| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
ANIMAL NUTRITION |
,4
* Department of Animal Science, University of Arkansas Division of Agriculture, Fayetteville 72701;
and
Animal Science Section, Arkansas Cooperative Extension Service, University of Arkansas Division of Agriculture, Little Rock 72204; and
and
Department of Horticulture, University of Arkansas Division of Agriculture, Fayetteville 72701
 |
Abstract |
|---|
 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
0.002) across sampling dates for leaf, stem, and whole-plant tissues. Conversely, percentages of the total N pool within NDIN and ADIN fractions generally increased over sampling dates in mostly linear patterns. For crabgrass, the immediately soluble portion of the total N pool (fraction A; overall mean = 54.6% of N) was greater (P < 0.001) than for all control hays. Crabgrass exhibited a more rapid N disappearance rate (overall mean = 0.093/h; expressed as a proportion disappearing/h) than that of bermudagrass (0.046/h; P < 0.001), but the disappearance rate for alfalfa N (0.223/h) was considerably faster (P < 0.001) than for crabgrass. The effective ruminal disappearance of N was greater (P < 0.001) for crabgrass (overall mean = 85.4%) than for the alfalfa (83.3%), bermudagrass (72.3%), or orchardgrass (76.0%) control hays. For alfalfa, the ruminal disappearance rate of NDIN (0.150/h) was more rapid (P < 0.001) than for crabgrass (overall mean = 0.110/h); however, the disappearance rate for crabgrass was faster than that for bermudagrass (0.072/h; P < 0.001) or for orchardgrass (0.098/h; P = 0.010). Effective ruminal disappearance of NDIN was greater (P < 0.001) for crabgrass (overall mean = 72.0%) than for the bermudagrass (69.0%) or alfalfa hays (50.5%), but there was no difference (P = 0.865) between crabgrass and orchardgrass (72.1%). Although crabgrass forages exhibited concentrations of total N that were comparable with those of alfalfa and rates of ruminal N disappearance that were <50% of those for the alfalfa hay control, improvements in N use efficiency relative to alfalfa are questionable because of the excessively large Fraction A for crabgrass.
Key Words: crabgrass • in situ disappearance kinetics • neutral detergent insoluble nitrogen
|
INTRODUCTION |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
demonstrated that crabgrass offers a clear advantage over many perennial warm-season grasses in both of these respects, and the excellent nutritional value of the stem tissue appears to contribute heavily to these traits. Crabgrass may be a legitimate alternative to perennial warm-season grasses and may provide a partial remedy for the poor BW gains that are common throughout the late summer months in the southeastern United States.
The creation of new feeding models for livestock (Sniffen et al., 1992; NRC, 1996) has resulted in a need for in-depth knowledge of forage proteins. Knowing the distribution of N within plant fiber and understanding the ruminal disappearance kinetics of forage N are important considerations to acquire the greatest benefit from these feeding models. If common crabgrass is to be considered an alternative forage that can potentially replace perennial warm-season grasses in the diets of livestock with high nutrient demands throughout the southeastern United States, more knowledge is needed about the nutritive value and ruminal disappearance properties of crabgrass proteins.
Our objectives in this study were 1) to determine the concentrations of N, neutral detergent insoluble N (NDIN), and ADIN in leaf, stem, and whole-plant crabgrass tissues; 2) to evaluate ruminal in situ disappearance kinetics of N and NDIN for common whole-plant crabgrass forages harvested at weekly intervals during the summer; and 3) to compare these kinetic measurements with those of bermudagrass, alfalfa, and orchardgrass control hays determined simultaneously.
|
MATERIALS AND METHODS |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
) and are summarized briefly in Table 1.
|
; Elementar Americas, Inc., Mt. Laurel, NJ) on samples ground through a 1-mm screen. Subsamples ground similarly were digested in neutral (Van Soest et al., 1991) or acid detergent (Official Method 973.18; AOAC, 1990), and the N within the associated residues (NDIN and ADIN, respectively) was quantified by combustion procedures identical to those described for total N. Both NDIN and ADIN were calculated and reported on a percentage of total forage N basis. Both sodium sulfite (Van Soest et al., 1991; Licitra et al., 1996) and heat-stable 
-amylase were omitted from the neutral detergent solution. Concentrations of all fiber components within leaf, stem, and whole-plant crabgrass have been described previously (Ogden et al., 2005).
