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ANIMAL NUTRITION |
Division of Animal and Veterinary Sciences, West Virginia University, Morgantown 26505-6108
Abstract
To evaluate the influence of mass of ruminal contents on voluntary intake and ruminal function, five ruminally cannulated steers (550 kg) were fed an orchard grass hay diet ad libitum in a 5 x 5 Latin square experiment. The mass of ruminal contents was altered by adding varying weights of modified tennis balls to the rumen before the initiation of each 15-d experimental period. Treatments consisted of 50 balls with a specific gravity of 1.0, 1.1, 1.2, 1.3, or 1.4; the total weight of the balls was 7.45, 8.50, 9.25, 10.55, and 11.55 kg, respectively. Increasing the specific gravity of the balls added to the rumen decreased DMI and particle passage rate (P < 0.05) in a linear manner. A second experiment was conducted to evaluate the influence of mass of ruminal contents on voluntary intake and ruminal function of both forage and concentrate diets. Five ruminally cannulated steers (580 kg) were fed a 70% concentrate (DM basis) or an orchardgrass hay diet ad libitum in a 5 x 5 Latin square experiment. The mass of ruminal contents was altered as in the first experiment. Treatments consisted of 0 balls added to the rumen of steers fed concentrate diet (control), 75 balls with a specific gravity of 1.1 given to steers fed a concentrate diet, 75 balls with a specific gravity of 1.4 given to steers fed a concentrate diet, 75 balls with a specific gravity of 1.1 given to steers fed a hay diet, and 75 balls with a specific gravity of 1.4 given to steers fed hay diet. The addition of balls to the rumen of steers fed the concentrate diet decreased DMI (P < 0.05) compared with the 0-ball treatment, and increasing specific gravity of balls also decreased DMI (P < 0.01) for both concentrate and hay diets. Adding balls to the rumen of steers fed the concentrate diet decreased particle passage rate (P < 0.05), whereas increasing specific gravity of balls decreased particle passage rate for both concentrate and hay diet. The results of this study suggest that the density of ruminal digesta can have an influence on voluntary intake of both forage and concentrate diets.
Key Words: Beef Cattle • Concentrates • Forage • Voluntary Intake
Introduction
A decrease in DMI of ruminants has been reported by adding or by displacing ruminal space of animals fed forage diets with inert bulk (Anil et al. 1993
; Dado and Allen, 1995; Schettini et al., 1999). These results led to the conclusion that gastrointestinal fill is an important consideration in the voluntary intake of ruminants fed a forage diet. Contrary to these findings with forage diets, Loerch et al. (1991) found that feed intake was not changed for steers fed a 100% concentrate diet when ruminal volume was increased by adding an inert bulk.
The influence of ruminal fill on voluntary intake of ruminants has been fairly well documented; however, the influence of mass of ruminal contents on voluntary intake has not been studied extensively. Schettini et al. (1999) demonstrated that mass, in addition to the volume of the inert bulk added to the rumen, had significant effects on DMI of steers fed a forage diet. They reported that for each kilogram of weight added to the rumen of the steers, a depression of 112 g in DMI was observed. It was our hypothesis that as the weight of inert bulk added in the rumen increases, DMI will decrease, but to different degrees depending on whether it is a forage- or a concentrate-based diet. The objective of this study was to ascertain whether ruminal location of the added inert bulk influenced the results previously reported (Schettini et al., 1999). In addition, the effect of density of ruminal contents on DMI and other ruminal variables was examined in steers fed a high-concentrate diet.
Materials and Methods
Experiment 1: Animals, Treatments, Diet, and Experimental Design
Five steers of predominantly Angus breeding (Bos taurus) averaging 550 kg BW were used to determine the effect of weight of ruminal contents on DMI and digesta passage of a forage diet. The steers were surgically fitted with 10-cm (inner diameter) ruminal cannulas (Bar Diamond Inc., Purma, ID) approximately 6 mo before the initiation of the experiment. All surgical procedures and animal care were approved by the University Animal Care and Use Committee and followed procedures outlined in the Consortium (1988). The steers were injected i.m. with vitamins A, D, and E, and treated for internal parasites with Ivomec (MDS AG VET, Rahway, NJ) 1 wk before starting the experiment.
The experimental design was a 5 x 5 Latin square with five treatments consisting of inert bulk of five different masses with the same volume placed in the rumen. The five treatments consisted of placing 50 modified tennis balls, as described by Schettini et al., (1999) with an approximate specific gravity (SG) of 1.0, 1.1, 1.2, 1.3, or 1.4 into the rumen on d 1 of the experimental period. The actual weight of the 50 balls added to the rumen was 7.45 kg for 1.0 SG; 8.50 kg for 1.1 SG; 9.25 kg for 1.2 SG, 10.55 kg for 1.3 SG; and 11.55 kg for 1.4-SG balls. The total volume for the 50 balls was 7.25 L.
