EFFECTS OF DIET CONCENTRATE LEVEL AND SODIUM BICARBONATE ON SITE AND EXTENT OF FORAGE FIBER DIGESTION IN THE GASTROINTESTINAL TRACT OF WETHERS l

EFFECTS OF DIET CONCENTRATE LEVEL AND SODIUM BICARBONATE ON SITE AND EXTENT OF FORAGE FIBER DIGESTION IN THE GASTROINTESTINAL TRACT OF WETHERS l Karen J. Wedekind, Russell B. Muntifering and Kerry B. Barker University of Kentucky 2, Lexington 40546-0215 ABSTRACT Four adult wethers (45 kg) with permanent ruminal and abomasal cannulae were used in a repeated measures Latin-square arrangement of treatments to quantitate the effects of diet concentrate level and. sodium bicarbonate (NaHCO3) on site and extent of forage fiber digestion in the gastrointestinal tract. Experimental diets consisted of Kentucky-31 tall fescue hay, soybean meal and a semi-purified concentrate mixture in ratios of 95:5:0, 76:4:20, 57:3:40 and 38:2:60; NaHCO 3 represented 0 or 7.5% of the concentrate mixture. Ruminal digestion (% of intake) of neutral detergent fiber (NDF) and hemicellulose decreased linearly (P<.05), whereas acid detergent fiber (ADF) digestion responded in a cubic (P<.05) fashion to increasing concentrate level; NaHCO3 improved ruminal digestion of NDF (P<.10) and ADF (P<.05), but not bemicellulose. Post-ruminal digestion (% of tureen non-degraded) of NDF and ADF tended to increase, whereas hemicellulose digestion responded in a cubic (P<.05) fashion to increasing concentrate level; NaHCO 3 decreased (P<.05) post-ruminal digestion of all fiber fractions. Total tract digestion of NDF and ADF showed a cubic (P<.05) response, whereas hemicellulose digestion responded in a quadratic (P<.05) fashion to increasing concentrate level; NaHCO3 had no effect on total tract digestion of any fiber fraction. Correlations of ruminal hemicellulose digestion with mean ph (r =.33; P=.07) and minimum ph (r=.30; P=.09) were attained in a 24-h feeding cycle. Path coefficient analysis revealed forage hemicellulose degradability to be more ph-sensitive and less responsive to NaHCO 3 than ADF degradability when concentrate was fed. Concentrate feeding and NaHCO3 altered the primary sites of forage fiber digestion in the gastrointestinal tract, but did not systematically influence the extent of total tract utilization. (Key Words: Concentrates, Fiber, Forage, Rumen Digestion, Sodium Bicarbonate.) Introduction The potentially depressing effect of starch and other rapidly fermented carbohydrates upon fiber digestion in ruminants is welldocumented, and represents a negative associative effect of mixed diet feeding (Mould et al., 1983a). Many reports of this phenomenon in the literature are based on fiber digestion coefficients in the total gastrointestinal tract, which in turn form the basis for untested inferences regarding fiber digestion in the recticulorumen. Using this approach, interpretations regarding ruminal digestive function are necessarily confounded by compensatory post-gastric fermentation, which can be significant when fiber diges- IThis paper (No. 85-5-149) is published with the approval of the Director of the Kentucky Agr. Exp. Sta. 2Dept. of Anita. Sci. Received July 29, 1985. Accepted November 24, 1985. tion in the rumen is incomplete (Hoover, 1978). In addition, negative associative effects are commonly assumed to have resulted from depression of forage fiber digestibility because concentrates are overlooked as significant fiber sources (Van Soest, 1982). Cell walls from concentrates in fact show greater digestibility depression in vitro (Jeraci et al., 1980). Depression in the forage component is a lesser effect in vitro, and has only recently been investigated in vivo (Miller and Muntifering, 1985). Fiber digestibility depressions are often accompanied by depression of ruminal ph, and may be partially alleviated by compounds possessing buffering capacity (Emmanuel et al., 1970; Erdman et al., 1982; Mould et al., 1983a). Whether fiber digestion response to buffering compounds is entirely attributable to increased ruminal disappearance has not been adequately tested. The following study was conducted to quantitate the effects of diet concentrate level and sodium bicarbonate (NaHCO3) on site and extent of forage fiber degradability in the gastrointestinal tract. 1388 J. Anim. Sci. 1986.62:1388--1395

