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Effect of grain processing and silage on microbial protein synthesis and nutrient digestibility in beef cattle fed barley-based diets^1,2

Agriculture and Agri-Food Canada, Research Centre, Lethbridge, AB, Canada T1J 4B1

³ Correspondence:
P.O. Box 3000 (phone: 403-317-3341; fax: 403-382-3156; E-mail:
koenig{at}agr.gc.ca).

	Abstract

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Effects of the extent of grain processing and the percentage of silage in barley-based feedlot diets on microbial protein synthesis and nutrient digestibility were evaluated using four steers (initial BW of 442 ± 15 kg) with ruminal and duodenal cannulas. The experiment was a 4 x 4 Latin square with four periods of 21 d each. Dietary treatments were arranged as a 2 x 2 factorial with two levels of barley silage (20 and 5% DM basis) and two degrees of barley grain processing (coarsely and flatly steamrolled to a processing index [PI] of 86 and 61%, respectively). The PI was quantified as the volume weight of the barley grain after processing, expressed as a percentage of the volume weight prior to processing. Digesta flow (Yb) and microbial (¹⁵N) markers were continuously infused into the rumen for a period of 13 d. Ruminal, duodenal, and fecal samples were collected at various times over the last 6 d of marker infusion. Diurnal ruminal pH was measured for 48 h. Intake of DM averaged 1.8% of BW, and was not different among the dietary treatments (P > 0.10). Ruminal starch digestibility was higher (P < 0.05) for the more extensively processed grain and tended (P < 0.10) to be highest when the more extensively processed grain was combined with 5% barley silage. In contrast, ruminal fiber digestibility for the 5% silage diets was reduced (P < 0.05) when the grain was more extensively processed. There was, however, no effect of grain processing on ruminal OM digestibility (P > 0.10), and hence, no inhibitory effect on microbial N flow to the intestine (P > 0.10). There was also no effect of the level of silage on microbial N flow (P > 0.10), but there was a tendency for improved efficiency of microbial protein synthesis for the 20% silage diets (P = 0.072). Ruminal escape of nonmicrobial N (P = 0.003) was greater, and thus, protein flow to the intestine was greater for the 5% silage diets. Diurnal ruminal pH was lower (P < 0.05) for 11 of the 24 hourly time points in steers fed the 5% silage diets than those fed the 20% silage diets. In conclusion, barley grain rolled to a PI of 86 to 61% and combined with 20 and 5% barley silage had little effect on microbial protein supply. Microbial protein supply was not inhibited when the barley grain was extensively processed (PI of 61%) and the silage was limited to only 5% of the diet DM, but feed intake of steers in this study was lower than would be expected in the feedlot.

Key Words: Barley • Beef Cattle • Digestibility • Grain • Microbial Protein • Silage

	Introduction

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It is well known that the utilization of barley grain as a ruminant feed is improved with some form of processing. In a summary of experiments comparing the feeding value of whole vs. dry-rolled barley grain for cattle, OM and starch digestibility were reduced by an average of 16 and 37%, respectively, when the whole grain was fed (Mathison, 1996). Rates of gain were reduced by 5 to 50% and the feed required for gain increased by 15 to more than 100% when whole unprocessed barley grain was fed (Mathison, 1996). Processing of barley grain is necessary to disrupt the fibrous hull and pericarp and to permit access of microbial enzymes to the internal structures. The intent of grain processing is to optimize energy or starch availability in the rumen by maximizing the extent of ruminal carbohydrate digestion while controlling the rate at which it is digested.

The energy derived from carbohydrate digestion in the rumen drives microbial protein synthesis, and in turn, the supply of protein to the growing animal. However, feeding extensively processed grain with rapid and complete digestion can create unfavorable conditions within the rumen that limit microbial protein synthesis and fiber digestion. The optimal extent of grain processing will, however, be influenced by other components of the diet, most notably the forage amount and physical form. Our hypothesis was that the optimal extent of barley grain processing would vary depending on the percentage of barley silage in high concentrate, barley grain-based diets fed to beef cattle. The objectives of the study were to determine the effects of the degree of barley grain processing and the percentage of barley silage in the diet on ruminal fermentation, microbial protein supply, and site and extent of nutrient digestibility in beef cattle.

	Materials and Methods

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The experiment was reviewed and approved by the Animal Care Committee of the Lethbridge Research Centre and was conducted in accordance with the guidelines of the Canadian Council on Animal Care (1993).