The diet consisted of a low-quality orchardgrass (Dactylis glomerata L.) hay (Table 1) harvested at full bloom and ground through a 3-cm screen in a hammer mill before feeding. The hay was offered ad libitum twice daily at 0700 and 1900 with a 10 to 15% refusal level throughout each experimental period. The steers were maintained in individual pens (3 x 3 m) in an environmentally controlled building with a room temperature of 15.5°C. Trace mineral salt (Table 1) and water were available at all times.
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Dry Matter Intake
The DMI of hay was determined over a 5-d period, and was sampled at feeding on d 8 through 13 of the trial period. Orts were weighed before each feeding on d 9 through 14, and a 10% sample was collected at each feeding schedule. Feed and orts samples were placed in forced-air ovens (65°C) immediately after collection, dried to a constant weight, and allowed to air equilibrate. Composite samples then were ground in a Wiley Mill (Thomas Scientific, Swedesboro, NJ) through a 1-mm screen, subsampled, and stored in plastic containers until they were analyzed. Analysis of these samples included DM, CP, (AOAC, 1990), ADF, and NDF using the method of Goering and Van Soest (1970) as modified by Van Soest et al. (1991). The difference between the amount of DM, CP, ADF, and NDF offered and that refused determined the daily intake of forage constituents.
Ruminal Kinetics of the Particulate Phase
To estimate particulate passage rate (PPR), portions of the dietary hay were labeled with a Yb marker according to procedures of Varga and Prigge (1982), with some modifications. The hay was soaked for 48 h in the Yb solution (2.5 g YbCl3•7 H2O), stirred every 12 h, and then washed once every hour for 6 h in deionized water, and subsequently dried in a forced-air oven at 65°C. Steers were dosed in the dorsal rumen with 100 g (DM basis) of Yb-labeled hay placed on top of the digesta mat immediately before the 0700-feeding schedule on d 14 of the trial. Ruminal digesta samples were collected from the dorsal and ventral rumen at specific locations using the technique described by Prigge et al. (1993) at 6, 12, 18, 24, 36, 48, 60, 72, 84, and 96 h after dosing. Digesta samples were dried in a forced-air oven at 65°C, and ground through a 1-mm screen. For determining Yb concentrations, samples were solubilized in diethylenetriaminepentaacetic acid prepared as described by Karimi et al. (1986), and held at room temperature overnight. Samples were filtered through No. 41 Whatman filter paper, and the solution was stored in screw-cap plastic conical tubes at 4°C until analyzed. Atomic absorption spectroscopy (model 5000, Perkin-Elmer, Norwalk, CT; nitrous oxide-acetylene flame) was used to determine Yb concentration of both the dosed hay and digesta samples.
The PPR from the rumen was estimated using the following equation:
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where t = time in hours relative to dosing, Y(t) = concentration of Yb in ruminal DM, A = Yb intercept at time zero, and k = fractional rate constant for disappearance of Yb-marked particles leaving the rumen. Ruminal DM pool was estimated by dividing the Yb doses by Yb concentration at time zero (A). Mean ruminal retention time was calculated as the reciprocal of the particulate fraction passage rate constant.
Ruminal Fill
On d 14 at 1900, the entire ruminal contents of the steers were emptied to determine the ruminal digesta weight. The balls were removed from the steers at the time of ruminal evacuation, after the approximate location (dorsal, ventral, and midsection of the reticulorumen) of the balls at each site was recorded on a schematic diagram of the rumen for each animal. Ruminal contents were evacuated by hand into two tared 200-L containers, one for predominantly fluid digesta and the other for predominantly solid. Ruminal contents in each container were mixed, weighed, and sampled for DM determination. The remaining digesta was then returned to the rumen of the same steer in preparation for the next experimental period.
Statistical Analyses
Intake variables, ruminal kinetics, and ruminal fill results were analyzed as a 5 x 5 Latin square design for linear, quadratic, and cubic effects. Location of the balls within each treatment was analyzed by the 
2 goodness-of-fit test, assuming an even distribution of the balls in the ruminal locations (dorsal, middle, and ventral). The 2 test of homogeneity was used to determine whether ruminal distribution of the balls differed among treatments. The GLM procedures of SAS (SAS Inst., Inc., Cary, NC) were used for Latin square analyses and FREQ procedures of SAS were used for the 2 analyses.
