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Effect of various levels of imbalance between energy and nitrogen release in the rumen on microbial protein synthesis and nitrogen metabolism in growing double-muscled Belgian Blue bulls fed a corn silage-based diet¹

Faculté universitaire des Sciences agronomiques, Passage des Déportés, 2 B-5030 Gembloux, Belgium

	Abstract

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Seven double-muscled Belgian Blue bulls (initial BW: 341 ± 21 kg) with cannulas in the rumen and proximal duodenum were used in an incomplete replicated Latin square. The study examined the effect of imbalance between energy and N in the rumen on microbial protein synthesis and N metabolism by giving the same diet according to 3 different feeding patterns. The feed ingredients of the diet were separated into 2 groups supplying the same amount of fermentable OM (FOM) but characterized by different levels of ruminally degradable N (RDN). The first group primarily provided energy for the ruminal microbes (12.5 g of RDN/kg of FOM), whereas the second provided greater N (33.3 g of RDN/kg of FOM). These 2 groups were fed to the bulls in different combinations with the aim of creating 3 levels of imbalance (0, 20, and 40 g/ kg of DM) between energy and N supplies in the rumen. Imbalance was measured by the variation of the degradable protein balance (OEB value in the Dutch system) of the diet between the 2 meals each a day. Diurnal variations in ruminal NH₃-N concentrations and plasma urea concentrations were greatly influenced by the feeding patterns of the diet. Introduction of imbalance affected neither microbial N flow at the duodenum (P = 0.97) nor efficiency of growth (P = 0.54). The feeding patterns of the diet had no negative impact on NDF degradation in the rumen (P = 0.33). Nitrogen retention was not affected by imbalance (P = 0.74) and reached 49.7, 52.0, and 51.3 g/d, respectively for 0, 20, and 40 g of OEB/kg of DM imbalance. It seems that introduction of an imbalance between energy and N supplies for the ruminal microbes by altering the feeding pattern of the same diet does not negatively influence the microbial activity in the rumen nor N retention of the animal. Nitrogen recycling in the rumen plays a major role in regulating the amount of ruminally available N and allows a continuous synchronization of N and energy-yielding substrates for the microorganisms in the rumen. Therefore, imbalance between dietary energy and N created over a 24-h interval was not detrimental to rumen microbial growth for the animal as long as the level of imbalance did not exceed 40 g of OEB/kg of DM. Thus, these feeding patterns of the diet can be used under practical feeding conditions with minimal impact on the performance of ruminant animals for meat production.

Key Words: energy • imbalance • nitrogen • nitrogen balance • rumen fermentation

	INTRODUCTION

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Recent efforts to enhance productive performance of ruminants through synchronization of carbohydrate and N to improve fermentation in the rumen did not result in detectable benefits for the animals (Richardson et al., 2003). Furthermore, the introduction of a 12-or 24-h imbalance between N and energy supplies in the rumen did not negatively influence microbial protein synthesis nor N use in sheep (Ludden et al., 2002a,b) or growing bulls (Valkeners et al., 2004). Both the ruminal microbes and their host animal possess the means to compensate for variations in kinetics of feed degradation to ensure a relatively continuous supply of nutrients to places of biosynthesis to optimize microbial growth (Dawson, 1999).

The movements of N across the gut play a major role in regulating the amount of ruminally available N, and provide overall plasticity to allow a rapid response to any changes in metabolic status (Lapierre and Lobley, 2001). During periods of N excess in the rumen, NH₃ is absorbed in portal blood circulation and detoxified to urea by the liver. This N may then be recycled back to the rumen during subsequent periods of N shortage. Nevertheless, N recycling probably has limitations dependent on the time and the level of the imbalance created between energy and N release in the rumen and cannot totally buffer important temporary N excess.

The objective of the current study was to examine the effect of the level of imbalance between energy and N release in the rumen on digestion and N metabolism of double-muscled Belgian Blue bulls. This was achieved by feeding the same diet according to different feeding patterns, because it has been suggested that differences due to imbalance may be attributed to some factors associated with the manipulation of the level or type of ingredients (Dewhurst et al., 2000). Therefore, increasing levels of imbalance between ruminal supplies of energy and N were tested.

