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ANIMAL PRODUCTION |
Department of Animal Sciences, University of Kentucky, Lexington 40546-0215
Abstract
This experiment was conducted to determine the effects of tall fescue hay maturity on intake, digestion, and ruminal fermentation responses to different supplemental energy sources fed to beef steers. Twelve ruminally cannulated, crossbred steers (initial BW = 228 ± 21 kg) were used in a split-plot experiment with a 3 x 4 factorial treatment arrangement. Steers were assigned randomly to three supplement treatments: 1) no supplement, 2) pelleted soybean hulls, or 3) coarse cracked corn. The second treatment factor was fescue hay maturity: 1) vegetative (VEG), 2) boot-stage (BOOT), 3) heading-stage (HEAD), and 4) mature (MAT). Supplements were fed once daily at 0.67% of BW (OM basis) and tall fescue hay was offered once daily at 150% of average intake. Supplement type x forage maturity interactions were not detected (P 
0.25) for forage, total, or digestible OM intake, which generally decreased (P < 0.01) with advancing forage maturity. Supplementation decreased (P < 0.01) forage and increased (P < 0.01) total OM intake. Supplement type had no effect (P = 0.56) on substitution ratio (unit change in forage intake per unit of supplement intake). Digestible OM intake was increased (P < 0.01) by supplementation and was greater (P = 0.05) with soybean hulls than with corn. Supplement type x forage maturity interactions (P 
0.10) were observed for OM and NDF digestibilities and N retention. Increases in digestibility with soybean hulls relative to corn were greater and supplementation elicited greater increases in N retention with more mature forages. Compared with soybean hulls, corn supplementation resulted in greater (P < 0.01) negative associative effects on OM digestibility. Supplementation did not affect (P 0.10) ruminal pH, total VFA concentrations, or acetate:propionate ratio. Corn supplementation decreased (P 0.07) ruminal NH3-N concentrations compared with control and soybean hulls; however, decreases in ruminal NH3-N concentrations were not consistent with the presence of negative associative effects. Thus, mechanisms not involving ruminal pH or NH3-N concentration seem responsible for negative associative effects observed with corn supplementation. Within the range of forage quality in this study, increases in digestible OM intake from starch- or fiber-based supplements were independent of forage maturity. When fed at similar levels of OM, soybean hull supplementation provided an average of 6% greater digestible OM intake than corn supplementation.
Key Words: Associative Effects • Digestibility • Forage Intake • Supplementation
Introduction
Cattle consuming diets solely comprised of forages are often unable to meet desired levels of production. Considerable research has been done to determine optimal supplementation strategies under various conditions. With high-quality forages, this typically involves the addition of energy supplements to the diet. However, feeding starch-based energy supplements such as cereal grains has been shown to cause depressions in forage intake as well as negative associative effects on fiber digestibility (Chase and Hibberd, 1987
; Pordomingo et al., 1991). Energy supplements high in digestible fiber, such as soybean hulls, have shown potential to reduce or alleviate these negative associative effects (Anderson et al., 1988; Martin and Hibberd, 1990). Comparisons of effects of starch- vs. fiber-based energy supplements on forage intake have resulted in either little difference between supplement types (Garces-Yepez et al., 1997; Elizalde et al., 1998) or a greater depression in forage intake with fiber-based supplements (Galloway et al., 1993; Hess et al., 1996, Garces-Yepez et al., 1997). One possible factor contributing to the wide range of responses measured with energy supplements is variation in forage quality. Horn and McCollum (1987) suggested that supplementation-induced depressions in forage intake were greater with increasing forage quality. However, few reports detail interactions between forage quality and supplemental energy. Although Matejovsky and Sanson (1995) tested for such interactions, their work was conducted with sheep and did not evaluate effects of supplemental energy source. This study was designed to evaluate and characterize interactions between tall fescue forage maturity and source of supplemental energy on intake, digestibility, and ruminal fermentation characteristics by beef steers.
