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Supplementing barley or rapeseed meal to dairy cows fed grass-red clover silage: II. Amino acid profile of microbial fractions¹

MTT Agrifood Research Finland, FIN-31600, Jokioinen, Finland

² Correspondence:
phone: +358 3 4188 3664; fax: +358 3 4188 3661; E-mail:
mikko.korhonen{at}mtt.fi.

	Abstract

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Four ruminally cannulated dairy cows were used to examine the effect of diet on the AA composition of rumen bacteria and protozoa, and the flow of microbial and nonmicrobial AA entering the omasal canal. Cows were offered grass-red clover silage alone, or that supplemented with 5.1 kg DM of barley, 1.9 kg DM of rapeseed meal, or 5.1 kg DM of barley and 1.9 kg DM of rapeseed meal according to a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. During the first 10 d of each period, cows had free access to silage and, thereafter intake was restricted to 95% of ad libitum intake. Postruminal digesta flow was assessed using the omasal canal sampling technique in combination with a triple marker method. Liquid- (LAB) and particle- (PAB) associated bacteria were isolated from digesta in the reticulorumen and protozoa from digesta entering the omasal canal. Microbial protein flow was determined using ¹⁵N as a microbial marker. Flows of AA entering the omasal canal were similar in cows fed silage diets supplemented with barley or rapeseed meal. However, rapeseed meal increased nonmicrobial AA flow while barley increased the flow of AA associated with LAB and protozoa. Diet had negligible effects on the AA profile of microbial fractions. Comparison of AA profiles across diets indicated differences between LAB and PAB for 10 out of 17 AA measured. Rumen bacteria and protozoa were found to be different for 14 out of 15 AA measured. For grass silage-based diets, energy and protein supplementations appear to alter postruminal AA supply through modifications in the proportionate contribution of microbial and nonmicrobial pools to total protein flow rather than as a direct result of changes in the AA profile of microbial protein.

Key Words: Amino Acids • Dairy Cows • Grass Silage • Rumen Bacteria • Rumen Protozoa

	Introduction

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Within current protein evaluation systems for ruminants (Vérité and Peyraud, 1989; Madsen et al., 1995; NRC, 2001), protein requirements and supply are based on the digestible total AA. The supply of an individual AA may, however, also be important for maximizing milk protein synthesis (Schingoethe, 1996). Thus, developing protein evaluation systems on the basis of the supply of individual AA may result in improvements in the utilization of dietary protein for milk production. Further progress in the development of accurate protein evaluation systems requires information concerning the supply of AA derived from the diet and rumen microbes for a range of feeds because the first limiting AA for milk production is dependent on the basal diet.

Microbial protein consists of liquid- (LAB) and particle-associated bacteria (PAB), and protozoa. The contribution of each fraction to total microbial flow and differences in AA and marker concentrations between microbial pools are affected by diet (Cecava et al., 1990; Faichney et al., 1997). Therefore, the use of an average AA profile may result in erroneous estimates of microbial AA flow (Clark et al., 1992). It is assumed that differences due to diet are related to variations in energy and protein content. To improve the accuracy of estimates of postruminal microbial AA flow, both the AA profile of individual fractions as well as their contribution to total postruminal digesta flow need to be measured. A recently developed sampling technique that allows digesta to be collected from the omasal canal (Huhtanen et al., 1997; Ahvenjärvi et al., 2000) represents an alternative to duodenal sampling with the advantages that endogenous N secretions are minimized (Ørskov et al., 1986) and digesta can be separated into various microbial fractions.

The objective of the present study was to examine the effect of protein and energy supplements on the AA profile of individual microbial fractions and total postruminal AA flow.

	Materials and Methods

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Experimental Design and Treatments
The experimental design was conducted according to a 4 x 4 Latin square with a 2 x 2 factorial arrangement of treatments. Each experimental period lasted for 21 d, the last 3 d of which were used for sample collection. Treatments consisted of grass-red clover silage alone, grass-red clover silage supplemented with 5.1 kg DM of barley, 1.9 kg DM of rapeseed meal, or 5.1 kg DM of barley and 1.9 kg DM of rapeseed meal.

