J. Anim. Sci. 2006. 84:1468-1480
© 2006 American Society of Animal Science
Effects of corn condensed distillers solubles supplementation on ruminal fermentation, digestion, and in situ disappearance in steers consuming low-quality hay1
T. C. Gilbery,
G. P. Lardy2,
S. A. Soto-Navarro3,
M. L. Bauer and
J. S. Caton
Department of Animal and Range Sciences, North Dakota State University, Fargo 58105
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Abstract
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Two metabolism (4 x 4 Latin square design) experiments were conducted to evaluate the effects of corn condensed distillers solubles (CCDS) supplementation on intake, ruminal fermentation, site of digestion, and the in situ disappearance rate of forage in beef steers fed low-quality switchgrass hay (Panicum virgatum L.). Experimental periods for both trials consisted of a 9-d diet adaptation and 5 d of collection. In Exp. 1, 4 ruminally and duodenally cannulated steers (561 ± 53 kg of initial BW) were fed low-quality switchgrass hay (5.1% CP, 40.3% ADF, 7.5% ash; DM basis) and supplemented with CCDS (15.4% CP, 4.2% fat; DM basis). Treatments included 1) no CCDS; 2) 5% CCDS; 3) 10% CCDS; and 4) 15% CCDS (DM basis), which was offered separately from the hay. In Exp. 2, 4 ruminally and duodenally cannulated steers (266.7 ± 9.5 kg of initial BW) were assigned to treatments similar to Exp. 1, except forage (Panicum virgatum L.; 3.3% CP, 42.5% ADF, 5.9% ash; DM basis) and CCDS (21.6% CP, 17.4% fat; DM basis) were fed as a mixed ration, using a forage mixer to blend the CCDS with the hay. In Exp. 1, ruminal, postruminal, and total tract OM digestibilities were not affected (P = 0.21 to 0.59) by treatment. Crude protein intake and total tract CP digestibility increased linearly with increasing CCDS (P = 0.001 and 0.009, respectively). Microbial CP synthesis tended (P = 0.11) to increase linearly with increasing CCDS, whereas microbial efficiency was not different (P = 0.38). Supplementation of CCDS to low-quality hay-based diets tended to increase total DM and OM intakes (P = 0.11 and 0.13, respectively) without affecting hay DMI (P = 0.70). In Exp. 2, ruminal OM digestion increased linearly (P = 0.003) with increasing CCDS, whereas postruminal and total tract OM digestibilities were not affected (P 
0.37) by treatment. Crude protein intake, total tract CP digestibility, and microbial CP synthesis increased (P 
0.06) with increasing level of CCDS supplementation, whereas microbial efficiency did not change (P = 0.43). Ruminal digestion of ADF and NDF increased (P = 0.02 and 0.008, respectively) with CCDS supplementation. Based on this data, CCDS used in Exp. 2 was 86.7% rumen degradable protein. The results indicate that CCDS supplementation improves nutrient availability and use of low-quality forages.
Key Words: beef cattle • corn condensed distillers solubles • digestibility • fermentation • forage • protein supplementation
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INTRODUCTION
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The ethanol industry is currently in the midst of a considerable expansion in the upper Midwest (RFA, 2005
). Consequently, availability of ethanol by-products is also expanding. Interest among cow calf producers in using corn condensed distillers solubles (CCDS) is growing. Corn condensed distillers solubles are relatively high in CP (15 to 25%; DM basis), which makes the product an attractive supplement for low-quality forages. Low-quality forages and crop residues are an abundant, valuable feed resource for ruminant animals (NRC, 1983). However, to achieve optimal use of these forages and meet animal nutritional requirements, it is often necessary to provide supplemental nutrients (Caton and Dhuyvetter, 1997).
Specifically, rumen degradable protein (RDP) generally improves forage intake, digestion, and animal performance (Guthrie and Wagner, 1988; Del Curto et al., 1990; Köster et al., 1996). When feeding low-quality forages, it is common to have inadequate ruminal N concentrations. Deficient ruminal NH3 may limit microbial CP synthesis and growth, thereby limiting microbial fermentation of fiber, digesta outflow, and forage intake (Maeng et al., 1976; Egan, 1980). Burroughs et al. (1950) and Chen et al. (1976) reported improved cellulose digestion with the addition of either dried distillers solubles or corn distillers solubles, respectively. However, little is known about optimum levels of CCDS in low-quality forage-based diets and subsequent effects on ruminal fermentation, digestion, and ruminal metabolism. Plant to plant variations in nutrient content of by-products can be large (Spiehs et al., 2002).
The objectives of this study were to evaluate the influence of CCDS supplementation on intake and site of digestion in beef steers fed low-quality grass hay. We conducted 2 independent studies, using CCDS from 2 different sources, which differed widely in CP and fat content, whereas other nutrient characteristics were similar (Table 1).
