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Condensed tannin concentration of rhizomatous and nonrhizomatous birdsfoot trefoil in grazed mixtures and monocultures. (Forage & Grazing Lands).

TANNINS ARE POLYPHENOLIC NATURAL PRODUCTS that can precipitate proteins (Reed, 1995). Hydrolyzable tannins have small molecular weights and include simple phenolic acids and glucose. Condensed tannins are large oligomeric proanthocyanidins formed by condensation of hydroxyflavanol units (Hemingway, 1989).

In forage legumes such as Lotus spp., condensed tannins can be detrimental to ruminant livestock production (Waghorn et al., 1999). If they occur at concentrations above 60 g CE [kg.sup.-1] DM, condensed tannins reduce voluntary feed intake and depress digestion efficiency (Barry and Duncan, 1984; Barry and Manley, 1986). If they are absent, bloat can occur (Waghorn and Jones, 1989). At moderate concentrations, however, condensed tannins can be beneficial to ruminant livestock production. Among some of the beneficial effects, condensed tannins complex with soluble proteins in the rumen and permit subsequent absorption of amino acids in the lower digestive tract (Barry and Manley, 1986), thereby facilitating ruminal escape protein utilization (Waghorn et al., 1999).

Concentrations of condensed tannins considered to be optimal for intake, digestion efficiency, and general animal performance are not universally established. However, concentrations between 20 and 40 g CE [kg.sup.-1] DM have been suggested as optimal for forage crops in general (Aaerts et al., 1999), with a smaller range suggested for big trefoil (L. uliginosis Schkur; Barry et al., 1986) and a larger range suggested for birdsfoot trefoil (Miller and Ehlke, 1994). Lack of agreement regarding optimal concentrations of condensed tannins may result from differences in composition of proanthocyanidins among Lotus spp., their affinities for protein (Foo et al., 1996), and laboratory methodology (Hedqvist et al., 2000).

Condensed tannins have been quantified in several BFT breeding nurseries. In a survey of germplasm accessions, concentrations ranged from 0 to 132 g CE [kg.sup.-1] DM (Roberts et al., 1993). In another diverse collection, concentrations ranged from 0 to 85 g CE [kg.sup.-1] DM (Miller and Ehlke, 1994). Condensed tannins were recently quantified in a spaced-plant nursery of RBFT; concentrations were found to be higher in RBFT than in BFT (Gebrehiwot et al., 2002). According to all of these studies, condensed tannin concentration in BFT was not only affected by genetics, but also by season of harvest; concentrations decrease from late summer through the autumn.

While it is important for plant breeders to know the concentrations of condensed tannin in BFT and RBFT spaced plants, it is equally important for other researchers to know concentrations as they occur in normally managed pastures. In the USA, Italy, Sweden, Uruguay, Germany, and other countries, BFT is usually grazed continuously or in long rotations and is typically grown in combination with a grass (Blumenthal and McGraw, 1999). It is probable that RBFT, like BFT, will also be mixed with a grass and grazed in long rotations.

There is no published report of condensed tannin concentration in grazed pastures of BFT and RBFT in monoculture and mixed pastures. There is reason to believe condensed tannin concentration of BFT and RBFT will be affected by grazing as well as by growing with a companion grass. Studies have shown that when BFT is grown with tall fescue, chemical composition of the BFT component are different from those found in BFT sampled from pure stands (Beuselinck et al., 1992).

On the basis of the studies above and lack of information, therefore, our objectives were (i) to investigate the condensed tannin concentrations in grazed BFT and RBFT, (ii) to determine if concentrations in BFT and RBFT from pure stands differ from the BFT and RBFT component in tall fescue mixtures, and (iii) to determine if concentrations fluctuate through the spring grazing season.

