J. Anim. Sci. 2006. 84:678-685
© 2006 American Society of Animal Science
Influence of soybean hull supplementation on rumen fermentation and digestibility in steers consuming freshly clipped, endophyte-infected tall fescue1
C. J. Richards2,
R. B. Pugh and
J. C. Waller
Department of Animal Sciences, The University of Tennessee, Knoxville 37996
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Abstract
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Six steers (288.6 ± 2.1 kg of BW) fitted with rumen and duodenal cannulas were used in a crossover design to evaluate intake, rumen fermentation, and site of nutrient digestion of freshly clipped, endophyte-infected (E+) Kentucky 31 tall fescue with or without soybean hull (SH) supplementation at 0.60% of BW (OM basis). Steers were placed in metabolism units within an environmentally controlled room and provided with free-choice access to fresh forage, water, and a vitamin/mineral supplement. The spring growth of E+ tall fescue was harvested daily during the experiment. Supplement was fed at 0700 with approximately 65% of the estimated daily forage. To maintain a fresh forage supply, additional forage was stored in a cooler and fed at 1900. Periods were 21 d with 14 d of adaptation and 7 d of digesta sample collection. Chromic oxide was used as a marker of duodenal digesta flow. Duodenal samples were taken 4 times daily with times shifting by 1 h each day to represent all 24 h of a day. Treatments were considered significant at P < 0.05. Supplementation of SH decreased forage OM intake from 1.64 to 1.41% of BW but increased total OM intake from 1.64 to 2.01% of BW. Apparent percentages (53.1%) and quantities (2,786 g/d) of rumen OM disappearance were not affected by supplementation. Percentages of total tract OM disappearance were not different (70.8%). Percentages of apparent rumen NDF disappearance also were not different (65.6%). Percentages of N disappearance were not different. Supplementation of SH resulted in increased total N (34.1 g/d) and microbial N (17.1 g/d) flowing to the duodenum. Rumen pH (6.5) was not affected, and rumen ammonia concentrations exhibited a time x treatment interaction in which SH decreased ammonia for 12 h after supplementation. Total VFA concentrations (103.9 mM) were unaffected. Liquid dilution rate (12.7%/h) and rumen OM fill (4.3 kg) were not different between treatments. Supplementation of SH at a rate of 0.60% of BW (OM basis) to calves consuming fresh E+ tall fescue decreased forage consumption but resulted in greater total intake, greater flow of N to the duodenum, and increased total tract OM disappearance.
Key Words: cattle • digestibility • forage intake • supplementation • tall fescue
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INTRODUCTION
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The goals of supplementing cattle on pasture include increasing animal performance, improving forage use, extending forage resources when forage quantity is limited, and increasing stocking rates. Often, feeding high levels of low-fiber, starch-based energy supplements has been shown to reduce forage intake and digestion (Goetsch et al., 1991
; Galloway et al., 1993; Moore et al., 1995). Supplementation with high-fiber energy sources, such as soybean hulls (SH), could potentially increase ruminally available energy without greatly altering the rumen environment. By maintaining a rumen environment suitable for fibrolytic microorganisms, consumption and use of forage nutrients may be sustained. Because several cool season forages are high in rumen degradable protein, an additional advantage of increasing ruminally available energy may be to enhance incorporation of rumen ammonia into microbial N. Therefore, the objective of this experiment was to determine the influence of supplemental pelleted SH on fresh forage intake, rumen fermentation, site and extent of nutrient disappearance, and microbial N production in beef steers consuming freshly clipped, endophyte-infected (E+) tall fescue.
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MATERIALS AND METHODS
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Animal Management and Surgery
Six Angus and Angus x Hereford steers (288.6 ± 2.1 kg), selected from The University of Tennessee Knoxville Experiment Station calf crop and acclimated to human interaction, were used in a replicated crossover design experiment. Steers were housed in individual pens (2.5 x 5.0 m) within a temperature-controlled facility (23°C) with 16 h of light and 8 h of dark. Steers had continuous access to fresh water and a vitamin/mineral mix (18.3% Ca, 12.0% P, 18.3% salt, 0.55% K, 1.25% Mg, 4,500 ppm of Mn, 1,500 ppm of Cu, 5,000 ppm of Zn, 30 ppm of Co, 50 ppm of I, 30 ppm of Se, 440 IU of vitamin E/kg, 132,000 IU of vitamin D3/kg, and 528,000 IU of vitamin A/kg). Before the experimental periods, dehydrated alfalfa cubes were fed at 1.7% of BW (DM basis) and split between 2 equal feedings at 0800 and 1630.