Experiment 2: Animals, Treatments, Diet, and Experimental Design
The influence of mass of ruminal contents in steers fed either a concentrate or a forage diet on DMI, digestion coefficients, digesta passage, and other variables related to ruminal function were assessed. Angus steers (580 kg BW) used in Exp. 1 were also used in Exp. 2. Animal care followed procedure as described in Exp. 1.
The experimental design was a 5 x 5 Latin square with a 2 x 2 factorial (two diet types and balls of two SG) arrangement of treatments and a control concentrate diet with no mass added. Diets consisted of hay only, or a 70% concentrate-based diet (Table 1
). The diets were offered ad libitum at 0700 and 1900, with a 10 to 15% refusal level. The hay was ground through a 3-cm screen in a hammer mill before feeding. The same hay also served as the roughage source for the concentrate diet. During the experiment, steers were maintained individually in pens and trace mineralized salt (Table 1) and water was available at all times as in Exp. 1. Each experimental period within the Latin square lasted 18 d, with a 10-d treatment adaptation followed by a sample collection period from d 11 to 18. Because of the length of the experiment, quality of the hay diet, and animal care concerns, a corn-silage based diet (11% CP) was fed to each steer between experimental periods (7 to 10 d). Experimental treatments consisted of 0 balls (control) given to steers fed a concentrate diet, 75 balls of approximately 1.1 SG given to steers fed a concentrate diet, 75 balls of approximately 1.4 SG given to steers fed a concentrate diet, 75 balls of approximately 1.1 SG given to steers fed a hay diet, and 75 balls of approximately 1.4 SG given to steers fed a hay diet. The total volume occupied by the 75 balls was 10.8 L. The actual weight of the balls was 12.75 kg for 1.1-SG balls and 17.46 kg for 1.4-SG balls. The balls were removed from the rumen via the fistula at the end of the last day of each experimental period. Ruminal contents were evacuated, analyzed and placed back into the rumen as described for Exp. 1.
Dry Matter Intake and Digestibility
The feeding procedure was the same as in Exp. 1. Apparent digestibility coefficients for DM, NDF, ADF, and CP were estimated by a 5-d total collection using fecal collection bags. Collection bags were exchanged and feces weighed and sampled (10% of the total) twice daily at 0700 and 1900 on d 10 through 15 of the experimental period. Feed, orts, and fecal samples were placed in forced-air ovens (65°C) immediately after collection, dried to a constant weight, and allowed to air equilibrate. Composite samples then were ground in a Wiley Mill (Thomas Scientific, Swedesboro, NJ) through a 1-mm screen, subsampled, and stored in plastic containers until analyzed. Samples were analyzed as described in Exp. 1.
Particulate, Fluid Passage, and Fill of Ruminal Contents
Ruminal kinetics of particulate phase were determined using the same procedure as those described in Exp. 1. Digesta sample collections for these analyses were on d 13 to 17 of the experimental period.
Ruminal fluid kinetics were estimated using cobalt EDTA by dosing 25 g of CoEDTA in 500 mL (Uden et. al., 1980) of deionized water immediately before feeding (0700) in the ventral rumen on d 13 of the trial. Fluid samples were collected as described by Prigge et al. (1993) at 6, 9, 12, 18, 24, and 48 h after dosing. Samples were strained through four layers of cheesecloth, and the fluid fractions were stored at –1°C, thawed, and centrifuged at 10,000 x g for 15 min to remove sediment before analyses. Cobalt was measured by atomic absorption spectroscopy (model 5000, Perkin-Elmer, Norwalk, CT). Ruminal fluid dilution rate was estimated using the same equation as that used for PPR, where t = time in hours relative to dosing, Y(t) = concentration of Co in ruminal fluid, A = Co concentration at time zero, and k = fractional rate constant for disappearance of Co from ruminal fluid.
Particle Size of Ruminal Digesta and Feces
Particle sizes of ruminal digesta and feces were determined using the wet-sieving technique and sampling procedures described by Neel Stetter et al. (1995). Ruminal digesta samples were collected on d 17 at specific locations, at 3, 7, and 12 h after feeding, as reported by Prigge et al. (1993). For fecal particle size determination, the composite sample of feces collected for digestibility measurements was subsampled. Geometric mean diameter (GMD) was calculated for particles as described by Ensor et al. (1970) and modified by Neel Stetter et al. (1995), using the average size of the successive screens. Six nested sieves with a 5-cm diameter and pore sizes of 1.905, 1.130, 0.447, 0.250, and 0.104 mm were used.