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MATERIALS AND METHODS
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Experiment 1
Animal Diets and Treatments.
All animal care, handling techniques, and surgical procedures were approved by the North Dakota State University Animal Care and Use Committee before initiation of the research. Four ruminally and duodenally cannulated Holstein steers (561 ± 53 kg of initial BW) were used in a 4 x 4 Latin square design to evaluate the effects of increasing level of CCDS on intake, ruminal fermentation, and digestion, as a supplement fed to steers consuming low-quality hay. Steers were housed in an enclosed barn in individual stanchions (1.2 x 2.2 m).
Steers were fed switchgrass hay (Panicum virgatum L.) and CCDS at 0700 and 1900 daily (hay and the CCDS were offered separately to the steers at each feeding) and were allowed free access to water and trace mineralized salt blocks (minimum 4.0 g of Zn, 1.6 g of Fe, 1.2 g of Mn, 0.33 g of Cu, 0.10 g of I, and 0.04 g of Co/kg; North American Salt Company, Overland Park, KS). Hay was offered to ensure ad libitum intakes and 10% feed refusal daily. At each feeding, CCDS was offered first; if any remained after 15 min, it was placed in the rumen through the cannula. Hay was offered after CCDS was either consumed or placed in the rumen.
The switchgrass hay was harvested in eastern Clay County, MN, approximately 35 km east of Fargo, ND. The hay was mature forage, which had been used to produce grass seed. The hay was baled after the seed had been harvested. Before being fed, the hay was chopped in a tub grinder through a 3.8-cm screen. The nutrient content of the switchgrass hay and CCDS used in this study is provided in Table 1
. The CCDS was obtained from a dry-milling ethanol plant in northeast South Dakota. Steers were supplemented with 1 of 4 levels of CCDS, which were: 1) control, no CCDS; 2) 5% CCDS; 3) 10% CCDS; and 4) 15% CCDS supplementation of forage intake (DM basis).
Sample Collection
Each experimental period was 14 d in length, allowing 9 d for adaptation to the diet and 5 d for sample collection. Orts were collected on d 8 to 14, weighed, and composited. Animal weights were recorded at the beginning and end of each experimental period. Chromic oxide (8 g), placed in gelatin capsules (Torpac Inc., Fairfield, NJ), was dosed intraruminally on d 6 to 14 at 0700 and 1900. Samples of duodenal fluid (200 g) were collected on d 11 to 14 in a manner to achieve a sampling point every other hour between feedings (0700 and 1900). Samples were composited within steer for each period. Fecal trays were placed behind steers for total fecal collection during d 10 through 14. Fecal material was scraped directly into the tray on an hourly basis. Fecal output was weighed, subsampled (10% of wet weight), and composited across days within steer for each period. Samples were stored frozen (– 20°C) until dried in a forced-air oven for 48 h (50°C; Model SB-350, Grieve Corp., Round Lake, IL.).
Estimates of in situ disappearance of grass hay were determined on d 10 to 14. Forage for in situ work was ground through a 2-mm screen in a Wiley mill (Model 3, Arthur H. Thomas, Philadelphia, PA). Switchgrass hay (approximately 5 g) was placed in Dacron bags (10 x 20 cm, 50 ± 15 µm pore size; Ankom, Fairport, NY). In situ samples were ruminally incubated for 0, 2, 5, 9, 12, 24, 36, 48, or 96 h. Bags were inserted into the rumen in reverse order. Removal of all bags occurred at 0 h. Bags were rinsed to remove particulate matter from the surface before rinsing by a top loading washing machine (model WJXR2080TSWW, General Electric, Louisville, KY). During the rinsing process, the machine was filled with 45 L of cold water, and bags were agitated on the delicate cycle for 1 min. After each cycle, the washing tray was drained and spun for 2 min; this cycle was repeated 5 times. Bags were dried in a forced-air oven for 48 h and stored at room temperature before laboratory analysis.
On d 12 of each period, ruminal fluid samples were collected at – 2, 0, 2, 4, 6, 8, 10, and 12 h after feeding. After the – 2 h collection, the rumen was dosed with 200 mL of CoEDTA to determine ruminal liquid dilution rate (Uden et al., 1980). A volume of ruminal fluid (200 mL) was drawn using a suction strainer, and the pH was recorded using a pH meter and combination electrode (model 2000, Beckman Instruments Inc., Fullerton, CA). A 3-mL sample of ruminal fluid was retained, and 1 mL of 25% HPO3 (wt/vol) was added to the sample in a 12 x 75 mm culture tube. Samples were stored frozen (– 20°C) until analysis for NH3 and VFA.
On d 14, ruminal evacuations were performed at 1800 to determine DM fill. Ruminal contents were removed, weighed, and mixed with a subsample retained for DM analysis. A 4-kg sample was taken, and 2 L of 3.7% formaldehyde/0.9% NaCl (wt/vol) was added (Zinn and Owens, 1986) for isolation of bacterial cells and analysis for DM, ash, N, and purines. Samples were stored (– 20°C) until analysis.