Each steer was surgically fitted with a rumen cannula and a double-L-shaped duodenal cannula (Streeter et al., 1991) under a protocol approved by The University of Tennessee Institutional Animal Care and Use Committee. Before ruminal or duodenal cannulation surgeries, steers were withheld from feed and water for 48 or 24 h, respectively. Rumen surgeries were conducted under local anesthesia with steers standing. Daily rectal temperatures and twice-daily antibiotic (Penicillin G, 2.2 mL/100 kg of BW; Butler Animal Health Supply, Dublin, OH) treatments followed surgeries for 5 and 3 d, respectively.
For the duodenal cannulation, steers were allowed at least 14 d to recover from rumen surgery before general anesthesia was induced. Steers were prepared for aseptic surgery, and a duodenal cannula was placed 10 cm posterior to the pylorus. After surgery, diet intake, fecal output, and rectal temperature were monitored. Steers were also given antiinflammatory medications (Banamine, 4.4 mL/100 kg of BW; Schering-Plough, Kenilworth, NJ) and antibiotics (Naxcel, 4.4 mL/100 kg of BW; Pharmacia & Upjohn, Kalamazoo, MI) for 5 and 4 d, respectively.
Forage Harvest and Management.
Spring growth of fresh forage was harvested daily from grass plots at the Knoxville Experiment Station, Plant Sciences Unit, Knoxville, TN. The plots consisted of 0.72 ha of E+ Kentucky 31 tall fescue seeded in 1999 and 0.44 ha of E+ Kentucky 31 tall fescue seeded in 1990. These plots were determined to be 92 to 96% E+ (Hiatt et al., 1997).
Forage for feeding in this experiment was harvested from April 12 to May 23. Forage was harvested at approximately 1300 each d with a 1954 Gravely 2-wheeled tractor with a 0.91-m sickle bar mower head (Arians Co., Brillion, WI). The forage was clipped at a height of approximately 7.62 to 10.16 cm from the surface of the ground to avoid collection of underlying dead forage material. Forage was collected with hand rakes and transported to the animal facilities. Forage was weighed into 2 portions per steer to be fed at the next 2 feedings. After weighing, the forage was loosely placed into open-top plastic bags to allow for air circulation and placed in a dark walk-in cooler (4°C) to ensure freshness of the forage. Forage growth was maintained in a vegetative state by clipping to a height of 25.4 cm as needed. Once an area had been harvested it was not harvested again.
Experimental Periods and Sampling Procedures.
The experiment consisted of two 21-d periods with a 14-d diet adaptation and 7-d digesta sample collection. Steers were paired by weight and then randomly assigned to 1 of 2 treatments. Treatments were free-choice, freshly clipped E+ Kentucky 31 tall fescue (control), or the same forage free choice with pelleted SH supplemented at 0.60% of BW (OM basis; supplemented). During d 1 to 8, steers were housed in individual pens (2.5 x 5.0 m). To allow for total fecal collection, during d 9 to 21, steers were housed in individual metabolism stalls. Supplementation amounts were based on BW taken at the beginning of each period. Spring growth of tall fescue was harvested and provided at an initial rate of 110% of their previous 5-d average intake. Supplement was fed at 0700 with approximately 65% of the estimated daily forage. To maintain a fresh forage supply, the remaining quantity of forage was supplied at 1900. To ensure steers had free-choice access at all times, additional quantities of forage were available and supplied if needed.
On d 9 to 20, gelatin capsules (Torpac #07 capsules; Torpac Inc., Fairfield, NJ) containing 9 g of chromic oxide were intraruminally dosed at the 0700 and 1900 feedings. Chromic oxide was utilized as an indigestible duodenal digesta flow marker. Soybean hull samples (0.11 kg/d), fresh forage samples (0.454 kg/d), and total orts were collected on d 12 to 18. Orts were composited within day and period by taking 10.0% (as-fed basis) of the orts from each steer at the 0700 and 1900 feedings. Dry matter and nutrient intakes were determined by subtracting orts from forage and supplement offered. Feed samples were frozen (–20°C) until dried at 55°C in a forced air oven to a constant weight and then ground to pass a 1-mm screen in a Wiley mill (Model 3; Arthur Thomas Company, Philadelphia, PA).
