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Effects of cereal grain supplementation on apparent digestibility of nutrients and concentrations of fermentation end-products in the feces and serum of horses consuming alfalfa cubes¹

H. S. Hussein^*,2, L. A. Vogedes^*, G. C. J. Fernandez and R. L. Frankeny

^* Department of Animal Biotechnology and and Nevada Agricultural Experiment Station, University of Nevada-Reno, Reno 89557 and and Comstock Large Animal Hospital, Reno, NV 89511

	Abstract

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Twenty geldings (five groups; similar age and BW) were used in a completely randomized design experiment to determine effects of grain supplementation of an alfalfa-cube diet on apparent nutrient digestibility and hindgut fermentation. The geldings were housed individually, fed their diets in two equal meals (0600 and 1800), and adapted to five dietary treatments over 6 wk. The treatments were alfalfa cubes (1% of BW; DM basis) without (control) or with one of four rolled cereal grains (i.e., barley, corn, naked oats, or oats) to provide a target level of 0.4% of BW as total nonstructural carbohydrates (TNC). Due to acute laminitis, three geldings (one in the control group and two in the barley group) were excluded. Because of this and multiple incidents of gas colic, TNC level was decreased to 0.2% of BW to ensure the geldings’ health throughout the adaptation (7 d) and sample collection (5 d) periods. Grain intakes varied (P < 0.05) and reflected the different TNC concentrations. Apparent digestibilities of DM, OM, CP, NDF, ADF, and cellulose were not affected (P > 0.05) by grain supplementation and averaged 63.2, 63.1, 79.5, 42.7, 39.9, and 50.3%, respectively. Regardless of the source, grain supplementation increased (P < 0.05) apparent digestibility of TNC (from 85.6 to 94.6%) and decreased (P < 0.05) fecal pH (from 7.04 to 6.74). Fecal concentrations of total VFA (mg/g of DM) were greatest for the barley and naked oats diets (averaging 11.73), intermediate for the oats diet (8.00), and least for the control and corn diets (averaging 5.00; P < 0.05). Fecal concentrations of lactate (µg/g of DM) were greatest for the barley diet (254), intermediate for the oats diet (138), and least for the remaining diets (averaging 100; P < 0.05). Fecal concentrations of NH₃ N (mg/g of DM) were greatest for the naked oats diet (1.68), intermediate for the barley and oats diets (averaging 0.86), and least for the remaining diets (averaging 0.63; P < 0.05). Serum concentration of lactate was 46% higher (P < 0.05) for the control than for the grain diets (averaging 0.05 mg/100 mL). Feeding barley, corn, naked oats, and oats contributed to 13, 15, 8, and 20% higher (P < 0.05) serum NH₃ N concentrations than the control diet (0.25 mg/100 mL). Higher (P < 0.05) serum concentrations of urea N (mg/100 mL) were detected for the control, barley, and naked oats diets (averaging 25.28) than for the corn or oats diets (averaging 22.21). Results suggest that horses consuming alfalfa cubes could be supplemented with rolled barley, corn, naked oats, or oats at levels not exceeding 0.2% of BW as TNC without affecting nutrient digestion or overall health negatively.

Key Words: Cereal Grains • Digestion • Fermentation • Horses • Volatile Fatty Acids

	Introduction

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Under certain production or performance conditions, the nutrient requirements of horses exceed those provided by forage alone (NRC, 1989) and therefore, cereal grains (e.g., barley [Hordeum vulgare], corn [Zea mays], and oats [Avena sativa]) are fed to meet the nutrient needs. Naked oats (Avena nuda) is a new variety of oats with higher DE, CP, P, AA, and fat contents than conventional oats (Kohnke et al., 1999). Meyer et al. (1993) created a list of the factors affecting starch digestion of grains, including the source, its processing method, amount consumed, source and timing of forage feeding, and animal-to-animal variations. For example, higher apparent starch digestibility was reported (Radicke et al., 1991) for oats than for corn (98 vs. 71%). Grinding of corn increased apparent starch digestibility from 29 to 45% (Meyer et al., 1993), whereas wet processing (e.g., steam-rolling, steam-flaking, or extrusion) improved apparent starch digestibility to 90% (Pagan, 1998). Rapid ingestion of large amounts of starch (i.e., grain overload) decreased its small intestinal digestion significantly (Potter et al., 1992). As a result, starch escapes digestion and becomes available for hindgut fermentation.