MATERIALS AND METHODS

Pasture Establishment

In April 1997, BFT and RBFT were sown as pure stands and tall rescue mixtures at the University of Missouri South Farm near Columbia, MO. Pastures were 0.53 ha, and the soil type was a heterogenous mixture of Mexico silt loam (fine, montmorillinitic, mesic, Udollic Ochraqualf), Moniteau silt loam (fine-silty, mixed, mesic, Typic Ochraqualf) and Mandeville silt loam (fine-loamy, mixed, mesic, Typic Hapludalfs). Before planting treatment pastures, residual stubble of the existing smother crop, sudangrass [Sorghum bicolor (L.) Moench], was sprayed with glyphosate [N-(phosphonomethyl) glycine] at a rate of 387 mL a.i. [ha.sup.-1].

Four pasturetreatments were seeded with a no-till drill at the following rates. The experimental design was a randomized complete block with four replications. The first two treatments included pure stands of BFT (cv. Norcen) and RBFT (cv. ARS-2620) sown at 6.72 kg [ha.sup.-1]. The other two treatments were mixtures of BFT-tall fescue mixture (cv. Phyter) and RBFT-tall fescue sown at 6.72 kg birdsfoot trefoil [ha.sup.-1] and 13.44 kg tall fescue [ha.sup.-1]. The tall fescue seed did not contain the fungal endophyte, Neotyphodium coenophialum (Morgan-Jones and Gams) Glenn, Bacon, and Hanlin comb. nov. Seed of BFT and RBFT were inoculated with Bradyrhizobium loti before planting.

Pasture Management

Soils were tested for pH, phosphorus (P), and potassium (K), and pastures were fertilized as needed each year according to recommendations from the University of Missouri Soil Testing Laboratory. Except for 33.7 kg N [ha.sup.-1] applied at planting, no N fertilizer was applied.

To control grass weeds in pure stands, hexazinone [3-cyclohexl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H) dione] was applied at 875 mL a.i. [ha.sup.-1] in February 1998 and 1999. Further control of escape weeds occurred with a 205 mL [L.sup.-1] solution of glyphosate applied each year with a ropewick applicator.

Pastures were grazed in continuous stocking each year to maintain an 8- to 10-cm stubble height by the put-and-take grazing method (Bransby, 1989; Hoveland et al., 1991). In 1998, the grazing season began 5 May and ended 30 June. In 1999, the season occurred from 19 April to 14 July. The grazing season was terminated when pasture growth failed to support at least two steers per experimental unit of 0.53 ha. In Missouri, pastures fail to grow during late summer and early autumn as they enter the summer dormant period. Pure stands of birdsfoot trefoil often produce below the minimum dry matter requirement to support cattle grazing during this period. Therefore, this study did not include the late summer or autumn.

Sample Collection and Analysis

Samples of BFT and RBFT were hand clipped from monoculture and mixed pastures when grazing began, then at 14-d intervals. These samples were clipped at 4 cm above ground and at 25 random sites within the paddock. The 14-d interval permitted five harvest dates in 1998 and seven in 1999. Clipped samples were placed on dry ice in the field, then stored at -20[degrees]C until lyophilization (Roberts et al., 1993). Dried samples were ground to pass a 1-mm screen with a cyclone-type grinder and stored in sealed plastic bags at -20[degrees]C until condensed tannin analysis.

Eighty-two ground samples were analyzed in triplicate for condensed tannin concentration according to the colorimetric vanillin-HCl procedure reported by Burns (1971), with the modified sample blank reported by Price et al. (1978), aqueous acetone extractant recommended by Petersen et al. (1989), and final modifications reported by Miller and Ehlke (1994). Reference data from the 82 samples were used to develop a spectral prediction equation for condensed tannins by NIR according to the procedure of Roberts et al. (1993) and software developed by Infrasoft International (Port Matilda, PA). Samples were scanned with near infrared radiation from 1110 to 2490 nm with a Pacific Scientific 5000, scanning monochromator (NIRSytems, Silver Spring, MD). Log 1/reflectance (log 1/R) was recorded and converted to second derivative spectra. Reference data were regressed against derived spectral data using modified partial least squares regression, and every fourth sample was reserved for cross validation. Optimum equations were selected on the basis of high values for squared correlation coefficients and low standard errors of calibration and validation (Table 1).