Animals and Experimental Design
Four Jersey steers (initial average BW = 442; SD =15 kg) with ruminal (10-cm diameter opening, Bar Diamond, Parma, ID) and duodenal T-type (placed proximal to the bile and pancreatic ducts approximately 10-cm distal to the pylorus) cannulas were used. The experiment was designed as a 4 x 4 Latin square with four periods of 21 d (9 d for dietary adaptation and 12 d of measurements). The steers were housed in individual tie stalls on comfort mattresses bedded with wood shavings. The steers were released to an outdoor pen at 1300 for 1 h of exercise each day, except for a period of 48 h during continuous pH measurements. Body weight was measured at the beginning of period 1 (d 1) and at the end of each of the four periods (d 21) at the same time on each day.

Treatment Diets
The dietary treatments were arranged as a 2 x 2 factorial with two levels of forage and two degrees of barley grain processing. The forage source was barley silage and the percentage of silage in the diet was either 20 or 5% (DM basis). The fermentability of the carbohydrate was varied by steamrolling the barley grain to a coarse and flat thickness to achieve processing indices (PI) of 86 and 61%, respectively. The PI was calculated as the volume weight of the barley after processing and was expressed as a percentage of the volume weight before processing. The barley grain for the experiment was from a single source and was processed as a single batch for each PI prior to the start of the experiment. The barley was first screened to remove chaff and small kernels, and then steamrolled under high pressure for 5 min and rolled through a mill to the desired PI. Samples of approximately 2 kg of the barley grain were collected before cleaning and after processing for measurement of volume weight (0.5-L cup, Seedburo Equipment Co., Chicago, IL). Kernel thickness was measured using micrometer calipers (Electronic Digital Calipers, VWRbrand, VWR Canlab, Mississauga, ON) on 20 kernels for each of the processed barley grains. The diets were fed as total mixed rations (TMR) and contained sufficient minerals and vitamins to meet NRC (1996) requirements (Table 1).

Ruminal Fermentation Characteristics
Diurnal ruminal pH was measured using indwelling industrial pH electrodes (model PHCN-37; Omega Engineering, Stamford, CT) for a period of 48 h, beginning at the time of feeding (1500) on d 10 and continuing until d 12. The electrode was inserted into the rumen of each steer through the cannula. A weight was attached to the electrode to ensure that it remained positioned within the ventral sac. A protective shield with openings was placed around the electrode to permit ruminal fluid to percolate freely and to prevent the electrode from coming in contact with the ruminal epithelium. The pH was measured every 5 s, and then an average of the readings was recorded for every 15-min interval using a data logger. After 24 h, the electrodes were removed from the rumen and recalibrated.

Whole ruminal contents (approximately 2 L) were collected from four sites within the rumen (reticulum, ventral sac, caudal sacs, and the mat layer) at eight time points (0100, 0400, 0700, 0900, 1200, 1400, 1800, and 2100) between d 16 and 21. Approximately 250 g of the whole contents were squeezed through one layer of cheesecloth and a 5-mL volume of the filtrate was combined with 5 mL of protozoal staining solution (methylgreen formalin saline; Ogimoto and Imai, 1981). The preserved sample was stored in darkness at room temperature until protozoal enumeration. Another 250 g of ruminal contents was squeezed through four layers of cheesecloth. Five milliliters of the filtrate was combined with 1 mL of 25% wt/vol of meta-phosphoric acid, and another 5 mL was combined with 1 mL of 0.18 M H₂SO₄, and each was stored frozen (-30°C) until VFA and lactate, and NH₃ nitrogen analysis, respectively.

Microbial Protein Synthesis, Digesta Flow and Nutrient Digestibility
Digesta flow and nutrient digestibilities were determined using Yb as the external digesta marker. Microbial protein synthesis was measured using ¹⁵N as the ruminal microbial marker. The marker solution was prepared daily by dissolving 8.8 g of YbCl₃•6H₂O (approximately 2 g of Yb; Rhodia Inc., Shelton, CT) and 4 g of [¹⁵N](NH₄)₂SO₄ (10.7% atom percentage of ¹⁵N, Isotec, Inc., Miamisburg, OH) in 2 L of deionized water. The markers were continuously infused using an automatic pump at a rate of 2 L/d into the rumen via the ruminal cannula for the last 13 d of the period. A prime dose, containing half of the daily dose of the markers, was administered to the rumen on d 9, just prior to the initiation of the infusion.