	MATERIALS AND METHODS

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Animals

Seven double-muscled Belgian Blue bulls (initial BW = 341 ± 21 kg; final BW = 480 ± 32 kg), fitted with a ruminal cannula (67 mm i.d.) and a T-type cannula at the proximal duodenum, were individually penned (1.5 x 2.5 m). Fresh water was available at all times. The experimental protocol was approved by the Gembloux Agricultural University Animal Care and Use Committee, and followed the guidelines issued by the committee on care, handling, and sampling of the animals.

Diet and Feeding Management

Bulls received the same diet according to 3 different feeding patterns at an intake level of 76 g of DM/kg^0.75. Offering feed at this level was roughly 97.5% of ad libitum consumption (Fiems et al., 1999). Adaptation to the diet lasted 20 d. Feed was offered twice a day in equal amounts at 0830 and 2030. The diet was made up of 53% concentrate and 47% corn silage on a DM basis. The diet supplied 83 g of intestinal digestible proteins (DVE) and 7.5 MJ of NE for fattening (NEF) per kg of DM according to the Dutch system (Van Es and Van der Honing, 1977; Tamminga et al., 1994). The degradable protein balance (OEB value in the Dutch system) of the diet amounted to 0.4 g/kg of DM and was close to zero, which corresponded to the optimum OEB value according to the Dutch system. This OEB value is equivalent to a ratio between ruminally degradable N (RDN) and fermentable OM (FOM) of 24 g/kg, which is close to the optimal ratio of 25 g of N/kg of OM truly digested in the rumen proposed by Czerwaski (1986). The OEB value estimates the (im)balance between ruminal degradable CP and energy extracted during anaerobic fermentation in the rumen for microbial protein synthesis (Tamminga et al., 1994). When the OEB value is positive for a diet, potential loss of N from the rumen occurs, whereas a negative value means a shortage of ruminally degradable N with a consequent impaired microbial activity.

The feed ingredients of the diet were separated in 2 groups supplying the same amounts of DVE, NEF, and FOM, but with different amount of RDN. Ingredient composition, actual chemical composition, and tabular nutritive value (CVB, 2000) of the 2 groups of feeds are presented in Table 1. No forage was fed separately. The first group (HRDN) provided all the major protein sources (coarsely ground peas, coarsely ground lupins, and urea) of the experimental diet, with a tabular OEB value of 40.2 g/kg of DM, which corresponded to an RDN:FOM ratio of 33.3 g/kg. This part of the diet supplied RDN in excess in comparison with the available FOM to the ruminal microbes. The second group (LRDN) was characterized by an OEB value of –39.4 g/kg of DM, which corresponded to an RDN:FOM ratio of 12.5 g/kg. According to the Dutch system, this part of the diet had a low RDN content relative to the available FOM.

Digestibility was measured using Cr₂O₃ as an indigestible flow marker. From 5 d before sample collection and continually thereafter, 3.5 g of Cr₂O₃/kg of DMI was administered to each bull twice daily (just before each meal) intraruminally via gelatin capsules (Milton et al., 1997). Bulls were weighed at the beginning of each treatment period before the 0830 feeding and feed offered was adjusted to the metabolic weight of each animal and fixed during the period.

Experimental Protocol and Sample Collection

The 7 bulls were allocated to 3 treatment periods in an incomplete replicated Latin square, separated by a 20-d rest period. Each treatment period lasted 18 d and included collection of digesta (duodenal and fecal), urine, blood, and ruminal fluid. Refusals were collected daily before the morning feeding, weighed, and dried at 60°C. Samples of each group of feed ingredients were obtained each time LRDN and HRDN were mixed, ground to pass a 1-mm screen (Cyclotec 1.093, Foss Tecator AB, Höganäs, Sweden), and pooled by period on an equal weight basis.

Urine was collected during the first 10 d of each treatment period with an adaptation of the apparatus of Veenhuizen et al. (1984). During collection, urine was acidified with 4 N H₂SO₄ to pH 3 at 0900, 1700, and 2100 to avoid N losses. Total urine collection was weighed daily at 0900, sampled, and frozen at –20°C. The majority of the fecal output was collected simultaneously into containers placed underneath each animal, sampled daily at 0830, and frozen at –20°C.