Hindgut fermentation of starch creates environments in which serious pathological conditions, such as colic (King, 1999; de Fombelle et al., 2001) and laminitis (Garner et al., 1977; Mansmann and King, 2000), may develop. Starch fermentation by amylolytic bacterial (Streptococcus and Lactobacillus) species increases lactate production (Garner at al., 1977; 1978), decreases hindgut pH, fiber digestion, and VFA production (Pagan, 1998; Kohnke et al., 1999), and has the potential to release endotoxins (Sprouse et al., 1987; Clarke et al., 1990). Radicke et al. (1991) illustrated that grains vary in lactate production in the horse’s hindgut. We hypothesized that certain cereal grains could be included in alfalfa-based diets without exposing horses to digestive or health risks. The objective of this study was to determine effects of grain supplementation of an alfalfa-cube diet on apparent nutrient digestibility and hindgut fermentation by geldings.

	Materials and Methods

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Animals and Diets
Twenty mature geldings (initial BW = 437.1 ± 59.6 kg; initial age = 11.7 ± 6.0 yr) were divided into five groups of similar number, age, and BW before being used in a completely randomized design experiment (Steel et al., 1997). The geldings were vaccinated against Streptococcus equi (intranasal; live culture), rhinopneumonitis Types 1 and 4, and influenza A₁ and A₂ (Fort Dodge Animal Health, Fort Dodge, IA). They were also dewormed (Zimecterin [1.87% ivermectin paste]; Farnam Co., Inc., Omaha, NE) against large strongyles, small strongyles, pinworms, roundworms, hairworms, neck threadworms, large-mouth stomach worms, and bots. The geldings were examined by a veterinarian for overall health, and their teeth were deemed to be in good condition. Visual appraisal of the geldings’ body condition revealed that they were in a good condition based on a five-point (i.e., poor, moderate, good, fat, and very fat) scale (Kohnke et al., 1999). The geldings were housed individually in 3.6 m x 7.2 m dry paddocks at the University of Nevada-Reno Equestrian Center (Reno, NV) and were fed their diets in two equal meals at 0600 and 1800.

The geldings were offered diets containing alfalfa (Medicago sativa) cubes (1% of BW; DM basis) and graded levels (0.45 to 0.90 kg/d) of rolled oats to allow for gradual adaptation to dietary starch inclusion over a 3-wk period. The 1% alfalfa cube level was based on recommendations to meet the nutrient requirements of mature horses when a high-quality forage is fed (Harper, 1979). The five groups of geldings were then allotted at random to five dietary treatments: alfalfa cubes without (control) or with one of four rolled grains (barley, corn, naked oats, or oats). The chemical composition of dietary ingredients is presented in Table 1.

The geldings were allowed to move freely in their paddocks, were taken out daily for a 30-min exercise period between meals, and had ad libitum access to fresh water and trace-mineralized salt blocks (Morton Salt, Chicago, IL) containing 0.35% Zn (ZnO), 0.28% Mn (Mn₃O₄), 0.175% Fe (FeCO₃), 0.035% Cu (CuO), 0.007% I (iodized salt), 0.007% Co (CoCO₃), and 98% NaCl. The nutrients provided by the grain diets exceeded those recommended by the NRC (1989) for geldings. However, the nutrients provided by the control diet were below those recommended by the NRC (1989) for DE (9.2 vs 14.5 Mcal/d) and P (9.9 vs 12.1 g/d). Because the geldings rested during the study, no signs of deficiency were observed. Among the treatment groups (i.e., control, barley, corn, naked oats, and oats), the number of geldings (i.e., 3, 2, 4, 4, and 4, respectively), initial BW (i.e., 427, 500, 437, 441, and 441 kg, respectively), and final BW (i.e., 413, 500, 438, 443, and 442, respectively) varied because three geldings were excluded from the study. These data showed that initial BW was slightly decreased for the geldings in the control group compared with the geldings in other groups. All experimental procedures were according to a protocol that was approved by the Institutional Animal Care and Use Committee.

Sample Collection
During the sampling period, each gelding was equipped with a harness bag for total fecal collection. The bags (Equisan Marketing Pty Ltd., Melbourne, Victoria, Australia) were designed to allow geldings to urinate freely without contamination of feces. The bags were removed, weighed, and emptied twice daily at 0600 and 1800. The geldings were groomed, checked for skin irritations, washed between the legs, and dried before the clean bags (washed and dried) were reattached. At each collection time, the fresh feces were mixed for each gelding, pH (Accumet Portable AP61 pH meter with a flat-surface electrode; Fisher Scientific, Pittsburgh, PA) was measured, and subsamples (10%) of the mixed feces were taken for storage (–20°C). At the end of the 5-d sampling period, the frozen samples from each gelding were thawed and mixed on an equal weight basis. Approximately 10 g of feces (wet basis) were mixed with 50 mL of 1 N HCl and stored at 4°C for analysis of fecal concentrations of VFA, lactate, and NH₃ N. The remaining mixture of feces for each gelding was freeze-dried, ground (1-mm screen), and a subsample was taken for chemical analysis. Feed (alfalfa cubes and cereal grains) samples were collected daily during the 5-d sampling period, composited by ingredient, ground (1-mm screen), and a subsample of each ingredient was taken for chemical analysis. On 2 d (i.e., d 2 and 5 of the sampling period), jugular blood samples were collected from each gelding in the morning immediately before feeding and continued every 2 h until 2 h before the evening feeding (i.e., 0600, 0800, 1000, 1200, 1400, and 1600) to allow for monitoring circulating end products of hindgut fermentation between meals. Blood samples were allowed to clot on ice before centrifugation (1,800 x g at 4°C for 20 min), and serum was harvested and stored at –20°C for later analysis.