The ‘Affinity’ alfalfa (Medicago sativa L.), common bermudagrass (Cynodon dactylon [L.] Pers.), and "Benchmark" orchardgrass (Dactylis glomerata L.) control hays were harvested as small square bales at the University of Arkansas Forage Research Area in Fayetteville. All bales were made during Spring and Summer 2002, and they exhibited no evidence of spontaneous heating or molding. The orchardgrass hay was taken from a second cutting on June 19, 2002 and was entirely vegetative regrowth. Flakes were taken from the center of each bale and ground through either a 1-or 2-mm screen as described previously. For samples ground through a 1-mm screen, all methods of analysis were identical to those described for crabgrass. Concentrations of NDF and ADF were reported without correction for residual ash (Ogden et al., 2005). The bermudagrass hay (62.1% NDF, 27.0% ADF, and 2.74% N) had excellent nutritional value, but the alfalfa (51.9% NDF, 38.7% ADF, and 2.58% N) and orchardgrass (67.2% NDF, 34.9% ADF, and 1.97% N) hays exhibited only moderate nutritional value for these species.
In Situ Procedures
Five 383 ± 22.7-kg ruminally cannulated crossbred (Gelbvieh x Angus x Brangus) steers were used to evaluate in situ disappearance kinetics of N and NDIN for crabgrass and the control hays. All procedures involving the steers were approved by the University of Arkansas Animal Care and Use Committee. Steers were housed in individual 3.4- x 4.9-m pens with concrete floors that were cleaned regularly.
Steers were offered a diet of alfalfa hay (2.58% N, 51.9% NDF, and 38.7% ADF) and a corn-based supplement containing 94.7% cracked corn, 3.0% molasses, and 2.3% trace mineral salt (94.0% NaCl, 3,500 mg of Zn/kg, 2,000 mg of Fe/kg, 2,000 mg of Mn/kg, 300 mg of Cu/kg, 70 mg of I/kg, and 50 mg of Co/kg). On an as-fed basis, the basal diet contained 85.0% alfalfa hay and 15.0% supplement and was offered in equal portions at 0630 and 1430 at a total of 2.0% of BW daily; this level of intake was maintained by all steers throughout the adaptation period without refusal. Steers had ad libitum access to fresh water and were adapted to the basal diet for 10 d before initiating the experiment.
In situ procedures were consistent with the standardized techniques described by Vanzant et al. (1998). Dacron bags (10 x 20 cm; 50 ± 10-µm pore size; ANKOM Technology, Corp., Fairport, NY) were filled with 5 g of each experimental forage that had been ground previously through a 2-mm screen and heat-sealed with an impulse sealer (Type TISH-200; TEWI International Co., Ltd., Taipei, Taiwan). All dacron bags for each test period were placed in 35- x 50-cm mesh laundry bags and preincubated in tepid (39°C) water for 20 min to decrease the lag time associated with wetting. Samples were then inserted into the ventral rumen prior to the morning feeding and incubated for 3, 6, 9, 12, 24, 36, 48, 72, or 96 h. Subsequently, dacron bags containing in situ residues were rinsed in a top-loading washing machine (Model LXR7144EQ1, Whirlpool Corp., Benton Harbor, MI) using procedures outlined by Coblentz et al. (1997). A separate set of bags was preincubated and rinsed without ruminal incubation (0 h). To quantify the residual DM in each bag, after rinsing, all dacron bags containing forage residues were dried to a constant weight under forced air at 50°C and then allowed to equilibrate with atmospheric moisture before weighing.
To assess levels of microbial contamination before calculating disappearance kinetics, a subset (n = 50) of individual in situ residues that were selected with equal representation from all steers, forages, and incubation periods were analyzed independently for concentrations of purines by the method of Zinn and Owens (1986). Purine concentrations were found to be negligible (overall mean = 0.12 ± 0.042% of DM), and no corrections for N from microbial contamination were made before determining disappearance kinetics.
