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Production Papers

Illinois Bundleflower Forage Potential in the Upper Midwestern USA

II. Forage Quality

Dep. of Agron. and Plant Genetics, Univ. of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108

^* Corresponding author (peter072{at}umn.edu)

Received for publication May 21, 2004.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS DISCUSSION AND CONCLUSIONS REFERENCES

 
Illinois bundleflower [Desmanthus illinoensis (Mich.) MacMill] is a warm-season perennial legume native to the central USA. Little is known about its forage quality in the upper midwestern USA. Two experiments were established at four Minnesota locations in 2000 to evaluate the effects of N fertilization, maturity at harvest, and residual height of cutting on the acid detergent fiber (ADF), neutral detergent fiber (NDF), crude protein (CP), in vitro true digestibility (IVTD), in vitro dry matter digestibility (IVDMD), and leaf proportion of three Illinois bundleflower (IBF) ecotypes. Whole-herbage forage quality was greatest (P < 0.05) at early flower in mid-July with average ADF, NDF, and CP concentrations of 315, 352, and 180 g kg^–1, respectively. Fiber values increased to 412 and 479 g kg^–1, respectively, while CP decreased to 129 g kg^–1 at late pod in mid-August. Leaf proportion decreased (P < 0.05) from 618 g kg^–1 at early flower to 335 g kg^–1 at late pod while leaf CP decreased (P < 0.05) from 216 to 147 g kg^–1. Whole-herbage IVDMD and IVTD concentrations decreased (P < 0.05) from 470 and 648 g kg^–1 at early flower to 390 and 560 g kg^–1 at late pod, respectively. Increasing residual cutting height from 15 to 35 cm decreased (P < 0.05) ADF and NDF concentrations by an average of 50 g kg^–1 and increased (P < 0.05) IVTD, IVDMD, and CP concentrations by 43, 39, and 20 g kg^–1, respectively. Illinois bundleflower can provide good quality summer forage in the upper midwestern USA, but more research is needed to ascertain the implications of its low IVDMD on animal performance.

Abbreviations: ADF, acid detergent fiber • CP, crude protein • DM, dry matter • IBF, Illinois bundleflower • IVDMD, in vitro dry matter digestibility • IVOMD, in vitro organic matter digestibility • IVTD, in vitro true digestibility • NDF, neutral detergent fiber

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS DISCUSSION AND CONCLUSIONS REFERENCES

 
ILLINOIS BUNDLEFLOWER is a warm-season legume native to the central plains of the USA. There is interest in mixing IBF with native warm-season grasses in the upper midwestern USA because of its potential for supplying fixed N to ruminants. Research in the grasslands of Texas, Oklahoma, and Kansas has indicated that IBF has significant potential as a forage legume. It has been successfully introduced into existing grass swards and increased dry matter (DM) yield (Dovel et al., 1990; Posler et al., 1993). In addition, IBF has a vigorous seedling that establishes well in monoculture and binary mixtures when planted into prepared seedbeds (Piper, 1998; Beran et al., 2000).

Despite a growing body of research evaluating IBF's agronomic potential, little is known about its forage quality potential or how management practices affect it, particularly in northern regions of adaptation such as Minnesota. In Kansas, IBF in binary mixture with sideoats gramma [Bouteloua curtipendula (Michx.) Torr.], indiangrass [Sorghastrum nutans (L.) Nash], or switchgrass (Panicum virgatum L.) increased CP concentration of the forage mixture over grass alone (Posler et al., 1993). However, the addition of IBF decreased the IVDMD concentration of the forage compared with grass alone. Adjei and Pitman (1993) reported leaf CP concentration of 230 g kg^–1 for IBF grown on a phosphatic clay mine-spoil in Florida. Despite relatively high leaf CP concentration, in vitro organic matter digestibility (IVOMD) concentrations averaged across three harvest frequencies were quite low, with leaf IVOMD of only 400 g kg^–1 and stem IVOMD of 350 g kg^–1. Interestingly, the IVOMD of IBF actually increased with plant maturity, which the authors suggest was due to dilution of antiquality components, possibly tannins. In Texas, despite an ADF concentration of only 214 g kg^–1, IBF in sacco DM and N degradability averaged only 422 and 450 g kg^–1, respectively, considerably less than five other forages with which it was compared (Packard et al., 2003).