FORAGE FIBER DIGESTION 13 8 9 Experimental Procedures Four adult crossbred wethers (mean initial weight, 45 kg) with permanent ruminal and abomasal cannulae were used in a repeated measures Latin-square arrangement of treatments. Experimental diets consisted of mature, ground (2.54-cm screen) Kentucky-31 tall fescue hay (IFN 1-09-188; table 1), soybean meal (IFN 5-04-612) and a semi-purified concentrate mixture (table 2) in ratios of 95:5:0, 76:4:20, 57:3:40 and 38:2:60. Soybean meal was fed in a fixed proportion to forage to yield isonitrogenous diets (10.2% crude protein in dry matter) and to ensure that microbial fermentation would not be limited by low ruminal ammonia concentration. The semi-purified concentrate mixture had the desirable feature of containing no fiber, which might otherwise confound the interpretation of treatment effects on forage fiber degradability. The first phase of each period in the Latin square included a 2-wk diet adjustment followed by 6-d total fecal collection. Each wether received 1 kg daily of its assigned diet in equal feedings at 0900 and 2100 h. Five grams of trace mineralized salt 3 were provided at each feeding and water was available continuously. In the second phase of each period, NaHCO 3 was fed at the rate of 7.5% of the concentrate mixture by replacing 6 and 1.5 percentage units of corn starch and dextrose, respectively. A 10-d adjustment to buffered diets was followed by 6-d total fecal collection. A 10% aliquot of daily fecal output for each wether was dried at 50 (2 for 48 h in a forced-air oven. Feed samples were taken daily and composited at the end of each collection period. Orts were collected daily, weighed and composited on an individual animal basis. Samples (approx 100 g) of ruminal fluid and abomasal digesta were taken during collection periods at 12-h intervals advanced 2 h daily. Ruminal fluid ph was measured immediately after collection using an Expandomatic SS-2 ph meter 4 equipped with a combination electrode 5. Abomasal digesta was collected into plastic vials, frozen (-10 C) and lyophilized after pooling thawed samples into individual animal 3provided (mg/kg diet):naci, 9,800; Zn, 35.0; Mn, 28.0; Fe, 17.5; Cu, 3.5; I,.7 and Co,.7. 4Beckman Instruments, Inc., Fullerton, CA 92634. SCorning Science Products, Medfield, MA 02052. composites. Dried, ground (1-mm screen), airequilibrated feed and fecal samples and freezedried abomasal samples were analyzed for neutral detergent fiber (NDF), acid detergent fiber (ADF) and acid detergent lignin (ADL) according to procedures of Goering and Van Soest (1970). Hemicellulose was defined as the difference between NDF and ADE Dry matter (DM), ash and N determinations were made according to AOAC (1975) procedures. Ruminal digestion coefficients for cell wall constituents (CWC) were calculated by reference to CWC:ADL ratios in feed and abomasal digesta. Post-ruminal digestion coefficients were calculated on the TABLE I. CHEMICAL COMPOSITION OF KENTUCKY-31 TALL FESCUE HAY USED IN EXPERIMENTAL DIETS Item % Dry matter 92.0 Composition of dry matter Organic matter 92.8 Crude protein 7.8 Neutral detergent fiber 67.0 Acid detergent fiber 41.9 Hemicellulose 25. I Lignin 5.2 TABLE 2. COMPOSITION OF SEMI-PURIFIED CONCENTRATE MIXTURE USED IN EXPERIMENTAL DIETS Item %a Ingredient Corn starch (IFN 4-02-889) 76.0 Dextrose (IFN 4-02-125) 12.9 Soybean meal (1FN 5-04-612) 7.0 Urea (IFN 5-05-070) 2.0 Dicalcium phosphate (IFN 6-28-335) 1.3 Magnesium sulfate (IFN 6-02-758).5 Potassium sulfate (IFN 6-06-098).3 Chemical composition Dry matter 89.3 Composition of dry matter Organic matter 95.1 Crude protein 10.2 aas-fed basis.