Functional Specific Gravity of Ruminal Digesta
Samples for measuring FSG of ruminal digesta from the dorsal part of the rumen (anterior dorsal sac, caudodorsal blind sac) and the ventral part of the rumen (anterior ventral sac, caudo-ventral sac, and reticulum) were collected at the same times as ruminal particle size samples. Two 2-L separatory vessels, as described by Hooper and Welch (1985), were used to isolate the SG fraction. Vessels were filled with one of the two SG solutions (1.1 and 1.4) were made using deionized water and appropriate amounts of CaCl2. Immediately after collection, duplicate samples of wet ruminal digesta (50 g) from each location were placed into each of the separatory vessels, stirred for 10 s, and allowed to settle for 2 min. Digesta in the upper and lower portions of the separatory vessel were collected in separate 500-mL plastic containers, as described by Neel Stetter et al. (1995), and stored under (5°C) refrigeration until filtered. Each sample was poured under vacuum into a tared filter made of polyvinyl chloride reducing pipe fitted with a 53-µm screen to separate solid from soluble matter. After rinsing three times with 100 mL of distilled water, the filters and contents were dried and weighed. The percentage of ruminal DM lighter or heavier than the respective specific gravity solution of the sample was calculated.
Ruminal pH, Osmolality, and NH3-N and VFA Concentrations
Ruminal pH, osmolality, and concentrations of NH3-N and VFA were determined in strained ruminal samples collected for the fluid dilution determinations on d 13 at 3, 6, 9, and 12 h after feeding. Ruminal NH3-N samples (50 mL) were preserved by adding 1 mL of a 50% H2SO4 solution. Ruminal NH3-N was determined according to AOAC (1990) procedures using automated Kjeldahl equipment (Tecator Inc., Herndon, VA). A pH meter (Corning 130; Corning Sciences Products, Medfield, MD) with a combination electrode was used to determine ruminal pH. Osmolality was determined by a freezing-point technique (Osmette A, automatic osmometer, Precision Systems, Inc., Natick, MA). Samples for VFA were prepared and analyzed as described by Supelco (1975). Concentrations of VFA were determined using a gas chromatograph (3300 Varian gas chromatograph, Walnut Creek, CA).
Statistical Analyses
Intake, digestibility coefficients, and ruminal kinetic results were analyzed as a 5 x 5 Latin square design with a control and a 2 x 2 factorial arrangement of treatments. Treatment contrasts included control vs. balls within the concentrate diet, concentrate vs. hay diet, and SG of the balls (1.1 vs. 1.4) and the interaction of diet x SG of the balls. Changes in ruminal particle size and FSG were analyzed in a split-split-plot design within a Latin square with time (3, 7, and 12 h) effect as the split-plot and ruminal location (dorsal and ventral) effect as the split-split-plot. Ruminal fermentation variables were analyzed as a split-plot within a Latin square with time effect considered as the split-plot. Time after feeding for the fermentation variables was analyzed for linear, quadratic, and cubic effects, as well as interactions. The GLM procedures of SAS were used for all analyses.
Results and Discussion
Experiment 1: Forage Composition, Intake, and Rate of Passage
The relatively low CP (7.0%) and high NDF (70.3%) and ADF (40.12%) components of the hay fed in this experiment are indications of a forage considered to be of low quality.
As SG of the balls added to the rumen increased from 1.0 to 1.4, a linear decrease in DMI, NDFI, and ADFI (P < 0.05; Table 2) was evident. Regression analyses were conducted to estimate these intakes and resulted in the following regression equations:
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Digesta PPR from the rumen of steers decreased linearly (P < 0.05) when mass added to the rumen increased (Table 3), which resulted in the following linear equation: PPR = 3.47 – 0.18SG; r2 = 0.65; RMSE = 0.037, P = 0.060. The slower PPR for steers with increasing mass added to the rumen may be related to the lower DMI observed for those treatments. In a previous study (Schettini et al., 1999), the rate of ruminal digesta DM passage was unaffected by increasing SG of the mass added to the rumen of steers fed a similar quality hay diet as in this study. However, Okine et al. (1989) observed an increase in PPR in restricted forage-fed steers when weight was added to the rumen.