Duodenal (300 mL) and fecal samples (100 g wet weight from the rectum) were collected at 21 different time points over the last 6 d of the period (d 16 to 21). The samples were collected at 6-h intervals within a 24-h period, with sampling times moved ahead by 1 h each day. The pH of the duodenal contents was measured using a pH meter to ensure that the pH was <3, indicating that the cannula was positioned appropriately and that there was an absence of any backflow of digesta. Duodenal samples were composited by steer and period as collected, and immediately frozen. The pooled duodenal samples were later freeze-dried and ground through a 1-mm screen for chemical analysis. Fecal samples were first dried (55°C for 48 h in a forced-air oven), ground through a 1-mm screen, and then composited by steer and period for chemical analysis. Duodenal and fecal samples were analyzed for analytical DM, OM, N, ¹⁵N (duodenal), starch, NDF, ADF, and Yb.

For the determination of microbial N flow to the duodenum, mixed ruminal bacteria were isolated from whole ruminal contents collected at the eight time points from d 16 to 21 as described previously for ruminal fermentation characteristics. Ruminal contents (approximately 750 mL) were combined with an equal volume of 0.9% saline, homogenized for 2 x 30-s periods in a Waring blender (Waring Products Division, New Hartford, CT) to dislodge particulate-associated bacteria, and then squeezed through four layers of cheesecloth. The filtrate (2 x 500 mL) was centrifuged (800 x g for 15 min at 4°C) to remove feed particles and protozoa and then the supernatant was centrifuged (20,000 x g for 40 min at 4°C) to obtain the mixed ruminal bacterial pellet. Bacterial pellets from each time point were freeze-dried and analyzed for N and ¹⁵N. A composite sample of bacteria was made by combining an equal weight of bacterial DM from all time points for each steer in each period for analysis of analytical DM and OM.

Ruminal contents were manually removed at 1300 on the last day of each period. The contents were weighed, sampled (2 x 1 kg), and DM content determined by oven drying (55°C). The ruminal contents from each steer were then transferred to the next steer assigned to receive the same treatment diet to hasten the adaptation to the diets.

Chemical Analysis
All chemical analyses were performed on each sample in duplicate, and where the coefficient of variation for the replicate analysis was >5%, the analysis was repeated.

Ruminal VFA were quantified using crotonic acid as the internal standard, and gas chromatography (model 5890, Hewlett Packard, Little Falls, DE) with a capillary column (30 m x 0.25 mm i.d., 1µ phase thickness, bonded PEG, Supelco Nukol, Sigma-Aldrich Canada, Oakville, ON) and flame-ionization detection. The oven temperature was 100°C for 1 min, which was then ramped by 20°C/min to 140°C, and then by 8°C/min to 200°C, and held at this temperature for 5 min. The injector temperature was 200°C, the detector temperature was 250°C, and the carrier gas was helium. Lactate was determined as the methyl ester by gas chromatography (Supelco bulletin 856, Sigma-Aldrich Canada). Ruminal NH₃ nitrogen concentration was determined by the salicylate-nitroprusside-hypochlorite method using a flow injection analyzer (Sims et al., 1995). Ciliate protozoal numbers were determined by counting protozoa present in a known volume of the sample using a standard counting chamber (Fuchs-Rosenthal, Hausser Scientific Partnership, Horsham, PA) and a light microscope. Ruminal VFA, lactate, NH₃ nitrogen concentration, and protozoal numbers were averaged for the eight time points for each steer in each period for statistical analysis.

Analytical DM was determined by drying the samples at 135°C for 2 h, followed by hot weighing. The OM content was calculated as the difference between 100 and the percentage of ash (AOAC, 1995; Method 942.05). Starch was determined by the enzymatic procedure described by Herrera-Saldana et al. (1990) with the following modifications. The enzymatic hydrolysis was extended to 1 h using -amylase (Termamyl; Novo Nordisk, Bagsvaerd, Denmark); amyloglucosidase (No. 208-469, Boehringer Mannheim, Laval, Canada) replaced glucoamylase, and the quantities of the sample and reagents were reduced for colorimetric determination at 490 nm using a plate reader. The NDF and ADF were determined in the Ankom²⁰⁰ fiber analyzer (Ankom Technology Corp., Fairport, NY) using heat-stable -amylase and sodium sulfite. The particle length of the barley silage was determined using the Penn State particle size separator (Nasco, Fort Atkinson, WI). The physical effectiveness factor of the silage was calculated as the sum of the percentage of DM retained on the two sieves of the particle size separator (Lammers et al., 1996).