Chemical Analyses
Diluted fecal samples were prepared for VFA analysis by the procedure of Erwin et al. (1961). Concentrations of VFA were determined using a gas chromatograph (Varian model 3800, Varian Inc., Walnut Creek, CA) equipped with a glass column (180 cm x 4 mm i.d.) packed with GP 10% SP-1200/1% H₃PO₄ on 80/100 Chromosorb W AW (Supelco, Bellefonte, PA). Helium was used as a carrier gas with a flow rate of 85 mL/min. The oven, injection port, and detector (flame ionization) port temperatures were 125, 175, and 180°C, respectively. Concentrations of lactate (Barker and Summerson, 1941) and NH₃ N (Chaney and Marbach, 1962) in the feces were measured colorimetrically. Serum samples were also analyzed for lactate (Barker and Summerson, 1941) and both NH₃ N and urea N (Chaney and Marbach, 1962).

Feed and fecal samples were analyzed for DM and OM contents by drying (at 105°C for 24 h) and ashing (at 500°C for 16 h) the ground samples in a forced-air oven and a muffle furnace, respectively. The NDF (Van Soest et al., 1991) and both ADF and ADL (Goering and Van Soest, 1970) contents of these samples were also determined. Hemicellulose (NDF – ADF) and cellulose (ADF – ADL) concentrations were calculated by difference. The N content of feed and fecal samples was determined using the Kjeldahl procedure (AOAC, 2000). These samples were also analyzed for ether extract (AOAC, 2000) and TNC (Smith, 1969).

Statistical Analyses
Data on intake and apparent digestibility of nutrients and on fecal measurements (i.e., pH and concentrations of VFA, lactate, and NH₃ N) were analyzed by ANOVA using the GLM procedure of SAS (Release 8.2; SAS Inst., Inc., Cary, NC). The treatment means were separated using protected (P < 0.05) Fisher’s LSD. Data on blood measurements (i.e., serum concentrations of lactate, NH₃ N, and urea N) were analyzed as a randomized complete block (i.e., sampling day) design experiment using Proc MIXED of SAS for repeated-measure, two-way (five dietary treatments x six sampling times) ANOVA. No dietary treatment x sampling time interactions (P > 0.05) were detected for any of the measurements evaluated. The means of dietary treatments were separated using protected (P < 0.05) Fisher’s LSD, whereas orthogonal contrasts (Steel et al., 1997) were used to test for linear and quadratic responses to feeding the treatment diets over time. During adaptation of the geldings to the intended TNC target level (i.e., 0.4% of BW; 6 g of starch•kg of BW^–1•d^–1), three geldings (i.e., one consuming only alfalfa cubes and two consuming the barley diet) developed acute laminitis and were therefore excluded from the study before data collection. The tabulated values are least squares means.

	Results

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Intake and Digestion of Nutrients
All geldings maintained their initial body condition throughout the study. Daily intakes of dietary ingredients, DM, OM, CP, ether extract, TNC, and fiber components are summarized in Table 2. Intakes of alfalfa cubes (DM basis) were not different (P > 0.05) among diets because they were fed at a fixed level (i.e., 1% of BW) and averaged 4.14 kg/d. Intakes of grains, however, varied (P < 0.05) and reflected their different TNC concentrations (Table 1). Intakes of DM, OM, CP, ether extract, TNC, and hemicellulose were lowest (P < 0.05) for the control diet reflecting the absence of grain supplementation. Intakes of OM, CP, ether extract, TNC, and hemicellulose, however, showed slight variations (P < 0.05) among the grain diets and reflected the different concentrations of these components in the grains (Table 1). It is worth noting that intakes of NDF, ADF, and cellulose did not differ (P > 0.05) among diets because these fiber components were mainly from alfalfa cubes that were fed at a fixed level.