Little is known about the forage quality of northern accessions of IBF grown in the upper midwestern USA. DeHaan et al. (2003) determined the ADF, NDF, and CP concentrations of 20 accessions of IBF at three locations in Minnesota. Averaged across locations and accessions, whole-herbage IBF forage quality was favorable compared with that reported for other more common forages (Sanderson and Wedin, 1989; Sheaffer et al., 2003) with 329, 367, and 180 g kg^–1 ADF, NDF, and CP, respectively. However, in the DeHaan et al. (2003) study, forage quality was determined on spaced plants with an average density of 0.9 plants m^–2 to characterize genotypic diversity. In forage production systems, IBF will likely be grown at much greater densities in monoculture or in mixture with warm-season grasses. Greater plant density is likely to affect the growth form of IBF and, consequently, its forage quality (Volenec et al., 1987). In addition, understanding the effects of N fertilization, plant maturity, and cutting height on the forage quality of IBF is essential to designing productive forage systems for the upper midwestern USA.

Although IBF nodulates with indigenous and applied Rhizobium spp., a positive yield response to N fertilization indicates the symbiosis may not be fully effective in satisfying the N needs of the plant (Byun et al., 2004; Fischbach et al., 2005). Thus, in some environments, N fertilization may be required to maximize total forage production of IBF grown in monoculture or in mixture with warm-season grasses. Nitrogen fertilization can increase the height of cool-season legumes, resulting in lower leaf proportions and greater fiber concentration. It can also increase the N content of the leaves, resulting in greater CP levels (Fishbeck and Phillips, 1981; Redfearn et al., 2001). It is unknown how N fertilization may affect forage quality of IBF.

Forage quality of most temperate legumes is affected by cutting height and maturity at harvest (Sheaffer et al., 2003). Cool-season legumes harvested before flowering or at early flower have greater forage quality than when harvested at late pod (Sanderson and Wedin, 1989). A higher cutting height tends to increase the leaf-to-stem ratio of the harvested forage and, thus, results in greater forage quality (Buxton et al., 1985). The magnitude of the increase depends on basal leaf retention and growth form of the forage species. Because of IBF's tall growth in dense stands (Fischbach et al., 2005), a higher residual cutting height should increase forage quality by increasing the leaf proportion of harvested forage.

The objective of this study was to evaluate the effects of N fertilization, maturity at harvest, and cutting height on forage quality of three northern ecotypes of IBF grown in monospecific stands in the upper midwestern USA.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS DISCUSSION AND CONCLUSIONS REFERENCES

 
Field Experiments
Experiment 1 was established at three University of Minnesota agricultural experiment stations at Rosemount, St. Paul, and Becker, MN, in spring 2000. Locations were selected to represent a range of potential environments in which IBF might be grown in the upper midwestern USA. The trials were seeded 7 June 2000 at Rosemount (44°53' N, 93°13' W) on a Tallula silt loam (coarse-silty, mixed, mesic Typic Hapludoll) with pH 6.9, 30 mg kg^–1 P, and 67 mg kg^–1 K; 8 June 2000 at St. Paul (44°99' N, 93°18' W) on a Waukegan silt loam (fine-silty over sandy, mixed Typic Hapludoll) with pH 6.8, 288 mg kg^–1 P, and 445 mg kg^–1 K; and 25 May 2000 at Becker (45°38' N, 93°89' W) on a Hubbard loamy sand (fine-silty over sandy, mixed Typic Hapludoll) with pH 7.1, 35 mg kg^–1 P, and 90 mg kg^–1 K.