For the measurement of N and ¹⁵N enrichment, samples were ground using a ball mill (Mixer Mill MM2000; Retsch, Haan, Germany) to a fine powder. Nitrogen was quantified by flash combustion with gas chromatography and thermal conductivity detection (Carlo Erba Instruments, Milan, Italy), and ¹⁵N enrichment was measured by flash combustion with isotope ratio mass spectrometry (VG Isotech, Middlewich, England). Nitrogen and ¹⁵N enrichment measured in the eight bacterial samples were averaged for each steer in each period. Ytterbium was determined by inductively coupled plasma emission spectrometry (SpectoCiros^CCD, Specto Analytical Instruments, GmbH & Co., Kleve, KG, Germany) after dry ashing and extraction of the Yb into 0.32 M HNO₃.

Calculations and Statistical Analysis
Ruminal pH data were summarized for each steer in each period as daily mean pH, minimal and maximal pH, the amount of time the pH was below 5.8 and 5.5, and the area under the curve (AUC) for pH below 5.8 and 5.5. The area was calculated by adding the absolute value of the negative deviations from the pH curve and each threshold pH for each 15-min interval and was expressed as pH units x h. The amount of time that the pH remained below a threshold of 5.8 and 5.5 indicates the duration of subclinical acidosis, whereas the area between the curve and the pH threshold indicates the severity of subclinical acidosis.

Flow of DM to the duodenum and DM excreted in feces were calculated by dividing the amount of Yb infused (g of Yb/d) by the Yb concentration (g of Yb/kg DM) in duodenal digesta and feces, respectively. Microbial N flow for each steer was determined by multiplying total N flow to the intestine by the ratio of ¹⁵N enrichment (atomic percent excess) in duodenal contents to enrichment in mixed ruminal bacteria. Microbial OM flow was calculated by multiplying microbial N flow by the ratio of OM:N in ruminal bacteria. True ruminal OM and N digestibilities were then computed by subtracting the microbial OM and N flows from total duodenal OM and N flows, respectively.

The data were analyzed as a mixed linear model (SAS Inst., Inc., Cary, NC) to account for the fixed effects of silage, PI, and the silage x PI interaction, and the random effects of period and steer. When the interaction was significant, differences among treatments were compared using Fisher’s protected LSD test adjusted with the Tukey-Kramer option. Data for the diurnal hourly measurements of pH were analyzed as a mixed linear model as described above with hour included as a repeated measure and using the compound symmetry variance-covariance error structure. Differences among treatments for each hour were compared using Fisher’s protected LSD test. Differences were determined significant at P < 0.05 and trends or tendencies were discussed for P < 0.10.

	Results and Discussion

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Carbohydrate Digestion
The objective of grain processing is to obtain a balance between maximizing the extent of ruminal starch fermentability and, perhaps more importantly, to control the rate of starch fermentation, to avoid digestive and metabolic disturbances. In the present study, grain was processed by steamrolling to achieve a coarse roll with a PI of 86% and a flat roll with a PI of 61%. Typically, the recommendation for the feed industry is to process barley grain to achieve a PI between 80 to 85% to maximize feed efficiency and ADG (Mathison, 2000). Feeding coarsely rolled grain, while often reducing the rate of starch fermentability in the rumen (Yang et al., 2001), also reduces the energy available for microbial protein synthesis and, therefore, protein supply to the growing animal. On the other hand, feeding extensively processed grain with rapid and complete ruminal degradation can reduce ruminal pH creating an unfavorable environment in the rumen for synthesis of microbial protein and digestion of fiber. Other dietary factors, in addition to grain processing, can also influence the ruminal environment; the most notable is the amount and physical form of the forage. In the present study, the percentage of barley silage was 20 or 5% of dietary DM and was combined with 80 or 95% concentrate containing 69 or 84%, respectively, of coarsely or flatly rolled barley grain.