Animals and Management
Four multiparous rumen cannulated Finnish Ayshire cows averaging 167 d in milk (SD = 18), and 571 kg BW (SD = 66) were used. Cows were housed in individual stalls, fed at 0600 and 1800, milked at 0700 and 1700, and had continuous access to water. Grass silage was prepared from secondary growths of timothy grass (Phleum pratense) and red clover (Trifolium pratense) sward. Once cut, herbage was wilted and harvested using a precision chopper and ensiled using a formic acid based additive (AIV-2; Kemira-Agro, Helsinki, Finland) applied at a rate of 5 L/t grass. Cows were given ad libitum access to silage during the first 10 d of each period and, thereafter, intake was restricted to 95% of free intake to minimize between- and within-day variations in intake. A 250-g mineral mixture (Viher Hertta-Minera Muro; Suomen Rehu Oy, Helsinki, Finland) was given daily. Composition of the mineral mixture is presented elsewhere (Ahvenjärvi et al., 2002). All cows that participated in the current study were managed according to legislation documented in the Finnish Animal Welfare Act (247/96), the Order using vertebrate animals for scientific purposes (1076/85), and the European convention for the protection of vertebrate animals used for experimental or other scientific purposes, Appendix A and B, implemented under the auspices of the local Animal Use and Care Committee.

Sampling and Chemical Analysis
Feed intake was recorded daily throughout the experiment, and feed samples were taken on each collection period for analysis. Chemical composition of feedstuffs is reported in a companion paper (Ahvenjärvi et al., 2002). Amino acid composition of feeds is shown in Table 1.

For the determination of AA (except Met and Cys) concentrations, feedstuffs, reconstituted digesta, protozoal, LAB, and PAB samples were hydrolyzed (6 M HCl, 23 h, 110°C) prior to AA analysis. For the determination of Met and Cys concentrations, a similar hydrolysis was conducted with the exception that samples were oxidized with performic acid before acid hydrolysis. Amino acid concentrations were analyzed following acid hydrolysis according to Directive 98/64/EC (European Commission, 1998), using an AA analyzer (Biochrom 20; Pharmacia Biotech, Cambridge, UK) equipped with a 90 x 4.6 mm PEEK Sodium Pre-Wash Column, and a 250 x 4.6 mm Bio 20 PEEK Sodium Main Column. Analysis was conducted according to the manufacturer’s recommendations, with minor changes in running time and temperature. The reported AA data are not corrected for possible incomplete recovery of AA due to incomplete hydrolysis or loss of AA because of possible destruction during hydrolysis.

Calculations and Statistical Analysis
Flows of individual AA entering the omasal canal were calculated based on AA concentrations in reconstituted digesta and estimates of omasal canal DM flow. Calculations of omasal canal N flow in bacterial fractions (LAB and PAB) were based on ¹⁵N enrichment in bacterial fractions. Protozoal N flow was determined quantitatively based on estimates of liquid and nonammonia N (NAN) flow, and protozoal DM content.

Adequacy of AA supply (the ratio of supply to requirement) was evaluated based on estimates of total digestible AA flow, measured milk production, and maintenance and production AA requirements according to the Nordic protein evaluation system (Madsen et al., 1995). Within this system, maintenance requirement for absorbed AA is assumed to be 3.25 g/W^0.75 per day, lactation requirement is defined as 45 g/kg energy-corrected milk, and the digestibility of undegraded feed and microbial protein is assumed to be 82 and 85%, respectively. Because, AA of microbial origin is the major component of duodenal AA supply for grass silage-cereal based diets, the intestinal digestibility of AA was estimated to be 84%.

The effect of supplementation of silage with barley and rapeseed meal on omasal canal AA flow, AA profiles of microbial fractions, and the contribution of each fraction to microbial protein flow was assessed by the following statistical model using the MIXED procedure of SAS (Littell et al., 1998):

where A is a normally distributed random effect of animal, and P, B, and R are fixed effects of period, barley, and rapeseed meal, respectively. Differences in AA profiles between microbial fractions (LAB, PAB, and protozoa) were also evaluated with MIXED procedure using the following model for repeated measurements:

where M is the fixed effect of microbial population.