The experiment was designed as a randomized complete block in a split-split plot arrangement with four replications. Treatments were imposed beginning in 2001, the year after establishment. Whole-plot treatments were either no N added or 110 kg N ha^–1 yr^–1. Subplot treatments were two stages of maturity at harvest. In 2001, an early-flower harvest was made around 17 July and a late-pod harvest around 17 August. Harvest treatments were repeated in 2002 at all locations; however, the mid-August 2001 treatment harvested at late pod was not repeated at St. Paul or Rosemount in 2002 due to a lack of persistence. Sub-subplot ecotype treatments were three relatively persistent northern ecotypes of IBF selected from the University of Minnesota Native Perennial Legume Collection by DeHaan et al. (2003). Accession 3 (Ecotype 3), from Gordner Lake, Stevens County, MN (45°30'49'' N, 96°00'38'' W), has early to midseason maturity with low to moderate seed yield and short plant stature. Accession 8 (Ecotype 8), from Spirit Lake, Dickinson County, IA (43°28'31'' N, 95°41'40'' W), has midseason maturity with moderate to high seed yield and tall height. Accession 10 (Ecotype 10), from Cottonwood Lake, Spink County, SD (44°46'41'' N, 98°41'40'' W), has early maturity with high seed yield and moderate height. All three accessions mapped to Cluster 1 in the genetic analysis of Dehaan et al. (2003). See Fischbach et al. (2005) for a complete description of the experimental design and treatments.

Experiment 2 was established in spring 2000 at three University of Minnesota agricultural experiment stations in southern Minnesota representing a range of potential environments in which IBF might be grown in the upper midwestern USA. Trials were seeded 7 June 2000 at Rosemount (same soil as in Experiment 1), 25 May 2000 at Becker (same soil as in Experiment 1), and 8 June 2000 at Lamberton (44°15' N, 95°19' W) on a Normania loam (fine-loamy mixed, mesic Aquic Hapludoll) with pH 7.5, 20 mg kg^–1 P, and 189 mg kg^–1 K. The experiment was designed as a randomized complete block in a split-split plot arrangement with three replications. Treatments were imposed in 2002, 2 yr after seeding. Whole-plot treatments were three stages of maturity at harvest. An early-flower harvest was made around 15 July, an early-pod harvest around 22 July, and a late-pod harvest around 13 August. Subplot ecotype treatments were the same three northern ecotypes of IBF used in Experiment 1, selected from the University of Minnesota Native Perennial Legume Collection by DeHaan et al. (2003). Sub-subplot cutting height treatments were either a 15- or 35-cm residual height and were selected to represent a range of potential residual heights to which IBF might be cut or grazed. See Fischbach et al. (2005) for a complete description of the experimental design and treatments.

Forage Quality Analysis
In Experiment 1, random 1-kg samples were hand-clipped from each sub-subplot at all locations immediately before early-flower and late-pod harvests and dried at 60°C for 96 h. Since leaves would most likely be grazed preferentially where IBF is used as pasture (Muir, 2002), selected locations and treatments were separated into leaf and stem components to gain information on leaf proportion and leaf quality with what resources were available. In 2001, samples from Rosemount were analyzed as whole herbage. All samples from St. Paul were separated into leaf and stem components following determination of total dry weight to quantify leaf proportion. Forage quality analysis was performed on only the leaf component. At Becker, all samples were separated into leaf and stem components to determine leaf proportion, recombined, and analyzed as whole herbage. In addition, at Becker, a sample taken at the late-pod harvest was separated into leaf and stem components, and forage quality analysis was performed on the leaf component. In 2002, harvested samples from all locations were analyzed as whole herbage.

In Experiment 2, random 1-kg samples were hand-clipped from each sub-subplot at all locations immediately before harvest and dried at 60°C for 96 h. All samples were left intact and analyzed as whole herbage except at Lamberton where Ecotype 3 samples were separated into leaf and stem components and forage quality analysis was performed on only the leaf component.

Samples from both experiments were ground with a Thomas–Wiley Laboratory Mill Model 4 (Thomas Scientific,¹ Swedesboro, NJ) to pass a 10-mm screen, hand-mixed, and then ground to 1 mm with a Cyclotec 1093 Sample Mill (Foss Tecator,¹ Eden Prairie, MN). All samples were scanned with a NIRSystems 6500 scanning monochrometer with a range of 400 to 2500 nm (NIRSystems Inc.,¹ Silver Springs, MD) to determine DM, NDF, ADF, CP, IVTD, and IVDMD concentrations. In vitro true digestibility was determined in addition to IVDMD to provide an alternative measure of digestibility and to begin to gain information about IBF's fiber digestibility. To develop calibration equations, 50 samples from Experiment 1 and 10 samples from Experiment 2 were chosen using WINSI II software (Infrasoft International,¹ Port Matilda, PA). Samples were selected based on spectral differences and represented leaves and whole herbage from harvests across four locations, three maturities, and 2 yr. Prediction equations were developed from the calibration sets by performing a modified partial least squares regression using the "Global Calibration" function of the WINSI II software. Equation statistics are shown in Table 1.