1 3 90 WEDEKIND ET AL. basis of amounts of each constituent recovered at the abomasum and appearing in the feces. Data were analyzed by repeated measures analysis of variance for a Latin-square design (SAS, 1979) with NaHCO3 as a sub-plot effect. Independent variables were animal, period, concentrate level and the three-way interaction as the whole-plot error term. The sub-plot consisted of NaHCO3, NaHCO 3 concentrate level interaction and residual as the sub-plot error term. Significant concentrate level effects were further tested for linear, quadratic and cubic components using orthogonal contrasts. Correlation coefficients were derived for all possible combinations of dependent variables using procedures of SAS (1979). Path coefficient analysis (Kempthorne, 1973) was performed to determine the proportion of variability (r 2) in ruminal NDF digestion attributable to digestion of the ADF and hemicellulose fractions. Results and Discussion Daily DM intake averaged 887-4- 32 g (mean _ SE), and was unaffected (P>. 10) by treatment; however, DM intake tended to be greater for diets containing the semi-purified concentrate mixture. Intake of forage DM equalled 660, 701,526 and 351 g/d, respectively, for diets containing O, 20, 40 and 60% concentrate. Fecal recovery of lignin in this experiment averaged 94.9 _+ 4.6% across all treatments. Ruminal digestion of NDF (% of intake and g/d) decreased linearly (P<.05) with increasing concentrate level (table 3), and was improved (P<.10) by NaHCO 3. Except for the highest level of concentrate addition, post-ruminal NDF digestion (g/d and % of NDF entering the small intestine) tended to increase with increasing concentrate level, and was decreased (P<.05) by NaHCO 3. Addition of NaHCO3 increased (P<.05) fractional NDF disappearance from the rumen (79.5 vs 66.4% of total tract NDF digestion) and consequently decreased (P<.05) the fraction of total tract NDF digestion attributable to post-ruminai disappearance (20.5 vs 33.6%). Brink and Steele (1985) observed deci'eased ruminal digestion and increased post-ruminal digestion of NDF in steers when level of corn grain in a corn silage-based diet increased. Limestone (a compound with buffering capac- TABLE 3. EFFECTS OF CONCENTRATE LEVEL AND SODIUM BICARBONATE (NaHCO3) ON SITE AND EXTENT OF NEUTRAL DETERGENT FIBER (NDF) DIGESTION NaHCO3: - + Item Concentrate level, %: 0 20 40 60 0 20 40 60 SE a Intake, g/d b 431 469 360 236 462 464 353 232 18 Ruminal digestion g/d t~ 184 160 108 61 196 184 124 76 11 %~ 43.4 34.3 30. I 26. I 42.7 39.7 35.1 32.7 3.0 % of total tract NDF digestion a 82.7 68. I 56.6 58.1 85.8 83.3 71.0 78.0 4.1 Entering small intestine, g/d 246 309 252 174 266 280 229 156 19 Post-ruminal digestion g/d a 52 69 82 44 39 36 50 23 11 %d 16.4 20.4 32.8 24.6 12.3 12.0 22.1 14.3 2.8 % of total tract NDF digestion a 17.3 31.9 43.4 41.9 14.2 16.6 29.0 21.9 4.1 Total tract digestion g/d b 236 229 191 105 235 220 174 98 14 %e 53.0 48.8 53.0 44.6 50.0 47.5 49.4 42.3 2.6 astandard error of mean bmain linear effect due to concentrate level (P<.05). CMain effect due to NaHCO3 (P<.I0) dmain effect due to NaHCO3 (P<.05). ~Main cubic effect due to concentrate level (P<.05).

FORAGE FIBER DIGESTION 1391 ity) did not affect ruminal digestion, but tended to increase post-ruminal NDF digestion in their study. Total tract NDF digestion coefficients showed a cubic response (P<.05) to increasing concentrate level. Similarly, Joanning et al. (1981) and Mould et al. (1983a) noted either quadratic or cubic responses (i.e., associative effects) for total tract nutrient digestion coefficients as grain levels increased in forage-based diets. Specifically, Joanning et al. (1981) observed non-linear response patterns for nearly all nutrients examined: DM, energy, N, starch and ADF; a linear depression was observed for NDF digestibility. Mould et al, (1983a) noted quadratic responses for both DM and organic matter digestion in hay-grain diets, but did not test for these effects in other nutrient fractions. Differences in total tract NDF digestibility due to NaHCO3 were not significant in the present study. Kovacik et al. (1984) concluded that rumen ph and fibrolytic activity are altered by NaHCO3, but that total tract nutrient digestibilities may be unchanged. Ruminal, post-ruminal and total tract digestion of the ligno-cellulose fraction (ADF) responded to increasing concentrate level and NaHCO3 addition in a manner resembling that of NDF digestion. Ruminal ADF digestion coefficients showed a cubic (P<.05) response to increasing concentrate level, and were increased (P<.05) by NaHCO3 (table 4). Several other studies have demonstrated decreased ruminal cellulose digestion in response to increasing concentrate level in the diet (EI-Shazly et al., 1961; Mitchell et al., 1967; MacRae and Armstrong, 1969; DeGregorio et al., 1982; Kovacik et al., 1984). Emmanuel et al. (1970) and Hall and Thomas (1984) observed increased in vitro cellulose digestion with buffer additions. Post-ruminal ADF digestion (g/d and % of ADF entering the small intestine) was decreased (P<.05) by NaHCO 3. Addition of NaHCO3 increased (P<.05) fractional ADF disappearance from the rumen (85.2 vs 72.4% of total tract ADF digestion) and consequently decreased (P<.05) the fraction of total tract ADF digestion attributable to post-ruminal disappearance (14.8 vs 27.6%). Total tract ADF digestion coefficients showed a cubic response (P<.05) to increasing concentrate level, in agreement with results of Joanning et al. (1981), but contrary to those of van der Linden et al. (1984) and Henning et al. TABLE 4. EFFECTS OF CONCENTRATE LEVEL AND SODIUM BICARBONATE (NaHCO3) ON SITE AND EXTENT OF ACID DETERGENT FIBER (ADF) DIGESTION Item NaHCO3: - + Concentrate level, %: 0 20 40 60 0 20 40 60 SE ~ Intake, g/d b 276 291 229 145 299 287 229 144 12 Ruminal digestion g/d b~ 110 86 76 45 124 103 88 45 5 %~d 40.2 29.6 33.4 31.0 41.2 36.7 38.5 31.5 2.1 % of total tract ADF digestion c 86.1 66.9 66.7 69.9 90,2 87.3 86.0 77.4 4,5 Entering small intestine, g/d 166 206 253 101 176 185 141 98 l I Post-ruminal digestion g/d c 28 42 38 20 18 15 16 14 7 %c 11.7 19.9 25.0 17.9 7.8 7.5 11.4 13.8 3.7 % of total tract ADF digestion c 13.9 33.0 33.3 30.1 9.8 12.7 14.0 22.6 4.5 Total tract digestion g/d b 138 128 115 64 141 118 104 59 10 %d 47,5 43.9 50.1 44.2 45.9 40.8 45.3. 41.1 2.6 astandard error of mean. bmain linear effect due to concentrate level (P<.05). CMain effect due to NaHCO 3 (P<~.05). amain cubic effect due to concentrate level (P<.05).

1 3 92 WEDEKIND ET AL. (1980). These latter researchers observed decreased total tract digestion of cellulose as grain level in the diet increased. DeGregorio et al. (1982) reported no significant differences in total tract ADF digestion with increasing concentrate level. Total tract ADF digestion was not affected by addition of NaHCO3 in the present study, in agreement with Kovacik et al. (1984) and Williams et al. (1984). In other studies (Erdman et al., 1982; Mould et al., 1983a; James and Wohlt, 1985) tendency for increased total tract ADF digestion with NaHCO3 addition was noted. Apparent ruminal digestion of hemicellulose (g/d and % of intake) decreased linearly (P<.05) with increasing concentrate level, and was unaffected by NaHCO 3 (table 5). MacRae and Armstrong (1969) observed decreased ruminal digestion of non-glucose reducing sugars (pentosans) as concentrate level in the diet increased. Post-ruminal hemicellulose digestion (% of hemicellulose entering the small intestine) showed a cubic response (P<.05) to increasing concentrate level, and was decreased (P<.05) by NaHCO3. Addition of NaHCO3 in- creased (P<.05) fractional hemicellulose disappearance from the rumen (69.9 vs 55.3% of total tract hemicellulose digestion) and consequently decreased (P<.05) the fraction of total tract hemicellulose digestion attributable to post-ruminal disappearance (30.1 vs 44.7%). Fractional hemicellulose disappearance from the rumen decreased (P<.05), whereas fractional post-ruminal digestion of hemicellulose increased (P<.05) in response to increasing concentrate level, in agreement with MacRae and Armstrong (1969). Consequently, fractional digestion of hemicellulose in the ruminal and postruminal segments of the digestive tract showed a NaHCO 3 concentrate level interaction (P<.IO); i.e., differential response of site of digestion to increasing concentrate level in the presence or absence of NaHCO3. Across all treatments, larger percentages of the digestible hemicellulose fraction disappeared post-ruminally compared with ADE Other studies dealing with the effects of feed processing on sites of nutrient digestion have likewise shown greater post-ruminal digestion of hemicellulose compared with cellulose (Beever et al., 1972; Thom- TABLE 5, EFFECTS OF CONCENTRATE LEVEL AND SODIUM BICARBONATE (NaHCO3) ON SITE AND EXTENT OF HEMICELLULOSE DIGESTION NaHCO~: - + Item Concentrate level, %: 0 20 40 60 0 20 40 60 SE ~' Intake, g/d b 155 178 131 91 163 176 124 88 6 Ruminal digestion g/d b 74 72 32 16 70 