Proportion of birdsfoot trefoil in the mixed pastures was monitored throughout the experiment. Ratios of BFT:TF and RBFT:TF were determined by NIR reflectance spectroscopy according to the procedure of Moore et al. (1990). Samples of BFT, RBFT and tall fescue were hand clipped from each paddock at each harvest date in 1998 and 1999. Hand-clipped samples were placed on dry ice in the field, lyophilized, ground to pass a 1-mm screen with a cyclone-type grinder and stored in sealed plastic bags. They were mixed in proportions ranging from 0 to 100% legume, and a regression equation was developed according to the protocol described above (Table 1). The equation was applied to spectra of mixed samples clipped at 4 cm above the ground from randomly placed strips (0.76 by 7.6 m) in each paddock.

Statistical Analysis

The experimental design was a randomized complete block in a split-split plot arrangement with four replications. Main plots were pasture composition (BFT, BFT+TF, RBFT and RBFT+TF). Years were considered subplots, and sampling dates within years were sub-subplots. Analysis of variance was conducted on pasture composition, years, sampling dates and all possible interactions by means of the analysis outlined by Steel and Torrie (1980). When the F test was significant (P < 0.05), separation of means was conducted using Fisher's protected LSD (Steel and Torrie, 1980) at [alpha] = 0.05 unless noted otherwise.

RESULTS AND DISCUSSION

Condensed Tannin Concentrations in BFT from Grazed Pastures

Averaged across harvest dates, condensed tannin concentration in pure stands of BFT was 5.2 g CE [kg.sup.-1] DM in 1998 (Fig. 1) and 17.5 g CE [kg.sup.-1] DM in 1999 (Fig. 2). Differences in tannin concentration between years occur frequently (Roberts et al., 1993) because concentrations are influenced by many environmental effects, such as climate, season, and soil fertility (Roberts et al., 1993). Concentrations this low, however, were not expected. As reported in other studies, condensed tannin in BFT typically ranges from 0 to more than 85 g CE [kg.sup.-1] DM, and populations often average more than 35 g CE [kg.sup.-1] DM (Roberts et al., 1993; Miller and Ehlke, 1994).

[FIGURES 1-2 OMITTED]

One reason why concentrations in this study were lower than in studies cited above relates to management. In this study, BFT was continuously grazed; in the studies cited above, BFT was allowed to grow in spaced-plant nurseries. The constant leaf removal that occurred in this study by continuous grazing would greatly lower concentrations of condensed tannin since tannin levels are highest in the leaves (Donnelly, 1954; Jackson and Barry, 1996).

Another reason why condensed tannin concentrations were lower in this study relates to germplasm of BFT. The BFT analyzed in this study was the cultivar Norcen. In the studies cited above, the BFT germplasm included diverse accessions with a wide range of characteristics; some of these accessions contained extremely high concentrations of condensed tannin (Roberts et al., 1993; Miller and Ehlke, 1994). Other studies show that Norcen contains low concentrations of tannins; in a side-by-side comparison conducted by Broderick and Albrecht (1997), Norcen contained 3.9 tannic acid equivalents (TAE) while another nonrhizomatous cultivar, Viking, contained 7.0 TAE.

Condensed Tannin in BFT vs. RBFT

Condensed tannin concentrations of birdsfoot trefoil differed greatly among in pasture composition types (P < 0.05), an exception being the similarity between RBFT in pure stands and mixtures. The most notable difference occurred between BFT and RBFT (Fig. 1 and 2). In 1998, concentrations in BFT were five times lower than in RBFT (Fig. 1), and in 1999, they were more than twice as low (Fig. 2). The pasture composition x year interaction was not significant.