Samples of duodenal digesta were collected on d 11 to 13 to cover a 24-h period in 1.5-h steps. After removal of the cap of the duodenal cannula, accumulated digesta were discarded, and when the flow seemed normal, 400 mL of duodenal contents was collected. For each collection, a 100-g sample of duodenal contents was immediately used to extract liquid-associated bacteria according to the method described by Poncet and Rémond (2002). Moreover, the solid-adherent bacteria were extracted according to the method developed by Legay-Carmier and Bauchart (1989) for rumen samples. The remainder of each duodenal sample was frozen at –20°C.

On d 14, central venous catheters (polyurethane aliphatic, No. 67262J17, Plastimed, Le Plessis Bouchard, France) were inserted into one of the external jugular veins of each bull after an intravenous injection of analgesic (xylazine 2%, 0.08 mL/100 kg of BW). On d 15, blood samples (20 mL) were slowly collected into 2 heparinized (143 IU/10 mL) tubes every 2 h for 24 h. On the last 3 d of each treatment period, rumen digesta were sampled 0, 1, 2, 3, 4, 6, 8, and 10 h after each meal to cover a 24-h period.

Laboratory Analyses

Concentrate, corn silage, and feed refusals pooled per animal at each treatment period were ground to pass a 1-mm screen (Cyclotec 1.093, Foss Tecator AB) before DM, OM, ether extract, Kjeldahl N (AOAC, 1990), NDF, and ADF (Van Soest et al., 1991) analyses were conducted. The NDF was determined without use of sodium sulfite but with the addition of heat-stable amylase (Thermamyl 120, NOVO Nordisk, Bagsvaerd, Denmark) for the last 30 min of boiling. A small part of these samples was ground to pass a 0.5-mm screen for starch determination (Faisant et al., 1995). After freeze-drying and grinding (1 mm), DM, OM, N, NDF, and Cr₂O₃ (François et al., 1978) were measured in feces pooled per animal and per period. Urinary N was also analyzed (AOAC, 1990).

Blood was centrifuged at 1,200 x g for 15 min immediately after sampling. Plasma was removed and frozen at –20°C until further analyses. After thawing, plasma urea N (PUN) was analyzed using the coupled urea-glutamate dehydrogenase enzyme system (Talke and Schubert, 1965). This analysis was automated by using a Vitalab 200 and 200R (Vital Scientific NV, AC Dieren, The Netherlands).

The pH of ruminal fluid was measured (Microprocessor pH-meter, pH 320/SET, WTW, Weilheim, Germany) immediately after sampling and filtration (250 µm). Supernatant from 1,200 x g centrifugation for 12 min was frozen at –20°C after H₂SO₄ acidification to pH 3 until NH₃-N determination. The NH₃-N concentration was determined by steam distillation with MgO using a 2,300 Kjeltec Analyser Unit (Foss Tecator AB).

Duodenal samples were freeze-dried, ground to pass a 1-mm screen, and pooled per animal and period before DM, OM, N, NH₃-N, NDF, and Cr₂O₃ determinations on the whole digesta. A small part was ground to pass a 0.5-mm screen for starch and purine determinations. Purine content was measured following the method of Zinn and Owens (1986) adapted for hydrolysis of nucleotides by perchloric acid (Makkar and Becker, 1999), and for wash solution used for washing the precipitate of free purines with silver nitrate (Obispo and Dehority, 1999). Duodenal bacteria samples were freeze-dried, ground with a coffee grinder, and pooled per animal and period before N, OM, and purine content determinations.

Calculations and Statistical Analyses

Nutrient flows at the duodenum and fecal output were calculated by reference to Cr₂O₃. The proportion of microbial N flowing at the duodenum was calculated by dividing the duodenal purine:N ratio by the corresponding purine:N ratio in duodenal microbes. An endogenous N flow of 2.98 g/kg of OM to duodenum (Kim et al., 2001) was applied for the fractionation of the estimated duodenal nonmicrobial N into endogenous and undegraded dietary N.

Statistical analyses on N balance, total tract and ruminal digestibility, and microbial protein synthesis were conducted using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC). Data were analyzed by ANOVA for an incomplete replicated Latin square design. Model sums of squares were separated into bull, period, and treatment. Least squares means were separated into significant main effects by the PDIFF option of SAS. Because of animal illness, the last experimental period could not be completed and the statistical analysis was done with missing values in place of the actual data. Therefore, the SEM for n = 6 are presented. For data collected over time within each sampling period (ruminal pH, ruminal NH₃-N concentration, and PUN), a repeated-measures analysis was performed using the REPEATED statement within the GLM procedure of SAS. Effects in the model included bull, period, treatment, time, and treatment x time interaction. When treatment x time interactions were significant, variables were analyzed within time periods. Differences among treatments were considered significant when P < 0.05, whereas when P > 0.05 but P < 0.10, differences were considered to indicate a trend toward a significant effect.