Materials and Methods
Twelve ruminally cannulated, crossbred steers (initial BW 228 ± 21 kg; final BW 276 ± 27 kg) were used in three simultaneous 4 x 4 Latin squares. Three supplement treatments were combined with each of four forage maturities in a 3 x 4 factorial arrangement. Experimental and surgical protocols were approved by the University of Kentucky Institutional Animal Care and Use Committee (protocol number 31A2000). Steers were blocked by weight (four weight blocks) and assigned to one of three supplement treatments: 1) no supplement; 2) pelleted soybean hulls; or 3) coarse cracked corn, each of which was represented by a square in the replicated Latin square design. The second treatment factor was tall fescue hay maturity: 1) vegetative (VEG); 2) boot-stage (BOOT); 3) heading (HEAD); and 4) mature (MAT). Hay was harvested from alternate windrows in endophyte-infected tall fescue pastures on four dates (May 4, May 24, June 14, and July 7) during the 2000 grazing season in an attempt to provide four distinctly different hay maturities. Before feeding, hay was coarsely ground through a tub grinder to provide smaller and more uniform particle size (5 to 10 cm). Chemical composition of the four hays and two supplements is presented in Table 1. Supplements were fed once daily at 0.67% BW (OM basis) just before offering tall fescue hay at 150% of the previous 5-d average intake. This level of supplementation is considered to be on the higher end of generally recommended supplementation levels with stocker cattle grazing fescue. We used this level to allow for maximum opportunity to detect interactions with forage maturity, where they might exist. Forage was offered at 150% of ad libitum intake because, at lower levels of offering, there was substantial sorting with the more mature forages. Subsequent analysis (data not shown) indicated that treatment differences in the NDF and N concentrations in the forages actually consumed were consistent with differences in the forages offered. All steers had continuous access to fresh water and consumed 40 g of a commercial mineral mix daily (Ca, 13%; P, 6.2%; salt, 18%; Mg, 3.0%; S, 1.0%; K, 0.8%; Zn, 2,300 ppm; Fe, 175 ppm; Mn, 2,200 ppm; Cu, 1,070 ppm; I, 55 ppm; Co, 11 ppm; Se, 11 ppm; vitamin A, 661,387 IU/kg; vitamin E, 276 IU/kg; Burkmann Feeds, Danville, KY).
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Supplements were offered daily at 0730, and feed pans were removed when supplement had been consumed. In the rare instance when supplements were not completely consumed within 1 h, the remaining portion was placed in the rumen via the rumen cannula. After feeding supplements, orts were removed from hay feeders, weighed, and sampled. Hay feeders were filled daily after orts determination. A daily sample of each hay maturity and supplement was retained. Supplement, hay, and ort samples were dried at 55°C in a forced-air oven to a constant weight and then ground in a Wiley mill (Model 4, Thomas Scientific, Sweedesboro, NJ) to pass a 1-mm screen. All supplement and hay samples were composited within period after grinding by combining equal amounts of the retained sample from each day. Ort samples for each steer were composited by taking a set percentage of each day’s total orts amount.
After feeding, the previous 24-h fecal output was weighed and 1% of the total wet material was sampled, dried, and ground in the same manner as described for feed samples. Fecal material was removed from stalls four times daily to prevent contamination and keep animals clean. After grinding, fecal samples were composited for each steer within each period. Total urine output was collected daily prior to fecal collections, by fitting each steer with a neoprene funnel (Belko Corp., Kingsville, MD) connected to a vacuum line, using stainless steel containers (7.5 L; SABCO Ind., Toledo, OH) as collection reservoirs. For each steer, a constant percentage of the total daily output was sampled and frozen (-20°C) as a composited sample. The total sample size ranged from 1 to 3% of the daily output, depending on the steer’s total output volume, and was targeted to weigh approximately 100 g. Sample size was kept at a constant percentage of output for each steer within a period. During the first two periods, 6 N hydrochloric acid was used to acidify the urine. Because of corrosion on vacuum fittings, 44.6 N ortho-phosphoric acid was used in the final two periods. The quantity of acid added to containers was adjusted daily (100 to 500 mL) to ensure a final pH of 3.0 or less.
For measurement of liquid dilution rate, each steer was dosed intraruminally in numerous sites with a pulse dose of 360 mL of Cr:EDTA solution (Binnerts et al., 1968) before feeding. Using a suction strainer, ruminal fluid samples were taken from three locations in the ventral rumen before dosing (0 h) and at 3, 6, 9, 12, and 24 h after dosing. Ruminal fluid samples were measured for pH within 20 min of collection with a portable pH meter fitted with a combination electrode (IQ 150; IQ Scientific Instruments, Inc., San Diego, CA) and then subsampled, combined with 25% (wt/vol) metaphosphoric acid (8 mL of ruminal fluid to 2 mL of acid) and frozen (-20°C) for VFA and NH3-N analyses. Additional samples of ruminal fluid (16 mL) were frozen without any additives and analyzed for Cr concentration.