Though tannin concentrations differed among these treatments, their concentrations might not affect livestock performance differently. Assuming the optimum condensed tannin concentration for ruminant livestock occurs between 20 and 40 g CE [kg.sup.-1] DM, as proposed by Barry and Manley, 1986; Aerts et al., 1999, neither BFT nor RBFT appeared to have a comparative nutritional advantage. Depending on the year, they both contained concentrations outside the range of 20 to 40 g CE [kg.sup.-1] DM. In 1998, RBFT averaged 31.8 g CE [kg.sup.-1] DM; BFT contained less than the minimum concentration for ideal ruminal escape protein digestion (Fig. 1; Tanner et al., 1994; Broderick and Albrecht, 1997). In 1999, RBFT contained concentrations that could inhibit voluntary feed intake and digestive efficiency (Fig. 2; Barry and Duncan, 1984; Barry, 1985; Chiquette et al., 1988; Aerts et al., 1999), while BFT contained concentrations between 20 and 40 g CE [kg.sup.-1] DM.

Condensed Tannin in BFT and RBFT from Monocultures vs. Mixtures

When BFT was grown in combination with tall fescue, condensed tannin concentration of the BFT component increased (Fig. 1 and 2). In 1998, concentrations were 100% higher when BFT was grown in a mixture instead of a pure stand (P < 0.10). In 1999, condensed tannin concentrations were 55% higher (P < 0.05) in the BFT from a mixed stand than BFT from a pure stand. Condensed tannin concentration of RBFT was not affected by growing in a tall fescue mixture, possibly because concentrations were already very high in RBFT (Fig. 1 and 2).

The fact that tall fescue affected BFT tannins but not RBFT tannins was not likely related to differences in legume proportion. In 1998, the grass:legume ratio was the same; BFT and RBFT proportion was 32 and 31%, respectively. In 1999, they were both below 15% in the field; BFT was 14% and RBFT was 9%.

An increase of condensed tannin in BFT as a result of being grown with tall fescue or other grasses has not been reported in the literature. To our knowledge, a companion-mediated increase in condensed tannins has not even been studied, and our reason for determining this was merely to ensure that we were analyzing BFT as it is normally used--under grazing and in a grass mixture (Blumenthal and McGraw, 1999). Though the magnitude of this increase in concentration was not expected, some sort of response due to a grass might have been anticipated. In a previous study, crude protein and fiber constituents of a BFT component were affected when BFT was grown in combination with tall fescue (Beuselinck et al., 1992).

We can only speculate as to why condensed tannin concentration of BFT increased when incorporated into tall fescue. Many studies have shown the effect of one plant on another within a mixture. In the example cited earlier, forage quality of BFT is affected by the presence of a tall fescue companion grass (Beuselinck et al., 1992). In another example, the wheat (Triticum aestivum L.) component, when grown in combination with hairy vetch (Vicia villosa Roth), contained more total crude protein than when grown alone (Roberts et al., 1989). The explanation for the increase in wheat protein was related to soil fertility and plant nutrition; as N was fixed by the vetch, nitrates were taken up by the wheat.

The higher concentrations of condensed tannins in BFT from mixed pastures compared with concentrations in BFT from monocultures might be explained by grazing and changes in morphology. In the mixed pasture, BFT grew shorter than the tall fescue canopy. Therefore, it may have been more difficult to graze than BFT in the pure stands, thereby reducing leaf removal. A reduction in BFT leaf removal would have resulted in a higher leaf:stem ratio of the BFT component, and thus, a higher concentration of condensed tannins. As stated earlier, tannin concentrations are highest in the leaves (Donnelly, 1954; Jackson and Barry, 1996).

In addition to changes in morphology, changes in microenvironment may account for increased tannins in the BFT component. Research reported by Bryant et al. (1992) showed that light limitation could affect the balance of N-based and C-based secondary metabolites. Other authors reported that temperature also affects production of condensed tannins (Roberts et al., 1993; Miller and Ehlke, 1994). The effects of light penetration and canopy temperature on condensed tannin production of a BFT component warrant further study.