	RESULTS

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Ruminal Fermentation Characteristics and Plasma Urea N

A level of imbalance x time interaction (P < 0.001) was observed in ruminal pH (Figure 1). However, this interaction was largely due to differences in ruminal pH at specific time points and probably not biologically significant, because a pH of 6.2 is generally considered the pH threshold below which the growth of fiber-utilizing microbes and digestibility of fiber and OM are decreased (Pitt et al., 1996).

View larger version (13K): [in this window] [in a new window]   Figure 1. Ruminal pH in bulls fed the same diet but offered in 3 feeding patterns to induce different levels of imbalance between energy and nitrogen supplies for ruminal microbes measured by the variation of the degradable protein balance (OEB) between the 2 meals of a day, with 0 (), 20 (), or 40 (•) g of OEB/kg of DM. Feeding times are indicated on the x-axis by x. Error bars indicate SEM for n = 6.

View larger version (14K): [in this window] [in a new window]   Figure 2. Ruminal NH3-N concentrations in bulls fed the same diet but offered in 3 feeding patterns to induce different levels of imbalance between energy and nitrogen supplies for ruminal microbes measured by the variation of the degradable protein balance (OEB) between the 2 meals of a day, with 0 (), 20 (), or 40 (•) g of OEB/kg of DM. Feeding times are indicated on the x-axis by x. Error bars indicate SEM for n = 6.

View larger version (11K): [in this window] [in a new window]   Figure 3. Plasma urea N in bulls fed the same diet but offered in 3 feeding patterns to induce different time periods of imbalance between energy and nitrogen supplies for ruminal microbes measured by the variation of the degradable protein balance (OEB) between the 2 meals of a day, with 0 (), 20 (), or 40 (•) g of OEB/kg of DM. Feeding times are indicated on the x-axis by x. Error bars indicate SEM for n = 6.

Intakes of OM, NDF, and starch were not different among treatments (Table 2). Total tract digestibility of OM and NDF were greater (P < 0.048 to 0.049) in bulls on L40 compared with those on L0, with L20 being intermediate (Table 2). Ruminal digestibility of OM, NDF, and starch were not influenced by the feeding patterns. The ratio of ruminal:total tract digestibility of NDF were high and not significantly different between the feeding patterns. The feeding patterns of the diet did not affect intake or digestibility of N (Table 3). Moreover, apparent ruminal digestibility of N may be considered equal to zero according to a conformity test of the mean (t_observed < t_0.975).

Level of imbalance between energy and N supplies for the ruminal microbes had no effect on NH₃-N, non-NH₃-N, and microbial N flows at the duodenum (Table 3). Efficiencies of microbial protein synthesis reached 29.2, 25.8, and 24.6 g of N/kg of OM apparently digested in the rumen and 21.1, 19.4, and 18.6 g of N/kg of OM truly digested in the rumen, respectively for L0, L20, and L40, and were not significantly affected by the level of imbalance.

Nitrogen Balance

Fecal and urinary outputs of N, as well as absolute and relative amounts of N retained did not differ significantly due the feeding pattern (Table 3).