Rumen evacuations were done on d 21, before (0 h) and 6 h after feeding supplements and hay. Ruminal contents of each steer were removed manually, weighed, mixed by hand, subsampled in triplicate, and then returned to the rumen. Samples of ruminal contents were dried and ground in the same manner described for feed and ort samples.
Forage, supplement, ort, and fecal samples were analyzed in duplicate for DM, OM (AOAC, 1990), and CP (by gas N analysis using a Leco FP-2000 N analyzer; Leco Corp., St. Joseph, MI). Neutral detergent fiber, ADF, and ADL concentrations were determined sequentially according to Van Soest et al. (1991), with the exclusion of sodium sulfite and decalin from the procedure. Fiber analyses were accomplished using an Ankom 200 Fiber Analyzer (Ankom Technology Corp., Fairport, NY). Degradable intake protein (DIP) concentrations of forages and supplements were determined using the Streptomyces griseus protease procedure described by Coblentz et al. (1999), using 0.33 protease activity unit/mL and incubating for 48 h at 39°C. The S. griseus protease (P-5147; Sigma Chemical Co., St. Louis, MO) used for this analysis contained 5.2 units of enzyme activity units per milligram of solid. Ergovaline concentrations in the forage samples were analyzed by HPLC according to the procedure of Rottinghaus et al. (1993). Urine samples were thawed and analyzed for N using the Leco FP-2000 N analyzer and for allantoin concentration as described by Young and Conway (1942). After thawing, ruminal fluid samples were centrifuged at 30,000 x g for 5 min, and supernatant fluid was removed for chemical analysis. Ruminal VFA concentrations were determined by gas chromatography (model 6890 Hewlett-Packard, Avondale, PA) by the procedure described by Vanzant and Cochran (1994). Ruminal NH3-N concentrations were determined using glutamate dehydrogenase (171-B; Sigma Chemical Co.) and a method adapted for use on a COBAS FARA II centrifugal analyzer (Roche Diagnostic Systems, Montclair, NJ). Chromium concentration was determined using an atomic absorption spectrophotometer (ATI Unicam 929, Cambridge, U.K.) with an air-acetylene flame. Fluid dilution rate was calculated by regressing the natural logarithms of Cr concentrations against sampling times (Warner and Stacy, 1968). Ruminal DM and liquid contents were determined directly from manual evacuation of ruminal contents. The average content weight, DM, and OM concentrations of ruminal contents from each evacuation were used to determine ruminal OM and liquid fill.
Calculations.
Associative effects were estimated as the difference between the observed apparent OM digestibility (OMD) and OMD estimated from forage and supplement intakes using an additive model. For each of the forages, the average OMD from the unsupplemented, control treatment was used as the OMD estimate. For corn and soybean hulls, TDN values listed in the NRC (1996) feed composition tables (90% for corn and 77% for soybean hulls) were used as OMD estimates, after converting to an OM basis. For each steer consuming supplement, within each forage maturity, the predicted, or additive, OMD was calculated by the following equation:
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where FOMI = forage OM intake; OMDF = forage OMD; SOMI = supplement OM intake; OMDS = supplement OMD; and TOMI = total OM intake. The deviation between predicted and observed OMD (dOMD) was used as a measure of associative effects.
Substitution ratios were calculated as the unit change in forage intake per unit of supplement intake as follows:
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where FOMIS = forage OM intake of supplemented steers (% of BW); FOMIC = average forage OM intake (% BW) of unsupplemented steers consuming the same forage; SOMI = supplement OM intake (% BW).
Statistical Analysis.
The MIXED procedure of SAS (SAS Inst. Inc., Cary, NC) was used to evaluate all data using a model for a factorial treatment arrangement in a split-plot design. The model included terms for supplement type (i.e., square), forage maturity, supplement type x forage maturity, period, and steer, where steer within supplement was specified as a random effect and period as a repeated effect. Because all squares were conducted simultaneously, this approach did not confound supplement type with time effects. Fermentation characteristics were analyzed using a model including the above terms along with sampling time, which was included as a repeated effect. The error covariance of repeated measures was modeled with an autoregressive correlation structure. When significant time x treatment interactions occurred, means within sampling time were evaluated to determine the consistency of treatment responses across sampling times. Protected (P 0.10) Fisher’s LSD were used to separate treatment means.