Laboratory Analysis
Diet, orts, ruminal content, and fecal samples were dried in a forced air oven at 50°C for 48 h. Dried samples were ground through a Wiley mill with a 2-mm screen. Duodenal samples were lyophilized (Virtis Genesis 25LL, Virtis Co. Inc., Gardiner, NY) and ground in a blender (Osterizer Galaxie Pulse Matic I6, Sunbeam, Purvis, MS).
Diet, orts, fecal, and duodenal samples were analyzed for DM, ash, N (AOAC methods 4.1.06, 4.1.10, 4.2.10, respectively; AOAC 1997), and ADF and NDF (Ankom, Fairport, NY). Ruminal content samples were analyzed for DM (AOAC method 4.1.06; AOAC, 1997). Duodenal samples were analyzed for Cr by the spectrophotometer method of Fenton and Fenton (1979). Chromium concentrations were used to calculate digesta flow. Digestibility was calculated by subtracting flow rate from intake and dividing by intake. In situ forage samples were analyzed for DM and CP (AOAC method 4.1.06, 4.2.10, respectively; AOAC, 1997), ADF, and NDF (ANKOM, Fairport, NY).
Ruminal fluid was centrifuged 20,000 x g, 20 min. Liquid was filtered through a 0.45-µm filter, and the supernatant was analyzed for NH3 (Broderick and Kang, 1980). Ruminal VFA concentrations were determined by gas chromaphotography (5890A Series II GC, Hewlett Packard, Wilmington, DE) and separated on a capillary column (Nukol, Supelco, Bellefonte, PA) using 2-ethyl butyric acid as the internal standard (Goetsch and Galyean, 1983).
Bacterial cells were isolated from formalized ruminal contents. Ruminal contents were blended (Model 37b119, Waring, New Hartford, CT), and the mixture was strained through 2 layers of cheesecloth. Feed particles and protozoa in the ruminal samples were removed through centrifugation at 500 x g for 20 min. The sample was then centrifuged at 30,000 x g for 20 min to collect the bacteria from the supernatant. Isolated bacteria were frozen, lyophilized, and analyzed for DM, ash, N (AOAC methods 4.1.06, 4.1.10, 4.2.10, respectively; AOAC, 1997), and purines (Zinn and Owens, 1986).
Calculations
Microbial organic matter and N leaving the abomasum were calculated using purines as microbial markers (Zinn and Owens, 1986). Organic matter fermented in the rumen was calculated as OM intake minus the difference between the amount of total OM reaching the duodenum and microbial OM reaching the duodenum. Feed N escape to the small intestine was calculated by subtracting microbial N from total N and thus includes any endogenous and NH3-N contribution. Liquid dilution rate was calculated by regressing the natural logarithm of the Co concentration on sampling time. Ruminal DM, NDF, and ADF disappearance (%/h) of switchgrass hay were estimated using the model described by Mertens and Loften (1980). The CP kinetic parameters of switchgrass hay were estimated using the model described by Ørskov and McDonald (1979) and did not account for potential microbial contamination.
Statistical Analysis
Data were analyzed as a 4 x 4 Latin square using MIXED procedures of SAS (version 9.1, SAS Inc., Cary, NC). The model included CCDS level and period as fixed effects, and steer as a random effect. Ruminal data over time were analyzed as repeated measures using the AR(1) first-order autoregressive covariance structure in the MIXED procedures of SAS. Other covariance structures were evaluated, but the AR(1) structure fit the data the best (Wang and Goonewardene, 2004). The model included CCDS level, period, time, and the interaction of CCDS level x time as fixed effects, and steer nested within period x CCDS level as a random effect. Orthogonal contrasts for linear, quadratic, and cubic effects of CCDS level are discussed when a significant (P < 0.10) treatment F-test was detected.
Experiment 2
Animals, Diets, and Treatments.
All surgical procedures, animal care, and handling procedures were approved by the North Dakota State University Institute of Animal Care and Use Committee before initiation of the study.
Four ruminally and duodenally cannulated beef steers (266.7 ± 9.5 kg of initial BW) were used in a 4 x 4 Latin square design to evaluate the effects of increasing level of CCDS. Steers were housed in an enclosed barn in individual stanchions (1.2 x 2.2 m). Steers were offered hay and CCDS at 0700 and 1900 and were allowed free access to water and trace mineralized salt blocks (minimum 4.0 g of Zn, 1.6 g of Fe, 1.2 g of Mn, 0.33 g of Cu, 0.10 g of I, and 0.04 g of Co/kg; North American Salt Company, Overland Park, KS). Dietary treatments were offered to ensure ad libitum intakes and 10% feed refusal daily.