Feed intake was not affected (P > 0.10; Table 2) by the percentage of barley silage (20 vs. 5%) or the degree of grain processing (PI of 86 vs. 61%) in the diets and averaged 8.7 kg/d and 1.8% of BW. Beauchemin et al. (2001) also reported no effect of barley grain processing (tempered-rolled to a PI of 82, 75, 70, and 65%) on DMI. This is in contrast, however, to results obtained in a feeding trial with cattle fed all-concentrate diets (96.7% steamrolled barley grain) varying in the extent of grain processing, where feed intake decreased as the thickness of the roll was decreased from whole, through to coarse and medium, and flat (Hironaka et al., 1992). In another feeding trial with bulls fed 90% concentrate diets (85% barley grain), feed intake was higher for the diet with grain kernels that were just cracked, but there were no differences for grain that was processed to a medium or fine roll (Mathison et al., 1997). Whereas it is often desirable to maximize feed intake in growing and finishing cattle, higher feed intake that is observed when feeding whole or very coarsely rolled barley grain can be offset by a reduction in nutrient digestibility and poorer feed:gain ratios (Hironaka et al., 1992; Mathison et al., 1997). The results of the present study suggested that the inclusion of 5% barley silage in high-concentrate diets with more extensively processed grain was sufficient to ameliorate digestive disturbances and the associated reduction in feed intake. However, it should be noted that DMI and performance of ruminally and duodenally cannulated cattle, are often compromised relative to that of growing and finishing cattle in the feedlot. In the present experiment, DMI (1.8% of BW) was lower than the DMI of 2.0 to 2.5% of BW that is typically observed in feedlot cattle, and hence, treatment differences in DMI, pH, or other related factors could have been lessened at the lower feed intake. Body weight of the steers increased through each period from a mean BW of 464 kg in Period 1 to 517 kg in Period 4. Silage and PI both influenced BW, with higher BW attained on the 5% silage diets and the less extensively processed grain diets (P < 0.05), but it should be cautioned that the duration of the periods was only 21 d.

Reducing the percentage of silage in the diet from 20 to 5% and increasing the percentage of concentrate from 80 to 95% increased (P = 0.045; Table 2) the daily intake of starch. With more starch entering the rumen of the steers fed the 5% barley silage diets, there was also a greater amount of starch digested in the rumen (P = 0.025). When ruminal digestibility was expressed as a percentage of starch intake, starch digestibility was higher (P = 0.009) for the more extensively processed grain and tended to be highest (P < 0.10) when the flatly rolled barley grain was combined with 5% barley silage. Thus, the combination of a lower proportion of silage and more extensively processed grain increased the availability of starch in the rumen. Conversely, the percentage of starch digested postruminally was lower (P = 0.016) for the more processed grain and tended (P < 0.10) to be lowest when combined with 5% barley silage. Although postruminal digestion was considerably less (8.8 to 13.4%) than ruminal starch digestion (85.6 to 90.7%), the starch that did enter the intestine was extensively digested (91 to 95%). Total-tract digestibility of starch was higher for the diets with a greater proportion of grain (5% silage, P = 0.032) and for the more extensively processed grain (PI of 61%, P = 0.006), but the differences were numerically very small, such that total-tract starch digestion ranged from 98.6 to 99.5%, and they are probably not of practical significance. Beauchemin et al. (2001) also reported only small improvements in total-tract starch digestibility with more extensive grain processing, and Hironaka et al. (1992) reported no effect of the extent of grain processing (coarse, medium, flat roll) on total-tract digestibility of starch from all-concentrate diets fed to beef cattle.

The observed NDF concentration of the 5% silage diets was lower than expected. Based on the NDF concentration of the barley grain and silage, the 20% silage and 5% silage treatments were calculated to have differed by about four percentage units for NDF (31.1 vs. 27.5%), but the numerical difference observed between the 20 and 5% silage TMR was only about two percentage units (32.1 vs. 30.3%; P > 0.10; Table 1). The composition of the NDF fraction (cellulose, hemicellulose, and lignin) was altered, and this was reflected in the ADF content (cellulose and lignin; 11.0 vs. 7.1% for the 20 and 5% barley silage diets, respectively). However, perhaps what is more important than total dietary NDF or ADF in terms of ruminal function, is the difference between the 20 and 5% barley silage treatments in terms of the percentage of forage NDF (33 vs. 8.7% of dietary NDF for the 20 and 5% silage diets, respectively).