Results
Forage CP concentration decreased with advancing maturity (Table 1
). The two earlier maturity forages (VEG and BOOT) contained nearly twice the CP concentration of the later maturity forages (HEAD and MAT). Neutral detergent fiber and ADF concentrations of the forages increased with advancing maturity. There was little variation in ADL concentration among forage maturities. Forage DIP concentration, as a percentage of the total CP, decreased as forage maturity increased. Soybean hulls contained greater concentrations of CP, NDF, and ADF than corn. However, the portion of the CP that was estimated to be ruminally degradable was nearly identical between the two supplements. Ergovaline concentrations were greatest in the VEG hay.
The ratio of DIP to digestible OM intake (DOMI) exhibited a significant (P = 0.02) supplement type x forage maturity interaction (Figure 1). With the exception of HEAD, there was no difference (P 0.42) between corn and soybean hull supplementation in depressing (P 0.10) DIP:DOMI in relation to the control. Energy supplementation reduced DIP:DOMI by 3.5 and 3.4 percentage units for VEG and BOOT, respectively, but for MAT the reduction was only 1.3 percentage units.
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0.25) for total OM intake (OMI), forage OM intake (FOMI), or DOMI, which all decreased (P < 0.01) with advancing forage maturity from VEG through HEAD and were greater (P < 0.01) for MAT than HEAD (Table 2). Individual treatment means for intake and digestibility are shown in a table in the appendix at the end of this article. In addition, no supplement type x forage maturity interactions (P 0.60) were observed for substitution ratio (unit change in FOMI per unit supplement OMI), dOMD, or allantoin excretion. Substitution ratio decreased (P 0.03) with advancing forage maturity, with the exception that the substitution ratio for MAT was intermediate (P 0.27) between VEG and BOOT. The magnitude of dOMD was independent (P = 0.74) of forage maturity. Urinary allantoin excretion was nearly 70% greater (P 0.01) with the earlier two forage maturities than with the two more mature forages.
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). Similarly, no difference (P = 0.53) was detected between corn and soybean hull supplementation with regard to FOMI, with both supplements depressing (P < 0.01) FOMI by an average of 14%. Corn supplementation increased (P < 0.01) DOMI by 20% over the control treatment. However, soybean hull supplementation increased (P = 0.05) DOMI by an additional 6% over corn. Substitution ratios were not affected (P = 0.56) by supplement type. Corn supplementation resulted in greater (P < 0.01) dOMD than did soybean hull supplementation. Corn supplementation decreased OMD an average of 6.4 percentage units below values estimated from an additive model, whereas dOMD calculated for soybean hull supplementation was not different from zero (P = 0.12). Supplementation tended (P = 0.14) to increase urinary allantoin.
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). For VEG, OMD averaged 68.7% and was unaffected (P 0.21) by supplementation. However, for BOOT, supplementation increased (P 0.07) OMD from 62.1% for the control treatment, to an average of 65.4% with supplementation, with no differences (P = 0.24) between corn and soybean hull supplementation. For both HEAD and MAT, OMD was greatest (P 0.03) with soybean hulls, and was greater (P < 0.01) with corn than with no supplement. A supplement type x forage maturity interaction was also observed (P = 0.04) for NDF digestibility (NDFD; Figure 3). As with OMD, there were no differences (P 0.27) among supplement treatments for VEG, with an average NDFD of 76.9%. For BOOT, NDFD was decreased (P 0.06) with corn compared to either soybean hull or no supplementation. Within HEAD, corn supplementation resulted in the least (P 0.02) and soybean hulls the greatest (P < 0.01) NDFD. For MAT, corn supplementation did not (P = 0.23) suppress NDFD compared to the control treatment, whereas soybean hull supplementation increased (P < 0.01) NDFD by 4.1 percentage units.
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). Within VEG, no differences (P 0.28) were found between supplement treatments, with a mean N retention of 42.7 g/d. Supplementation increased (P 0.08) N retention over the control treatment for BOOT and HEAD, whereas, with HEAD, soybean hull supplementation tended (P = 0.16) to increase N retention compared to corn supplementation. For MAT, soybean hull supplementation increased N retention over the control treatment (P = 0.02), and corn supplementation yielded intermediate values.
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0.10), either a three-way interaction between supplement type, forage maturity, and time (propionate, valerate, and acetate:propionate ratio), a supplement type x time interaction (pH, NH3-N, total VFA, acetate, butyrate, isobutyrate, and isovalerate), or a forage maturity x time interaction (butyrate). Although interactions with time were detected, treatment effects in all cases were fairly consistent at the different sampling times. Thus, time effects and their interactions have not been presented and emphasis has been placed on main effects and their interactions (Tables 4 and 5).
