| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
ANIMAL NUTRITION |
* Department of Animal Science, Iowa State University, Ames 50011 and
and
Department of Animal Science, North Carolina State University, Raleigh 27695-7621
Abstract
Soybean hulls have been successfully fed to ruminant animals as an economical substitute for hay. This feedstuff is a source of highly digestible fiber that does not contain starch. The purpose of this trial was to evaluate soybean hulls as a replacement fiber in horse diets. Four cecally cannulated Quarter Horse geldings, aged 6 to 10 yr and averaging 502 kg, were used in a 4 x 4 Latin square design with 21-d periods. Diets consisted of alfalfa/bromegrass hay (14.4% CP, 58.1% NDF, 39.1% ADF; DM basis) with the replacement of either 0, 25, 50, or 75% (as-fed basis) unpelleted soybean hulls (13.1% CP, 60.6% NDF, 43.7% ADF; DM basis). Diets were offered at 1.8% of BW (DM) daily and body weights were measured weekly. Cecal samples (90 min after feeding) and total fecal collections (3 d) were taken at the end of each treatment period. Fecal collection bags were emptied every 6 h and 10% of the total amount was frozen for later analysis. Total cecal VFA production increased linearly (P = 0.02) from 70 mM to 109 mM as proportions of soybean hulls in diets increased. Proportions of propionate increased linearly (P < 0.01) with means of 15.7, 18.0, 16.6, and 21.9 mol/100 mol total VFA for the 0, 25, 50, and 75% soybean hulls diets respectively. Proportions of butyrate decreased linearly (P < 0.01) from 5.3 to 3.9 mol/100 mol total VFA. The acetate:propionate ratio decreased linearly (P = 0.02) and cubically (P = 0.03) with means of 4.9, 4.2, 4.9, and 3.3. Apparent digestibility of DM (P = 0.95), OM (P = 0.70), NDF (P = 0.34), ADF (P = 0.31), cellulose (P = 0.93), and hemicellulose (P = 0.25) did not differ among treatments. Apparent digestibility of N decreased linearly (P < 0.01) as concentrations of soybean hulls increased in the diet, and this response was associated with increased cecal fermentation and microbial biomass production. Cecal pH decreased linearly (P = 0.01) from 7.00 to 6.45 as the level of soybean hulls increased, but there was no change (P = 0.68 for the linear effect) for cecal ammonia (mean concentration of 3.85 mM). Soybean hulls stimulate cecal fermentation and are a suitable replacement for hay in equine diets.
Key Words: Cecum • Forage • Horse • Soybean Hulls
Introduction
Soybean hulls (soyhulls) are rapidly degraded in the equine digestive system by microorganisms in the cecum and large intestine (Moore-Colyer et al., 2002
); however, little is known about the use of the feedstuff as a replacement fiber source. Due to thick cell walls, most of the polysaccharides are found in the NDF fraction of the soybean plant. The NDF fraction of soyhulls contains large amounts of cellulose and hemicellulose; however, it contains relatively little lignin, allowing extensive bacterial fermentation (Miron et al., 2001). Soyhulls are high in pectins and other soluble fibers, but they contain relatively little starch (Quicke et al., 1959). Therefore, soyhulls provide adequate energy without some of the common management problems associated with high-grain diets in both ruminant animals and horses. Due to the fibrous nature of the feedstuff, digestion of soyhulls occurs primarily in the cecum (Hintz et al., 1971). Fiber in equine diets is important to maintain a stable hindgut environment that is less susceptible to acidosis (Moore-Colyer et al., 2000).
Soyhulls have been successfully fed to ruminant animals as an economical substitute for grain and hay. Results of studies in beef cattle (Grigsby et al., 1992), meat goats (Moore et al., 2002a,b), and dairy cows (Slater et al., 2000) indicate favorable responses when soyhulls are substituted for hay in their diets, including increased total ruminal VFA and increased digestibility of DM, OM, and cell walls. The purpose of our experiment was to evaluate the use of soyhulls in increasing levels as a replacement fiber source for the horse, specifically evaluating effects of this feedstuff on digestibility and cecal environment.
Materials and Methods
The Iowa State University Animal Care and Use Committee approved all procedures used in this experiment.
Horses and Treatments.