Statistical Analysis
Analyses of variance (ANOVA) of forage quality data were performed using the PROC GLM procedure of SAS (SAS Inst., 2001) using a split-split plot model (Gomez and Gomez, 1984). In Experiment 1, N fertilization, maturity at harvest, and ecotype were the whole-plot, split-plot, and split-split plot factors, respectively. In Experiment 2, maturity at harvest was the whole-plot factor, ecotype was the split-plot factor, and cutting height was the split-split plot factor. Results from each experiment, location, and year were analyzed separately due to heterogeneity among environments. Significant interaction effects were sporadic and inconsistent across locations and years; they are presented only in the text where relevant response variables are discussed and footnoted in tables. Main-effect means were separated with the Least Significant Differences test with a significance level of P = 0.05. Where treatment effects are discussed in the following sections, they are at the P = 0.05 level of significance.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS DISCUSSION AND CONCLUSIONS REFERENCES

 
Experiment 1
Leaf Proportion
Of all factors studied, maturity at harvest had the greatest effect on leaf proportion of harvested IBF forage (Table 2). Averaged for St. Paul and Becker, IBF leaf proportion decreased from 618 g kg^–1 at early flower to 335 g kg^–1 at late pod, an 85% decrease in leaf proportion from mid-July to mid-August. The influence of N fertilization on leaf proportion was biologically insignificant (i.e., statistically significant at St. Paul 2001 but not practically significant overall). Leaf proportion among IBF ecotypes was similar at Becker with an average of 551 g kg^–1. At St. Paul, a maturity at harvest x ecotype crossover interaction occurred because Ecotype 3 had the greatest leaf proportion at early flower, but Ecotype 8 had the greatest at late pod (data not shown).

Crude Protein
Whole-herbage CP concentration of IBF averaged 160 g kg^–1 while leaf CP concentration averaged 190 g kg^–1. Whole-herbage CP concentration decreased by an average of 27 g kg^–1 from early flower to late pod at Rosemount and Becker (Table 2). Over the same period, leaf CP concentration decreased by 69 g kg^–1 at St. Paul in 2001.

Nitrogen fertilization had inconsistent effects on CP concentration of IBF (Table 2). There was no effect on whole-herbage CP at Becker in 2001 and 2002. At Rosemount in 2001, there was an N x maturity interaction for CP concentration (data not shown); at early flower, N fertilization increased CP concentration by 38 g kg^–1, but there was no N effect at late pod. In 2002 at Rosemount, N fertilization decreased CP concentration by 22 g kg^–1. Leaf CP concentration increased 27 g kg^–1 at St. Paul in 2001 in response to N fertilization.

Ecotypes of IBF did not consistently differ in CP concentration across locations (Table 2). Averaged across all treatments in 2001, Ecotype 10 had greater whole-herbage CP concentration than Ecotypes 3 and 8 at Becker, but at Rosemount, Ecotype 3 had greater CP concentration than Ecotype 10. In 2002, Ecotype 3 had the greatest CP concentration at Rosemount, but there was no difference among ecotypes at Becker or St. Paul. Leaf CP concentration differed among ecotypes at St Paul where Ecotype 8 had the greatest CP concentration and Ecotype 3 had the lowest. There were no ecotype differences in leaf CP concentration at Becker.

In 2001, nearly 75% of total harvested N was in IBF leaves. At Becker, an average of 3.9 Mg DM ha^–1 was harvested at late pod (Fischbach et al., 2005). This biomass had whole-herbage CP concentration of 151 g kg^–1 (Table 2). Therefore, a single harvest at late pod contained 94 kg N ha^–1. Harvested herbage had an average leaf proportion of 447 g kg^–1 with a CP concentration of 205 g kg^–1. Thus, of the 94 kg N ha^–1 harvested at late pod, 57 kg N ha^–1 was contained in the leaves. This value is similar to N yield at St. Paul at early flower where harvested leaves yielded an average of 62 kg N ha^–1.

