the consumer outlet can be thought of as the sink. The transfer from source to sink is considered a translocation in a concentration gradient (Williams 1964; Moser 1977; Smith and Nelson, 1985).
Expanding young leaves use most of the carbohydrates they produce. Primarily in the form of sucrose (Duffus and Duffus, 1984), young leaves also import some carbohydrate from older leaves that are exporting (Smith and Nelson, 1985). If there is an abundance of supply (photosynthesis) storage of reserves will occur. Highly productive forage grasses elevate their tissue deposition rate at some time during the growth period. This places their upper leaves in a higher position in the canopy. The result is more direct sunlight and ,thus, maximum food production (Dietz ,1975). Reserves are located in the roots and rhizomes of perennial grasses and in the seeds and roots of many annual grasses. These reserves help provide energy when the plant is in need, such as a growing period. In perennial species, this may occur after the grass has been grazed or cut. The grass is in a vulnerable position, and the recovery is dependent on how close the cutting occurred. The new growth has to occur from a dormant bud situated close to the crown. This is similar to what happens in the spring when the plant is beginning to green up from winter dormancy.
Defoliation
Defoliation refers to a grass being clipped or grazed and has many effects on a stand of grass. The emphasis in this paper is directed to grazing. If the carbohydrates in the roots and rhizomes of perennial grasses have been left depleted from the previous year the current stand will have a low survivability rate. El Hassan and Krueger (1980) believe carbohydrate reserves play a significant role in the phenology, vigor, development and productivity of plants. Exhaustion of reserves as a result of excessive defoliation has generally been associated with reduction in vigor and ,ultimately, range deterioration.
The effect of defoliation of a grass tiller in a vegetative stage will depend on the portion of the shoot taken (Walton, 1983; Valentine, 1990). In a study completed by Lacey et al. (1994), the effect of defoliation and competition on total non-structural carbohydrates of spotted knapweed (Centaurea maculosa Lam.) were evaluated. Plants were defoliated at different rates (control, single defoliation, and multiple defoliation). While the carbohydrate concentrations were similar for the control and the single defoliation, the multiple defoliation concentrations declined. Regrowth and intensity of grazing have an impact on forage storage reserves for continued regrowth similar to that of winter carbohydrate reserves on early spring growth. If there is adequate leaf area, the carbohydrate storage does not need to be used. But, if severe defoliation has occurred the reserves are needed to help restore the plants needs. A clipping trial provided by Waller et al. (1985) compared plants clipped at two inches and at nine inches. The nine inch clipping grew twice as tall as the two inch clipping within the six-day regrowth period.
The time of year and frequency of defoliation can make a difference. Mullahey et al. (1991) examined two grasses, sand bluestem (Andropogon gerardii yar. paucipilus (Nash) Fern.) and prairie sandreed (Calamovilfa longifolia (Hook) Scribn.) over a three-year period. Three years of continuous defoliation depleted the whole stand, but one defoliation or two defoliations per season increased the yield, tiller number, and bud number. The single cuttings should be taken earlier in the spring and the two cuttings are best if they come from June and August. Very heavy stocking rates could be detrimental to the carbohydrate reserves, but moderate to heavy stocking rates have a positive significant effect on carbohydrate concentration and TNC (total non-structural carbohydrate) concentrations for both the crowns and roots.
Management seems to be the largest factor for avoiding depletion of a stand. Some key points made by Waller et al. (1985) are to delay initial grazing period, permit ample photosynthetic area to remain at the conclusion of grazing period, allow adequate rest intervals following grazing periods, and allow plenty of time for carbohydrate reserves and bud development to replenish. Some of the morphological characteristics that can be taken advantage of are a lower growing point, vegetative shoot production over reproductive shoot formation, and a deep and expansive root system (Valentine, 1990).
Lignin
Plant cell walls contain non-carbohydrate substances that are resistant to digestion, particularly lignin (Van Soest, 1982). Lignins are strengthening components that occur within the cellulose and other polysaccharides of certain cell walls (Salisbury and Ross, 1992). Growth can be described, according Gardner et. al (1985) as cell enlargement, cell division and cell differentiation. All of these processes are irreversible and involve many types of substances and syntheses. During the plant cell development, the primary cell wall is a thin wall and composed of cellulose microfibrils. The cellulose microfibrils are embedded in a matrix which includes compounds like hemicellulose and pectic substances. The secondary cell wall is much thicker than the primary wall. These walls consist of cellulose, hemicellulose, and lignins. The lignin makes the structure resistant to changes in form and compression. The secondary wall is much more rigid compared to the primary wall and has an elastic composition similar to a balloon (Salisbury and Ross, 1992).