With only small numerical differences between the 20 and 5% barley silage treatments for NDF concentration, there were no differences in NDF intake among the dietary treatments (P > 0.10; Table 2). For ADF, intake was higher (P < 0.05) for the diets with 20% barley silage. In contrast to ruminal starch digestibility, fiber digestibility (NDF and ADF) in the rumen was lower (P < 0.05) when feeding the more extensively processed barley grain when combined with only 5% barley silage. However, grain processing had no effect (P > 0.05) on ruminal fiber digestion when combined with 20% barley silage. Beauchemin et al. (2001) reported no effect of grain processing on NDF digestion in the rumen when the grain processing was not as extensive (PI ranged 82 to 65%) and the percentage of silage in the diet was 10% of dietary DM. Postruminal and total tract digestibility of NDF were similar (P > 0.10) across all dietary treatments, with total-tract NDF digestibility averaging 64.7%. Beauchemin et al. (2001) also reported no effect of grain processing (PI of 65 to 82%) on total-tract NDF digestibility, although the total-tract digestibility reported (53.4%) was lower than that observed in the present study.

There was a tendency (P = 0.088) for greater ADF digestion postruminally for the 20% barley silage diets when expressed as a percentage of ADF entering the intestine, but not when expressed as a percentage of intake (P > 0.10); although numerically, the percentage of ADF digested on the 20% barley silage diet was double the percentage digested on the 5% barley silage diet (19.5 vs. 8.5%). Grain processing had no effect (P > 0.05) on total-tract ADF digestibility when combined with 20% barley silage. However, more extensive grain processing reduced (P < 0.05) total-tract ADF digestibility when combined with 5% barley silage and was a reflection of the reduction in ruminal ADF digestibility. Zinn (1993) reported no effect of grain processing (90% coarse vs. fine steamrolled barley grain-based concentrate) on ADF digestibility in the rumen (21.3%) and total tract (40.7%). Based on these results, it would appear that it is only when barley grain is extensively processed (to a PI of 61%) and combined with barley silage at a level of 5% or less of dietary DM that ruminal fiber digestibility is impaired when cattle are fed typical barley-based finishing diets (with PI ranging from 61 to 86%, and silage at 5 to 20% of dietary DM).

Results of in vitro studies indicate that the optimal pH for cellulose digestion by major cellulolytic ruminal bacteria is near a pH of 6.5, and that ruminal fiber digestion decreases as pH of ruminal fluid declines, particularly when it decreases below 6.0 (Strobel and Russell, 1986). In the present experiment, the mean ruminal pH was less than 6.0 for all dietary treatments, and the amount of time that the pH was less than 5.8 averaged 10.5 h/d (Table 3). Grain processing did not affect any of the measures of ruminal pH (mean, minimum, maximum, time below a pH of 5.8 and 5.5, AUC for pH below 5.8 and 5.5, diurnal; P > 0.10; not all data presented). However, decreasing the silage from 20 to 5% tended (P = 0.071) to lower the mean ruminal pH (Table 3) and the diurnal ruminal pH for 11 of the 24 hourly time points (P < 0.05; Figure 1), although there was no effect of the percentage of silage on any of the other measures of pH (P > 0.10; not all data presented). These results may indicate that the low diurnal ruminal pH observed for the 5% silage-based diets contributed to the lower ruminal fiber digestibility when 5% silage was combined with the more extensively processed grain; however, when 5% silage was combined with the less extensively processed grain, there was no detrimental effect on fiber digestibility. Beauchemin et al. (2001) reported no effect of grain processing on ruminal pH and fiber digestibility in beef cattle, but as noted previously, the grain was not processed as extensively and silage comprised 10% of the diet DM. Yang et al. (2001), reported that in lactating dairy cattle, ruminal pH was lower for the more extensively processed grain (coarse vs. flat), but the amount of forage (forage to concentrate ratios of 35:65 and 55:65) and the forage particle length (7.6 and 6.1 mm) were without effect.

View larger version (17K): [in this window] [in a new window]   Figure 1. Diurnal ruminal pH in steers fed high-concentrate, barley grain-based diets with 20 and 5% barley silage. Arrows indicate the time of feed allocation. Each point represents the mean of four observations. The pooled SE was 0.14. *P < 0.05 and P < 0.10.

