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a Agronomy Dep., Iowa State Univ., Ames, IA 50011-1010
b Dep. of Plant and Soil Science, Texas Tech Univ., Lubbock, TX 79409-2122
c USDA-ARS, Dep. of Agronomy and Horticulture, Univ. of Nebraska-Lincoln, Lincoln, NE 68583-0937
d Dep. of Range, Wildlife, and Fisheries, Texas Tech Univ., Lubbock, TX 79409
* Corresponding author (dphilipp{at}iastate.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: ADF, acid detergent fiber • CP, crude protein • DDM, digestible dry matter • DMD, dry matter digestibility • MSC, mean stage count • MSW, mean stage weight • NDF, neutral detergent fiber • NIR, near-infrared absorption technique • OWB, old world bluestem • PET, potential evapotranspiration • PLS, pure live seed • TNC, total nonstructural carbohydrates • WW, Woodward
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Nutritive value of forages is influenced by many factors including soil fertility, growth stage, species, and photosynthetic pathway (Hodges and Bidwell, 1993). Investigations by Sanderson et al. (1999), Dewald et al. (1995), and Niemann (unpublished data, 2000, Texas Tech University) suggested that nutritive value of old world bluestems (OWB) was influenced by species, environmental conditions, management, and physiographic location. Morphological characteristics of forages influence and can help in predicting nutritive value (Mitchell et al., 2001).
Available moisture as precipitation and irrigation is directly related to growth and total productivity of these OWB species (Philipp, 2004). In the Texas High Plains, water for irrigation is declining and additional land will likely be converted from cropland to grassland. While Bothriochloa species have been widely adopted, little information is available regarding their nutritive value and morphological responses to a variety of water regimes when grown in the climatic conditions of Texas High Plains. Thus, our objectives were to determine nutritive value and growth characteristics of B. caucasica, B. ischaemum, and B. bladhii under the semiarid environmental conditions of the southern High Plains of Texas as influenced by amount of irrigation water.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Existing stands of ‘Caucasian’ (B. caucasica), ‘WW Spar’ (B. ischaemum), and ‘WW-B. Dahl’ (B. bladhii) old world bluestems were used. There were three replicates of each forage (0.07 ha per replicate) in a complete randomized block design. Forages, established in 1996, were used in a grazing experiment with lambs (Ovis aris) during 3 yr (1998–2000; Niemann et al., 2001). Grazing was terminated in September 2000, before initiation of the current experiment in 2001.
Four water treatments imposed in a split-plot arrangement within each forage species replication included dryland, and low, medium, and high irrigation levels. Thus, there were 36 treatment plots and each was 10 by 15 m. Amount of water applied in the high level was 100% replacement of potential evapotranspiration (PET; Allen et al., 1998) minus precipitation. Medium and low levels were calculated as 66 and 33% of the high level; the dryland treatment received no irrigation (0%). Irrigation water was applied through a surface drip irrigation system. Details on derivation of PET data were published in Philipp (2004).
Average water applied (irrigation and/or precipitation) during the growing season (May to October) was 154, 421, 685, and 928 mm for dryland, low, medium, and high levels, respectively. Additionally, all plots received an average of 182 mm precipitation during dormant period (November–April; Philipp, 2004). In Year 1, treatments began in June after installation of irrigation system was completed. Years 2 and 3 represented full season irrigation from April through October.
In June 2001, plots, except those assigned as dryland, were equipped with 10 lines (1-m spacing) of 16-mm irrigation tubing (Eurodrip, San Diego, CA) with a 457-mm emitter spacing and a delivery rate per emitter of 1.52 L h–1. About 207 kPa pressure was maintained by valves connected to flowmeters that allowed precise monitoring of delivered water amounts. Water treatments began in spring with emergence of photosynthetically active tissues and ended in autumn with occurrence of the first frost, with the exception of Year 1, which began 21 June 2001, following installation of the irrigation system.