Cecally fistulated Quarter Horse geldings (n = 4) from the Iowa State University Horse Farm were utilized in a 4 x 4 Latin square design with 21-d treatment periods. Horses ranged in age from 6 to 10 yr, and BW averaged 502 ± 4.0 kg. Treatments consisted of four diets containing alfalfa/bromegrass hay with the inclusion of either 0, 25, 50, or 75% unpelleted soyhulls (as-fed basis). Diets were offered at 1.8% of BW (DM) daily. Feedstuffs were offered at the same time but in separate containers. Body weight was measured weekly, and diets were adjusted accordingly for the treatment period. Horses were fed twice daily and housed in 3.7 x 3.7 m individual stalls, receiving approximately 4 h of free exercise daily. Water and a trace mineral salt block (12% Ca, 12% P, 12% NaCl, 0.05% Mg, 100 ppm I, 28 ppm Se, 515 ppm Zn, 125 ppm Cu, 500 ppm Mn, 220,000 IU/kg vitamin A, 11,000 IU/kg vitamin D, 44 IU/kg vitamin E) were provided ad libitum. No additional vitamins were supplemented. Vaccinations and deworming practices were consistent with farm protocol. Horses were vaccinated annually for tetanus, Western equine encephalitis, Eastern equine encephalitis, West Nile, influenza, equine herpesvirus (EHV-1, EHV-4), strangles, and rabies. Horses were dewormed every 3 mo with non-boticide in the spring and summer and a boticide in fall and winter.
Sample Collection.
Cecal samples were collected on the first day and on d 17 of each period. Samples were obtained approximately 90 min after the morning feeding. The cannula was opened and cecal contents (solid and liquid) were collected in an insulated container. Cecal samples were immediately analyzed for pH using a Corning 314 hand-held pH meter (Corning, Inc., New York). Cecal fluid samples were then frozen until thawed for subsequent analysis at North Carolina State University.
Total fecal collections were obtained for the last 3 d of each treatment period; collections began on d 18, 39, 60, and 81 of the trial. During the 3-d collection period, horses were fitted with fecal collection bags and housed individually in tie stalls. Fecal samples were removed from the bags every 6 h. For each 24 h, a 10% subsample of feces (by weight) was collected, placed in individual bags, and frozen for later analysis. Samples of soyhulls, hay, and orts were also obtained throughout the total collection period and stored for subsequent analysis.
Preparation of Samples for Analysis.
Hay collected during the total collection period was ground through a 5-mm screen in a Wiley Mill (Thomas Scientific, Swedsboro, NJ) before being subsampled and ground through a 1-mm screen in the same mill. Soyhulls and fecal samples were ground through the 1-mm screen without first being ground through a 5-mm screen.
Cecal samples were thawed, strained for liquid content, and centrifuged at 3,600 x g for 10 min. Supernatant fluid was mixed with a 25% (vol/vol) metaphosphoric acid solution using a 5:1 ratio of cecal fluid to acid. This mixture was covered and set aside at room temperature for approximately 30 min. The mixture was then centrifuged again at 3,600 x g for 10 min and analyzed for ammonia and VFA content.
Chemical Analyses.
Hay, soyhulls, diet refusal, and fecal samples were analyzed for DM, OM, and Kjeldahl-N according to AOAC (1995) protocols. Crude protein was calculated as the percentage of Kjeldahl-N x 6.25. All samples were analyzed for NDF, ADF, cellulose, ADL, and hemicellulose according to procedures described by Van Soest et al. (1991) and modified (Komarek et al., 1994) for use in an Ankom fiber apparatus (Ankom Technology, Fairport, NY) with the omission of sodium sulfite.
Cecal VFA concentrations were determined by a Varion 3800 gas chromatograph (Varian Chromatography Systems, Walnut Creek, CA). The gas chromatograph was fitted with a Nikol fused silica capillary column (15 m, 0.53 mm i.d., 0.5 µm film thickness; Supelco, Bellefonte, PA). Cecal ammonia concentrations were determined via colorimetric procedures used for Kjeldahl-N (AOAC, 1995).
Statistical Analyses.
Data were analyzed by ANOVA using the PROC GLM procedure of SAS (SAS Inst. Inc., Cary, NC) with diet, period, and horse in the model statement. Data from all animals were included in the analysis. Linear and quadratic effects were tested in the form of contrasts. In all cases, an alpha level of 0.05 was used for determination of statistical significance. Probability values between 0.05 and 0.15 were deemed to be trends toward significance.
Results and Discussion
Diet Composition.
Compositions of hay, soyhulls, and diets used throughout the experiment are listed in Table 1. Soyhulls utilized in this trial had a chemical composition similar to published values (12.2% CP, 66.3% NDF, and 49.0% ADF; NRC, 1989), indicating there was little contamination with soybean meal (NRC, 1989). Soyhulls are often variable in composition, making comparisons between trials difficult. The composition of hay used in this trial was consistent with values typical for a medium-quality grass-legume mix, and chemical composition of the hay was comparable to the soyhulls product with the exception that soyhulls had 70% less lignin.
|
Intake and Body Weight.