	Results

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Chemical composition of feedstuffs, DMI, and energy status are reported in a companion paper (Ahvenjärvi et al., 2002). Estimates of AA supply relative to requirements, based on estimates of AA flow and intestinal digestibility and predicted requirements (Madsen et al., 1995) were 0.93, 1.02, 1.02, and 1.07 for silage alone, or that supplemented with barley, rapeseed meal, barley and rapeseed meal, respectively.

Flows of individual AA entering the omasal canal are presented in Table 2. Barley and rapeseed meal increased (P < 0.05) total AA, EAA, NEAA and BCAA flow, and increased (P < 0.05) or tended to increase (P < 0.10) the flow of individual AA.

	Discussion

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His is the first limiting AA for milk production in animals fed grass silage based diets (Vanhatalo et al., 1999; Kim et al., 2000; Korhonen et al., 2000), while Met has also been suggested as a potential candidate (Kim et al., 2000). In the present study, the relative flow of these AA was in direct contrast to previous study (Korhonen et al., 2000) with cows fed a grass silage-cereal diet. Postruminal flow of His has been shown to be higher than that of Met in sheep fed grass silage as the sole feed or when supplemented with barley (Thomas et al., 1980). This also appears to be true for cattle fed grass silage supplemented with rapeseed meal (Jacobs and McAllan, 1992). Differences in analytical methods and digesta sampling site may explain the apparent discrepancies between these findings. As such, comparison of flow measurements between studies has to be conducted with caution. Differences in the relative flow of individual AA between the current and an earlier study (Korhonen et al., 2000) that are based on the same analytical methods are consistent with the findings of Kim et al. (2000). Milk production responses to AA infusions in animals fed a similar basal diet were variable, suggesting that AA supply from the basal diet may not be constant. Grass silage containing red clover was found to enhance microbial protein production compared with grass silage alone (Vanhatalo et al., 1995). Variations in the relative flows of His and Met between the current and earlier study (Korhonen et al., 2000) may be related to the presence or absence of red clover in silage. If the variation in AA flow between studies in animals offered grass silage based diets diet is true, then the ranking order of limiting AA may also be different between studies.

Effect of Diet on AA Profiles of Different Microbial Fractions and Microbial Protein
In agreement with Hvelplund and Hesselholt (1987), Martin et al. (1996), and Volden et al. (1999), diet affected some but not all AA concentrations in microbial fractions (Tables 3, 4, and 5). Despite being statistically significant, numerically these differences were small. Changes in nutrient supply may affect bacterial species present in the rumen (Cecava et al., 1990; Martin et al., 1996), that may have an influence on the AA profile of harvested bacteria.

Clark et al. (1992) pointed out that using the mean AA composition of ruminal bacteria instead of that of bacteria isolated for individual diets may result in an erroneus estimation of microbial protein and AA supply. Dietary effects on AA profile were determined for microbial protein (Table 6), using the AA profile and contribution of individual microbial fractions to omasal NAN flow. Despite an effect on the proportion of individual microbial fractions in total omasal NAN flow (Ahvenjärvi et al., 2002) and differences in AA profiles between the fractions (Table 7), the AA profile of microbial protein appeared to be similar between diets. These and previously published data suggest that differences within study are relatively small, such that the AA profile of microbial protein is independent of diet. It appears that the origin of variation in AA profile of bacteria reported by Clark et al. (1992) is most likely to be explained by between-study variations arising from differences in analytical techniques. This suggestion is supported by the observation that formaldehyde treatment used to prevent bacterial lysis altered both total AA concentration and AA profile of rumen bacteria (Volden and Harstad, 1998).

Differences in AA Profiles of Microbial Fractions
The present study confirms the conclusions from previous studies (Martin et al., 1996; Chiquette and Benchaar, 1998; Volden et al., 1999), that AA profiles of various microbial fractions are markedly different. Particle-associated bacteria had higher total AA concentration (total AA, % of N) than LAB (Table 7), in agreement with Volden and Harstad (1998) and Volden et al. (1999). In contrast to the study of Volden et al. (1999) that noted higher AA concentrations in protozoa compared with bacteria, no differences were observed in the current study. This discrepancy may be related to assumption that Phe and Tyr concentrations were similar to those reported in previous study (Volden et al., 1999). According to values reported in the studies mentioned above, LAB appeared to have lower Arg, Leu, Phe, and EAA concentrations and a higher Thr concentration than PAB. However, differences in His, Lys, Met, and Val concentrations are inconsistent. Higher Ile, Lys, and EAA concentrations and lower Thr, and Val concentrations in protozoa than that of bacteria, in addition to differences in Leu concentrations between microbial fractions (Table 7), are consistent with the differences observed by Martin et al. (1996) and Volden et al. (1999). These authors observed no differences in Met concentration between bacteria and protozoa. Based on the data of Martin et al. (1996), the concentration of Arg is lower in protozoa and the concentration of His is similar between protozoa and bacteria.