Condensed Tannin over the Spring Grazing Season

In 1998, condensed tannin concentrations in BFT and RBFT fluctuated in the spring and early summer grazing season (Fig. 3). There was no three-way interaction between year x pasture composition x harvest date. However, there was a year x harvest date interaction; condensed tannin concentration did not change across the season in 1999 (Fig. 4). This was the only significant treatment interaction in the study.

[FIGURES 3-4 OMITTED]

It is possible that high tannin concentrations at the beginning of the 1998 season were caused by a high leaf:stem ratio. These samples were collected at onset of grazing, before leaf defoliation. It is also possible that the increase from 2 June to 30 June was caused by an increase in temperatures that occurred in late spring and early summer. However, there is no good explanation for the low concentrations at onset of grazing in 1999. Nor is there a good understanding of why condensed tannin concentration was constant over the spring of 1999.

Seasonal fluctuation of condensed tannin concentration in BFT has been reported by Roberts et al. (1993) and Miller and Ehlke (1994). In both reports, condensed tannin concentration decreased through the season. However, both of those studies reported fluctuations in the fall, not the spring. In addition, they reported concentrations in spaced-plant nurseries rather than BFT from grazed pastures.

CONCLUSIONS

We concluded that grazed pastures of RBFT contain condensed tannin concentrations two to five times higher than concentrations in Norcen BFT. According to some researchers, the concentrations reported in this study would be considered near optimal for intake, digestion efficiency, and general animal performance. If they were not optimal, the concentrations in RBFT were slightly too high in one year, while those in BFT concentrations were slightly too low the other year. We also concluded that condensed tannin concentrations in grazed BFT were higher when grown in combination with tall fescue.

The differences in condensed tannin concentration between BFT and RBFT can be readily explained by genetics (Fig. 1 and 2). Differences between BFT in pure stands and that from grass mixtures, however, cannot be fully explained without further investigation of morphological, physiological, and ecological factors. Finally, we concluded condensed tannin concentrations can fluctuate through the spring grazing season as they do throughout the autumn in spaced-plant nurseries.
Table 1. Calibration and validation statistics for near-infrared
reflectance spectroscopy determination of condensed tannin
concentration in birdsfoot trefoil from grazed pastures, as well
as proportion of birdsfoot trefoil in mixed tall rescue pastures.

                               [R.sup.2]
                         N    ([dagger])   1 - VR ([dagger])

Condensed tannin         82      0.86            0.81

Botanical composition   136      0.97            0.96

                                   SEC           SECV
                        Mean   ([section])   ([paragraph])

                                 -- g CE/kg DM# --

Condensed tannin        26         6.8            8.6

                            -- % Birdsfoot trefoil --

Botanical composition   50.0       4.4            4.6

([dagger]) Calibration coefficient of determination achieved in
modified partial least squares regression of spectra against
chemically-derived data.

([dagger]) 1 - VR = 1 minus the variance ratio calculated in cross
validation.

([section]) SEC = standard error of calibration calculated in modified
partial least squares regression.

([paragraph]) SECV = standard error of cross validation calculated in
modified partial least squares regression.

# CE = catechin equivalent.


Abbreviations: BFT, birdsfoot trefoil without rhizomes; RBFT, birdsfoot trefoil with rhizomes; CE, catechin equivalents; DM, dry matter; NIR, near infrared reflectance spectroscopy.

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L. Wen, C. A. Roberts, J. E. Williams, R. L. Kallenbach, P. R. Beusenlinck, and R. L. McGraw

L. Wen and J.E. Williams, Dep. of Animal Sciences; C.A. Roberts, R.L. Kallenbach, and R.L. McGraw, Dep. of Agronomy; P.R. Beuselinck, USDA-Agricultural Research Service, Univ. of Missouri, Columbia, MO 65211. Received 15 Mar. 2002. * Corresponding author (RobertsCr@missouri.edu).
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Author:Wen, L.; Roberts, C.A.; Williams, J.E.; Kallenbach, R.L.; Beuselinck, P.R.; McGraw, R.L.
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Date:Jan 1, 2003
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