Digestibility
Whole-herbage IVTD and IVDMD concentrations of IBF averaged 630 and 475 g kg^–1, respectively. In 2001, whole-herbage IVTD decreased by an average of 100 g kg^–1 from early flower to late pod at Becker (Table 4). At Rosemount in 2001, IVTD decreased by 31 g kg^–1. Leaf IVTD and IVDMD decreased by an average of 31 g kg^–1 from early flower to late pod at St. Paul in 2001.

There were slight digestibility differences among ecotypes (Table 4). Ecotype 3 had greater whole-herbage IVTD than Ecotypes 8 and 10 at Rosemount in 2001. Ecotype 8 had greater leaf IVTD and IVDMD than Ecotypes 3 and 10 at St. Paul. There was a three-way N x maturity x ecotype interaction for whole-herbage IVTD and IVDMD at Becker in 2001. Ecotype 10 had greater digestibility than Ecotypes 3 and 8 at early flower in unfertilized plots, but there were no differences among ecotypes at early flower in fertilized plots (data not shown). At late pod, Ecotype 10 was less digestible than Ecotypes 3 and 8 in unfertilized plots, but in N-fertilized plots, Ecotypes 10 and 8 had similar digestibility.

Experiment 2
Fiber Concentration
Whole herbage of IBF averaged 398 and 452 g kg^–1 ADF and NDF, respectively. Whole-herbage fiber concentrations increased from early flower to late pod at all locations (Table 5). Acid detergent fiber increased from 340 g kg^–1 at early flower to 450 g kg^–1 at late pod while NDF increased from 370 g kg^–1 at early flower to 520 g kg^–1 at late pod. A 35-cm residual cutting height decreased ADF and NDF concentrations at all locations by an average of 50 g kg^–1 compared with the 15-cm cutting height. There were no ecotype differences in fiber concentration except at Lamberton where Ecotype 10 had lower ADF and NDF concentrations than Ecotype 8. Leaf fiber concentrations, analyzed only for Lamberton, were unaffected by any treatment factor.

Digestibility
Whole-herbage IVTD and IVDMD concentrations of IBF averaged 604 and 386 g kg^–1, respectively, across locations, and decreased by an average of 107 g kg^–1 from early flower to late pod (Table 7). The largest decline in digestibility occurred at Becker where IVDMD concentration declined by 170 g kg^–1 from early flower to late pod. There was no significant change in leaf digestibility despite a trend toward lower digestibility with advancing maturity (Table 7). Both measures of digestibility were affected by cutting height. Leaving a higher residual cutting height increased whole-herbage IVTD and IVDMD at all locations by an average of 41 g kg^–1. There were no differences in ecotypes except at Lamberton where whole-herbage IVTD and IVDMD were greater for Ecotype 10 than for 8.

	DISCUSSION AND CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS DISCUSSION AND CONCLUSIONS REFERENCES

Whole-herbage and leaf fiber concentrations in IBF compared favorably with cool-season legumes. The experiment average of 360 g kg^–1 whole-herbage ADF concentration is similar to a season average of 370 g kg^–1 for alfalfa (Medicago sativa L.) in Minnesota (Sheaffer et al., 2000) and 350 g kg^–1 for stockpiled birdsfoot trefoil (Lotus corniculatus L.) harvested in mid-August in Wisconsin (Collins, 1982). The NDF concentration is also similar, if not less, with 410 g kg^–1 for IBF compared with 440 g kg^–1 for alfalfa.

In mid-July, whole-herbage CP concentration of IBF averaged 180 g kg^–1 but dropped to 130 g kg^–1 by mid-August. The mid-July CP concentration at early flower is about 20 g kg^–1 less than that measured in alfalfa cut at early flower in Minnesota (Sheaffer et al., 2000) but similar to the season-average CP concentration of cool-season legumes in a two-cut system (Sheaffer and Marten, 1991).