Approximately 200 g of duodenal digesta was collected 4 times per day at 6-h intervals on d 15 to 20. Sampling times were advanced 1 h each day to account for flow in each hour of a 24-h period. Duodenal samples were composited within steer and period by manually mixing individual samples and then removing approximately 100-g subsamples. Composite samples were stored at –20°C until lyophilized. Fecal samples were collected on d 15 to 20. Total fecal output was collected once daily, weighed, and mixed manually before a 2.5% subsample (as-is basis) was obtained and stored at –20°C. Fecal samples were thawed, and subsamples equaling 1% of the total fecal output were composited by steer within period and then weighed, lyophilized, and reweighed to determine DM. Duodenal and fecal samples were ground to pass a 1-mm screen in a Wiley Mill.
On d 19, steers were intraruminally pulse-dosed with 5 g of CoEDTA in a 150-mL aqueous solution (Uden et al., 1980) as a marker for liquid passage kinetics before the 0700 feeding. Cobalt-EDTA was poured through a stainless steel funnel and Tygon tubing into multiple random areas of the rumen. Rumen fluid samples (approximately 100 mL) were collected from 3 locations in the ventral rumen using a suction strainer device (Raun and Burroughs, 1962). Collections were made before dosing and at 3, 6, 9, 12, and 24 h postdosing. Immediately after each collection, rumen fluid pH was measured with a pH meter fitted with a combination electrode (Accumet Basic AB15 pH Meter; Fisher Scientific). Two aliquots (25 mL and 5 mL) of each rumen fluid sample were immediately stored at –20°C until analysis for Co, VFA, and ammonia.
Manual evacuation of the rumen contents from each steer (Lesperance et al., 1960) occurred 4 h after the 0700 feeding on d 21. After evacuation, total rumen contents were weighed, mixed thoroughly by hand, and subsampled in triplicate (approximately 400 g, as-is basis). An additional 2-kg sample of total rumen contents was collected into a plastic bucket and mixed with 2 L of cold (4°C) 0.9% (wt/vol) NaCl. Remaining rumen contents were returned to the respective steers. The 400-g rumen content samples were weighed, dried in a forced-air oven (55°C), reweighed for DM determination, and composited within steer by period. The 2-kg sample mixture was refrigerated (4°C) for at least 24-h before being mixed by hand and was homogenized in a blender (Waring Products, New Hartford, CT) at high speed for 1 min and strained through 4 layers of cheesecloth to remove large feed particles. The liquid fraction was centrifuged (800 x g; 10 min, 4°C) in 250-mL centrifuge bottles to separate protozoa and feed particles from bacteria. Supernatant was decanted into additional 250-mL centrifuge bottles and bacteria pelleted by centrifuging (16,000 x g; 15 min, 4°C). The supernatant was decanted and discarded, leaving only the bacterial pellet. The bacteria pellet was resuspended with 100 mL 0.9% (wt/vol) NaCl and centrifuged (16,000 x g; 15 min, 4°C). Through this process, bottles of bacteria from a steer were combined into a single sample. This resulted in each initial bacteria sample receiving 3 rinses. Bacteria were then frozen (–20°C), lyophilized, and ground with a mortar and pestle before analysis.
Composite forage, SH, orts, rumen contents, duodenal digesta, and fecal samples were analyzed for DM, OM, and N (AOAC, 2002) along with NDF without sodium sulfite and ADF (Ankom 200 Fiber Analyzer; Ankom Co., Fairport, NY). Bacteria were analyzed for DM, OM, and N. Duodenal and fecal samples were prepared (Williams et al., 1962) for Cr determination by atomic absorption spectroscopy (nitrous oxide/acetylene flame). Chromium concentrations of samples were used with corresponding nutrient concentrations to determine flow of nutrients throughout the gastrointestinal tract (Merchen, 1988). Purine content of rumen bacteria and duodenal digesta was determined according to Zinn and Owens (1986) with modifications from Obispo and Dehority (1999). Flow of bacterial N at the duodenum was estimated by dividing the average bacterial N:purine ratio of harvested bacteria by the N:purine ratio of the duodenal digesta and multiplying the quotient by the total N flow at the duodenum.