The switchgrass hay (Panicum virgatum L.) was harvested in eastern Clay County, MN, approximately 35 km east of Fargo, ND. The hay was mature forage that had been used to produce grass seed. The hay was baled after seed had been harvested. Before being fed, the hay was chopped in a tub grinder through a 3.8-cm screen. The nutrient content of the switchgrass hay and CCDS used in this study is provided in Table 1. The CCDS was obtained from a dry milling ethanol plant in west central Minnesota.
Steers were supplemented with 1 of 4 levels of CCDS, which were 1) control, no CCDS (3.3% dietary CP; DM basis); 2) 5% CCDS (4.4% dietary CP; DM basis); 3) 10% CCDS (5.3% dietary CP; DM basis); and 4) 15% CCDS (6.1% dietary CP; DM basis) supplementation of total intake. The treatments that included CCDS were blended with the switchgrass hay in a Roto-Mix mixer (Model # 84-8 stationary, Dodge City, KS). Application of CCDS into the ration was accomplished via a 3.8-cm Roper pump (Model # 2835, Commerce, GA) through a 2.5-cm hose line across 2 flood-jet spray tips calibrated to deliver 2 gallons per minute of CCDS. Sample collection, laboratory analysis, calculations, and statistics were the same as outlined for Exp. 1.
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RESULTS AND DISCUSSION
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Experiment 1
Hay DMI was not affected by level of CCDS supplementation (Table 2). By design, CCDS intake (kg/d) increased linearly (P < 0.001); as a result, total DMI tended (P = 0.11) to increase linearly when expressed as kg/d; however, when expressed as percentage of BW, no effects were detected (P = 0.20). Similarly, other researchers (Freeman et al., 1992; Köster et al., 1997) have reported no response to supplemental RDP when low-quality forages were fed. In contrast, providing RDP to ruminants fed low-quality forages has been shown to increase forage intake (Mathis et al., 1999; Olson et al., 1999; Bodine et al., 2001). No differences were detected in ruminal DM fill (% of BW; P = 0.30). This is similar to Olson et al. (1999), who reported no differences in ruminal DM fill with protein supplementation. However, others have reported increased ruminal DM fill with protein supplementation (DelCurto et al., 1990; Sunvold et al., 1991). Because no differences were detected in DMI, differences in fill were not expected.
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Table 2. Effect of corn condensed distillers solubles (CCDS) level on DMI, ruminal fill, and fluid dilution rate in beef steers consuming low-quality hay (Exp. 1)
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No treatment effect (P = 0.58) for fluid dilution rate (FDR; 2.18 ± 0.17%/h) was observed (Table 2). Response of FDR to protein supplementation when feeding low-quality forages has varied among authors with some reporting increases in FDR (Freeman et al., 1992; Hannah et al., 1991), whereas others did not report increases in FDR (Köster et al., 1997; Bandyk et al., 2001). Because there was no intake response to increasing levels of CCDS supplementation, increases in FDR were not expected.
Organic matter intake, total OM flowing to the small intestine, and microbial OM flowing to the small intestine did not change (P = 0.13 to 0.15) with increasing CCDS level (Table 3). Similarly, there was no effect on ruminal, postruminal, or total tract OM digestibility (P = 0.21 to 0.59). Increasing DMI in response to supplemental RDP is not universal in ruminants fed low-quality forages. Freeman et al. (1992) noted no increase in intake for cattle fed 5.6% CP prairie hay and provided a cottonseed supplement. Similarly, Judkins et al. (1991) reported no difference in intake for Holstein steers fed 6.1% CP hay and provided a cottonseed meal supplement (fed at 0.16% of BW). The lack of a forage intake response to supplementation resulted in little change to OM intake and associated digestibility thereafter.
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Table 3. Effect of corn condensed distillers solubles (CCDS) level on OM digestion in beef steers consuming low-quality hay (Exp. 1)
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Hatch et al. (1972) reported no differences in DM digestibility when 2.5% corn distillers solubles were included in a liquid supplement offered as part of a corn-based diet for beef steers. Burroughs et al. (1950) reported increased cellulose digestion in artificial rumens with the addition of dried distillers solubles, whereas Chen et al. (1976) also reported increased cellulose digestion in vitro with the addition of either screening processed distillers solubles or centrifuge processed distillers solubles.
Total CP intake (P = 0.002) and total tract CP digestibility increased linearly (P = 0.03), whereas microbial protein synthesis tended (P = 0.11) to increase linearly with increasing level of CCDS supplementation (Table 4). However, microbial efficiency (11.6 ± 2.66 g of N/kg of OM truly fermented) was not affected by treatment (P = 0.38). The low NH3-N concentrations and no change in true ruminal OM and NDF digestion we observed suggest that microbial growth was not enhanced by CCDS supplementation. Optimal NH3-N concentrations ( 3.5 mM; Satter and Slyter, 1974; Hoover, 1986) for improvement of microbial efficiency were never reached in this study. Although our NH3-N concentrations were low, the concentrations we observed were in the same range of values published by other researchers (Hannah et al., 1991; Freeman et al., 1992) feeding low-quality forages and providing CP supplementation. Hatch et al. (1972) reported decreased ruminal NH3- N concentrations in steers when 2.5% corn distillers solubles were added to a liquid supplement. However, these researchers also reported increased N retention in the same study.