80 36 30 8 %~' 48. I 39.4 24. I 17.8 44.0 44.9 29.2 33.2 5.3 % of total tract hemicellulose digestion b':d 77.0 65.2 40.7 38,3 76,2 77.3 50.5 75,4 7.0 Entering small intestine, g/d 81 106 99 75 93 96 88 58 9 Post-ruminal digestion g/d c 25 29 44 25 24 22 35 9 5 %~de 27.6 23.3 44.5 33.4 24. I 22. I 39. I 15.6 3.0 % of total tract hemicellulose digestion ~d 23.0 34.8 59.3 61.6 23.7 22.7 49.5 24.5 7.0 Total tract digestion g/d b 99 I01 76 41 94 102 71 39 6 %f 62.6 56.6 58.0 45. I 57.5 57.8 57.0 43,7 3.4 "Standard error of mean. bmain linear effect due to concentrate level (P<.05). ~Main effect due to NaHCO~ (P<.05), ~ concentrate level interaction (P<.I0). emain cubic effect due to concentrate leve] (P<.05). fmain quadratic effect due to concentrate level (P<,05).

FORAGE FIBER DIGESTION 13 93 son et al., 1972). Total tract digestion coefficients for hemicellulose showed a quadratic response (P<.05) to increasing concentrate level in the diet, and were not affected by NaHCO 3 addition. Similarly, van der Linden et al. (1984) and Henning et al (1980) observed decreased total tract hemicellulose digestion with increasing concentrate level in the diet. Mean and minimum ph values attained in a 24-h feeding cycle decreased linearly (P<.05) with increasing concentrate level, and were increased (P<.05) by NaHCO3 (table 6). Mould et at. (1983b) observed decreased ruminal ph and fiber digestion as diet concentrate level increased, and were able to alleviate fiber digestibility depressions with ruminal infusion of bicarbonate salts. These authors attributed decreased forage fiber and DM degradation to low ruminal ph associated with rapid grain fermentation, as well as to preferential use of readily available carbohydrate by ruminal microorganisms. Several other studies have reported increased ruminal ph and improved fiber digestibility in response to buffer additions (Emmanuel et al., 1970; Erdman et al., 1982; Ha et al., 1983; James and Wohlt, 1985). In addition to mean and minimum ph, two other ph variables were calculated. Hours below ph 6.7 simply represents the amount of time in a 24-h feeding cycle during which ruminal ph remains below 6.7. The other variable is an integration of the area under the ph-time curve below 6.7 (ph-hours), and represents the combined effects of magnitude of ph depression after feeding and time of exposure to lower ph on the growth rate and metabolic activity of the microbial population. A reference ph of 6.7 was chosen because the mean ph observed for the 0% concentrate diet (forage only) equalled 6.7. In addition, Cheng et al. (1955), Terry et al. (1969) and Stewart (1977) have reported that the optimum ph for fiber digestion is between 6.7 and 7.1. Hours below ph 6.7 tended to increase with increasing concentrate level, and were decreased (P<.05) by NaHCO3. Similarly, Kovacik et al. (1984) measured the percentage of time in the day during which ruminal ph remained below 6.6, 6.2, 5.8, 5.4 and 5.0, and reported that the percentage of time ph remained below 6.2 and 5.8 was decreased when NaHCO3 was fed. Values for ph-hours increased linearly (P<.05) with increasing concentrate level, and were decreased (P<.05) by NaHCO 3. While a direct effect of ph on cellulase activity may explain part of the depression in fiber digestion when mixed forage-concentrate diets are fed, it does not account for all of the decrease. Preferential substrate utilization or other catabolite regulatory mechanisms may also be operative (Russell and Baldwin, 1978). Concentrate feeding and(or) ph may likewise influence major determinants of microbial competition in the rumen, including maintenance energy requirements (Russell and Ba!dwin, 1979b), substrate affinities (Russell and Baldwin, 1979a) and growth rates (Russell et al., 1979). Van der Linden et al. (1984) observed no significant TABLE 6. EFFECTS OF CONCENTRATE LEVEL AND SODIUM BICARBONATE (NaHCO3) ON RUMEN ph MEASUREMENTS NaHCO3: - + Item Concentrate level, %: 0 20 40 60 0 20 40 60 SE a Mean ph ~d 6.66 6.62 6.32 6.31 6.70 6.61 6.52 6.42.03 Minimum ph ~ 6.37 6.28 5.80 5.70 6.49 6.29 6.07 5.77.07 Hours below ph 6.7 ~ 11.9 15.8 21.7 21.9 10.6 13.4 18.9 17.1 1.2 ph-hours ~df 2.05 2.70 9.04 9.38 1.59 3.06 4.94 6.97.51 astandard error of mean. bmain effect due to NaHCO3 (P<.05). CMain linear effect due to concentrate level (P<.05). dna[.-ico3 x concentrate level interaction (P<.05). eamount of time in a 24-h feeding cycle during which ruminal ph remains below 6.7. flntegration of the area under the ph-time curve below 6.7.