Rumen fluid samples were prepared by thawing under refrigeration (4°C) and centrifuging (16,000 x g, 10 min at 4°C). Supernatant from the 25-mL sample was used for Co analysis (AOAC, 2002) by atomic absorption using an air/acetylene flame (Unicam 969 AA Spectrometer; TJA Unicam, Cambridge, United Kingdom). Supernatant from the 5-mL sample (1.0 mL) was mixed with 0.2 mL of 25% orthophosphoric acid with 25 mM 2-ethylbutyrate as an internal standard. The sample was placed in an ice bath for 1 h before centrifuging at 16,000 x g for 15 min. The supernatant was analyzed for VFA by gas chromatography (Bock et al., 1991). One milliliter of supernatant from the 5-mL sample was acidified with 0.2 mL of 25% (wt/vol) metaphosphoric acid, centrifuged for 15 min at 16,000 x g, and analyzed for rumen ammonia as described by Broderick and Kang (1980).
Statistical Analysis
Data was analyzed as a crossover design utilizing the MIXED procedure of SAS (SAS Inst., Inc., Cary, NC). The model included steer, period, and treatment. Steer was included as a random effect. The response variables of interest involved 1) forage OM, total OM, NDF, and N intake; 2) rumen, intestinal, and total tract digestibilities of OM, NDF, and N; 3) duodenal microbial N flow; and 4) liquid dilution rate, rumen fluid volume, and rumen OM fill. Rumen fluid pH, ammonia, and VFA concentrations measured over a 24-h period were analyzed as just described with sampling time added as a repeated measure and the error covariance modeled with an autoregressive correlation structure. Repeated analysis of individual and total VFA concentrations and pH did not result in treatment x time interactions (P > 0.10). These measures were therefore averaged across time and analyzed as just described.
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RESULTS AND DISCUSSION
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Nutrient composition of the tall fescue during both experimental periods and the single lot of SH fed are presented in Table 1. The forage maintained a similar quality throughout the experiment, except that the N concentration of the forage in period 2 declined by 0.5%. The initial forage quality and changes over time are similar to clipped samples that were taken by Dubbs et al. (2003) to characterize tall fescue forage quality. In period 1, the nutrient content (NDF and N) was similar for the forage feed and orts (data not shown). In period 2, the orts were numerically lower in quality (NDF increased 5.33 and N decreased 1.12 percentage units) than the forage offered. This would indicate that some sorting of forage by the steers was occurring. This was most likely associated with greater quantities of senescent material in the forage due to the clipping of plots to maintain the forage in a vegetative state.
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Table 1. Nutrient composition of freshly clipped, endophyte-infected Kentucky 31 tall fescue and soybean hulls, DM basis
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Difficulties exist in measuring intakes of grazing animals because of the necessity of using some form of marker and digestibility estimate. Experiments with confined animals eliminate the concerns of the need for markers and digestibility estimates for total tract digestibility determination, but in turn present concerns of being able to maintain intakes that are representative of those in grazing animals. Chromium recovery rates were not different (P > 0.44; SEM = 0.02) at 89.3 and 91.7% for control and supplemented steers, respectively. No differences in Cr recoveries support that disappearance and nutrient flow data were equally represented between treatments. The control steers in our experiment consumed 1.64% of BW of forage OM (Table 2), whereas the supplemented steers consumed 1.41% of BW plus an additional 0.60% of BW of SH OM, which increased the daily OM intake to 2.01% of BW. These are within the range of values published for cattle grazing E+ pastures or consuming E+ tall fescue hay. Fieser and Vanzant (2004) reported an average intake of 2.11% of BW of OM intake for nonsupplemented steers over a wide range of hay qualities. Nonsupplemented E+ fescue grazing intakes have been reported from 2.34% of BW of OM intake (Elizalde et al., 1998) to 1.02% of BW of DM intake (Parish et al., 2003). Similar intakes to those in the current experiment were reported by Forcherio et al. (1995) for calves grazing spring E+ pastures and by Vanzant et al. (2002) for calves grazing stockpiled E+ pastures.