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0.30) for any of the ruminal characteristics reported in Tables 4 and 5, with the exception of butyrate (P = 0.07) and valerate (P = 0.01). However, the nature of the interactions for butyrate and valerate was such that supplement effects differed only in magnitude, and the ranking of supplement treatments was similar among the various forage maturities. Liquid dilution rate decreased (P 0.08) with advancing forage maturity from VEG through HEAD maturities (Table 4), and no difference (P > 0.10) was detected between BOOT and MAT forage maturities. Ruminal liquid fill tended (P = 0.11) to increase and ruminal OM fill increased (P = 0.04) with advancing forage maturity.
Ruminal pH increased (P < 0.01) with increasing forage maturity from VEG through HEAD maturities, with no further increase (P = 0.41) between HEAD and MAT forages. Ruminal NH3-N was greatest (P < 0.01) for BOOT and least (P 0.02) for MAT, with no difference (P = 0.46) between the other two forage maturities. Total VFA concentrations were greater (P < 0.01) for VEG and BOOT than for HEAD and MAT, with no differences (P 0.27) within either pair of forage maturities. Average total VFA concentration for the two early forage maturities was 122.2 mM, whereas the average for the two later maturities dropped to 105.5 mM. The average acetate:propionate ratio was 4.48 for VEG and BOOT, which was greater (P 0.09) than the average ratio for HEAD and MAT (4.25). Molar proportion of acetate was affected in a similar manner, with the two earlier maturities supporting greater (P 0.07) acetate proportions than the two later maturities. The VEG and BOOT maturities had lesser (P 0.06) molar proportions of propionate than HEAD, whereas MAT was intermediate. Boot-stage forage had the least (P 0.04) molar proportion of butyrate, followed by VEG (P 0.04), with the greatest (P 0.02) butyrate proportions found with HEAD and MAT. There was no difference (P = 0.61) between VEG and BOOT with regard to molar proportion of valerate and the least (P < 0.01) molar proportion of valerate was found with MAT. Vegetative forage and MAT resulted in lesser (P 0.03) proportions of isobutyrate and isovalerate than BOOT and HEAD.
Supplementation had no effect (P = 0.19) on liquid dilution rate, which averaged 9.0%/h (Table 5). Supplementation reduced (P = 0.01) ruminal liquid fill by nearly 30 mL/kg BW, but had no effect (P = 0.44) on ruminal OM fill. Supplementation had no effect (P = 0.35) on ruminal pH, which averaged 6.33 across all supplement treatments. No difference (P = 0.20) was observed between the control treatment and soybean hull supplementation for ruminal NH3-N, with a mean of 9.10 mM. Corn supplementation decreased (P 0.07) ruminal NH3-N by 33%, compared to the mean of the other two treatments. Supplementation did not affect (P = 0.27) total VFA concentrations, which averaged 113.8 mM. The acetate:propionate ratio was unaffected (P = 0.36) by supplementation, although the unsupplemented treatment resulted in the greatest (P 0.05) proportion of acetate, and corn supplementation resulted in the least (P 0.02). Molar proportions of propionate and isobutyrate were unaffected by supplementation (P 0.12). Corn supplementation resulted in the greatest (P 0.01) proportions of butyrate, followed by soybean hulls (P < 0.01), and then control (P < 0.01). Molar proportions of valerate were greatest (P 0.10) for soybean hull supplementation at 0.72%, followed by corn (P 0.10), whereas proportions were least (P 0.10) with the control treatment. Supplementation increased (P < 0.01) molar proportions of isovalerate by 28%.
Discussion
Generally, forage quality decreased with advancing maturity, although little difference existed in the chemical composition between HEAD and MAT. Unexpectedly, ergovaline concentrations were greatest in the VEG hay. However, these concentrations were below values typically expected to influence the responses measured in this experiment (Stamm et al., 1994; Emile et al., 2000), particularly in the absence of heat stress, as in this study. Voluntary intake is often considered to be an excellent integrated measure of overall forage quality (Reid, 1961). Using unsupplemented intake as an index, forage quality was slightly greater for MAT than for HEAD. The range of forage quality represented by the forages in this study was at or below the range that we would expect for stocker cattle grazing summer fescue pastures. Dubbs et al. (2003) reported concentrations of CP ranging from 14 to nearly 25% of OM, 55 to 70% ash-free NDF, and 65 to 75% ruminally degradable protein, as a percentage of the total dietary CP, in masticate samples from steers grazing fescue between April and October.