Chemical Constituents
Lignin is a highly condensed phenylpropanoid polymer of high molecular weight. According to Van Soest (1982) lignin can vary widely according to the maturity of the plant due to the varying proportions of different monomers. The phenolic monomer units that are covalently linked to the cell wall are divided into core and non-core components (Jung, 1989). Core lignins are considered to contain two or more covalent linkages between phenolics. Three monomers which account for most of the core lignin molecules are p-coumaryl, coniferyl, and sinapyl alcohols (Van Soest, 1982 ; Church, 1988). These alcohols react as free radicals under the influence of phenol oxidase to form the polyphenols which make up core lignin.
The cinnamic acids of non-core lignin are synthesized by the shikimic acid pathway and serve as precursors for core lignin biosynthesis. The non-core lignin contains ester-linked p-coumaric and ferulic acids (Jung, 1989; Buxton and Russell, 1988) and are most prominent in grass forages (Church, 1988).
Lignin and Fiber Digestion
Lignin is the chemical constituent that is associated with nutrient indigestibility and facilitated with the determination of fiber digestibility. Jung and Vogel (1992) indicated that switchgrass and big bluestem differ in both detergent fiber composition and degradability. The lower degradability of big bluestem stem fiber is due to the greater degree of lignification of big bluestem. Griffin and Jung (1983) found similar results with 'Blackwell' switchgrass and 'NY 1145" big bluestem. The switchgrass seems to contain more NDF (neutral detergent fiber) than big bluestem. Buxton and Russell (1988) proved that lignin concentrations on a cell wall basis doubled with maturity in grass stems, but when the stems are immature, they are almost twice as digestible as mature stages on a cell wall-lignin basis. These digestibility concentrations could be related to the various monomers within the lignin.
Protein
Protein is a large component in the consideration of a diet. Many producers are concerned with the crude protein and also the amino acid composition of their forage. According to Mullahey et al. (1992), whole plant crude protein concentrations were higher for smooth bromegrass than switchgrass at each harvest date and declined with maturity. Plant nitrogen sources can be divided into two different categories, protein and non-protein nitrogen. Plant proteins can be broken into two different categories, proteins of the leaf and stem, and storage protein of the seed. Van Soest (1982 concluded that leaf and stem protein represents the actively metabolizing matter of living plants and leaves contain a high level of proteins. Although the leaves contain a larger amount of protein, some proteins are present as insoluble heat- denatured cytoplasmic and chloroplastic proteins, and they can be recovered by NDF. Therefore, the nitrogen content of NDF is greatly increased by heating of feeds, but not necessarily in ADF (acid detergent fiber) which requires various other reactions to render the protein recoverable in ADF. Additional heating can make protein more highly insoluble and recoverable in the ADF, also, but a protein that is insoluble in NDF and soluble in ADF appears to have higher digestibility, although it may digest at a slower rate.
Solubility and By-Pass Protein
Solubility has received attention due to its affect on digestion efficiency. Soluble compounds are degraded faster and at different rates than insoluble proteins due to microbial activity within the digestion system. Protein reaching the lower digestive tract without significant degradation is called escape protein. In recent years, research has been focused in this area because escape, or by-pass protein can be utilized more efficiently in some cases and, therefore, maximize gains in post-ruminal digestion (Anderson et al., 1988; Blasi et al., 1991; Hafley et al., 1993; Redfearn et al., 1995; Van Soest, 1982). Mullahey et al., (1992) reported that both switchgrass and smooth bromegrass had measurable levels of escape protein concentrations. Switchgrass escape protein concentration declined with maturity while smooth bromegrass by-pass protein concentration increased with fall growth.
An important implication in ruminal protein disappearance, reported by Redfearn et al. (1995) is the difference in species. Due to these differences, generalizations regarding protein degradability should not be made. Other factors such as, plant maturity, degradation rate, animal type, and other plant characteristics, should be considered when evaluating and developing strategies for protein supplementation.
Summary
Management is a key item which can be used to efficiently generate a productive system. Grazing models have been established to account for pasture size, stocking rates, grazing dates, animal nutrition, and forage needs. When establishing a system, the two most important factors to consider are animal type and forage needs. These factors are determined according to the needs of each individual farm or ranch.
Location, climate, and temperature can have a direct affect on the forage supply. Forage supply can, in turn, have an impact on the stocking rates. Forage yield and quality are all dependent on the circumstances of forage carbohydrate status, lignin amount, and protein availability reported in this paper. Extent of defoliation of leaves and the status of many essential nutrients are factors that can serve as measuring tools to help determine the growth demands of forage and its ability to supply herbage for a grazing operation. Once the forage supply has been determined, grazing systems, such as rotational, deferred, or continuous, are taken into account. The paddocks or pastures of a system can then be put into action. The animal factors of a system can follow a similar determination of supply and demand requirements. Once all the needs have been resolved, a balance of the two systems is set into action.