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Table 4. Effect of corn condensed distillers solubles (CCDS) level on CP digestion in beef steers consuming low-quality hay (Exp. 1)
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Treatment did not affect total tract ADF (51 ± 6.2%) or NDF (51 ± 6.5%) digestibility (P = 0.58 and 0.68, respectively; Table 5). Given the relatively low forage quality used in this study, increases in ADF and NDF digestibility were expected with supplemental CCDS (Caton and Dhuyvetter, 1997). Although fat supplementation (3 to 5% of DM) has been reported to decrease fiber digestibility (Chalupa et al., 1984), negative effects on fiber digestion were not expected because the 15% CCDS treatment only provided 0.63% of dietary fat from the hay plus CCDS (calculated).
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Table 5. Effect of corn condensed distillers solubles (CCDS) level on ADF and NDF digestion in beef steers consuming low-quality hay (Exp. 1)
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Level of CCDS did not affect pH (P = 0.68; 6.41 ± 0.10), ruminal NH3 (P = 0.34; 1.95 ± 0.48 mM), or total VFA concentration (P = 0.55; 38.8 ± 2.5 mM; Table 6). However, acetate molar proportion ratio decreased linearly (P < 0.001), whereas molar proportion of butyrate increased linearly (P < 0.001) with increasing level of CCDS supplementation. Time x treatment interactions were observed for acetate (P < 0.001) and butyrate (P < 0.001). However, these interactions were largely due to magnitude of response and were not deemed biologically significant. Consequently, only main effects are reported (Table 6). Molar proportion of acetate decreased (P < 0.001), and butyrate increased (P < 0.001) with CCDS supplementation. Acetate and butyrate formation proceed through acetyl-CoA and are interconverted (Berger, 1990). Our data indicate acetate was being replaced by butyrate as indicated by no effect (P = 0.56) of CCDS on the ratio of acetate plus 2 times butyrate to propionate. This is presumably to remove additional reducing equivalents (Fahey and Berger, 1988) from increased (P = 0.10) fermentation of OM in the rumen (3.39, 4.32, 4.66, and 5.13 kg truly fermented/d for 0, 5, 10, and 15 CCDS, respectively).
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Table 6. Effect of level of corn condensed distillers solubles (CCDS) on ruminal pH, NH3 concentration, and VFA concentration in beef steers consuming low-quality hay (Exp. 1)
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Rate of in situ disappearance for hay DM (2.3 ± 0.39%/h; P = 0.80), NDF (2.4 ± 0.47%/h; P = 0.80), ADF (2.3 ± 0.42%/h; P = 0.99), and CP (2.7 ± 0.83%/h; P = 0.83) were not affected by CCDS supplementation (Table 7). Additionally, the hay CP soluble fraction (P = 0.31), slowly degradable fraction (P = 0.83), and extent of degradability (P = 0.86) were not affected by CCDS supplementation level. We hypothesized that CCDS supplementation would increase digestion rates of low-quality hay because of rumen degradable protein and N supply to ruminal bacteria; however, no differences were observed. Likewise, Arroquy et al. (2004) reported no effect of supplemental RDP (as NPN) on forage OM and NDF digestion; their hay quality was similar to ours. Certain microorganisms require amino acids for fiber digestion. Russell et al. (1992) observed the importance NH3 plays in the fermentation of structural carbohydrates, whereas nonstructural carbohydrate fermentation remained more responsive to peptides supplied in true proteins. In our study, ruminal NH3 did not change with CCDS supplementation during incubation of grass hay. This may have limited microbial fiber fermentation rates.