Serum Concentrations of Fermentation End Products
No dietary treatment x sampling time interactions (P > 0.05) were detected for serum concentrations of lactate, NH₃ N, or urea N. Therefore, effects of the main factors are presented in Tables 6 (dietary treatments) and 7 (sampling time). Serum concentration of lactate was 46% higher (P < 0.05) for the control than for the grain diets, which had similar (P > 0.05) concentrations (averaging 0.050 mg/100 mL). Feeding the barley, corn, naked oats, and oats diets resulted in 13, 15, 8, and 20% higher (P < 0.05) serum concentrations of NH₃ N than the control diet. Differences (P < 0.05) among the grain diets were detected only between the naked oats diet and the oats diet. Higher (P < 0.05) serum concentrations of urea N were detected for the control, barley, and naked oats diets (averaging 25.28 mg/100 mL) than for the corn or oats diets (averaging 22.21 mg/100 mL). Table 7 illustrates postprandial serum concentrations of lactate, NH₃ N, and urea N. A quadratic response (P < 0.05) to sampling time was detected for all the measurements evaluated. The highest serum concentration of lactate was detected at 1000, whereas the highest concentrations of NH₃ N and urea N were detected at 0800.

	Discussion

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Based on availability, cost, and nutritional characteristics, several grains (e.g., barley, corn, and oats) are commonly fed to horses. Oats are highly palatable and can be fed whole or processed (e.g., rolled or crimped). Because of the higher fiber and lower energy contents in oats than in other grains (Fahrenholz, 1998), they are considered a safer energy source for horses, especially when fed in excess (Evans, 1977). In comparison to oats, barley is considered a "heavy" feed due to its greater energy value (NRC, 1989). Because barley is harder than most grains, it requires processing before feeding. Corn has the highest energy value (NRC, 1989) and can be fed whole or processed. In recent years, naked oats was developed as a variety with 20 to 27% greater DE (i.e., 3.72 Mcal/kg of DM) than conventional oats (Jones et al., 1985; Hintz et al., 1991; Kohnke et al., 1999), and as a result, its use for feeding horses has increased gradually (Valentine, 1989). Dry or wet methods of processing grains are known to enhance energy availability to the horse. For example, rolling (as was done with the grains evaluated in this study) increases the surface area of the starch granules exposed to pancreatic and intestinal enzymes and therefore improves energy supply to the horse.

In comparison with other nonruminants, the horse has a limited ability to digest starch in its small intestine. For example, the horse produces only about 8 to 10% of the amount of amylase produced by the pig (Pagan, 1998). Because of this, a large proportion of ingested starch escapes digestion in the small intestine, especially when high levels of grains are fed (Potter et al., 1992; Julliand et al., 2001). Of the grains fed to horses, oats contain the most digestible form of starch, followed by corn and barley (Harper, 1979). Feeding oats vs. corn at a similar level of starch intake (i.e., 2.0 g•kg of BW^–1•d^–1) resulted in small intestinal starch digestibilities of 84 and 29%, respectively (Meyer et al., 1993). In the same study, small intestinal digestibility of cornstarch was increased to 45% by grinding. Because the horse is a nonruminant herbivore (having high rates of cecum and colon fermentation), grain overload can cause distinct physiological disturbances and/or metabolic disorders. Colic (Reeves et al., 1996; King, 1999), laminitis (Mansmann and King, 2000; Bailey et al., 2002), and postfeeding acidemia (Rowe et al., 1994; Ralston, 1995) are examples of life-threatening diseases and/or disorders induced by improper management of grain supplementation. Due to rapid fermentation of starch under these conditions, gas production is increased and bowel motility is decreased, resulting in distension of the hindgut and higher incidence of colic (Reeves et al., 1996; King, 1999). Laminitis (a serious foot problem associated with breakdown of the hoof laminae) is a complex disease that may be induced by individual or multiple factors. These factors are diet, body condition, activity level, disease, drugs, toxins, and foot and shoeing problems (Mansmann and King, 2000). Under grain overload conditions, two metabolic factors (i.e., amines and endotoxins) are implicated in inducing laminitis. The role of amines (produced by decarboxylation of AA by the amylolytic bacterial species) was attributed to their vasoactive properties (Sprouse et al., 1987; Hood et al., 1993; Bailey et al., 2002). It has been shown (Sprouse et al., 1987; Clarke et al., 1990; Kohnke et al., 1999) that very low hindgut pH (e.g., 6.2) environments (i.e., following grain overload) support the release of endotoxins.