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Table 2. Apparent OM disappearance in steers fed freshly clipped, endophyte-infected Kentucky 31 tall fescue with or without soybean hull supplement
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Supplementation of 0.60% of BW (average = 1,751 g/d; OM basis) reduced (P < 0.01; Table 2
) daily forage OM intake by 684 g/d (14.4%) but resulted in a greater quantity of dietary nutrients for potential digestion and metabolism than control steers by increasing (P < 0.01) the total OM intake by 1,069 g/d (22.6%). Supplementation resulted in SH constituting an average of 30.2% of the OM intake for supplemented steers and with a substitution ratio of –0.39 when calculated as supplemented forage intake minus control forage intake divided by the quantity of SH fed. Substitution ratios calculated as stated previously result in a negative number if forage intake is reduced by the supplemental treatment, and ratios become more negative as forage intake decreases relative to the supplementation rate. A –1 value indicates that forage intake reduction was equal to the supplementation rate. Horn and McCollum (1987) and Fieser and Vanzant (2004) determined that substitution rates generally became more negative as the forage quality increased. Reports of cattle being fed E+ hay (Jansen van Rensburg and Vanzant, 2003; Fieser and Vanzant, 2004) or grazing E+ pastures (Hannah et al., 1989; Elizalde et al., 1998, Vanzant et al., 2002) and supplemented with 0.3 to 1.0% of BW of feedstuffs high in digestible fiber averaged a substitution ratio of –0.38 with a range of 0.04 (Vanzant et al., 2002) to –0.78 (Elizalde et al., 1998). Nonsupplemented intake level may be another factor when projecting substitution ratios, particularly when nonnutritional factors may play a role in intake such as with E+ tall fescue. Hess et al. (1996) reported that steers grazing endophyte-free fescue had nonsupplemented intakes of 3.49% of BW (OM basis) which, when wheat bran was supplemented at 0.34 or 0.48% of BW (OM basis), resulted in decreased total OM intake and substitution ratios of –1.97 and –2.10, respectively.
Duodenal OM flow (P < 0.02) and fecal OM output (P < 0.01) were greater for supplemented steers. There was no difference in the percentage of rumen (average = 53.0%; P > 0.30), intestinal (average = 37.3%; P > 0.13), or total tract (average = 70.8%; P > 0.22) OM disappearance when calculated as a percentage of flow to the individual segment. Without a change in the percentages of apparent OM disappearance at any site, the greater intake for supplemented steers resulted in a tendency (P < 0.06) for 428 g/d more rumen, 352 g/d more (P < 0.05) intestinal, and 781 g/d more (P < 0.01) apparent total tract OM disappearance. Jansen van Rensburg and Vanzant (2003) reported increases in total tract OM disappearance with SH supplementation over a wide range of supplementation levels when feeding moderate quality fescue hay diets.
Quantity of NDF intake (P < 0.01) and fecal output (P < 0.02) increased with SH supplementation (Table 3) while duodenal flow tended to increase (P < 0.07). Percentages of apparent rumen (P > 0.57) and intestinal (P > 0.20) NDF disappearance were not different, but SH supplementation resulted in a small increase (1.2 percentage units; P < 0.02) in total tract disappearance. This indicates that the digestibility of the fiber in SH was at least as digestible as the fiber from the forage in this study and that the tendency for increased duodenal NDF flows was related to intake rather than digestibility differences. When expressed on a quantity basis, supplemented steers had 469 g/d more (P < 0.01) rumen NDF disappearance, no difference (P > 0.17) in the quantity of apparent intestinal NDF disappearance, and 597 g/d more apparent total tract NDF disappearance.