	DISCUSSION

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According to the Dutch system (Tamminga et al., 1994), the feeding pattern L0 reaches the target of a good balance of energy and N release for the microorganisms in the rumen. In practice, because the apparent ruminal digestibility of N (–9.3%) may be considered equal to zero, the amount of feed N degraded in the rumen was roughly equal to the amount used by the microorganisms. Thus, the recycling of urea N in the rumen balanced the amount of ruminal NH₃ absorbed through the rumen wall and transferred to the blood. The total tract and ruminal digestibility of OM and NDF measured with L0 were consistent with those generally measured on double-muscled Belgian Blue bulls (Fiems et al., 1997; Valkeners et al., 2004). The theoretical ruminal N degradability (in sacco values), estimated from the Dutch tabular values (CVB, 2000) averaged 50.2 and 72.4%, respectively, for LRDN and HRDN. For the feeding pattern L0, the estimated in sacco ruminal N degradability was 64.5%, which was close to the true ruminal digestibility of N measured with L0 (59.6%). Efficiency of microbial protein synthesis obtained with L0 (29.2 g of microbial N/kg of OM apparently digested in the rumen) is in agreement with the average value of 24 g of microbial N/kg of OM apparently digested in the rumen adopted for all diets by the Dutch system (Tamminga et al., 1994). From these results, it seems that the feeding pattern L0 created appropriate conditions in the rumen for the microorganisms and their activities. The N retention obtained during this experiment with L0 (49.7 g of N/d) was indicative of the ability of the double-muscled Belgian Blue bulls to grow rapidly (Hornick et al., 1998; Froidmont et al., 2000). Assuming 20.6% of CP in the gain (De Campeneere et al., 2001), N retention observed with L0 corresponded to an ADG of 1.51 kg/d., which is near the average ADG of 1.29 kg/d observed with the bulls on the whole experiment. The use of the N digested by L0 was high and attested that the supplies of NE for fattening and DVE were well balanced.

Over a 24-h period, no significant differences were observed between L20 and L40 average concentrations of ruminal NH₃-N compared with L0, although diurnal variation for this response was important. With L20 and L40, a succession of periods of excess or deficiency in N supply for the microorganisms was created. The group LRDN had a low CP content (95 g/kg of DM) and a theoretically low ruminal degradability of N (50.2%). The ingestion of LRDN at the morning meal by bulls fed L40 did not induce a postfeeding increase of ruminal NH₃-N concentration and led to a prolonged period (11 h over the 12 h after the morning meal; Figure 2) during which ruminal NH₃-N was below the concentration (5 mg/dL) generally considered as the minimum required for optimal microorganism growth in the rumen (Satter and Slyter, 1974). The average ruminal NH₃-N concentration over the 12 h after LRDN feeding amounted to 2.8 mg/dL, and concentrations of PUN decreased from 8.1 to 4.4 mg/dL (Figure 3), probably indicating N transfer from the blood into the rumen. In contrast, the group HRDN had a high CP content (175 g/kg of DM) and contained feedstuffs with high ruminal N degradability. Ingestion of L40 (evening meal) led to a large production of NH₃ in the rumen. Average ruminal NH₃-N concentration over the 12 h after HRDN feeding was 9.2 mg/ dL. It was very likely that large amounts of NH₃ were lost from the rumen fluid by absorption through the rumen wall and transferred by portal blood to the liver to produce urea because PUN increased for 12 h after HRDN ingestion. With L20, the groups LRDN and HRDN were fed in different combinations (3:1 and 1:3, respectively, for 0830 and 2030 feedings), which led to intermediate ruminal NH₃-N and PUN evolutions. Although each animal received the same diet on a 24-h basis, the feeding pattern of the diet and the nature of the meal ingested had a large influence on the diurnal evolution of the ruminal NH₃-N concentration during this period. Ruminal bacteria require NH₃ for microbial growth and activity. Low ruminal NH₃ has the potential to inhibit microbial activity and decrease rate of fiber digestion.

Intake was not affected by the feeding patterns. Average DMI was 73.2 g of DM/kg^0.75 and was near ad libitum, which is comparable to voluntary DMI of 78.2 g of DM/kg^0.75 reported by Fiems et al. (1999) for double-muscled Belgian Blue bulls fed a corn silage-based diet. Total tract digestibility of OM and NDF were not negatively influenced by the introduction of an imbalance between energy and N supplies for ruminal microbes and were surprisingly greater (P = 0.049 and P = 0.048, respectively, for OM and NDF) for L40 than L0. The ruminal digestibility of OM, NDF, and starch were not found to differ among treatments. Across all treatments, the NDF ruminal:total tract digestibility ratios were high (Table 2) and not influenced by the level of imbalance. The fiber degradation activities of the ruminal microbes were not affected by the perturbations experimentally created in terms of energy and N supplies for the ruminal microbes when an imbalance of 20 or 40 g of OEB/kg of DM was induced. Although the periods of low ruminal NH₃-N concentration were longer after the L20 and L40 morning feeding than after the L0 morning meal, it seemed that the N supplies in the rumen for the cellulolytic bacteria and the nonstructural carbohydrate-fermenting bacteria were satisfied even if the diet was fed with a level of imbalance of 20 or 40 g of OEB/kg of DM. Russell and Strobel (1987) found that in in vitro studies, NH₃ could be transported into microbes, reaching high intracellular NH₃ concentrations despite the fact that the NH₃-N concentration used in the media was very close to zero. This mechanism of active NH₃ transport is likely to be present in vivo, allowing ruminal cellulolytic microbes to obtain NH₃ from the medium at low ruminal NH₃ concentration.