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), the location of the balls in the rumen appeared to have been affected by their SG. The light balls were evenly distributed in the rumen, whereas the heavier balls were primarily located in the ventral part of the rumen and reticulum. It could be suggested that the depression in DMI was due to the ruminal location of the balls rather than due to the overall mass of the ruminal contents. In the current study, the ruminal distribution of balls with SG 1.0 in the dorsal, middle, and ventral site (36, 39, and 24%, respectively) was not different (P < 0.10) between ruminal sites. However, the mean ruminal distribution of the balls ranging from 1.1 to 1.4 SG was found in the middle (14%) and ventral part of the reticulorumen (86%), showing a difference (P < 0.001) in regard to their location from the 1.0-SG balls. However, the distribution of the 1.1, 1.2, 1.3, and 1.4 SG balls did not differ (P < 0.10) in ruminal location. Because of the linear effect of the balls’ SG on DMI and PPR and the similar distribution in the rumen of the 1.1 to 1.4 SG balls, it seems that SG, and not ruminal location of the balls, was the major factor in influencing these variables.
In the present study, the ruminal digesta DM pool was not influenced by treatments (P > 0.10; Table 3). Okine et al. (1989) demonstrated that the ruminal particulate pool was reduced when weight was added to the rumen of restricted-fed steers. Because of the lack of response in our current and previous studies (Schettini et al., 1999) on the ruminal digesta DM pool, it is suggested that passage rate compensated for changes in intake to maintain a constant fill. Schettini et al. (1999) found no effect of mass on passage although DMI was inhibited. They suggested that passage may be a function of DMI and not vice versa, and that mass added to the rumen could have a direct effect on intake independent of fill. This was reflected by the different amounts of DMI (112 vs. 157 g) that were shown to be depressed for each kilogram of weight or liter of volume added to the rumen of the steers, respectively. Thus, it is suggested that fill (volume) and mass (weight) independently affect DMI.
Experiment 2: Diet Composition
Composition of both the hay and the concentrate diet is reported in Table 1. The forage quality was similar to that of the hay used in Exp. 1 (Table 1). The concentrate diet was formulated (DM basis) for 10.5% CP, 0.55 Mcal/kg of NEg, 0.2% P, and 0.32% Ca, and was 30% hay and 70% concentrate, as shown in Table 1.
Dry Matter Intake
Dry matter intake was 58.7% lower (P < 0.01) for steers fed the hay diet (Table 4) than for those fed the concentrate diet. Increasing mass from 1.1 to 1.4 SG in the rumen of steers fed the concentrate and hay diets also decreased (P < 0.01; Table 4) DMI. An interesting observation for the concentrate diet regarding DMI was that the intakes for control and 1.1 SG ball treatments, which added 10.8 L of volume to the rumen, were similar (15.12 vs. 14.95 kg/d). This may suggest that the weight of digesta may have more impact on fill when concentrate diets are fed to ruminants. This supports the suggestion that the intake of cattle receiving a high-concentrate diet is not under the control of physical constraints influencing ruminal fill (Johnson and Combs, 1992; Van Soest, 1994; Dado and Allen, 1995). However, based on these results, it is possible that digesta density could be a major physical factor limiting intake of high-concentrate diets.
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) for animals on 1.1- vs. 1.4-SG ball treatments was decreased by 936 g of DM/kg of added weight to the rumen for the steers fed the concentrate diet and 215 g of DM/kg of added weight for steers fed the hay diet. These results also suggest that inert material added may be a strong physical inhibitor of intake on both hay and concentrate diets, although intake depression was somewhat less for the hay diet. Based on the literature, volume of digesta may be a more important inhibitor of intake only with high-fiber diets, for which periods of distention may be greater. As fiber increases in the diet, less energy is provided, so the animal eats more to compensate for the lack of energy. Tension receptors responsible for detecting distension in the rumen are located in the muscle layers of the rumen and in the visceral muscles that lie in series with the smooth muscle cells. Receptor activity is increased by heightened tension developed actively or passively in the muscle (Iggo, 1995).
Digestibility
Digestibility coefficients for DM, NDF, ADF, and CP (Table 4) were greater (P < 0.01) for the concentrate than hay diet as expected. Increasing mass of inert material in the rumen of steers fed either diet did not affect digestibility coefficients. Our results agree with Okine et al. (1989) and Schettini et al. (1999), who found that increasing mass in the rumen of steers fed a hay diet did not change digestibility coefficients. Nevertheless, Waybright and Varga (1991) observed that digestibility of OM, DM, ADF, and starch was reduced in wethers fed a 75% concentrate diet when an inert bulk (bladders) of a constant mass was added in the rumen.