The decrease in forage intake with energy supplementation is typical of other studies in which energy supplements have been fed (Chase and Hibberd, 1987; Martin and Hibberd, 1990; Hess et al., 1996; Elizalde et al., 1998). Although forage intake was decreased, the decrease was small enough that total OMI increased with energy supplementation. In agreement with other work conducted with endophyte-infected tall fescue (Elizalde et al., 1998), no difference was found between starch- or fiber-based energy supplementation on intake. Thus, substitution ratios observed in this study were not affected by supplement type but were affected by forage maturity. Although Galloway et al. (1993) found greater substitution with soybean hulls than with corn on bermudagrass and orchardgrass hays, others have seen little difference with starch, as compared to digestible fiber-based supplements (Garces-Yepez et al., 1997; Elizalde et al., 1998). Although effects of supplementation on intake measures were similar across forage maturities, larger (more negative) substitution ratios with higher-quality forages indicate that depressions in intake with energy supplementation were greater with higher-quality forages. However, other unidentified factors appear to be important as well, as the MAT forage did not fit this pattern. Based on a review of the literature, Horn and McCollum (1987) suggested that substitution ratios were increasingly negative with higher-quality forages. This relationship indicates that the influence of energy-satiety intake control mechanisms may increase in importance with increasing forage quality.
A greater opportunity exists to increase digestibility by supplementing low-quality, as compared with high-quality forages, and such a response is reflected in the current results and in research reported by Matejovsky and Sanson (1995). It has been suggested that responses to starch- vs. fiber-based supplements depend on forage quality (Bowman and Sanson, 1996). Although not apparent for DOMI, digestibility results from the present experiment support this contention. Effects of supplement type on total-tract OMD were not evident with higher-quality forages (VEG and BOOT), whereas OMD was greater with soybean hulls than with corn with the more mature forages (HEAD and MAT). This difference between supplement types was reflected in our calculated dOMD, although the interaction between supplement type and forage maturity was not significant. Greater NDFD was also seen with soybean hull as compared with corn supplementation. Although this is in agreement with the calculated dOMD, effects on NDFD cannot be attributed solely to associative effects (or lack thereof). Increases in NDFD with soybean hull, as compared with corn supplementation, could be a consequence of fewer negative associative effects on forage fiber digestion, or of dilution of the diet with a highly digestible NDF source. It is likely that the greater NDFD seen with soybean hulls was a combination of both of these factors. Interactions between forage maturity and supplementation recorded in the present study may help to explain the variable digestibility responses to supplementation reported in the literature.
Various mechanisms have been proposed to explain negative associative effects observed with starch-based feedstuffs. Based on numerous studies, Hoover (1986) reported that decreasing ruminal pH below 6.0 resulted in a precipitous loss of fibrolytic activity, with a complete cessation of fiber digestion between pH 4.5 and 5.0. In the present study, supplementation had no effect on ruminal pH or total VFA concentrations. Others have reported similar ruminal pH values and VFA concentrations on high-quality forages with (Hess et al., 1996) or without (Elizalde et al., 1998) differences between supplement types. Still others have found depressions in ruminal pH with both starch- and fiber-based supplements (Bodine et al., 2001; Beck et al., 1992; Vanzant et al., 1990), although in none of these cases was ruminal pH depressed to levels below 6.4. Thus, although low ruminal pH can decrease fiber digestion, it does not appear to be a primary factor in many practical situations in which energy supplements are added to forage-based diets.