In some of the earlier literature, protozoa were reportedly absent or substantially reduced in cattle fed high-grain feedlot diets (Eadie et al., 1970). In the present experiment, ruminal ciliate protozoal numbers were relatively high, ranging from 0.81 to 1.4 million cells/mL for the high concentrate diets (80 to 95%), and are in agreement with recent studies where cattle adapted to high-barley grain diets harbored protozoal numbers ranging from 0.47 to 18 million cells/mL (Beauchemin et al., 2001; Hristov et al., 2001). Protozoal numbers were not affected (P > 0.10; Table 3) by the percentage of silage in the diet, but when the barley grain was more extensively processed, protozoal numbers were lowered by 43% (P = 0.032). Hironaka et al. (1979) reported a decrease in the number of ruminal protozoa in cattle as the feed particle size (80% barley grain, 10% oats) was reduced, which was coincident with a reduction in ruminal pH. In the present study, however, barley grain processing had no effect on the severity of low ruminal pH.

Low ruminal pH can also contribute to chronic or subclinical acidosis, resulting in reduced feed intake and animal performance. Acidosis is not easily detected because animals often show no overt signs of illness, but lactate and ruminal pH may be used as diagnostic indicators. Lactate accumulation is associated with acute acidosis following an abrupt increase in carbohydrate consumption, although it may not necessarily be present with chronic acidosis (Owens et al., 1998). Lactate can have an even greater effect than VFA on decreasing ruminal pH. Ruminal lactate concentration in samples analyzed from the first period was less than 0.25 mM (data not presented), and therefore, lactate analysis was not performed on samples collected in the second, third, and fourth periods. The absence of any appreciable quantity of lactate in ruminal fluid indicated that the populations of lactate utilizers and lactate producers were adequately balanced and that it was the high concentration of VFA in ruminal fluid that depressed the mean ruminal fluid pH to less than 6.0 for all dietary treatments.

Nitrogen Metabolism and Microbial Protein Synthesis
Nitrogen intake was about 13% lower when steers were fed the 20% barley silage diets than when they were fed the 5% silage diets (P = 0.016; Table 4), reflecting the lower CP concentration of the barley silage compared to the barley grain (10.4 vs. 13.3%; Table 1). Barley silage typically contains a greater proportion of ruminally degradable intake protein than barley grain, and therefore ruminal NH₃ nitrogen concentration was higher (P = 0.004; Table 3), despite a lower N intake, for the 20% silage-based diets.

The amount of microbial protein synthesized in the rumen is largely driven by the amount of energy derived from ruminal fermentation of carbohydrate, but is also influenced by ruminal pH and the bacterial requirements for N (Russell, 1998). In the present experiment, microbial N flow accounted for the majority of the N flowing to the duodenum. Because the amount of N flowing to the duodenun was lower (P = 0.023) for the 20% barley silage diets, microbial N flow expressed as a percentage of duodenal N flow was greater for the 20% silage diets (70 vs. 58%; P = 0.011). There was, however, no effect (P > 0.10) of the grain processing or level of silage on the amount of microbial N flowing to the duodenum (157 g N/d). Increasing the extent of grain processing (PI of 61%) to increase the rate and extent of ruminal carbohydrate fermentation increased the ruminal digestibility of the dietary starch, but it lowered the ruminal digestibility of fiber when combined with the 5% silage, such that there was no effect on ruminal OM digestibility, and in turn, no inhibitory effect on microbial N flow to the duodenum.

Beauchemin et al. (2001), reported a 50% increase in microbial N flow when moving from a coarsely rolled barley grain (PI of 82%) to a medium-rolled grain (PI of 75%), but there was little improvement from further processing (up to a PI of 65%). The lower microbial N flow with the coarsely processed grain was likely due to the numerically lower ruminal OM digestibility. Zinn (1993) reported no effect of barley grain processing (coarse vs. flat) on microbial N flow.

There was no effect of the level of silage on microbial N flow to the intestine (P > 0.10), but there was a tendency for improved efficiency of microbial protein synthesis (g of microbial N/kg of OM truly fermented in the rumen) for the 20% barley silage diets (P = 0.072). The efficiency of microbial protein synthesis is affected by several factors, including the maintenance energy requirements of the bacterial populations (Russell et al., 1992) and the turnover rate of the solid and liquid pools (Rode and Satter, 1988). Maintenance requirements are higher for bacteria that ferment nonstructural carbohydrates (starch, pectin, and sugars; Russell et al., 1992), and thus would be expected to be higher for the bacterial populations in steers fed the 5% silage diets with a greater amount of ruminal starch digestion. Maintenance requirements may also have been higher for the ruminal bacteria in steers fed the 5% silage diets to regulate intracellular pH with the lower ruminal pH. Increasing the proportion of forage in the diet may or may not affect the solids dilution rate, but a larger ruminal pool of fiber can increase the liquid dilution rate by stimulating rumination and salivation (Rode and Satter, 1988). Rate of passage was not measured in the present study, but there was no effect of the dietary treatments on ruminal volume or the size of the DM and liquid pools (P > 0.10). Nitrogen limitation, when there is a large pool of readily fermentable energy, can lead to energy spilling and a reduction in microbial efficiency. In the present experiment, dietary CP concentration ranged from 14.3 to 15%. A combination of both urea and canola meal were used as supplemental protein sources to provide ruminal ammonia, peptides, and amino acids, and therefore it is unlikely that microbial protein synthesis was limited by N supply.