Whole-herbage and leaf IVDMD of IBF were low compared with reports for cool-season legumes. Whole-herbage IVDMD of cool-season legumes in a two-cut system in Minnesota ranged from 672 g kg^–1 for kura clover (Trifolium ambiguum M. Bieb.) to 554 g kg^–1 for birdsfoot trefoil (Sheaffer et al., 2003). In this study, IBF whole-herbage IVDMD averaged only 450 g kg^–1 at early flower in mid-July to 350 g kg^–1 at late pod in mid-August. Although both measures of digestibility were positively correlated with leaf concentration (IVDMD r = 0.89, P < 0.05; IVTD r = 0.93, P < 0.05), even the IVDMD of leaf samples was only 650 g kg^–1 compared with an IVDMD of alfalfa leaf of 800 g kg^–1.

As with other legumes (Sanderson and Wedin, 1989), IBF whole-herbage quality declined as plants matured from early flower to late pod. Crude protein concentration decreased by 38% from 180 to 130 g kg^–1 while ADF and NDF concentration each increased by about 34%. In vitro dry matter digestibility and IVTD decreased by averages of 29 and 17%, respectively. Between mid-July and mid-August, leaf proportion decreased 85%. The overall decline in forage quality with advancing maturity is most likely due to the precipitous decline in leaf proportion and concomitant increase in stem concentration as has been reported for a diversity of legumes (Sanderson and Wedin, 1989; Sheaffer et al., 2003).

The decline in leaf proportion was partly a result of normal leaf senescence associated with flowering and seed set. However, leaf proportion of harvested biomass was also affected by the variable and poorly understood occurrence of leaf drop. In both experiments, leaf drop from the lower two-thirds of the stem was sporadic across environments, occurring at any stage of maturity. The cause is unknown but may be due to micronutrient deficiencies, plant density, drought stress, leaf maturity, or foliar disease. A better understanding of the causes of leaf drop is needed to develop management schemes that limit leaf drop and, thus, maintain forage quality through July and August.

Adding N increased whole-herbage CP concentration at only one of five site-years and had no effect or decreased whole-herbage CP at four of five site-years. Nitrogen fertilization had no consistent, biologically significant affect on whole-herbage fiber concentration or digestibility. The lack of a consistent increase in CP concentration in response to N may be due to the relative unresponsiveness of IBF to N fertilization or the utilization of supplemental N for stem growth rather than leaf growth. The positive yield response to N fertilization (Fischbach et al., 2005) coupled with a decrease in leaf proportion in response to N seen in Experiment 1 supports the latter explanation.

A higher residual cutting height improved forage quality at all three site-years. Acid detergent fiber and NDF concentrations decreased by about 50 g kg^–1, CP increased by 20 g kg^–1, and IVTD and IVDMD both increased by 40 g kg^–1. In the high-density stands of our study, leaves senesced and dropped from the basal 30 cm of IBF stems by mid-July, perhaps due to shading. The higher residual cutting height did not include these leafless stems and, thus, effectively increased leaf proportion and whole-herbage quality. In Experiment 2, Ecotype 3 had greater fiber concentration and digestibility responses to cutting height than did Ecotypes 8 and 10. As the shortest of the three ecotypes, Ecotype 3 may have benefited more from having its leafless stem bases excluded from harvested forage. Since Ecotypes 8 and 10 were taller, the 35-cm cutting height did not alter the leaf proportion as in Ecotype 3. Given the resulting greater forage quality, greater regrowth, and the lack of a reduction in first-harvest forage yield associated with a 35-cm cutting height (Fischbach et al., 2005), the data suggest that IBF should be harvested with a relatively high residual cutting height to optimize yield and forage quality. Indeed, a 35-cm residual height may more closely approximate the height to which grazing animals are apt to defoliate the plant than a 15-cm residual.

The low IVDMD of IBF leaves and whole herbage measured in this study is consistent with existing literature. Posler et al. (1993) found that addition of IBF to pure grass forage decreased digestibility compared with grass alone. Adjei and Pitman (1993) reported IBF whole-herbage IVOMD of only 35%. Interestingly, in contrast to our study, digestibility increased with advancing maturity, which the authors suggest was due to dilution of antiquality components. Packard et al. (2003) reported only 422 g kg^–1 in sacco degradability for IBF. Percentage and species of legume in the diet of cattle in/from which digestibility estimates were made influenced IBF in sacco degradability, but to a limited extent.