A second potential mechanism for negative associative effects involves competition for available nutrients. Ruminal NH3-N is often cited as a potentially limiting nutrient. Satter and Slyter (1974) recommended a minimum ruminal NH3-N concentration of 5.0 mg/dL (3.6 mM) to support cellulolytic activity. Based on numerous papers reviewed by Hoover (1986), in diets greater than 6% CP, optimal NH3-N levels ranged from 3.3 to 8.0 mg/dL (2.4 to 5.7 mM). In the present study, reductions in ruminal NH3-N concentrations with corn supplementation corresponded to the reductions in NDFD. Although interaction means are not presented, average ruminal NH3-N concentrations were below 5 mM when corn was supplemented with either HEAD or MAT. However, we also detected decreased NDFD with corn supplementation to BOOT hay, which maintained the highest ruminal NH3-N concentrations, in excess of 11 mM. Another means of evaluating the sufficiency of ruminally available N to support ruminal fermentation is through the use of DIP:DOMI ratios. Mathis et al. (2000) found that intake and digestibility of forages was maximized when dietary DIP:DOMI was in the range of 8 to 13%, in general agreement with NRC (1996). The DIP:DOMI values in the present study agreed with ruminal NH3-N concentrations, suggesting that ruminally available N may have been limiting on the two more mature forages, but was likely well in excess with the BOOT hay, for which depressions in NDFD were observed. Thus, as with ruminal pH, changes in ruminally available N do not explain depressions in NDFD in each case.
Branched-chain VFA have been implicated as potentially limiting nutrients for ruminal fiber digestion (Hoover, 1986). Dehority et al. (1967) suggested that concentrations of isobutyrate and isovalerate required by ruminal bacteria in vitro were between 0.05 and 0.50 mM. In the present study, mean concentrations of these VFA for all treatments exceeded 0.50 mM, and increased with supplementation. Thus, it is unlikely that the availability of branched-chain VFA was responsible for negative associative effects observed with corn supplementation.
Other possible mechanisms by which rapidly fermented carbohydrate may decrease fiber digestion include end-product inhibition of cellulase activity (Huang and Forsberg, 1990), decreased microbial attachment to fibrous substrate (Piwonka and Firkins, 1993), induction of shifts in microbial populations away from cellulolytic species (Piwonka et al., 1994), or direct toxic effects of excess carbohydrate (Russell, 1998). In the present study, the rates of pH decline (data not shown) with soybean hulls and corn were nearly identical and the rate of increase in VFA concentrations following supplementation (data not shown) was more rapid with soybean hulls. This suggests that the rate of degradation for the soybean hulls was at least as rapid as the rate of degradation of the corn. Accordingly, rates of carbohydrate degradation listed in NRC (1996) are greater for soybean hulls than for cracked corn grain. Rapid digestion of soybean hulls would be expected to produce glucose and cellobiose, which can inhibit cellulase (Smith et al., 1973), decrease microbial attachment, or be directly toxic to ruminal bacteria. Thus, it appears that mechanisms associated with starch-induced negative associative effects may be independent of the rate of substrate fermentation. One possibility is that lactate production subsequent to starch digestion may play a pivotal role. Fay and Ovejero (1986) suggested that lactate-induced depressions in fiber digestibility could be potentiated by moderate depressions in ruminal pH, such that grain supplementation may produce sufficient lactate to inhibit fiber digestion in supplemented ruminants.
Digestible OM intake provides an integrated measure of intake and digestibility that closely represents digestible energy intake and, thus, is expected to closely correspond with expected performance. Although we did detect significant interactions between forage quality and supplement source for measures of digestibility, we did not find such interactions for intake of OM or digestible OM. Although DOMI was increased by approximately 20% with corn supplementation compared with the negative control, soybean hull supplementation at a similar level of OM yielded an additional 6% DOMI above that obtained with corn. Thus, these results suggest that, regardless of forage quality (within the range of qualities used in the present study), greater levels of digestible OM will be consumed when using soybean hulls compared with corn as an energy supplement. In contrast, neither Garces-Yepez et al. (1997) nor Elizalde et al. (1998) detected differences in DOMI for starch- vs. fiber-based supplements, and Pordomingo et al. (1991) found no benefit to DOMI from corn supplementation. Furthermore, the benefits we observed from soybean hulls vs. corn appear to be a result of a lack of negative associative effects on digestibility, as opposed to positive associative effects, per se, or to effects on forage intake.