Under grain overload conditions, the small intestine’s digestion capacity is exceeded, and various amounts of starch become available for hindgut fermentation (Potter et al., 1992; Kienzel, 1994). The increased supply of starch rapidly selects for amylolytic bacterial species (Garner at al., 1978; Julliand et al., 2001; Bailey et al., 2003) and results in production of various amounts of lactate (Garner et al., 1978; Goodson et al., 1998). Large amounts of lactate are known to irritate the gut lining, overwhelm normal buffering capacity of the hindgut, and decrease pH of its contents (Pagan, 1998; Kohnke et al., 1999; Julliand et al., 2001). Low hindgut pH decreases cellulolytic (Garner et al., 1978; Medina et al., 2002) and hemicellulolytic (Goodson et al., 1988) bacterial numbers, fiber digestion (Karlsson et al., 2000; Drogoul et al., 2001), VFA production (Hintz et al., 1971; Medina et al., 2002), and their absorption (Pagan, 1998; Kohnke et al., 1999). A shift toward a lower proportion of acetate and a higher proportion of propionate was also reported (Pagan, 1990; McLean et al., 2000). These changes in hindgut environment significantly decrease forage nutrient utilization (Karlsson et al., 2000; McLean et al., 2000).

The effects of feeding graded levels (i.e., 20, 40, and 60% of DM to provide 1.3, 2.5, and 3.8 g of starch•kg of BW^–1•d^–1) of whole oats on apparent total tract digestibility of nutrients were evaluated with geldings consuming grass hay (Karlsson et al., 2000). Regardless of the level, feeding oats increased (P < 0.05) apparent digestibility of DM (from 48 to 57%) and OM (from 49 to 58%). Apparent digestibility of NDF and ADF were not affected (P > 0.05) by oats supplementation until the highest starch level was fed. At that level, oats decreased (P < 0.05) apparent digestibility of NDF (from 37 to 26%) and ADF (from 28 to 12%). These observations are in agreement with earlier findings by Thompson et al. (1981), who reported that apparent digestibility of cellulose decreased from 37.5 to 17.5% when crimped oats were fed at 80% of dietary DM. In another investigation, feeding graded levels (i.e., 0, 30, and 50% of dietary DM) of rolled barley (i.e., not exceeding 4.0 g of starch•kg of BW^–1 •d^–1) to ponies (Drogoul et al., 2001) increased (P < 0.05) apparent digestibility of OM (i.e., 47.6, 55.0, and 63.7%, respectively) and decreased (P < 0.05) apparent digestibility of NDF (i.e., 46.1, 40.4, and 39.3%, respectively) and ADF (i.e., 42.4, 36.1, and 33.9%, respectively) in a linear fashion. In agreement with data of others (Karlsson et al., 2000; Drogoul et al., 2001), our results (Table 3) showed that grain supplementation (regardless of the source) tended to increase (P = 0.06) apparent digestibility of DM (from 55.0 to 65.2%) and OM (from 55.2 to 65.1%). Table 3 also showed that regardless of the source, TNC was almost completely digested (averaging 94.6%). Apparent digestibilities of NDF and ADF were not affected (P > 0.05) by grain supplementation (Table 3) and averaged 42.7 and 39.9%, respectively. The lack of detecting negative effects on fiber digestion in this study (Table 3) could be explained by the fact that TNC intakes (i.e., 2.9, 2.8, 2.9, and 3.2 g•kg of BW^–1•d^–1 for the diets containing barley, corn, naked oats, and oats, respectively) were less than those used by Karlsson et al. (2000). Hindgut pH (as illustrated in fecal pH) did not appear to be drastically lowered by grain supplementation (Table 4). Fecal pH of the geldings consuming grains averaged 6.72, which is close to the neutral pH detected for those consuming the control diet (Table 4). These pH environments are known to support cellulolytic and hemicellulolytic bacterial species and fiber digestion (Hungate, 1966).