1394 WEDEKIND El" AL. changes in total culturable, cellulolytic and xylanolytic numbers with increasing grain level in the diet. Likewise, Mackie et al. (1978) observed that numbers of total culturable and cellulolytic bacteria remained constant, regardless of forage:concentrate ratio. Leedle et al. (1982) reported no change in numbers of cellulolytic bacteria in the rumen over a range of concentrate levels fed, Based on these studies, it would not appear that decreased digestion of cellulose and hemicellulose in the presence of readily available carbohydrate can be explained simply on the basis of decreased numbers of fibrolytic bacteria. However, Henning et al. (1980) reported decreased numbers of cellulolytic bacteria in response to increasing concentrate level in the diet. In the present study, the hemiceilulose fraction was the only CWC whose ruminal degradability was correlated with any of the ph variables measured. Ruminal hemicellulose digestion (% of intake) was correlated with ph variables as follows: minimum ph (r=.30; P=.09, mean ph (r=.33; P=.07) and ph-hours (r= -.36; P=.04). Total tract hemicellulose digestibility also was correlated with minimum ph (r=.35; P=.05) and ph-hours (r=-.31; P=.09). Istasse and Orskov (1983) reported washed hay degradability in situ was related more highly with ph-hours below ph 6.0 than with either hours below ph 6.0 or minimum ph attained in a 24-h period. Contrary to their resuits, use of the ph-hours concept in the present study did not materially improve the correlation of ph measurements with total cell wall digestion. Van der Linden et al. (1984) and Henning et al (1980) were unable to show significant correlations between ph-hours and digestibility of either hemiceuulose or cellulose. These investigators used reference points of ph 6.5 and 6.0, respectively, and measured total tract digestion only. In the absence of NaHCO3, ruminal digestion of hemicellulose and ADF accounted for 35 and 21%, respectively, of the variability in ruminal NDF digestion. The remaining 44% of the variability was attributable to digestibility of ADF and hemicellulose fractions collectively. For the unbuffered treatments, correlation analysis revealed a significant relationship between ruminal hemicellulose digestion coefficients and ph-hours (r= -.45; P=.08); coefficents for ADF and NDF were not correlated to any of the ph variables measured. It would appear then, that hemicellulose was the CWC whose ruminal degradability was most depressed at low ph, therefore explaining a greater portion of the variability in NDF digestion. However, when diets were supplemented with NaHCO 3, ruminal hemicellulose digestion coefficients and ph-hours were no longer correlated (r= -.06; P>.10). When ph change was attenuated, ruminal hemicellulose digestion explained less of the variability (21%) in NDF digestion than did digestion of ADF (37%). Hemicellulose and ADF digestion collectively accounted for the remaining 42% of the variability in ruminal NDF digestion. In summary, increasing levels of a semipurified concentrate mixture in a fescue-based diet altered site and extent of forage CWC digestion in the gastrointestinal tract of mature wethers. Depressions in digestibility of forage CWC were not as severe as those reported in earlier digestion trials for mixed diets containing a composite of forage and grain CWC. The addition of NaHCO3 increased ruminal digestibility, as well as fractional digestion of forage CWC attributable to ruminal disappearance, but did not systematically improve total tract digestion coefficients of CWC. Hemicellulose was the only forage CWC whose digestion in the rumen or total tract was correlated with ruminal ph. 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