No clear reasons for the differences in AA profiles between microbial fractions have been offered. One possible explanation is due to differences in methods used to harvest rumen bacteria. Procedures used to separate PAB render only a minor proportion of the total adherent population (Martin-Orue et al., 1998), which raises the question as to whether the AA profile of detached bacteria is similar to that of the residual adherent bacteria and therefore representative of the entire microbial population. On the other hand, PAB are more sensitive to errors in AA composition measurements due to feed particle contamination (Martin et al., 1996). If contamination had been extensive and more prevalent in PAB than LAB samples, then concentrations of N and Lys would be expected to be higher for LAB than PAB, because the concentations of N and Lys are lower in feedstuffs compared with rumen bacteria. Particle-associated bacteria had higher concentrations of both N (95 vs 92 g/kg OM) and Lys (Table 7) than LAB, supporting earlier conclusions that differences in AA profile and total AA content between LAB and PAB are potentially explained as differences between bacterial species present in rumen liquid and solid phases (Cecava et al., 1990; Martin et al., 1996). Differences in AA profile between protozoa and bacteria fractions are most probably related to differences in nutrient metabolism and cell structure between bacteria and protozoa (Martin et al., 1996; Volden et al., 1999). Protozoa have a lower cell surface area relative to volume than bacteria, and their capability to synthesize AA and utilize intermediatery metabolites differs from that of bacteria (Wallace and Cotta, 1989).

Effect of AA Profiles and Proportions of Individual Microbial Fractions on AA Profile of Microbial Protein and Microbial AA Supply
Diet affected both microbial N flow and the contribution of individual microbial fractions to total microbial protein flow (Ahvenjärvi et al., 2002). Furthermore, the AA profiles of individual microbial fractions were different (Table 7). It has been suggested that both sources of variation need to be taken into account to improve the accuracy of postruminal microbial AA flow measurements (Volden et al., 1999). Assessing the AA profile and the contribution of each fraction to total microbial flow allowed the effect of these sources to be evaluated (Table 6). In spite of the effect of diet on LAP, PAB, and protozoa flow and differences in AA profiles between these fractions (Table 7), the AA profile of microbial protein remained relatively constant. The ¹⁵N enrichment of microbial fractions was also different (Ahvenjärvi et al., 2002). The mean microbial AA flow across diets was 941 g/d based on ¹⁵N enrichments and AA profile of all microbial fractions. Using mean values for LAB or a combined value of LAB and PAB gave corresponding values of 859 and 936 g/d. These values suggest that the omission of protozoa may not have a significant impact on total microbial AA flow. Furthermore, use of LAB and PAB appears to give more reliable estimates than PAB alone. Based on the flow values and AA profile of microbial protein measured for the current diets, it appears that the accuracy of microbial AA flow measurements is more dependent on a reliable assessment of total microbial protein flow than measurements of AA profiles and the contribution of individual microbial fractions.

	Implications

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The amino acid profile of microbial protein appears to be independent of diet despite differences in amino acid profile and the contribution of individual microbial fractions to total microbial protein flow. Separation of individual microbial fractions may have an influence on estimates of microbial amino acid flow through improving the accuracy of microbial protein flow measurements rather than due to a reliable assessment of amino acid profile for each fraction. On grass silage based diets, manipulation of the proportion of microbial and undegradable feed protein appears to be a suitable means of altering the composition of amino acid absorbed from the small intestine.

	Footnotes

 
¹ The authors thank Aino Matilainen and her staff for technical assistance and Vesa Toivonen and his staff for chemical analyses.

Received for publication October 22, 2001. Accepted for publication April 19, 2002.

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