Level of imbalance had no significant effect on non-NH₃ and microbial N flows at the duodenum. The efficiency of microbial protein synthesis obtained with L20 and L40 were not significantly different compared with L0. The in vitro studies of Newbold and Rust (1992) and the in vivo studies of Henning et al. (1993), Sinclair et al. (1993, 1995), and Richardson et al. (2003) with sheep have focused on the synchronization of ruminal N and energy to prevent periods of N deficiency during the day. According to their results, there is some disagreement as to whether synchronizing dietary energy and N supply to the rumen improves microbial protein growth. Newbold and Rust (1992), Henning et al. (1993), and Richardson et al. (2003) found no effect of dietary synchrony on microbial growth either in vitro or in vivo. However, Sinclair et al. (1993, 1995) have found microbial growth rates or efficiency to be improved by dietary synchrony. In the current experiment, there was no significant effect on the microbial protein synthesis when a 12-h imbalance of a daily OEB variation of 20 or 40 g of OEB/kg of DM was created. This lack of response to an increased level of imbalance between energy and N supplies for the ruminal microbes is in agreement with results obtained in a previous study (Valkeners et al., 2004), which demonstrated that a lack of imbalance in the diet between the energy and N supplies for ruminal microbes was not detrimental to their growth as long as nutrient supply was balanced on a 48-h basis and the level of imbalance did not exceed 30 g of OEB/kg of DM.

Because N retention obtained with L20 and L40 did not differ from L0, it seemed that animal growth was not affected by a lack of synchrony. The total tract N digestibility was not influenced by the feeding patterns. Feeding the diet with an imbalance of 20 or 40 g of OEB/kg of DM had no significant impact on use of N digested by the animal and therefore, the overall balance of nutrients for the tissues was not modified by the imbalance of nutrients for the rumen.

A large extent of NH₃-N excess in the rumen may be excreted as urea in the urine by growing ruminants (Dehareng and Ndibualonji, 1994). In this experiment, urinary excretion of N was not affected by the feeding patterns. Our results tended to show that the temporary excess of ruminal degradable N created after ingestion of the evening meal by L20 and L40 were not lost in urine by the bulls but could return in the rumen during temporary N shortages. The ruminal recycling of urea helps the animal and its ruminal microbes to overcome a deficiency in N supply and could minimize the effect of a lack of synchronization between energy and N supplies in the rumen (Dawson, 1999). The N recycling might be an adaptive mechanism to retain N within the system and might be needed to provide NH₃ continuously for microbial protein synthesis (Marini and Van Amburgh, 2003). In the current study, this mechanism has minimized the potential negative impact of feeding a diet with a level of imbalance between energy and N supplies in the rumen of 20 or 40 g of OEB/kg of DM on digestion and N metabolism of double-muscled Belgian Blue bulls.

	IMPLICATIONS

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Provided the overall balance between ruminally degradable nitrogen and ruminally fermentable organic matter in the daily intake is sufficient, there is no further advantage for the double-muscled Belgian Blue bulls in balancing the release of energy and nitrogen in the rumen over the short term. Thus, these feeding patterns of the diet can be used under practical feeding conditions with minimal impact on the performance of ruminant animals for meat production.

	Footnotes

 
¹ The research was sponsored by the Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA), Brussels, Belgium. The authors express their sincere thanks to C. Leirens for her assistance with sample collection and analysis associated with this project, and to J. Moreman for help with the preparation of the manuscript.0

² Current address: Agriculture and Agri-Food Canada, Lennoxville, QC, J1M 1Z3, Canada.

³ Corresponding author: beckers.y{at}fsagx.ac.be

Received for publication July 15, 2005. Accepted for publication November 18, 2005.

	LITERATURE CITED

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