Passage Kinetics of Ruminal Digesta and Fluid, and DM Ruminal Pool
Ruminal PPR was greater for steers fed the concentrate diet compared with the hay diet (P < 0.01; Table 5). Johnson and Combs (1991) suggested that increased PPR is most likely to occur when high-concentrate diets are fed, which require little particle size reduction. Addition of balls into the rumen of steers fed the concentrate diet (P < 0.05) and the increase in SG of the balls decreased PPR (P < 0.05) when compared with control steers (Table 5). Our results suggest that density of ruminal contents had a larger impact on rate of particulate passage for the concentrate compared to the hay diet. We observed a 31% decrease in PPR as SG of added balls increased from 1.1 to 1.4 for the concentrate diets vs. 8% difference in PPR when increasing SG of the balls for the hay diet. The decrease in PPR with increasing mass added into the rumen in our study may be a function of DMI, which also decreased when mass in the rumen of steers was increased. The lack of effect of the addition of mass on digestibility coefficients in spite of difference in PPR within the concentrate diet treatments might be explained by postruminal compensatory digestion, which can be significant when digestion in the reticulorumen is incomplete (Hoover, 1978). Passage rate of digesta was increased in cows and sheep fed concentrate diets when volume of ruminal contents was increased by addition of water-filled bladders (Waybright and Varga, 1991; Johnson and Comb, 1992; Dado and Allen, 1995). Okine et al. (1989) indicated that the addition of weight to the rumen of steers limit-fed a forage diet resulted in an increase in ruminal digesta passage, whereas no effect on rate of passage was found when weight was added into the rumen of steers fed ad libitum a hay diet (Schettini et al., 1999).
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). However, contradictory results have been reported (Owens and Goetsch, 1986) in which LDR was greater in cattle fed a high content of roughage in a diet as opposed to concentrate, reflecting an increasing stimulation of saliva secretion and mastication for the forage diets. In our study, the steers on the 1.1-SG treatments had an 8% greater (P < 0.01) LDR than the steers on the 1.4-SG treatments for both diets. Mass added to the rumen of steers appeared to have a greater influence on the change in LDR than did diet type. It has been reported that ruminal LDR was not affected by inserting inert bulk in the rumen of steers fed either a concentrate or hay diet ad libitum (Loerch, 1991; Schettini et al., 1999). However, LDR was linearly increased in sheep and cows fed a concentrate or mixed forage-concentrate diet when water-filled bladders were added to the rumen replacing between 25 to 66% of the ruminal volume (Johnson and Combs, 1991; Waybright and Varga, 1991).
Digesta DM pool size (Table 5) was measured using two methods: Yb marker concentration and ruminal emptying. Numerically, the results of both methods seemed to follow the same trend. However, the significance level for the DM pools was different between methods. Ruminal DM pool was greater for steers fed the concentrate diet when measured by both methods (P < 0.01), and was greater (P < 0.01) for control vs. balls within concentrate diet by the Yb method. The increased DMI for the concentrate diet treatments would be expected to contribute to the greater ruminal DM pool observed for the control treatment.
Okine et al. (1989) found that ruminal DM pool decreased in steers limit-fed a forage diet when mass (24 kg) was added to the rumen. An increase in ruminal DM pool was observed in wethers and cows fed concentrate diets (75 to 100% concentrate) with the addition of an inert volume into the rumen (Loerch, 1991; Waybright and Varga, 1991). However, other researchers (Johnson and Combs, 1991, 1992; Dado and Allen, 1995) showed a decrease in ruminal DM digesta pool in cows fed a mixed diet (60 to 65% concentrate) when an inert volume was added to the rumen.
Ruminal and Fecal Particle Size
The geometric mean diameter of ruminal digesta particles was not affected by either hour postfeeding or ruminal location (P < 0.10). The digesta particle GMD size was greater for the concentrate diet vs. the hay diet (P < 0.05; Table 6). For steers fed the concentrate diet, the GMD of the ruminal digesta particle size increased (P < 0.05) as mass was added to the rumen. This result is most likely related to the slower rate of passage for these treatments and could be the result of less chewing. Interestingly, the greater ruminal DM pool for the control steers on the concentrate diet may stimulate rumination or enhance chewing during eating, and as a consequence, may have reduced the GMD of ruminal digesta particle size compared to steers with balls. Rumination is substantially reduced when cattle are fed a concentrate as opposed to a forage diet (Van Soest, 1994); however, no observations were made in regard to rumination or chewing in this study. The proportion of large GMD ruminal digesta particles was found to increase as DMI increased in cattle fed a forage or a mixed diet (55% concentrate) (Luginbuhl et al., 1990; Okine and Mathison, 1991b; Kovacs et al. 1997).