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Table 7. Effect of corn condensed distillers solubles (CCDS) level on rate of DM, NDF, and ADF ruminal disappearance (%/h), and on CP kinetic parameters of grass hay in beef steers consuming low-quality hay (Exp. 1)
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Experiment 2
Hay DMI (kg/d) tended to increase quadratically (P = 0.08; Table 8). The greatest DMI occurred with the 10% CCDS treatment. When hay DMI was expressed as a percentage of BW, we noted no effect (P = 0.16) with increasing CCDS supplementation. By design, CCDS intake increased linearly (kg/d, DM basis; P < 0.001). In addition, total intake expressed in kilograms per day and percentage of BW increased linearly (P = 0.008 and 0.01, respectively). Cattle consuming low-quality forages often lack adequate concentrations of ruminal N necessary to support fiber digesting microbial populations (Maeng et al., 1976, Bandyk et al., 2001). The consequence of this situation is a reduction in ruminal fermentation, outflow of digesta, and ultimately, intake. In the current study, in which we fed low-quality forage (3.25% CP; DM basis), we expected to observe an increase in ruminal NH3-N with the supplementation of CCDS. The increases in NH3-N observed with supplementation of CCDS suggest this, in fact, did occur. Other studies in which forage CP was similar to ours (Hannah et al., 1991) have reported increases in DMI when supplemental RDP was provided. Due to the addition of supplements, total DMI increased 14, 34, and 20% with the addition of 5, 10, and 15% CCDS, respectively, over the control treatment (0% CCDS). Ruminal DM fill decreased linearly (P = 0.001) with increasing level of CCDS. This is in contrast to Olson et al. (1999) who reported no differences in ruminal DM fill between supplemented and nonsupplemented steers, and DelCurto et al. (1990) and Sunvold et al. (1991) who reported increased ruminal DM fill with protein supplementation. The reasons for these differences are not immediately clear. Because hay intake increased, we expected to observe an increase in DM fill as well.
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Table 8. Effect of corn condensed distillers solubles (CCDS) level on DMI, ruminal fill, and fluid dilution rate in beef steers consuming low-quality hay (Exp. 2)
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We observed no treatment effect on fluid dilution rate (FDR; P = 0.36; 8.71 ± 1.11%/h; Table 8). Owens and Goetsch (1988) hypothesized slow FDR may result in slow carbohydrate fermentation in diets that are limiting in ruminally available protein. With slow rates of passage, more digested energy is used for microbial maintenance (Russell et al., 1992). Therefore, the efficiency of synthesis of bacterial CP from digestible energy is reduced (NRC, 1996). This situation could lead to a high proportion of available energy being allocated to the maintenance of the bacterial population, and negatively affect rate of passage, thereby increasing retention time of ruminal content.
Organic matter intake increased linearly (P = 0.007) and tended to increase quadratically (P = 0.09; Table 9). Organic matter intake increased from 2.8 kg/d for the control to 3.8 kg/d for the 10% supplementation level and then decreased to 3.54 kg/d for the 15% treatment. It is possible that supplemental DIP in this study increased to the point that OM intake was maximized. Previous research has documented that increases in OM intake (Guthrie and Wagner, 1988; Köster et al., 1996) can be attained with the addition of RDP when feeding low-quality forages. The largest incremental response occurred with the 10% supplementation level. Total OM flowing to the small intestine increased quadratically (P = 0.005), whereas microbial OM flow tended to increase (P = 0.09), with increasing CCDS level.
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Table 9. Effect of corn condensed distillers solubles (CCDS) level on OM digestion in beef steers consuming low-quality hay (Exp. 2)
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We observed linear increases for apparent ruminal (P = 0.003) and true ruminal OM digestion (P < 0.001) with supplementation of CCDS (Table 9). The largest incremental response to treatment occurred at the 10% supplementation level. There was no difference among treatments for intestinal (P = 0.37) and total tract OM digestion (P = 0.68) across CCDS treatments. Burroughs et al. (1950) reported increased cellulose digestion in artificial rumens with the addition of dried distillers solubles, and Chen et al. (1976) also reported increased cellulose digestion in vitro with the addition of either screening processed distillers solubles or centrifuge processed distillers solubles.
Ruminal passage rate of OM (%/h; Table 9) increased linearly (P = 0.004) with CCDS supplementation. Whereas passage rate of OM increased with supplementation, total tract OM digestibility did not; this situation suggests that fermentation of OM continued postruminally on the control treatment, extending into the large intestine. Other research (Campling et al., 1962) in which oat straw (CP = 3%) was fed to cows supplemented with increasing levels of urea resulted in decreased mean retention time of straw residues in the whole gut. Coombe and Tribe (1963) reported decreased mean retention time of particulate matter in the reticulorumen for wethers supplemented with increasing urea levels when straw was provided as the forage (CP ranged from 2.7 to 3.3%). A search for more recent data on passage rate of low N and high fiber forages suggests a void in data on supplementation and its effects on digestion of the aforementioned forage types.