Horses readily consumed the soyhulls with few refusals of dietary treatments (Table 2); however, both the total intake and intake of the soyhulls portion (relative to the amount offered) tended to decrease (P < 0.08) with increasing percentages of soyhulls in the diet. Measured intake of soyhulls as a percentage of diet DM was 97% of what was offered for the 25% soyhulls diet and 99% for the 50% soyhulls diet; however, it was only 93% for the 75% diet, indicating this level may be at the upper end of what is acceptable to horses. Intake of soyhulls was most likely limited by palatability of the feedstuff. Orts consisted primarily of soyhulls suggesting horses selectively consumed hay.
|
). Linear regression on the diets containing increasing levels of soybean hulls was used to estimate the digestibility of the soybean hulls, and values obtained were as follows: DM = 64.9% (r2 = 0.02); OM = 65.5% (r2 = 0.34); NDF = 62.4% (r2 = 0.85); ADF = 58.5% (r2 = 0.84); cellulose = 62.4% (r2 = 0.01); hemicellulose = 66.2% (r2 = 0.50); and N = 53.4% (r2 = 0.89). Values calculated using linear regression do not take into account potential associative effects of the ingredients. Moore-Colyer et al. (2002) observed greater total-tract DM, OM, CP, ADF, and NDF digestibility of soyhulls compared with hay cubes used as the sole forage source. In that trial, total-tract digestibility was calculated using the mobile bag technique rather than total fecal collection, and the hay cubes fed were of a quality that was lower (8.2% CP, 62.3% NDF, 35.4% ADF) than that of the grass-legume hay fed in our trial. Digestibility of the hay in our study was greater than that of the hay cubes used by Moore-Colyer et al. (2002) to ponies, although the digestibility of soyhulls was similar. Thus, the higher quality hay fed in our study may explain the lack of observed differences in digestibility between hay and soyhulls.
|
). The decreased N digestibility is most likely due to increased production of microbial biomass in the cecum; microbial N is lost in the feces because the cecum is beyond the site of protein digestion. The increase in VFA production (Table 4) suggests that an increase in microbial biomass occurred.
|
observed an increase in the digestibility of NDF, ADF, N, OM, and DM in goats as proportions of soyhulls in diets increased. Goats in their experiment were offered diets similar to those fed to horses in our trial; however, in their study, feed was offered on an ad libitum basis. Grigsby et al. (1992) also observed an increase in the digestibility of DM, OM, and cell wall contents as soyhulls were added to cattle diets. Steers in their study were offered diets containing up to 60% soyhulls on a restricted basis. Garcia et al. (1999) also observed an increase in DM and NDF digestibility when soyhulls were compared with alfalfa hay as the sole forage source fed to rabbits. The rabbits in their study received ad libitum access to diets, and varying inclusion rates of soyhulls were not investigated; however, it is important to consider species and diet quality differences when comparing these data.
The relatively short fecal collection time (3 d) used in our study may have influenced digestibility values. This short collection period may have resulted in a greater end-period error compared with studies using longer collection periods (Blaxter et al., 1956). This potential error may account for our observed lack of differences in digestibilities; however, it is more likely due to other factors. Intake was restricted for horses on our trial. In addition, the chemical composition of hay and soyhull sources was more similar in our trial than in the previously mentioned trials. These improvements in digestibility observed in ruminants are most likely due to the lower quality of forage often fed to ruminants. In our study, the hay was a medium-quality alfalfa/bromegrass mix with a nutrient composition similar to that of the soyhulls. Grigsby et al. (1992) fed low-quality bromegrass hay and Moore et al. (2002b) used soyhulls higher in CP than ours as a result of contamination with soybean meal.
Cecal Fluid.
Total cecal VFA (P < 0.02) production increased as proportions of soyhulls in diets increased (Table 4). This suggests that soyhulls are indeed highly fermentable in the cecum, and this increased production of VFA would contribute to the energy needs of the horse. The increase in total VFA concentrations observed in our trial is similar to that seen in previous studies, with similar diets fed to ruminants (Moore et al., 2002b; Grigsby et al., 1992) and rabbits (Garcia et al., 2000). Total VFA produced from all diets on our trial were higher than those observed by Moore-Colyer et al. (2000) when feeding other by-product feeds (sugar beet pulp, oat hulls) in ponies. Similar cecal total VFA concentrations were observed in ponies fed either clover hay alone or clover hay with additional oats (Kern et al., 1973).