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). Other studies (Bae et al., 1980; Luginbuhl et al., 1990; Kovacs et al., 1997) indicated no difference in GMD of fecal particle size among cows fed hay or concentrate diets. When steers were fed a forage diet at different levels of intake, the GMD of fecal particles increased linearly (Luginbuhl et al., 1990; Okine and Mathison, 1991a) as intake increased. Increased proportion of large fecal particles at high intake was reported by Shaver et al. (1988), and it appeared to be related to a reduction in chewing per unit of feed consumed. Okine et al. (1990) demonstrated that the proportion of large fecal particles increased linearly with increasing weight (0, 9, 18 kg) placed in the rumen. Fecal particle size was greater when weight of ruminal contents was increased by addition of inert bulk to the rumen of steers fed a forage diet (Schettini et al., 1999). In our study, there was a tendency (P < 0.10) to increase fecal particle size for the concentrate diet in contrast to (P < 0.10) a decrease in fecal particle size for the hay diets as mass was increased in the rumen.
Functional Specific Gravity of Ruminal Digesta
No time after feeding x treatment interaction (P > 0.10) for the proportion of particles less than 1.1 functional SG (FSG) in the rumen was detected (Table 7). The proportion of particles less than 1.1 FSG was affected by diet (P < 0.01), with a greater proportion of these particles present in the rumen for the concentrate as opposed to the hay diet at all times after feeding (Table 7). Time after feeding did not influence (P > 0.10) the proportion of the light particles (< 1.1 SG) in the rumen. However, the proportion of light density particles increased markedly (P < 0.01) when mass was added to the rumen of steers fed the concentrate diet. The greater proportion of lighter particles for the ball treatments in the concentrate diet may indicate less chewing for these treatments. There was a ruminal location x diet interaction for the proportion of lighter particles in the hay diet (P < 0.05; Table 8). The proportion of lighter particles in the hay diet treatments was greater in the ventral site of the rumen than the dorsal site. Within the concentrate diet treatments, the proportion of light particles was greater (P < 0.01) for ball treatments vs. control in both the dorsal and ventral rumen. No difference (P > 0.10) was detected in the concentration of light particles for concentrate diet when mass was increased in the rumen by the addition of the 1.1 SG as opposed to the 1.4 SG balls. In agreement with these results, Schettini et al. (1999) found that the ruminal location of digesta with a FSG less than 1.1 was not affected when balls (1.1 and 1.3 SG) were added to the rumen of steers fed an all-forage diet.
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). An effect of time postfeeding was observed on both hay and concentrate diets in particles greater than 1.4 SG, which increased with time post feeding (P < 0.05) (data not shown). The proportion of heavier particles (> 1.4) was greater (P < 0.05) for hay diet compared with the concentrate diet at all times after feeding. In addition, a greater (P < 0.05) proportion of heavier particles in the rumen was observed at h 0 (collected immediately before feeds) compared with h 3 and 7 (data not shown), and for hay compared with concentrate diets. These results may be due to the slower change in FSG for hay diet particles compared with concentrate diet particles (Uden, 1992). The FSG of concentrate diet particles that enter the rumen already have a FSG closer to that required to exit the rumen compared with the hay diet (Siciliano-Jones and Murphy, 1991), and this may have contributed to a greater removal rate of the heavier particles on the concentrate diet.
Ruminal PH, Osmolarity, and Concentrations of VFA and NH3-N
Means of all sampling hours for osmolarity, ruminal pH and NH3-N are reported in Table 9. These variables were influenced by time after feeding (P < 0.01) as expected; however, no time x treatment interaction (P > 0.10) was observed, and therefore, time effects are not reported. Ruminal osmolarity was influenced by diet (P < 0.01) and was greater for the concentrate diet vs. hay diet treatment. Osmolarity of ruminal contents has been reported to increase from a prefeeding level of 250 to 300 mOsm/kg to as much as 500 mOsm/kg within a few hours after a large meal (Van Soest, 1994). When ruminal osmolarity was elevated to above 400 mOsm/kg, feed intake was depressed (Bergen, 1972). In our experiment, osmolarity averaged 300 mOsm/kg on the concentrate diet. The peak value was 366 mOsm/kg for the heavier balls within the concentrate diet. Johnson and Combs (1991) found that osmolarity was reduced in cows fed a mixed diet (60% concentrate) when water bladders were added in the rumen as an inert bulk. In our experiment, no effect of increasing mass added to the rumen on osmolarity was detected.