Moore et al. (1999) constructed empirical regression equations to predict substitution and associative effects resulting from supplementation. In their model, effects of supplements on forage intake are influenced by voluntary forage intake without supplement, by forage and supplement crude protein and TDN concentrations, and by forage type (temperate/tropical forages vs. native, mixed forages and straw) and supplement type (protein vs. molasses vs. grain/by-product vs. forage). Conversely, their model for predicting negative associative effects is influenced only by the expected (additive) TDN of the mixed forage/supplement diet. We used the equations developed by Moore et al. (1999) to determine their accuracy of predicting observations from the present study. Predicted substitution ratios (average for all four forage maturities, each predicted independently) were -0.83 and -0.59 for corn and soybean hull supplementation, respectively, compared with our observed values of -0.49 and -0.43. Thus, the model predicted somewhat greater substitution ratios with corn supplementation, although, in the present study, no differences were observed between the two supplements. Because the TDN-prediction model was unaffected by supplement composition, similar negative associative effects were predicted for the two supplement types (-4.2 and -4.0 percentage units for corn and soybean hull supplementation, respectively). The model did a reasonable job of predicting associative effects observed with corn supplementation (-5.9 percentage units), although it overpredicted observed effects with soybean hulls (0.3 percentage unit). This result is not surprising in light of the fact that the equations were developed from available literature reports, which were dominated by observations using cereal grains as energy sources. Additional information evaluating the influence of high-fiber, by-product supplements on forage intake and utilization is required to enhance current, empirical approaches for estimating these effects. Furthermore, a need exists for additional fundamental research to allow for the construction of more robust, mechanistic models to predict effects of supplementation on forage use.
The intake and OMD values measured can be used to predict effects of the supplements on gain of stocker cattle. Using the diet evaluation component of the NRC (1996) software, we calculated expected gains for 300-kg steers using our observed intake and digestibility values (using DOMI as an estimate of TDN intake; Heaney and Pigden, 1963). Additionally, to assess the effect of dOMD on performance estimates, we estimated gains assuming negative dOMD effects were absent for the corn supplement treatment. Ignoring dOMD effects, one would predict that the energy from the forage (average of four maturities) and corn supplement would support gains on the order of 1.11 kg/d. However, after accounting for dOMD (at a discount of 6 percentage units of TDN), predicted gains were lowered to 0.78 kg/d. Thus, by failing to account for associative effects of the magnitude measured in the present study, we would overpredict steer gain in this example by approximately 0.3 kg/d when supplementing with corn. Using the same approach, we estimated that soybean hulls fed at 0.67% BW (OM basis) would support stocker gains approximately 0.2 kg/d greater than corn fed at a similar level.
Another potential benefit of energy supplementation, particularly with high-protein forages, is increased capture of ruminally available N as microbial protein, and greater conversion of N into lean tissue, with subsequently less N excretion to the environment. Although urinary allantoin excretion is not useful for quantitative prediction of microbial CP production, it can be useful for detecting treatment differences in microbial CP (Johnson et al., 1998). Although we detected decreasing allantoin excretion with more mature forages, we did not detect differences due to supplementation, despite numerically greater values with supplemental feeding. However, N retention was improved with supplementation on all except the VEG forage. This response differs from what one would expect based on the DOMI response (which was not characterized by an interaction between forage maturity and supplement treatment) and expected gains based on DOMI, which should increase with supplementation regardless of forage quality. Because greater energy consumption in this situation was not accompanied by increases in N balance, these results indicate that one may expect a greater degree of fat deposition with supplementation, or that the efficiency of supplemental energy use is less with high- as compared with low-quality forages. Furthermore, results suggest that energy supplementation may not be an effective means of decreasing N excretion with high-quality forages.
Implications
Although interactions between forage quality and supplement type have been suggested, in this study, interactions were not detected for most responses across a range of forage quality in which degradable intake protein should not have been limiting. Despite similar substitution ratios, negative associative effects with corn supplementation resulted in greater digestible organic matter intake when supplementing with soybean hulls compared with corn grain. With starch-based energy supplements, negative associative effects must be accounted for to accurately estimate energy consumption, and subsequent performance. Factors other than ruminal pH and NH3-N concentration seem responsible for negative associative effects with starch-based supplementation. More research is needed to permit prediction of substitution ratios and associative effects with reasonable accuracy, to allow for better allocation of forage resources and prediction of performance responses to supplementation.
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Footnotes
1 This work is based on work supported by the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture, under Agreement No. 2001-34431-10409 and is publication No. 02-07-144 of the Kentucky Agric. Exp. Stn. The authors express their appreciation to K. B. Combs, S. Hamilton, J. Greenwell, S. Rudd, J. Piel, and R. B. Hightshoe for their expert assistance in data collection and cattle management, and to C. Schultz for alkaloid chromatography. 
2 Current address: 203 Animal Sciences Bldg., Stillwater, OK 74078.
3 Correspondence: 805 WP Garrigus Bldg. (phone: 859-257-9438; fax: 859-257-3411; e-mail: evanzant{at}uky.edu).
Received for publication August 28, 2002. Accepted for publication September 16, 2003.
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