This increase in total VFA production suggests that additions of soyhulls to horse diets increased microbial biomass in the cecum, contributing to increased fecal N output and reducing the apparent digestibility of N. Fecal N concentrations (1.4, 1.7, 1.8, and 2.5% N for diets containing 0, 25, 50, and 75% soyhulls, respectively) increased as dietary inclusion rates of soyhulls increased (P < 0.02). Increased fecal N was observed in rabbits receiving soyhulls as the sole forage source compared with those fed alfalfa hay (Garcia et al., 2000). Zervas and Zijlstra (2002) also observed increases in fecal N when soyhulls were fed to growing pigs at 15% of the diet.
Proportions of cecal propionate increased linearly (P < 0.01). Cecal propionate concentrations from horses fed the 0% soyhulls diet in our trial were similar to values observed in other trials when horses were fed grass-legume hay mixture (Willard et al., 1977). Addition of soyhulls to diets has resulted in increased propionate similar to that observed when concentrate is added to rations (Willard et al., 1977). Acetate:propionate ratios also decreased both linearly (P = 0.02) and cubically (P = 0.03) as the inclusion rates of soyhulls increased. These results are different than what has been observed in ruminants, which was an increase in acetate relative to propionate when soyhulls were added to ruminant diets (Grigsby et al., 1992; Moore et al., 2002a); however, it is important to consider species differences when comparing these data.
Butyrate concentrations decreased linearly (P < 0.01) as the proportion of soyhulls in diets increased. Cecal butyrate concentrations observed in our trial are in agreement with values reported by Kern et al. (1973) in ponies fed clover either with or without oats. In our trial, there were no differences between treatments for proportion of acetate, isobutyrate, isovalerate, and valerate (Table 4).
Increases in proportions of propionate and decreases in butyrate suggest changes in microbial populations based on changes in available substrate and pH of the cecum. Propionate is a gluconeogenic compound, which means an increase in the proportion of propionate produced represents a greater potential for gluconeogenesis for the horse. In addition, a shift to the production of more propionate results in less energy being lost as gas during the fermentation process. Cecal pH decreased (P = 0.01) as inclusion rates of soyhulls in diets increased. Cecal ammonia concentrations (P = 0.68) were not different with treatment (Table 4). Ammonia concentrations in our trial were higher than those observed by Kern et al. (1974) for ponies fed timothy hay. These differences were most likely due to higher percentages of CP in our diets compared with the timothy hay fed by Kern et al. (1974).
Cecal pH decreased as inclusion rates of soyhulls in diets increased; however, this decrease in pH was not large and did not approach levels indicative of subclinical acidosis (less than 6; Radicke et al., 1991). Similar pH values were observed in ponies fed timothy hay (Kern et al., 1974) and horses fed grass-legume mix hay (Willard et al., 1977). For ruminants, Moore et al. (2002a,b) observed decreases in ruminal pH in meat goats fed diets similar to those in our trial. Garcia et al. (2000) also observed decreases in cecal pH when soyhulls were fed to rabbits instead of alfalfa hay as the sole forage source.
Implications
Soyhulls seem to be an acceptable replacement for up to 75% (as-fed basis) of the total forage in diets for horses. This feedstuff is often economical, readily available, palatable, and digestible. Horses in our study readily consumed all experimental diets with no adverse reactions, but longer-term feeding experiments should be conducted to confirm the safety of these feeding recommendations. Compared with medium-quality alfalfa/bromegrass hay, soybean hulls seem to stimulate cecal fermentation and are a suitable replacement for hay in equine diets.
1 Correspondence: University of Georgia, Edgar L. Rhodes Center for Anim. and Dairy Sci., Athens 30602 (phone: 706-583-0398; fax: 706-542-0399; e-mail: jcover{at}uga.edu).
Received for publication June 30, 2003. Accepted for publication February 3, 2004.
Literature Cited
AOAC. 1995. Official Methods of Analysis of AOAC International. 16th ed. Assoc. Offic. Anal. Chem., Arlington, VA.
Blaxter, K. L., N. McC. Graham, and F. W. Wainman. 1956. Some observations on the digestibility of food by sheep, and on related problems. Br. J. Nutr. 10:69–91.[Medline]
Garcia, J., R. Carabano, and J. C. de Blas. 1999. Effect of fiber source on cell wall digestibility and rate of passage in rabbits. J. Anim. Sci 77:898–905.
Garcia, J., R. Carabano, L. Perez-Alba, and J. C. de Blas. 2000. Effect of fiber source on cecal fermentation and nitrogen recycled through cecotrophy in rabbits. J. Anim. Sci. 78:638–646.