By design, CP intake increased linearly (P = 0.002; Table 10). This increase is a reflection of the increase in dietary N supplied as level of CCDS increased. Köster et al. (1996) reported greater N intakes in cows when feeding increasing amounts of supplemental RDP in the form of casein. Total tract CP digestion increased linearly (P = 0.01) as well. Increased provision of supplemental RDP linearly increased (P = 0.002) total CP and microbial CP flow to the duodenum. Duodenal CP flow was greater (176 ± 21 g/d vs. 240 ± 12 g/d) than CP intake across treatments. The negative ruminal CP digestibilities we observed were similar to other researchers feeding unsupplemented low-quality forages (Hannah et al., 1991; Köster et al., 1996) due to N recycling (Bunting et al., 1989). Because microbial CP flow to the duodenum increased and there was a corresponding increase in OM intake and OM truly fermented in the rumen, microbial efficiency (g of microbial N/kg of truly OM fermented) was not different (P = 0.43). The microbial efficiencies we observed are similar to other studies (Lintzenich et al., 1995; Köster et al., 1996) that provided supplementation when feeding low-quality forages. Ruminal degradability of CCDS CP was calculated (Figure 1) by plotting grams of ruminal CCDS CP disappearance against CCDS CP intake (g/d). Ruminal hay CP disappearance was calculated based on hay CP intake and estimated hay CP degrability for the 0% treatment. Ruminal hay CP disappearance was subtracted from total ruminal CP disappearance to calculate ruminal CCDS CP disappearance. Ruminal CP disappearance was determined by subtracting nonmicrobial CP flow at the duodenum from total CP intake. From the resulting equation, CP degradability of CCDS was calculated to be 86.7 ± 13.2%. The intercept of the line was not different from zero (–4.2 ± 10.5 g/d; P = 0.70).
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Table 10. Effect of corn condensed distillers solubles (CCDS) level on CP digestion in beef steers consuming low-quality hay (Exp. 2)
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Figure 1. Estimated ruminal CP degradability of corn condensed distillers solubles (CCDS; Exp. 2). The slope of the line indicates ruminal CP degradability was 86.7 ± 13.2%, and the intercept was not different (P = 0.70) from zero (–4.2 ± 10.5 g/d).
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Intake of ADF (kg/d; Table 11) increased linearly (P = 0.04) with increasing CCDS level. This is a function of increased forage DM intake. We also observed a similar response in NDF intake, with a linear (P = 0.04) increase. Ruminal NDF digestion increased linearly (P = 0.008) with CCDS supplementation, whereas ruminal ADF digestion tended (P = 0.07) to increase with increasing CCDS levels. The results we observed were similar to other research in which increases in voluntary intake (McCollum and Galyean, 1985; Köster et al., 1996) and fiber digestibility occurred with N supplementation for ruminants fed low-quality forages.
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Table 11. Effect of corn condensed distillers solubles (CCDS) level on ADF and NDF digestion in beef steers consuming low-quality hay (Exp. 2)
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No treatment effects (P = 0.22) or time x treatment interactions (P = 0.99) were observed for ruminal pH (6.80 ± 0.07; Table 12). Likewise, McCollum and Galyean (1985) reported no change in ruminal pH when providing RDP to cattle consuming low-quality forages. Other researchers (Guthrie and Wagner 1988; Köster et al., 1996) have observed a decrease in pH when supplementing RDP. The similar pH values among treatments we observed correspond with similar total VFA concentrations that we observed across treatments when supplementing CCDS (Table 12). Concentration of total VFA did not change across treatments (P = 0.64; Table 12) and were very low (see later); therefore, we would not expect to measure changes in ruminal pH.
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Table 12. Effect of level of corn condensed distillers solubles (CCDS) on ruminal pH, NH3 concentration, and VFA concentration in beef steers consuming low-quality hay (Exp. 2)
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Ruminal NH3 concentration increased quadratically (P = 0.009) with increasing CCDS supplementation, with the greatest level occurring at 15% CCDS (Table 12). The NH3-N concentrations we observed were low when compared with the optimal concentrations to support microbial growth (Satter and Slyter, 1974). In other studies (Lintzenich et al., 1995; Köster et al., 2002) in which low-quality forages were fed in combination with RDP supplementation, similar values for NH3-N were reported. Likewise, those authors reported increases in NH3-N when providing RDP supplementation.
Total VFA concentration was not different (P = 0.64) among treatment groups. Time x treatment interactions were observed for molar proportions of acetate (P = 0.04) and butyrate (P = 0.014). However, these interactions were largely due to magnitude of response and were not deemed biologically significant, and only main effects are reported (Table 12). Acetate molar proportion decreased linearly (P < 0.001), whereas molar proportions of propionate and butyrate increased in a linear fashion (P 0.002) with increasing CCDS supplementation. Our data indicate acetate and butyrate were being replaced by propionate as indicated by a linear decrease (P = 0.001) in the concentration of acetate plus 2 times butyrate to propionate ratio. These data suggest that by supplementing CCDS, an increase in microbial fermentation did occur as evidenced by greater ruminal NH3 concentrations. Other research is in agreement with our study (Köster et al., 1996, Reed et al., 2004) in which ruminal NH3-N concentrations increased when providing RDP supplementation.