Effects of grain supplementation on equine hindgut fermentation has received much attention in recent years. For example, McLean et al. (2000) fed cecally cannulated ponies cubed grass hay without (a control) or with barley (i.e., rolled, micronized, or extruded) supplementation (i.e., 4.2 g of starch•kg of BW^–1•d^–1). Total VFA concentrations and molar proportions of butyrate were not affected (P > 0.05) by barley supplementation and averaged 51.2 mmol/L and 6.5 mol/100 mol, respectively. Of the three barley sources, only rolled barley decreased (P < 0.05) pH (from 6.50 to 6.26) and molar proportion of acetate (from 76.7 to 63.0 mol/100 mol) and increased (P < 0.05) molar proportion of propionate (from 17.2 to 30.2 mol/100 mol) and concentration of lactate (from 0.11 to 0.97 mmol/L) in the cecal contents. Thus, McLean et al. (2000) recommended micronized and extruded barley as safer forms of barley for horses requiring considerable amounts of starch in their diets (e.g., race horses). In two recent investigations (de Fombelle et al., 2001; Julliand et al., 2001), graded levels (i.e., 0, 30, and 50% of DM) of rolled barley (i.e., not exceeding 4.0 g of starch•kg of BW^–1•d^–1) were fed to cecally cannulated ponies. In the study by de Fombelle et al. (2001), increasing barley level did not affect (P > 0.05) cecal pH (averaging 6.56) or total VFA concentrations (averaging 88.3 mmol/L of cecal contents). Cecal lactate concentrations, however, increased (P < 0.05) linearly (i.e., 35.9, 88.6, and 128.4 mg/L of cecal contents, respectively) with increasing barley level. In the study by Julliand et al. (2001), barley supplementation (regardless of its level) decreased (P < 0.10) cecal pH from 6.7 to 6.4. However, cecal concentrations of total VFA and lactate were not affected (P > 0.10) and averaged 88.7 mmol/L and 238.7 mg/L of cecal contents, respectively. Regardless of the barley level, molar proportion of acetate decreased (from 71.6 to 67.1 mol/100 mol; P < 0.10) and that of propionate increased (from 19.1 to 24.7 mol/100 mol; P < 0.10) with supplementation. These changes in fermentation acids reflected the shift in the microbial ecology of the pony’s hindgut (Julliand et al., 2001). In a more recent investigation (Medina et al., 2002), feeding a very high level of starch (i.e., 6.8 g•kg of BW^–1•d^–1) from barley to ponies increased (P < 0.05) lactate concentrations in the cecal (from 167.9 to 407.7 mg/L) and colon (from 116.5 to 303.2 mg/L) contents but did not affect (P > 0.05) total VFA concentrations in the cecal or colon contents (averaging 67.3 and 92.9 mmol/L, respectively). Various responses, however, were detected for concentrations of individual VFA due to feeding barley (Medina et al., 2002). For example, acetate concentration decreased (P < 0.05) in the cecum (from 50.9 to 43.4 mmol/L) and was not affected (P > 0.05) in the colon (averaging 64.2 mmol/L). Propionate concentration increased (P < 0.05) in the cecum (from 12.8 to 17.7 mmol/L) and was not affected (P > 0.05) in the colon (averaging 18.9 mmol/L). Butyrate concentration, however, did not change (P > 0.05) in the cecum (averaging 3.8 mmol/L) but increased (P < 0.05) in the colon (from 6.3 to 9.3 mmol/L) with barley supplementation. Additional changes included a decrease (P < 0.05) in pH of the cecum (from 7.2 to 6.9) and colon (from 7.1 to 6.8).

In this study (Table 4), regardless of the source, grain supplementation decreased (P < 0.05) fecal pH from 7.04 to an average of 6.74. This observation is in agreement with the results of McLean et al. (2000), who reported a decrease in cecal pH when rolled barley was fed. In their investigation, cecal pH decreased at a much lower level than that observed in our study, which might be explained by the different levels of starch fed (i.e., 4.2 vs 2.9 g•kg of BW^–1•d^–1). Published reports on the effects of various grains on hindgut pH have been limited. However, in an investigation comparing corn to oats at 1.5, 2.5, or 2.5 g of starch•kg of BW^–1•d^–1 (Radicke et al., 1991), only the highest level of corn decreased (P < 0.05) colon pH (from 6.6 to 6.15). Colon pH, however, was not affected by the level of oats and averaged 6.7. In our study, feeding corn and oats at much higher levels (i.e., 2.8 and 3.2 g of TNC•kg of BW^–1•d^–1) did not affect (P > 0.05) fecal pH.