After ruminal incubation, a combustion technique identical to that described previously was used to quantify residual N and NDIN within in situ residues. These determinations of N were conducted on subsamples of approximately 0.1 g after careful mixing to ensure that all residual contents within in situ bags were represented adequately. Procedures for digestion of in situ residues in neutral detergent before quantifying NDIN were identical to those described for leaf, stem, and whole-plant crabgrass.
Data for ruminal disappearance of N and NDIN were fitted to the nonlinear regression model of Mertens and Loften (1980) by PROC NLIN of SAS (SAS Inst., Inc., Cary, NC). Nitrogen and NDIN were partitioned into 3 fractions based on relative susceptibility to ruminal disappearance (Nocek, 1988). Fraction A was defined as the immediately soluble portion, Fraction B represented that portion of N or NDIN that disappeared at a measurable rate, and Fraction C was defined as the portion of N or NDIN that was undegradable in the rumen. Fractions B and C, disappearance rate (kd), and the discrete lag time were determined directly by the nonlinear regression model. For each forage, Fraction A was calculated as 100% of N or NDIN – (B + C). For all forages, effective ruminal disappearance of N or NDIN was calculated as A + (B x [kd/(kd + kp)]), where kd = disappearance rate and kp = passage rate (Ørskov and McDonald, 1979). In this manuscript, both Kd and Kp are expressed as the proportion disappearing per hour. The kp for the basal diet within each steer was determined using acid detergent insoluble ash as an internal marker (Waldo et al., 1972); under the conditions described for this study, the mean calculated kp for the 5 experimental steers was 0.025 ± 0.0054/h (Ogden et al., 2005).
Statistical Methods
Effects of harvest date on N fractions (total N, NDIN, and ADIN) for the experimental crabgrass forages were analyzed as a randomized block design with field blocks (n = 4) as replications and the 7 harvest dates as treatment. Individual tissue types (leaf, stem, or whole plant) were analyzed by independent ANOVA. Harvest date means were separated by single degree of freedom orthogonal contrasts for linear, quadratic, cubic, and quartic effects of time. Disappearance kinetics of N and NDIN for whole-plant crabgrass harvested on the 7 dates and for the alfalfa, orchardgrass, and bermudagrass control hays were evaluated as a randomized complete block design with the 5 steers as experimental blocks. Single degree of freedom contrasts were utilized to evaluate the effects of harvest date on the disappearance kinetics of crabgrass and to compare crabgrass with each of the control hays.
|
RESULTS AND DISCUSSION |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
) declined linearly (P < 0.001) throughout the sampling period, but NDIN increased over sampling dates (P = 0.049, quartic) from 29.6 to 45.0% of N. Concentrations of ADIN increased (P = 0.049, quartic) over sampling dates, reaching a numerical maximum of 4.40% of N on August 15.
|
). Generally, these concentrations were relatively high compared with those observed for leaf-blade tissue sampled on the same date, ranging from 76.8 to 87.1% of those values. In past studies that have evaluated concentrations of N or CP independently within leaf and stem tissues for perennial warm-season grasses (Perry and Baltensperger, 1979; Griffin and Jung, 1983; Coblentz et al., 1998), percentages of N or CP in stem tissue generally are less than that observed for leaf blade, rarely exceeding 50% in harvests taken after July 1.
|
found that concentrations of NDIN, expressed as a proportion of total N, were 33 to 65% greater in leaf-blade tissue of eastern gamagrass [Tripsacum dactyloides (L.) L.] than in the associated stem. Although not compared statistically, concentrations of ADIN in crabgrass forages were comparable in magnitude and exhibited similar ranges in leaf-blade (2.65 to 4.40% of N) and stem tissue (2.22 to 4.13% of N).
Whole Plant.
Whole-plant concentrations of N declined linearly (P = 0.001), but in a relatively narrow range from 3.36 to 2.55% over the 7 weekly sampling dates (Table 4). This range is consistent with another report (White et al., 2001) in which pasture mixtures containing 90% crabgrass and 10% white clover (Trifolium repens L.) that were sampled during June and July in North Carolina exhibited a slightly greater mean concentration of N (3.54%). In the southeastern United States, crabgrass is likely to be used as an alternative to bermudagrass and may be utilized in monocultures or within mixtures with bermudagrass or bahiagrass (Paspalum notatum Flugge). By comparison, a survey study (Coblentz et al., 2004) indicated that concentrations of N ranged widely from 1.09 to 3.38% across warm-season (mostly bermudagrass) pastures sampled throughout northwest Arkansas.
|
). These differences also were observed between the alfalfa and bermudagrass hay controls evaluated in the current study; NDIN comprised only 16.8% of N for alfalfa compared with 50.8% of N for the bermudagrass (Table 4
).