No differences (P = 0.42) were observed for in situ DM disappearance (1.39 ± 0.42%/h) of forage (Table 13). Supplementation of RDP in low-quality forage-based diets often increases forage digestion (Köster et al., 1996; Bodine et al., 2000; Bandyk et al., 2001). Supplementation of CCDS did not increase in situ forage NDF disappearance (1.78 ± 0.35%/h); we detected no differences (P = 0.37). Likewise, Krysl et al. (1989) and Freeman et al. (1992) reported no differences in in situ NDF disappearance of forage when providing RDP supplementation. In situ degradation rate of CP (1.05 ± 0.10%/h) changed in a cubic fashion (P < 0.001) with the greatest disappearance occurring at the 5% supplementation level, which was 85% greater than the control (0% CCDS). Rate of CP degradation of forage on the 10% treatment declined 16.5% from the 5% treatment but remained 54% over the control levels. A similar decline in forage CP disappearance was observed between the 10 and 15% CCDS supplementation with the 15% treatment being 5.4% less than the 10% treatment. Forage CP disappearance on the 15% supplementation level was 46% greater than the control treatment. In situ disappearance rates for both CP and DM followed similar trends; CP in situ disappearance had a much lower level of variance and therefore was significant. We theorize that as more N was added to the diet from the 10 and 15% CCDS treatments, microbial bacteria were selecting the additional CCDS as a source of CP over hay CP because of a greater digestibility of CCDS. Mullahey et al. (1992) suggested that much of the protein in warm-season grasses might be protected structurally from extensive ruminal degradation because of its association with bundle-sheath cells.
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Table 13. Effect of corn condensed distillers solubles (CCDS) level on rate in situ of DM, NDF, and ADF ruminal disappearance (%/h), and on in situ CP kinetics (Exp. 2)
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The reasons for the differences in response between the 2 studies we conducted are not immediately clear but are likely related to 1 of 3 reasons: 1) feeding method (offering CCDS separately vs. part of a TMR mix), 2) differences in nutrient content of the 2 different CCDS products used in the studies, or 3) slight differences in forage nutrient content between the 2 studies. Offering the CCDS as a TMR with the forage may have resulted in improved synchrony and release of nutrients for the ruminal microorganisms. However, the research that has evaluated synchronizing release of energy and N in the rumen has indicated conflicting responses. In some cases, no response in microbial protein synthesis (Kim et al., 1999a) or animal growth (Richardson et al., 2003) has been observed. In others, increased microbial protein production has been reported because of a synchronized degradation of N and OM in the rumen (Kim et al., 1999b) when diets containing grass silage and a moderate amount of concentrate were fed. Maekawa et al. (2002) reported reduced minimum ruminal pH in dairy cows offered concentrate and forage separately compared with concentrate and forage offered together in a TMR. However, mean ruminal pH was not different between treatments. These authors also reported minimal differences in total salivary production when diets were consumed as a TMR or when components were fed separately. In our study, it is unlikely the responses we observed are related to synchrony of nutrient release because the basal forage was low in digestibility and slowly degradable. In addition, we did not observe a time x treatment interaction for ruminal NH3 concentration in Exp. 1, which indicates greater levels of CCDS had minimal impacts on the ruminal environment. If ruminal asynchrony of N and OM degradation were a major factor in our study, one would have expected a time x treatment interaction for ruminal NH3 with greater concentrations of NH3 occurring immediately after feeding with greater levels of CCDS. It is also possible that the increased level of CP or fat, or some other component of the CCDS that we did not analyze, contributed to the improved responses noted in Exp. 2. The CCDS products used in Exp. 1 and 2 may have differed in fermentation by-products (e.g., spent yeast cells, VFA, AA, peptides, or other compounds). In addition, the fact that the forage CP in Exp. 1 was 5.1% compared with 3.3% in Exp. 2 might have also contributed to some of the differences observed between the 2 studies. Because a direct comparison of the 2 products was not made under the same feeding conditions, it is difficult to draw conclusions with certainty as to why the differences in responses occurred.
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IMPLICATIONS
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The increase in dry matter intake and fiber digestion of low-quality hay when supplementing corn condensed distillers solubles indicates that corn condensed distillers solubles are similar to other rumen degradable protein supplements when fed in combination with forage-based diets. However, this seems to be only true when the corn condensed distillers solubles are fed in a totally mixed ration or when they contain greater levels of nutrients. Supplementing corn condensed distillers solubles at levels up to 15% (dry matter intake) resulted in no negative effects on intake or use of low-quality switchgrass hay. Based on our findings, corn condensed distillers solubles may be used as a protein supplement for forage-based diets; however, feeders need to be aware of the variable moisture, crude protein, and fat content of corn condensed distillers solubles.
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Footnotes
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1 Appreciation is expressed to the ND Corn Utilization Council for partial funding and support for this project. 
3 Current address: New Mexico State University, Las Cruces.
2 Corresponding author: glardy{at}ndsuext.nodak.edu
Received for publication August 26, 2005.
Accepted for publication January 24, 2006.
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Abstract
INTRODUCTION