Feeding barley or naked oats to our geldings resulted in the highest (P < 0.05) fecal concentrations of total or individual VFA (Table 4). The VFA concentrations were moderate (P < 0.05) for the oats diet and lowest (P < 0.05) for the corn diet, which was similar to the control diet. These observations suggest that more TNC were available for fermentation when barley or naked oats were fed. Molar proportion of acetate was lowest (P < 0.05) for the naked oats diet and was similar (P > 0.05) for the remaining diets (Table 5). Molar proportions of propionate and butyrate, and the acetate:propionate ratio were not affected (P > 0.05) by grain supplementation (Table 5). Fecal concentrations of lactate (Table 4) were highest for the barley diet, moderate for the oats diet, and lowest for the remaining diets (P < 0.05). It is worth noting that the fecal measurements (i.e., pH and concentrations of VFA, lactate, and NH₃ N) in Table 4 represent the response to nutrients (e.g., starch) entering the cecum, fermentation in the cecum and colon, and remaining end products after absorption in these sites. Unfortunately, no references were found correlating fecal measurements to those in the cecum and colon when grain-containing diets were fed to horses. Investigations of effects of starch from various grains on concentrations of VFA or lactate in the equine hindgut could not be found. These effects were determined in a limited number of investigations with barley that was rolled (McLean et al., 2000; de Fombelle et al., 2001; Julliand et al., 2001) or ground before being pelleted with remaining dietary ingredients (Medina et al., 2002). Contrary to our findings (Table 4), McLean et al. (2000) reported that total VFA concentrations and molar proportions of butyrate were not affected (P > 0.05) by feeding rolled barley at a higher level of starch than that used in our study (i.e., 4.2 vs 2.9 g•kg of BW^–1•d^–1). Rolled barley decreased (P < 0.05) molar proportion of acetate and increased (P < 0.05) molar proportion of propionate and concentration of lactate in the cecal contents (Medina et al., 2002). Our results (Table 4) indicated that rolled barley supported high lactate production in the hindgut. The different responses for rolled barley between the study of McLean et al. (2000) and this study may be explained by the different forages fed (i.e., grass vs. alfalfa, respectively). In agreement with the results of McLean et al. (2000), feeding graded levels (i.e., 30 and 50% of DM) of rolled barley (i.e., not exceeding 4.0 g of starch•kg of BW^–1•d^–1) did not affect (P > 0.05) cecal concentrations of total VFA (de Fombelle et al., 2001; Julliand et al., 2001). Cecal lactate concentration increased (P < 0.05) linearly in one investigation (de Fombelle et al., 2001) and was not affected (P > 0.05) in the other (Julliand et al., 2001). In the study by Medina et al. (2002), barley feeding (i.e., 6.8 g of starch•kg of BW^–1•d^–1) had no effect (P > 0.05) on total VFA concentrations in the cecum or the colon, but it increased (P < 0.05) lactate concentrations by 1.4 and 1.6 times in both sites, respectively. In the study by Radicke et al. (1991), lactate concentration in the horse’s hindgut was affected by the grain source. Radicke et al. (1991) reported that lactate concentrations in cecal contents increased from 6.1 to 19.3 mmol/L and from 3.7 to 11.6 mmol/L in response to feeding various levels (i.e., ranging from 1 to 4 g of starch•kg of BW^–1•d^–1) of oats and corn, respectively. Similar trends were found in Table 4. Higher fecal lactate concentrations were detected in geldings consuming oats than in those consuming corn (i.e., 137.6 vs. 96.6 µg/g fecal DM).

Dietary proteins escaping digestion in the small intestine are fermented in the hindgut to NH₃, which is utilized for bacterial protein synthesis (Evans, 1981). This bacterial protein, however, is not available to the horse because it is produced distal to the site (i.e., small intestine) of its digestion and absorption. The NH₃ produced but not utilized for bacterial protein synthesis is absorbed, detoxified in the liver to urea, and excreted in the urine. Horses with compromised liver or kidney functions may be at risk when fed high-CP diets. It is critically important to emphasize that our geldings not only consumed high-CP diets (i.e., 19.8, 18.2, 17.4, 19.6, and 17.3% of DM for the control, barley, corn, naked oats, and oats diets, respectively), but they also consumed large amounts of CP when grains were fed (Table 2). Our results (Table 4) indicate that fecal concentrations of NH₃ N were highest for the naked oats diet, intermediate for the barley and oats diets, and lowest for the control and corn diets (P < 0.05), reflecting the CP levels in the grains (Table 1). Because our diets contained equal amounts of CP from alfalfa cubes (Table 2), results in Table 4 suggested that naked oats CP reaching the cecum and colon was less resistant to microbial degradation, and therefore contributed to higher NH₃ N concentrations. The different rates and extents of microbial CP degradation have been illustrated for various cereal grains in ruminants (NRC, 1985). Except for one study (Medina et al., 2002), no data were found on NH₃ concentrations in fecal, cecal, or colon contents of horses consuming grains. Medina et al. (2002) reported that NH₃ concentration increased (P < 0.05) from 66.9 to 92.3 mg/L in the cecum and did not change (P > 0.05) in the colon (averaging 99.0 mg/L) due to barley feeding (i.e., 6.8 g of starch•kg of BW^–1•d^–1). Fecal concentrations of NH₃ N in the geldings consuming barley (Table 4) followed trends similar to those reported by Medina et al. (2002).