Soil at the site was a nearly level (0–1% slope) Pullman clay (fine, mixed, superactive, Thermic Paleustoll). Plots were fertilized equally across all treatments to meet or exceed soil test recommendations such that N and other nutrients were not limiting. In 2001, the initial year, N (60 kg ha–1) as urea was applied by hand in early August. In 2002 and 2003, N (60 kg ha–1) was applied at the beginning of the growing season and again in early August following hay harvest. Nitrogen application in spring was delivered in 38 mm water to all plots by means of an existing underground drip irrigation system. In August, N was applied by hand. After resuming irrigation treatments in spring 2002, the system was cleaned to remove algae with an N-containing solution, effectively increasing total N applied to 140 kg N ha–1. Thus, in 2002, only 30 kg N ha–1 was applied in August. All plots were harvested as hay at the end of July of each year (mean harvest date 31 July). Biomass was also removed in late winter before spring growth.
Plant material for forage nutritive value and morphology was sampled monthly within each treatment from May through October. Thus, a total of 36 plots were sampled six times during the growing season. Sampling in July occurred simultaneously with hay harvest. For measurements of forage nutritive value, six random forage samples were collected from each plot replication at each sampling date. Samples were taken at a clipping height of approximately 8 cm, composited, and dried at 55°C to a constant weight. Samples were ground in a Wiley mill (Comeau Technique Ltd., Vandreuil-Dorion, QC, Canada) to pass a 1-mm screen and ground forage samples were stored at room temperature for further analysis.
Plant samples for morphology were taken by clipping randomly six subsamples at an 8-cm height from each plot and transferred to the laboratory. Samples were separated into leaf blade, stem including leaf sheath, and dead fraction of tissue similar to the methodology used by Coyne and Bradford (1985). Separated samples were dried at 55°C to a constant weight and weight-based ratios among fractions were calculated. Additional samples were taken from WW-B. Dahl plots only to determine developmental morphology in June, July, late August, and October. These samples were taken by clipping randomly two 0.33- by 0.33-m quadrats from each plot. Tiller samples were placed in paper bags, stored in a cooler in the field, and transferred immediately to a freezer. One hundred tillers were chosen randomly and classified regarding their vegetative and elongated stages (Moore et al., 1991). The number of tillers in each class was counted. Each tiller class was weighed fresh, placed in paper bags, dried at 55°C, and the final constant weight determined. Because 100 tillers per sample were used for developmental morphology, a sampling area of 0.1 m2 was found not sufficient to obtain enough tillers to conduct a meaningful analysis, especially in early and late season when numbers of tillers per plant were greatly decreased. Thus, during 2002 and 2003, samples containing 500 to 1000 tillers were selected randomly from the entire plot area from which 100 tillers were again selected randomly.
Estimates of forage nutritive value included neutral detergent fiber (NDF), acid detergent fiber (ADF), crude protein (CP), and total nonstructural carbohydrates (TNC). These parameters were calculated by regression analysis based on near-infrared absorption patterns (near-infrared absorption technique, NIR) of all forage samples from each date of collection and a calibration subset of the sample population was analyzed by conventional wet chemistry procedures described below. Ground forage samples were scanned with a spectrophotometer (NIRSystems, Inc., Model #5000, Silver Spring, MD) and their absorption characteristics were determined by NIRS 2, Version 3.10-software provided by the company. For the subset of samples used for calibration, NDF, ADF, hemicellulose (NDF-ADF), cellulose, and lignin were determined on the basis of procedures described by Van Soest (1963) and Goering and Van Soest (1970). Percentage CP was estimated according to AOAC (1995). Dry matter digestibility (DMD) was estimated as DMD% = 88.9 – (0.779 ADF%) according to Linn and Martin (1989). Percentage TNC of samples was analyzed using the procedure described by Mounsif (1986) with modifications described by Philipp (2004).
Morphological characteristics of forages were predicted by NIR (Karnezos et al., 1993). A calibration set obtained by hand separation was used to predict percentage leaf blade, stem (including leaf sheath), and dead material of all samples originally scanned.
Economic returns for inputs of water in terms of pumping costs were calculated regarding amount of total digestible dry matter (DDM; Mg ha–1) obtained per unit of water supplied. A cost of $0.0381 was presumed for pumping 1 m–3 of water (Texas Cooperative Extension, 2004).