Concentrations of NDIN for annual whole-plant crab-grass were comparable with those reported (Coblentz et al., 1999a) for perennial warm-season grasses harvested over a wide range of maturities, such as switch-grass (30.1 to 33.6% of N; Panicum virgatum L.) and eastern gamagrass (32.9 to 41.3% of N), but slightly lower than that observed for big bluestem (40.9 to 46.2% of N; Andropogon gerardii Vitman) and native tallgrass prairie (35.5 to 44.9% of N). Concentrations of NDIN comprising even greater proportions of the total N pool have been reported for eastern gamagrass (51.1 to 63.5% of N; Coblentz et al., 1998) and for bermudagrass hays [48.6 to 58.1% of N (Coblentz et al., 2000) and 49.2 to 69.0% of N (Turner et al., 2002)].
Large proportions of NDIN also have been reported for tropical warm-season forages fertilized with 0 to 157 kg N/ha per harvest and grown in Florida (Johnson et al., 2001). In that work, proportions of the total N pool associated with the cell wall decreased with N fertilization across all species, ranging from 58.2 to 52.4% of N for bahiagrass, 47.5 to 36.6% of N for bermudagrass, and 48.5 to 32.8% of N for stargrass (Cynodon nlemfuensis Vanderyst). The orchardgrass control that was evaluated in the current study also partitioned a large proportion of the total N pool (56.4% of N) into forms insoluble in neutral detergent; however, this is in contrast to reports by Sanderson and Wedin (1989), who found that >80% of the total N pool in leaves, stems, and whole-plant timothy (Phleum pratense L.) and smooth bromegrass (Bromus inermis Leyss.) existed in forms that are soluble in neutral detergent.
Disappearance Kinetics of N
Fractions A, B, and C.
Fraction A for crabgrass forages comprised from 49.9 to 59.8% of total N and varied (P < 0.001, quartic) somewhat erratically over the entire sampling period (Table 5). The mean Fraction A over the 7 sampling dates (54.6% of N) was greater (P < 0.001) by 11.2 percentage units of N than that observed for alfalfa and was substantially greater (P < 0.001) than that observed for either grass hay control. Numerous in situ studies have reported high concentrations of Fraction A for alfalfa, ranging from approximately 27.0 to 62.5% of N, depending on experimental methodology, as well as forage variety, maturity, and processing (Hoffman et al., 1993; Coblentz et al., 1997, 1999a). This fraction, which disappears from dacron bags at an unmeasurably rapid rate, is largely assumed to be in the form of nonprotein N, but also contains water-soluble or small-particle-associated proteins that efflux from in situ bags during washing (Broderick, 1994). Although it may be presumptuous to assume that these washout proteins are completely degraded in the rumen (Nocek, 1988; Broderick, 1994), the large Fraction A observed for crabgrass in this study suggests that N from this forage may be used inefficiently within the rumen because of rapid conversion to ammonia.
|
). The mean Fraction B for crabgrass (39.0% of N) was less (P < 0.001) than that for any of the control hays, but differences from the bermudagrass and orchardgrass hay controls (23.8 and 25.1% of N, respectively) were especially notable. Fraction C, which is unavailable in the rumen, was very small for all of the forages evaluated, ranging from 5.2 to 11.8% of N. For crabgrass forages, Fraction C increased linearly (P < 0.001) over harvest dates. Fraction C for all 3 control hays differed (P < 0.001) from the crabgrass forages, but the magnitude of these differences for the 2 grass hay controls was very small.
Lag Time.
For crabgrass, lag times were relatively short (overall mean = 1.19 h) and were not affected (P 
0.227) by sampling date. Lag times for crabgrass forages and alfalfa did not differ (P = 0.428), but time delays before disappearance for bermudagrass and orchardgrass were longer (P < 0.001) by 1.14 and 3.67 h than that for crabgrass forages.
