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* U.S. Department of Agriculture-ARS, Forage and Range Research Laboratory, Utah State University, Logan, UT 84322–6300 and Rocky Mountain Research and Consulting, Center, CO 81125
3 To whom correspondence should be addressed. E-mail: njchatt{at}cc.usu.edu.
KEY WORDS: • carbohydrate • fructan • forage • equine • laminitis
Nonstructural carbohydrate (NSC)4 fractions found in forage may play a role in equine diseases that involve carbohydrate intolerance, such as laminitis. Starch in seed grains such as oats (Avena sativa L.), barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), and corn (Zea mays L.) has been used to induce equine laminitis in clinical studies (1).
Sugars in forage may adversely affect equines with dysfunctions of glucose metabolism. Insulin resistance has been associated with laminitis in equines (2), although the mechanisms by which excess sugars can trigger an episode are not yet understood. French and Pollitt (3) were the first to experimentally induce laminitis using fructans. Much of the existing data on NSC concentrations in forage use analytic methods that extract and quantify sugars and fructans collectively as water-soluble carbohydrates. In this study, purified hydrolytic enzymes were used in the analytic procedures to separate starch, fructan, and sugars, thereby facilitating quantification of individual carbohydrate fractions.
High concentrations of NSC, including sugars (glucose, fructose, and sucrose-GFS), fructan, and starch, must be considered in developing feed rations for horses and ponies prone to laminitis. Numerous factors are known to influence NSC concentrations in various plant parts. Nonstructural carbohydrate content and type depends on the plant species, plant part, stage of development, and environmental conditions such as root and shoot temperatures during growth as well as light intensity and duration, plant nutrient availability, and water status (4,5). Nonstructural carbohydrate concentrations vary through time, with lesser amounts being present during the morning than afternoon and early evening hours (6,7). It is often assumed that mature plants are higher in fiber and lower in NSC content than immature plants. Generally, any environmental condition that restricts growth (NSC utilization) to a greater extent than photosynthesis (NSC synthesis) results in increased amounts of NSC in plant herbage (8,9).
Cool-season grasses, those of temperate origin, grown under cool temperatures, accumulate soluble sugars, starch, and fructan, whereas warm-season grasses accumulate soluble sugars and starch but no fructan (10). Thus, cool-season and warm-season grasses have different metabolic pathways by which they fix and store carbon. Nonstructural carbohydrates are the sum total of GFS, fructan, and starch. As a cool-season grass that utilizes fructan as a storage carbohydrate, oat forage is an appropriate model for investigating the relationships that influence fructan concentration.
Fructans are water-soluble carbohydrate chains formed from the attachment of multiple fructose molecules (a few to hundreds or even thousands) to a sucrose molecule (11). The fructans that occur in most cool-season grasses, including the small grains wheat, barley, and oats, are called phleins. Those found in dicotyledonous plants are known as inulins (12). Although the roles of fructans in plant metabolism are not fully understood, they serve as a carbohydrate reserve (4). Accumulation of fructan occurs within cell vacuoles (13) and is often associated with conditions under which the rates of metabolism and plant growth are lower than the rate of photosynthesis (9). In contrast, starch generally accumulates within leaf chloroplasts in vegetative tissues.
A horse that ingests 10 kg/d of dry hay may be eating nearly 3 kg of sugars and starch (30% dry weight of diet). Thus, information on NSC content of forages is important in determining feed rations for carbohydrate-intolerant equines. Generalizations about NSC concentrations in forages are often difficult because of the many environmental and plant growth-related factors that influence carbohydrate metabolism and accumulation in forage (14). Generally, NSC is lost by respiration or leaching during drying of forage. Slow drying, as a result of cool or wet weather, generally increases NSC losses.
Because both environmental conditions and stage of plant maturity are thought to influence carbohydrate content of forages, an experiment was designed to quantify changes in carbohydrates in oats grown under field conditions and harvested for dry forage. Oats grown for forage are often planted and grown as an early first crop in the spring, but they are also grown as a late season crop. The objectives of this study were to quantify NSC concentrations at various times during development in oats planted in both spring and summer and to describe nutritional composition for both immature and mature plants grown in warm and cool environments.
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MATERIALS AND METHODS |
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 TOP MATERIALS AND METHODS RESULTSLITERATURE CITED |
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Plots were sprinkler irrigated as needed, generally every 3 d. Plant samples were harvested in the afternoon (1500–1700) from each of the replicated plots. Stage of plant growth was standardized across the 2 plantings in the 2 years by harvesting samples at specific developmental stages using the Feekes scale (15). Developmental stages are tiller, all leaf tissue, no stem elongation; joint, early stages of stem elongation; boot, stem has elongated, and reproductive tissue is developing but not emerged; flowering, stem fully elongated, reproductive organs present, but no seed development; milk, seeds are filled with milky fluid; soft dough, seed tissue is becoming solid; mature, green color absent from seeds. Planting dates, harvesting dates, and corresponding stages of growth at harvesting are listed in Table 1. Maximum and minimum air temperatures over the plant growth period are plotted in Figure 1.