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Table 3. Apparent NDF disappearance in steers fed freshly clipped, endophyte-infected Kentucky 31 tall fescue with or without soybean hull supplement
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Nitrogen intake was increased (P < 0.03; Table 4) with supplementation. The average N content of the diets consumed in this experiment ranged from 2.8 to 2.2% N; the diets that were consumed averaged 2.48 and 2.32% N (DM basis) for the control and supplemented treatments, respectively. Using the NRC (1996) TDN values for the fescue and weighted average of NRC (1996) TDN values of fescue and SH for the supplemented treatment based on the DM intake (data not shown), the average TDN:CP ratios were 3.9:1 and 4.5:1 for the control and supplemented treatments, respectively. According to estimations by Kunkle et al. (1999), these TDN:CP ratios indicate that both of these diets were adequate in CP. Total and microbial N flow at the duodenum were increased (P < 0.01 and P < 0.03, respectively) with SH supplementation. Similarly, Forcherio et al. (1995) increased duodenal microbial N flow with SH and corn supplements that also contained additional protein in E+ hay diets. Fieser and Vanzant (2004) reported numerical increases in urinary allantoin excretion with SH supplementation in E+ fescue hay diets, indicating that there was an increase in duodenal microbial N flow. Microbial efficiency was not different (P > 0.38) between the treatments and averaged 25.3 g of duodenal microbial N flow per kilogram of apparent rumen OM disappearance. These microbial efficiencies were lower than those determined by Elizalde et al. (1998), although they did not report an increase in OM intake or microbial protein flow at the duodenum with supplementation. Our microbial efficiencies were higher than those from Forcherio et al. (1995), who reported an increase in microbial efficiency with their supplements. There tended to be a decrease in quantity (P < 0.10) and percentage (P < 0.06) of apparent rumen disappearance with supplementation, but there was an increase in the quantity (27.6 g; P < 0.02) and a tendency (P < 0.09) for an increase in the percentage (6.3 percentage units) of apparent N disappearance in the intestine. If we assumed that the NPN and endogenous N flowing to the duodenum are not different due to supplementation, and then determined the quantity of N digested in the rumen by subtracting the nonmicrobial duodenal N flow from the N intake, both diets resulted in similar quantities of N digested (96 g/d) in the rumen. With these assumptions, the tendency for differences in the quantity of rumen disappearance are then related to the increased microbial N flow for the supplemented treatment. The tendency for the percentage of N disappearance (loss) in the rumen to decrease with supplementation is a combination of the percentages of nonmicrobial and microbial duodenal flow. The percentages of total tract apparent N disappearance were not different (P > 0.89), but the quantity was increased (P < 0.04) for the supplemented treatment. The responses for decreased rumen and increased intestinal percentages and quantities of N disappearance result in an increase in total duodenal N flow (34.1 g/d) that is approximately twice the increase in N intake (17.1 g/d); microbial N accounted for approximately one-half (17.1 g/d) of the increase in duodenal N flow. If we adjust our duodenal N flows for 15.6% NPN determined from duodenal N and AA flows of the control treatment of Elizalde et al. (1998), we have 480 and 659 g/d of metabolizable protein for our control and supplemented treatments, respectively. Using these estimates to project metabolizable protein allowable rates of gain for our calves with the NRC (1996) computer model, the control calves have sufficient metabolizable protein to sustain gains up to approximately 0.69 kg/d, whereas supplemented steers would have sufficient metabolizable protein to gain up to 1.34 kg/d.
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Table 4. Apparent N disappearance and microbial N flow in steers fed freshly clipped, endophyte-infected Kentucky 31 tall fescue with or without soybean hull supplement
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Although the quantity of rumen OM disappearance tended to increase, it did not affect the concentrations of total VFA (P > 0.11; Table 5). There was a tendency for the acetate:propionate ratio (P < 0.07) to increase. There was no difference in the molar proportions of acetate (P > 0.27), propionate (P > 0.66), or butyrate (P > 0.94). Others have reported that supplementation decreases proportions of acetate (Hess et al., 1996; Fieser and Vanzant, 2004). Because VFA molar proportions were not affected, this suggests that SH did not alter the fermentation characteristics from those of an all-forage diet. The molar ratios of 70:20:10 (acetate:propionate:butyrate) are similar to the ratios of 65:25:10 and 75:17:8 reported by Owens and Goetsch (1988) for forage diets and Fieser and Vanzant (2004) for tall fescue hay diets, respectively. There was a decrease in the molar proportions of isobutyrate (P > 0.01) and valerate (P > 0.03), but not isovalerate (P > 0.32). The small differences in the molar proportions of isobutyrate and valerate were probably not enough to have any biological significance. Furthermore, isobutyrate concentrations for both treatments were higher than suggested necessary for fiber digestion by Dehority et al. (1967).