Grigsby, K. N., M. S. Kerley, J. A. Paterson, and J. C. Weigel. 1992. Site and extent of nutrient digestion by steers fed a low-quality bromegrass hay diet with incremental levels of soybean hull substitution. J. Anim. Sci. 70:1941–1949.[Abstract]
Hintz, H. F., D. E. Hogue, E. F. Walker, Jr., J. E. Lowe, and H. F. Schryver. 1971. Apparent digestion in various segments of the digestive tract of ponies fed diets with varying roughage-grain rations. J. Anim. Sci. 32:245–248.
Kern, D. L., L. L Slyter, J. M. Weaver, E. C. Leffel, and G. Samuelson. 1973. Pony cecum vs. steer rumen: The effect of oats and hay on the microbial ecosystem. J. Anim. Sci. 37:463–469.
Kern, D. L., L. L Slyter, E. C. Leffel, J. M. Weaver, and R. R. Oltjen. 1974. Ponies vs. steers: Microbial and chemical characteristics of intestinal ingesta. J. Anim. Sci. 38:559–564.
Komarek, A. R., J. B. Robertson, and P. J. Van Soest. 1994. Comparison of the filter bag technique to conventional filtration in the Van Soest Analysis of 21 feeds. Page 78 in Proc. Natl. Conf. Forage Quality, Evaluation, and Utilization, Lincoln, NE.
Miron, J., E. Yosef, and D. Ben-Ghedalia. 2001. Composition and in vitro digestibility of monosaccharide constituents of selected byproduct feeds. J. Agric. Food Chem. 49:2322–2326.[Medline]
Moore, J. A., M. H. Poore, and J.-M Luginbuhl. 2002a. By-product feeds for meat goats: Effects on digestibility, ruminal environment, and carcass characteristics. J. Anim. Sci. 80:1752–1758.
Moore, J. A., M. H. Poore, A. T. Maye, J-M. Luginbuhl, and J. A. Booth. 2002b. Soyhulls as an alternative feed for small ruminants and horses. Pages 13–14 in Proc. 13th Annu. Inst. Nutr. Res. Symp., University of North Carolina, Chapel Hill, October 28, 2002.
Moore-Colyer, M. J. S., J. J. Hyslop, A. C. Longland, and D. Cuddeford. 2000. Intra-caecal fermentation parameters in ponies fed botanically diverse fibre-based diets. Anim. Feed Sci. Tech. 84:183–197.
Moore-Colyer, M. J. S., J. J. Hyslop, A. C. Longland and D. Cuddeford. 2002. The mobile bag technique as a method for determining the degradation of four botanically diverse fibrous feedstuffs in the small intestine and total digestive tract of ponies. Br. J. Nutr. 88:729–740.[Medline]
NRC. 1989. Nutrient Requirements of Horses, 5th rev. ed. National Academy Press, Washington, DC.
Quicke, G. V., O. G. Bentley, H. W. Scott, R. R. Johnson, and A. L. Moxon. 1959. Digestibility of soybean hulls and flakes and the in vitro digestibility of the cellulose in various milling by-products. J. Dairy Sci. 42:185–186.
Radicke, S., E. Kienzle, and H. Meyer. 1991. Preileal apparent digestibility of oats and corn starch and consequences for caecal metabolism. Pages 43–48 in Proc. 12th Equine Nutr. and Physiol. Symp., University of Calgary, Alberta, Canada.
Slater, A. L., M. L. Eastridge, J. L. Firkins, and L. J. Bidinger. 2000. Effects of starch source and level of forage neutral detergent fiber on performance of dairy cows. J. Dairy Sci. 83:313–321.[Abstract]
Van Soest, P. J., J. B. Robertson, and B. A. Lewis. 1991. Methods for dietary fiber, neutral fiber and non-starch polysaccharides in relation to animal nutrition. J. Dairy Sci. 74:473–481.[Abstract]
Willard, J. G., J. C. Willard, S. A. Wolfram, and J. P. Baker. 1977. Effect of diet on cecal pH and feeding behavior of horses. J. Anim. Sci. 45:87–93.
Zervas, S. and R. T. Zijlstra. 2002. Effects of dietary protein and fermentable fiber on nitrogen excretion patterns and plasma urea in grower pigs. J. Anim. Sci. 80:3247–3256.
This article has been cited by other articles:
|
|
 |
|
  N. Nicodemus, J. Garcia, R. Carabano, and J. C. De Blas Effect of substitution of a soybean hull and grape seed meal mixture for traditional fiber sources on digestion and performance of growing rabbits and lactating does J Anim Sci, January 1, 2007; 85(1): 181 - 187. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