Literature Cited
Anderson, S.J., T. J. Klopfenstein, and V.A. Wilkerson. 1988. Escape protein supplementation of yearling steers grazing smooth brome pastures. Journal of Animal Science 66:237.
Blasi, D.A., J.K. Ward, T.J. Klopenstein, and R.A. Britton. 1991. Escape protein for beef cows: III. Performance of lactating beef cows grazing smooth brome or big bluestem. Journal of Animal Science 69:2294.
Briske, D.D., and J.W. Stuth. 1986. Tiller defoliation in the moderate and heavy grazing regime. Journal of Range Management 35(4): 511-514.
Buxton, D.R., and J.R. Russell. 1988. Lignin constituents and cell-wall digestibility of grass and legume stems. Crop Science 28:553-558.
Church, D.C. 1988. The ruminant animal: digestive physiology and nutrition. Prentice-Hall Inc. Englewood Cliffs, NJ.
Dietz, Harland E. 1975. Special report: grass: The stockman's crop. How to harvest more of it. Range Conservationist Soil Conservation Service.
Duffus, C.M., and J.H. Duffus. 1984. Carbohydrate metabolism in plants. Longman House, New York, NY.
El Hassan, Babiker. and William C. Krueger. 1980. Impact of intensity and season of grazing on carbohydrate reserves of perennial ryegrass. Journal of Range Management 33(3): 200-203.
Gardner, Franklin P., R. Brent Pearce, Roger L. Mitchell. 1985. Physiology of crop plants. Iowa State University Press, Ames, IA.
Griffin J.L., and G.A. Jung. 1983. Leaf and stem forage quality of big bluestem and switchgrass. Agronomy Journal 75:723-726.
Hafley, J.L., B.E. Anderson, and T.J. Klopfenstein. 1993. Supplementation of growing cattle grazing warm-season grass with proteins of various ruminal degradabilities. Journal Animal Science 71:522-529.
Jung, H.G. 1989. Forage lignins and their effects on fiber digestibilities. Agronomy Journal 81:33-38.
Jung, H.G., and Kenneth P. Vogel. 1992. Lignification of switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii) plant parts during maturation and its effect on fibre degradability. J Sci Food Agric 59:169-176.
Lacey, John R., Kathrin M. Olson-Rutz, Marshall R. Haferkamp, and Gregory A. Kennett. 1994. Effects of defoliation and competition on total non-structural carbohydrates of spotted knapweed. Journal of Range Management 47:481- 484.
Mullahey, J.J., S.S. Waller, K.J. Moore, L.E. Moser, and T.J. Klopfenstein. 1992. In situ ruminal protein degradation of switchgrass and smooth bromegrass. Agronomy Journal 84:183- 188.
Mullahey, J.J., S.S. Waller, and L.E. Moser.. 1991. Defoliation effects on yield and bud and tiller numbers of two Sandhills grasses. Journal of Range Management 44(3):241-245.
Miller, Darrell A. 1984. Forage crops. McGraw-Hill, Inc., New York, NY.
Moser, L.E. 1977. In rangeland plant physiology. Range Sci Ser 4. Denver, Colo: Society of Range Management.
Redfearn, Daren D., Lowell E. Moser, Steven S. Waller, and Terry J. Klopfenstein. 1995. Ruminal degradation of switchgrass, big bluestem, and smooth bromegrass leaf proteins. Journal of Animal Science 73:598-605.
Salisbury, Frank B., and Cleon W. Ross. 1992. Plant physiology. 4th ed.. Wadsworth Publishing Company, Belmont, CA.
Smith, D., and C.J. Nelson. 1985. Forages: The science of grassland agriculture. 4th ed. Iowa State University Press, Ames, IA.
Valentine, John F. 1990. Grazing management. Academic Press Inc., San Diego, CA.
Van Soest, P.J. 1982. Nutritional Ecology of the ruminant. O & B Books, Inc., Corvallis, OR.
Waller, Stephen S., Lowell E. Moser, and Patrick E. Reece. 1985. Understanding grass growth: The key to profitable livestock production. Trabon Printing Co., Inc. Kanas City, MO.
Walton, Peter D. 1983. Production and management of cultivated forages. Reston Pub. Co., Reston, VA.
Williams, R.D. 1964. Outlook. Agr 4:136-142.