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) was influenced by diet (P < 0.01) and was lower for the concentrate vs. hay diet as expected, which could be explained by the expected fermentative characteristics of concentrate diets. A decrease in pH (P < 0.01) was observed as mass added to the rumen increased on the concentrate diet. Johnson and Combs (1991, 1992) in lactating cows and Waybright and Varga (1991) in whethers showed an increase in ruminal pH in those animals fed a concentrate diet when an inert bulk was added in the rumen. However, Dado and Allen (1995) found no difference in ruminal pH levels when an inert bulk was added into the rumen of cows fed a mix diet (60% concentrate). Schettini et al. (1999) reported that weight of mass added (1.1- and 1.3-SG balls) to the rumen of steers fed an all forage diet did not influence ruminal pH. From the literature, no clear effects of mass of ruminal contents on ruminal pH can be established.
There was a diet effect on ruminal NH3-N (P < 0.01) and levels were greater for concentrate vs. hay diet. This was most likely influenced by a greater CP concentration for the concentrate vs. the hay diets and a decreasing rate of absorption of ammonia through the ruminal walls as a result of a lower pH (ionized) (Owens and Zinn, 1988). Ruminal NH3-N (Table 9) decreased (P < 0.01) with increasing mass of ruminal contents for steers fed the concentrate diets (P < 0.01). Waybright and Varga (1991) observed an increase in ruminal NH3-N concentration in wethers fed a 70% concentrate diet when an inert bulk was inserted in the rumen. In our previous study (Schettini et al., 1999), we reported that the increasing mass added to the rumen of steers fed a forage diet did not influence ruminal NH3-N. In this study, the lower concentration of ruminal NH3-N for the heavier ball treatments in the concentrate diets may be related to the lower intake, smaller particle size reduction, and consequently lower extent of protein solubilization, resulting in less accumulation. Perhaps for those treatments, the smaller DM pool could also indicate a smaller value for ruminal NH3-N concentration.
There was a difference between diets (Table 10; P < 0.01) for the total VFA concentration, which was greater for the concentrate vs. hay diet and was expected since concentrate diets are more rapidly and extensively fermented in the rumen than the forage diet. A diet x SG of the balls interaction (P < 0.05) was observed for total ruminal VFA, which was greater for 1.1- vs. 1.4-SG treatment for the concentrate diet and essentially the same for the 1.4- and the 1.1-SG treatments in the hay diet (Table 10). The greater value observed for the 1.1-SG treatment for the concentrate diet may be related to the slower PPR and LDR assuming that ruminal VFA absorption across the ruminal wall was constant for these treatments. The slower PPR allows further fermentation of digesta and a slower fluid passage would result in less flow of VFA to the abomasum. Also, the lower intake and the greater ruminal particle size would suggest a lesser degree of fermentation for the 1.4- vs. 1.1-SG treatments. Previously (Schettini et al., 1999) it was shown that an increase in specific gravity of mass added to the rumen of steers fed a forage diet reduced total ruminal concentration of VFA. Waybright and Varga (1991) and Dado and Allen (1995) found no difference in VFA concentration when bulk was added into the rumen of cows or sheep fed a mixed diet (60 to 75% concentrate).
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). The molar proportion of propionic acid was also affected by diet (P < 0.05) and was greater for concentrate vs. hay diet, again as expected. Within the concentrate diet treatments, the proportion of propionate was greater (P < 0.05) for control vs. ball treatments, possibly as a result of the greater DM digesta pool for that treatment. Johnson and Combs (1991) found that the molar proportion of acetate increased and propionate decreased when bulk was added to the rumen of lactating cows fed a mixed-forage concentrate diet. There was a diet x SG interaction for the molar proportion of butyric acid (P < 0.05), which was greater for the concentrate diet than for the hay diet. The molar proportion of butyric acid was also greater for the 1.1-SG treatment for the concentrate diet, but was greater for the 1.4-SG treatment in the hay diet. Implications
The mass of ruminal contents can have a significant effect on voluntary intake of ruminants consuming both concentrate and forage diets. Ruminal mass could be an important physical factor not previously considered to affect dry matter intake of concentrate diets, which suggests that high-density feeds could have a negative effect on animal performance. Thus, by altering the diet density by formulation or processing, greater intake and performance could possibly be achieved by cattle on high-concentrate diets.
Footnotes
1 Published with the approval of the Director of the West Virginia Agric. and Forest Exp. Stn. as Scientific Paper No. 2870, Div. of Anim. and Vet. Sci. 
2 Correspondence—phone: 304-293-6131, ext. 4208; fax: 304-293-2232; e-mail: marcela.whetsell{at}mail.wvu.edu.
Received for publication January 16, 2003. Accepted for publication February 4, 2004.
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