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-glucosidase was used for digestion of sucrose to specifically measure GFS instead of the MegaZyme sucrase/ß-amylase/pullulanase/maltase enzyme specified in AOAC 999.03. Standard curves for the color reactions were prepared using glucose (for starch) or equal concentrations of glucose plus fructose for other measurements. Samples were analyzed for N using a LECO CHN-2000 Series Elemental Analyzer (LECO Corp.). Multiplying N x 6.25 established levels of crude protein (CP). Neutral detergent fiber (NDF) was determined using procedures described by Goering and Van Soest (16). An ANKOM-200 Fiber Analyzer (ANKOM Technology) was used to determine NDF. Mean values and standard deviations were determined using Microsoft Excel software.
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RESULTS |
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TOPMATERIALS AND METHODSRESULTSLITERATURE CITED |
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The amount of NSC present in harvested and grazed forages varied with several factors including date of planting, season of harvest, ambient temperatures, and plant maturity. Nonstructural carbohydrate contents should be considered in formulating feed for laminitic equines. The ideal approach is to chemically analyze all feeds. When feed analysis is not an option, practitioners should consider the following: 1) The concentrations of the various carbohydrate contents are not always inversely related to plant maturity. In fact, GFS (concentrations up to 15% dry weight) is the only carbohydrate fraction that always declines with plant maturity. 2) Because ambient temperatures at or just before harvest have been shown to influence fructan content in 
100 cool-season grasses when grown in controlled environments (10), the fructan contents of oat forage may also be related to seasonal changes in air temperatures. 3) Starch is present in vegetative tissues (up to 10% dry weight) and generally increases with maturity. 4) Fructan and starch are the major NSC components in harvested oat hay; however, concentrations of GFS may be high during the joint and boot stages of growth. 5) Environmental conditions may be as important as plant maturity in determining NSC content of oat hay.
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FOOTNOTES |
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2 Author disclosure: no relationships to disclose.
4 Abbreviations used: CP, crude protein; GFS, glucose, fructan, sucrose; NDF, neutral detergent fiber; NSC, nonstructural carbohydrates.
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LITERATURE CITED |
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TOPMATERIALS AND METHODSRESULTSLITERATURE CITED |
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1. Garner HE, Coffman JR, Hahn AW, Hutcheson DP, Tumbleson ME. Equine laminitis of alimentary origin: an experimental model. Am J Vet Res. 1975;36:441–4.[Medline]
2. Field JR, Jeffcott LB. Equine laminitis–another hypothesis for pathogenesis. Med Hypotheses. 1989;30:203–10.[Medline]
3. French K, Pollitt CC. Equine laminitis: loss of hermidesmosomes in hoof secondary epidermal lamellae to dose in an oligofructose induction model: an ultra structural study. Equine Vet J. 2004;36:230–5.[Medline]
4. Housley TL, Pollock CJ. The metabolism of fructan in higher plants. In: Suzuki M, Chatterton NJ, editors. Science and technology of fructans. Boca Raton, FL: CRC Press, Inc.1993; p.191–225.
5. Pollock CJ. Patterns of turnover of fructans in leaves of Dactylis glomerata L. New Phytol. 1982;90:645–50.
6. Fisher DS, Mayland HF, Burns JC. Variation in ruminants' preference for tall fescue hays cut either at sundown or at sunup. J Anim Sci. 1999;77:762–8.
7. Griggs T, MacAdam JW, Mayland HF, Burns JC. Nonstructural carbohydrate and digestibility patterns in orchard grass swards during daily defoliation sequences initiated in evening and morning. Crop Sci. 2005;45:1295–304.
8. Chatterton NJ, Thornley WR, Harrison PA, Bennett JH. Dynamics of fructan and sucrose biosynthesis in crested wheatgrass. Plant Cell Physiol. 1988;29:1103–8.
9. Housley TL, Pollock CJ. Photosynthesis and carbohydrate metabolism in detached leaves of Lolium temulentum L. New Phytol. 1985;99:499–507.
10. Chatterton NJ, Harrison PA, Bennett JH, Asay KH. Carbohydrate partitioning in 185 accessions of Gramineae grown under warm and cool temperatures. J Plant Physiol. 1989;134:169–79.
11. French AD, Waterhouse AL. Chemical structure and characteristics. In: Suzuki M, Chatterton NJ, editors. Science and technology of fructans. Boca Raton, FL: CRC Press, Inc.;1993. p. 41–81.
12. Suzuki M. History of fructan research: Rose to Edelman. In: Suzuki M, Chatterton NJ, editors. Science and technology of fructans. Boca Raton, FL: CRC Press, Inc. 1993; p. 21–39.
13. Wagner W, Keller F, Wiemken A. Fructan metabolism in cereals: Induction in leaves and compartmentation in protoplasts and vacuoles. Z Pflanzenphysiol. 1983;112:359–72.
14. Hoffman RM, Wilson JA, Kronfeld DS, Cooper WL, Lawrence LA, Sklan D, Harris PA. Hydrolysable carbohydrates in pasture, hay, and horse feeds: direct assay and seasonal variation. J Anim Sci. 2001;79:500–6.
15. Large EC. Growth stages in cereals. Plant Pathol. 1954;3:128–9.
16. Goering HK, Van Soest PJ. Forage fiber analysis (apparatus, reagents, procedures and some applications). USDA-ARS agricultural handbook 379. Washington, DC: U.S. Government Printing Office; 1970.
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