Neutral detergent fiber digestibility (NDFD) of IBF forage computed from IVTD and NDF measurements (NDFD = {1 – [(100 – IVTD)/NDF]} x 100) was consistently less than 5 g kg^–1 for Experiment 2 and negative for Experiment 1 (data not shown). These low levels further suggest that little digestion occurred in the rumen fluid incubation stage of the IVTD procedure and that IVTD values were determined primarily by the second-stage NDF extraction. These results agree with those of Packard et al. (2003), who reported 0 g kg^–1 ADF degradability of IBF measured in sacco.

It is unclear what caused low IBF digestibility in vitro; however, there is evidence that tannins may be responsible. Preliminary tannin analysis of IBF leaves in our laboratory, using the procedure of Terrill et al. (1990) as modified by Miller and Ehlke (1994), revealed 63 g catechin equivalent (CE) tannin kg^–1 DM compared with leaf values of 1 and 139 g CE kg^–1 DM for alfalfa and sericea lespedeza (Lespedeza cuneata G. Don ‘Interstate-76’), respectively. The presence of tannins in IBF, especially in leaves, is consistent with reports by Adjei and Pitman (1993) and W.R. Ocumpaugh (personal communication, 2003). However, the effect of tannins on digestibility of IBF forage remains unknown. Condensed tannin concentration is negatively related to IVDMD in birdsfoot trefoil (Miller and Ehlke, 1994). Lignin may also limit IBF digestibility (Packard et al., 2003). Additional research is needed to identify and quantify any other antiquality or antimicrobial compounds that may interfere with ruminal digestion of IBF herbage.

Forage quality for the three ecotypes in our study was slightly less than values reported for the same ecotypes in a spaced-plant study by DeHaan et al. (2003). In their study, ADF and NDF concentrations were greater by about 40 g kg^–1 at both early flower and late pod while CP concentration was 13 g kg^–1 less at early flower but nearly 40 g kg^–1 less at late pod. Growing IBF in the high-density monocultures of our study tended to increase the height of IBF plants, decrease the number of stems per plant, and decrease leaf concentration (Fischbach et al., 2005) compared with spaced plants in the DeHaan et al. (2003) study. These differences in IBF growth form may explain the differences in forage quality, especially the much larger decline in CP concentration between early flower and late pod in our study. Taller, more closely spaced plants may have shaded basal nodes and had relatively greater leaf senescence/drop, which by mid-August affected leaf and CP concentrations. However, environmental differences between the two studies may have contributed to forage quality differences as well.

In any one site-year in our study, differences among ecotypes were small. When averaged across all environments, however, Ecotype 3 tended to have the greatest CP and lowest fiber concentrations. Ecotypes 8 and 10 generally had similar forage quality. In spaced planting, Ecotype 3 tended to have the greatest CP and lowest ADF and NDF concentrations while Ecotype 10 had the lowest CP and greatest fiber concentrations (DeHaan et al., 2003). Differences among ecotypes, although small, can be explained by differences in growth form. As the shortest of the three ecotypes, Ecotype 3 tended to have the greatest leaf proportion in both studies and, thus, the greatest forage quality.

This study demonstrates that IBF can be a source of high-protein, low-fiber herbage during the summer slump in cool-season legume growth in the upper midwestern USA. However, the impact of IBF's limited in vitro digestibility on the performance potential of ruminants is unknown and needs further investigation.

	ACKNOWLEDGMENTS

 
The Land Institute (Salina, KS) partially funded this project through a graduate research fellowship to the senior author. Additional support was provided by the Minnesota Agricultural Experiment Station and the Rapid Agricultural Response Fund of the Minnesota Legislature. This project was made possible by the field assistance of Jennifer Abraham, Doug Swanson, and Joshua Larson. Special thanks to Jim Halgerson for assistance with NIRS and forage quality determination.

	NOTES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS DISCUSSION AND CONCLUSIONS REFERENCES

 
¹ Names are necessary to report factually on available data; however, the University of Minnesota neither guarantees nor warrants the standard of the product, and the use of the name by the University of Minnesota implies no approval of the product to the exclusion of others that may be suitable.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS DISCUSSION AND CONCLUSIONS REFERENCES

 

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