Ruminal Disappearance Rate.
To the best of our knowledge, ruminal disappearance rates of N for crabgrass have not been determined previously. A linear (P = 0.005) decline was observed across sampling dates for crabgrass forages (overall mean = 0.093/h; Table 5), but the overall range of N disappearance rates across these dates was only 0.022/h. Ruminal disappearance rates of N for crabgrass forages were intermediate relative to those observed for the control hays. Crabgrass N exhibited a more rapid ruminal disappearance rate than bermudagrass hay (P < 0.001), a slower (P < 0.001) rate than alfalfa hay, but did not differ (P = 0.120) from the orchardgrass hay control. Rates of N disappearance for these crabgrass forages were greater than have been observed for other perennial warm-season grasses, such as eastern gamagrass, switchgrass, big bluestem, native tallgrass prairie, and fall-stockpiled bermudagrass (range = 0.031 to 0.067/h; Coblentz et al., 1998, 1999a; Scarbrough et al. 2002).
Although crabgrass forages had concentrations of total N that were comparable with alfalfa and exhibited rates of ruminal N disappearance that were < 50% of those observed for the alfalfa hay control, improvements in the efficiency of N use relative to alfalfa are questionable because of the excessively large Fraction A in crabgrass, which contains nonprotein N, intact water-soluble proteins, and some feed particle-associated N that may efflux from the bag during washing (Broderick, 1994). Although complete ruminal degradation of these sources of N may be presumptuous (Broderick, 1994), they disappear from in situ bags at an immeasurably rapid rate and are widely assumed to be readily available to rumen microbes.
Effective Ruminal Disappearance.
The effective ruminal disappearance of N from crabgrass forages ranged from 87.4 to 83.1% of N over all sampling dates (Table 5). Although the overall range was relatively narrow, estimates decreased in a cubic (P = 0.012) pattern. The mean effective ruminal disappearance for crabgrass (85.4% of N) was greater (P < 0.001) by 2.1, 13.1, and 9.4 percentage units of N than observed for the alfalfa, bermudagrass, and orchardgrass control hays, respectively (Table 5).
Although the alfalfa hay control used in this study was only of moderate nutritional value, the estimate of effective N disappearance (83.3% of N) was comparable with many other estimates made with similar methodologies (Balde et al., 1993; Hoffman et al., 1993; Coblentz et al., 1999a) and with in vivo estimates (83.4% of N) made by Vanzant et al. (1996). Previous estimates of effective ruminal disappearance of N from bermudagrass forages have ranged generally between 40.0 and 65.0% of N (Coblentz et al., 2001, 2004; Scarbrough et al., 2002), which are considerably less than that observed in the current study.
Disappearance Kinetics of NDIN
Fractions A, B, and C.
The percentage of NDIN within Fraction A for crabgrass was not affected (P 0.226; Table 6) by sampling date, and the mean overall Fraction A for crabgrass (0.5% of NDIN) did not differ from that observed for either alfalfa (P = 0.071) or bermudagrass (P = 0.287), but was less than that observed for orchardgrass (P < 0.001). For all practical purposes, recovery of NDIN in all forages was nearly complete (97.3% of NDIN) within Fractions B and C, and any NDIN partitioned within Fraction A might have been associated more closely with an efflux of N from dacron bags that was associated with small particles of fiber, rather than the actual solubility of cell wall-associated N.
|
0.9% for all forages except orchardgrass, but Fraction A for the entire NDF pool ranged from 5.2 to 9.1% of NDF across all forages (Ogden et al., 2005). Fraction B for crabgrass varied (P = 0.006, quartic) over sampling dates; however, responses generally were static with a very minimal range (86.6 to 89.9% of NDIN). The overall mean Fraction B for crabgrass forages did not differ (P = 0.707) from that of orchardgrass, but was greater (P < 0.001) by 28.8 percentage units of NDIN than for alfalfa, and less (P < 0.001) by 5.6 percentage units of NDIN than for bermudagrass. Fraction C for crabgrass forages increased (P < 0.001, quartic) over sampling dates; however, NDIN partitioned within Fraction C was only 1.0 percentage unit of NDIN greater on August 22 than that observed on the initial July 11 sampling date. Thus, fiber-associated N of crabgrass was highly available, and this characteristic varied little over sampling dates.