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Table 5. Rumen VFA and fluid passage rates of steers fed freshly clipped, endophyte-infected Kentucky 31 tall fescue with or without soybean hull supplement
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Soybean hull supplementation did not change rumen pH (average = 6.5; P > 0.18). As noted by Fieser and Vanzant (2004), results have been variable when supplementing fiber-based supplements to forage diets, but as in this experiment, these do not commonly result in pH levels determined low enough (below 6.0) to reduce fiber digestion (Hoover, 1986) and cellulolytic bacterial growth (Ørskov, 1982). A notable exception is Hess et al. (1996), who reported pH values of 5.9 to 6.1 across their treatments. We only recorded one individual time point measure (data not shown) for either treatment in the current experiment that was below a pH of 6.0.
Rumen ammonia concentrations resulted in a treatment x time interaction (P < 0.01; Figure 1). The ammonia concentrations peaked in both treatments 3 h post-feeding and returned to lower than original levels by 12 h postfeeding. The reason for the variation between the 0 and 24 h concentrations is unclear. The several experiments supplementing SH have reported peak ammonia concentration times between 2 to 5 h (Martin and Hibberd, 1990; Grigsby et al., 1992; Jansen van Rensburg and Vanzant, 2003). In our experiment, the control steers had higher ammonia concentrations than the supplemented steers. Rumen ammonia concentration responses to supplementation with SH and other digestible fiber sources have been highly variable because they are affected by many factors including intake of ruminally available N, ruminally available energy, absorption, N recycled to the rumen, ammonia uptake by microbes, and passage out of the rumen. Additionally, N concentrations of SH can be highly variable. Within our laboratory, we have analyzed samples in a range of 11.4 to 18.5% CP (DM basis). The reduction in rumen ammonia in our experiment can be related, at least partially, to a decreased N concentration of the diet consumed with supplementation of SH. However, the tendency for greater quantities of apparent rumen OM disappearance and greater microbial N flows at the duodenum with the supplemented treatment indicates that more ammonia was also incorporated into microbial N. Rumen ammonia concentrations were lower than those reported by Hess et al. (1996) and Elizalde et al. (1998) for cattle grazing tall fescue pastures. The average ammonia concentrations were within a range considered optimal as determined by Hoover (1986), but concentrations at the 12 and 24 h measures were slightly below the range of 2.4 to 5.7 mM. With our rumen fluid sampling schedule, the 12-h sample was taken before refreshing the forage in the afternoon, and no additional samples were taken until just before the next morning feeding. Although it is difficult to determine what a normal intake pattern is for grazing animals, in our experiment the refreshing of forage in the afternoon did result in some meal activity at that point. Our rumen sampling protocol did not allow for detection of any increases in rumen ammonia that might have occurred with intake due to refreshing the forage. If refreshing the forage increased rumen ammonia concentrations above the 12 h measurement, additional sampling between 12 and 24 h would have resulted in an increased average ammonia concentration over the 24-h period. However, it does not seem the rumen ammonia differences affected cellulolytic activity; the percentage rumen NDF disappearance was not different between treatments.
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Figure 1. Rumen ammonia concentrations of beef steers fed freshly clipped, endophyte-infected Kentucky 31 tall fescue with or without 0.60% of BW soybean hull supplement. There was a treatment x time interaction (P < 0.01; SEM = 0.94; n = 6).
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No differences were detected for liquid dilution rate (P > 0.65; Table 5) or rumen fluid volume (P > 0.44), which are generally strong factors in controlling intake. Rumen OM fill was also not different (P > 0.57). These factors, along with the decrease in forage OM intake with supplementation, suggest that physical fill was responsible for controlling intake. However, decreases in forage OM intake were less than the increases in total OM intake with SH supplementation. This could be explained at least in part by the 5.7% numerical increase in fluid dilution rate and the 22.8% increase in duodenal OM flow or a factor other than fill limiting intake.
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Footnotes
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1 We express our appreciation to L. Miller, M. Dance, D. Stinson, and E. Jarboe from the Animal Science Department for their help in completing this project. 
2 Corresponding author: chris.richards{at}okstate.edu
Received for publication December 13, 2004.
Accepted for publication October 20, 2005.
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Abstract
INTRODUCTION