Although serum lactate concentrations in our geldings were higher for the control diet than for the grain diets (Table 6), the differences probably were of less biological significance. This is because lactate concentrations were very low (averaging 0.054 mg/100 mL). This observation suggests that small amounts of the lactate produced were absorbed and circulated. Additionally, blood lactate concentrations in the equine were depressed by ingestion of high-CP diets (Ott, 2001) such as the ones in this study (Tables 1 and 2). This response was attributed to decreased muscle glycogen use (i.e., glycogen sparing effect) and increased utilization of excess dietary AA for energy production (Ott, 2001). In our study, the highest serum lactate concentrations were detected at 240 min postfeeding (Table 7). Hoekstra et al. (2001) reported a rise in blood lactate at 150 min after feeding various forms (i.e., cracked, ground, or steam-flaked) of corn and attributed the response to bacterial fermentation of soluble carbohydrates in the nonglandular region of the stomach (Meyer, 1983). With few exceptions (Garner et al., 1977; Hoekstra et al., 2001), studies investigating effects of grain supplementation on serum lactate concentrations could not be found. Hoekstra et al. (2001) fed corn (i.e., cracked, ground, or steam-flaked) to horses (to provide 2.0 g of starch•kg of BW^–1•d^–1) and reported higher serum lactate concentrations (i.e., 3.6, 3.9, and 3.7 mg/100 mL, respectively) than those detected in Table 6. The higher concentrations reported by Hoekstra et al. (2001) could be explained by overfeeding the horses with grains in a single meal. Extreme effects (death or development of acute laminitis) of grain overload were illustrated (Garner et al., 1977) in horses consuming 14.9 g of starch•kg of BW^–1•d^–1 and having blood lactate concentrations ranging from 21.2 to 31.5 mg/100 mL.

In our study, geldings consuming the grain diets had 8 to 20% higher (P < 0.05) serum NH₃ N concentrations than those consuming the control diet (Table 6). This could be attributed to the higher CP intakes (Table 2) and the possibility of larger amounts of CP entering the hindgut for fermentation. Higher (P < 0.05) serum concentrations of urea N were detected for the control, barley, and naked oats diets than for the corn or oats diets (Table 6). This response is difficult to explain because it did not reflect dietary CP intakes (Table 2). Serum urea N concentrations in our geldings were 24 to 43% higher than those reported in others (Roose et al., 2001). The difference between the two investigations could be explained by the higher CP intakes (Table 2) in relation to the requirements (NRC, 1989) of our geldings. Although blood NH₃ and urea are derived from endogenous and exogenous sources (i.e., catabolism of excess dietary AA or tissue AA and absorbed NH₃), blood urea N were consistently higher in horses consuming high-CP diets but blood NH₃ N were not affected (Ott, 2001).

With regard to the three geldings that were excluded from our study due to laminitis, we suspect that diet and body condition were contributing factors. Overweight horses are known to be at increased risk for laminitis and are more likely to develop laminitis when other risk factors are present than those with ideal body condition (Mansmann and King, 2000). Individual differences in a horse’s ability to maintain weight or become overweight also may affect its susceptibility to certain clinical disorders, especially laminitis (Harris, 2001). In our study, the gelding in the control group that exhibited laminitis was slightly overweight which may have been a contributing factor to laminitis, especially when its diet contained low level of TNC (Table 1). However, highly palatable leafy forages, such as alfalfa, are known to be a risk factor for laminitis (Geor, 2001). For the two geldings consuming barley, it is evident that individual differences (Geor, 2001; Harris, 2001), along with the large amounts of lactate produced when barley was fed (Table 4), were factors involved in disposing them to acute laminitis.

	Implications

Top Abstract Introduction Materials and Methods Results Discussion Implications Literature Cited

 
Because of the inadequate supply of nutrients from forages to physiologically challenged or high-performance horses, grain supplementation is necessary. Under these conditions, strategies should be developed to select the source and to determine the level of grain supplementation that complement existing nutrients in the forage used. It is also critical to avoid or decrease dietary inclusion of grain sources that may induce digestive or metabolic disorders due to their rapid starch fermentation. Improper management of grain feeding could dispose horses to health risks, such as colic, laminitis, or postfeeding acidemia. Our results suggest that alfalfa diets could be supplemented with rolled barley, corn, naked oats, or oats at levels not exceeding 0.2% of body weight (dry matter basis) without affecting nutrient digestion negatively or exposing the horse to health problems. Naked oats seem to have a great potential because they not only can increase energy supply through starch, but also can improve protein supply to horses consuming low-protein forages such as grasses.

	Footnotes

 
¹ The authors thank R. Ferebee for providing the naked oats used in this study.

² Correspondence: Mail Stop 202 (phone: 775-784-1708; fax: 775-784-1375; e-mail: hhussein{at}agnt1.ag.unr.edu).

Received for publication June 11, 2003. Accepted for publication February 27, 2004.

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