Lag Time.
For crabgrass forages, a quartic (P = 0.010) relationship was observed between the time delay before ruminal disappearance of NDIN and sampling date. However, this was caused primarily by minimum and maximum estimates on the July 18 and August 1 sampling dates, respectively, with estimates on all other dates being relatively static. Overall, the range for lag time of NDIN disappearance for crabgrass over all sampling dates was 0.35 to 1.30 h, and the mean lag time (0.78 h) for all crabgrass forage NDIN was shorter (P 0.001) than that observed for any of the control hays.
Ruminal Disappearance Rate.
The disappearance rate of NDIN for crabgrass forages decreased in a linear (P = 0.001) pattern across sampling dates. The mean rate of NDIN disappearance for crabgrass (0.110/h) was faster than that observed for bermudagrass (P < 0.001) or orchardgrass (P = 0.010) hays, but slower than that observed for alfalfa hay (P < 0.001).
Our estimate of NDIN disappearance rate for alfalfa in this study (0.150/h) closely matched a previous estimate of NDIN disappearance for alfalfa (0.138/h; Coblentz et al., 1999b). The NDIN disappearance rates for crabgrass were faster than reported for whole-plant eastern gamagrass harvested at boot stage, anthesis, and at physiological maturity (0.075, 0.051, and 0.043/h, respectively; Coblentz et al., 1999b). Similarly, Juarez Lagunes et al. (1999) reported digestion rates for NDIN ranging from 0.029 to 0.095/h (mean = 0.059/h) for 15 tropical grasses grown in Mexico. Collectively, these results suggest that NDIN from crabgrass is available in the rumen at a rate that exceeds the most rapid rates reported for similar N pools for perennial warm-season or tropical grasses.
Effective Ruminal Disappearance.
The effective ruminal disappearance of NDIN for crabgrass forages was 70.3 to 74.2% (P 0.002, quartic) across sampling dates (Table 6). Therefore, fiber-associated N of crabgrass was degraded extensively within the rumen, regardless of plant age or growth stage. Within the context of the very narrow overall range of these responses, effective disappearance was essentially minimized by the August 1 sampling date, with only minor variability thereafter.
Crabgrass had a greater (P < 0.001) effective ruminal disappearance of NDIN than the alfalfa or bermudagrass hay controls. The mean effective disappearance for all crabgrass forages was 72.0% of NDIN compared with only 50.5% of NDIN for alfalfa; however, reduced ruminal availability of NDIN from alfalfa forages probably is of limited biological significance because alfalfa normally partitions extremely high proportions (83.9 to 90.0%; Coblentz et al., 1998, 1999b) of its total N pool into forms soluble in neutral detergent. In contrast to the wide discrepancy observed between alfalfa and crabgrass, there was no difference (P = 0.865) in effective disappearance of NDIN between crabgrass forages and orchardgrass. Although they differed statistically (P < 0.001), the mean disappearance for all crabgrass forages (72.0% of NDIN) exceeded that of bermudagrass by only 3 percentage units of NDIN.
Compared with bermudagrass, the faster rates of ruminal disappearance for N and NDIN observed for crabgrass were generally consistent with expectations for higher quality forages. Although the ruminal disappearance rates for crabgrass were distinctly slower than the extremely rapid rates observed for the alfalfa hay control, a large proportion of the total N pool in crabgrass was immediately soluble and probably highly degradable in the rumen. This may negate some of the apparent potential for improved efficiency of N use relative to alfalfa. Ogden et al. (2005) has suggested that crabgrass offers clear advantages over bermudagrass with respect to ruminal disappearance kinetics of DM and fiber and appears to be a promising forage alternative to perennial warm-season grasses for livestock producers in the southeastern United States. Generally, the results of this study do not negate this potential, and further work, including intensive measurement of livestock production, is warranted.
|
Footnotes |
|---|
3 Current address: North Carolina State University Mountain Research Station, Waynesville 28786.
4 Current address: Northwestern Oklahoma State University, Alva 73717.
2 Corresponding author: coblentz{at}uark.edu
Received for publication June 7, 2005. Accepted for publication October 3, 2005.
|
LITERATURE CITED |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