The digestibility of the N entering the intestine tended (P = 0.079) to be lower for the 20% silage diets. The lower digestibility of the N entering the intestine combined with a lower amount of N flowing to the duodenum (P = 0.023) meant that the amount of N digested postruminally was also lower for the 20% silage diets (P = 0.014). Postruminal N digestibility as a percentage of N intake, however, was not affected by diet and averaged 76.7%.

Apparent total-tract digestibility values, as reported here, are somewhat misleading since the fecal N fraction also contains N of microbial origin that arises from fermentation in the lower tract. As within the rumen, the amount of microbial N synthesized within the lower tract is driven by the amount of fermentable energy reaching the lower tract. The source of N for microbial synthesis in the lower tract is largely derived from the recycling of urea N from the blood. Therefore, true total-tract digestibility requires estimation of the N incorporated into microbial protein in the lower tract; however, measurement of this fraction is not readily attainable. In the present experiment, there was no effect (P > 0.10) of grain processing on total-tract N digestibility, suggesting that although the degree of grain processing influenced the amount of starch flowing from the rumen to the intestines (P < 0.05; data not shown), the starch was further digested and absorbed from the small intestine with very little reaching the lower tract. Total-tract OM (80.6 vs. 83.0; P = 0.098) and N (77.5 vs. 80.9%; P = 0.03) digestibility were lower for the 20 vs. 5% barley silage diets. Lower total-tract OM digestibility may reflect a potentially larger pool of fermentable energy available for lower tract microbial growth. A reduction in total-tract N digestibility as a result of lower tract fermentation and microbial growth is not necessarily a negative attribute. Volatile fatty acids produced during lower tract fiber fermentation contribute to the energy status of the animal, although only marginally. The N incorporated into microbial protein is, however, unavailable and is excreted in the feces. Shifting the route of excretion of urea N in blood from urine to feces may, however, lessen the impact of nutrient excretion from feedlot cattle on the environment by reducing the loss of NH₃ nitrogen via volatilization and improving the N to P balance of manure for crop production.

	Implications

Top Abstract Introduction Materials and Methods Results and Discussion Implications Literature Cited

 
The objective of grain processing is to obtain a balance between maximizing the extent of ruminal starch fermentability and controlling the rate of starch fermentation to avoid digestive and metabolic disturbances. The optimal extent of grain processing is also influenced by the amount and physical form of the forage in the diet. Limiting the percentage of barley silage in a barley grain-based diet increases the amount of energy and protein available to the animal; however, feeding cattle diets containing only 5% barley silage with extensively processed barley grain may increase the risk of digestive disturbances and decreased feed intake. To optimize nutrient digestibility and protein supply, our results indicate that barley grain should be coarsely rolled (up to a processing index of 86%) when diets are low in effective fiber, but when feeding up to 20% barley silage with adequate effective fiber, the barley grain can be more extensively rolled (to a processing index of 61%).

	Footnotes

 
¹ Contribution No. 38702020 of the Lethbridge Research Centre.

² This study was financially supported by the Canada/Alberta Beef Industry Development Fund (Edmonton, AB) and the Canada/Alberta Livestock Research Trust (Lethbridge, AB). The authors thank K. Andrews, R. Wuerful, D. Vedres, and R. Cadieu-Browne for their assistance with sample procurement and laboratory analyses, and the staff of the Lethbridge Research Centre Metabolism Unit for care of the steers.

⁴ Current address: Rosebud Technology Development, Ltd., 3302 Beauvais Place S., Lethbridge, AB, T1K 3J5.

Received for publication February 18, 2002. Accepted for publication December 13, 2002.

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