Effects of species and water treatments on response variables were determined by analyzing variances within and among treatments (Steel and Torrie, 1960). The possible intercorrelation that may have resulted from the systematic assignment of water treatments was addressed in the data analysis by using a Toeplitz variance-covariance structure (Barnett, 1990) for the water treatment effect (applied to each block/species combination) by the Mixed procedure in SAS (SAS Institute, 1998). Data collected monthly from each plot were analyzed as a repeated measures effect. Differences were considered significant at P < 0.05, unless stated otherwise.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Differences existed among species in CP concentration averaged across water treatments (Fig. 2A) . Higher CP concentration was observed in B. bladhii through the first half of the growing season compared with the other two species. In August, following the hay cut in late July, CP increased in all species, with B. bladhii being greater than B. ischaemum but similar to B. caucasica. By September and October, B. bladhii was again higher in CP than either B. caucasica or B. ischaemum. Bothriochloa caucasica was lower in CP than either B. bladhii or ischaemum in July, September, and October. Percentage CP declined in all three species with increasing maturity before the hay cut in July and dormancy in October.
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Total nonstructural carbohydrates declined with increasing irrigation amounts before hay cut in July (linear and quadratic effects; Fig. 3A) . Regrowth in August also decreased in TNC with increased irrigation treatment (linear effect). In September and October, however, TNC was higher in low and medium irrigation levels than either dryland or high irrigation (cubic effects). Concentration of TNC in all irrigated forages appeared to increase during the regrowth period while forage grown under dryland conditions generally declined in TNC.
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For 2002 and 2003, the 2 yr that received full season irrigation, total DDM production in July and October (Mg ha–1) differed among species (Fig. 4) . In 2002, B. caucasica and B. bladhii were similar but both were greater than B. ischaemum during July and October. In the following year, B. caucasica was higher in DDM than the other two species in July, September, and October. Additionally, B. caucasica was higher than B. ischaemum in June 2003. More DDM (Mg ha–1) was produced (about 77%) during the first growth period than during the second growth period after the hay cut regardless of irrigation level or species.
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Only B. bladhii was used to determine tiller dynamics. Year interactions were determined; however, water treatment effects did not differ in direction of response and thus, data were averaged. Tiller mean stage weight (MSW) increased linearly in June with increasing water treatments, and in August and October differences were greatest between dryland and irrigated plants (quadratic effects; Fig. 8A) . In August, following the hay cut in July, the primary difference was between dryland and irrigated plants with MSW of dryland plants only about 60% of irrigated plants (quadratic effect). The mean stage count (MSC) followed closely relationships of MSW (Fig. 8B). A linear increase in response to increased irrigation levels was observed in June but in July as plants approached maturity, this relationship was not significant. In August and October, linear effects of water treatments were again present but in regrowth following the hay cut in July, differences were primarily between dryland and irrigated forages.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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These irrigation relationships with nutritive value were visible during the entire growing season but were most consistent before the July hay harvest. Effects of irrigation on NDF and ADF during regrowth appeared related more to advancing growth stage and an increase in dead material than to a shift in leaf blade:stem-plus-sheath ratio. Following the July harvest, dryland effects on fiber components may be attributed to the fact that regrowth under these conditions was usually sparse, and forage samples included plants that were older along with fresh regrowth that occurred in response to sporadic precipitation.
Measured fiber values (660–730 g kg–1) were comparable to research conducted by other authors under conditions of more or less frequent defoliation of old world bluestems. Allen et al. (2000) indicated B. caucasica averaged 710 g kg–1 NDF under grazing in Virginia, whereas Niemann (D. Niemann, Texas Tech Univ., unpublished) obtained approximately the same results under herbivory in Texas. Londoño et al. (1981) reported NDF and ADF values of B. ischaemum hays of 740 and 440 g kg–1, respectively. Linn and Martin (1989) indicated that digestibility of NDF may vary from 200 to 800 g kg–1 across a range of forages, depending on plant age. Our data showed that NDF was lower early in the growing season independent of water treatments. It also appeared that water treatments had a smaller effect on NDF, ADF, and TCN in the first half of the growing season than in the second part. Thus, a greater nutritional advantage can probably be achieved by irrigation management during the later part of the growing season than in early season growth. However, because about 77% of dry matter (DM) was produced by July, it appears likely that when water resources are limited, investments in irrigation water during the early part of the growing season are more appropriate than irrigation after July.
Hay production or grazing of old world bluestems early in spring may also be optimal from the perspective of CP. While only slight numerical differences existed across water treatments, CP concentration fell from approximately 120 g kg–1 in May to 60 g kg–1 in July—a value that would limit intake in ruminants (Minson, 1990). The increase in CP in regrowth in August was likely due to physiologically younger plants and to the second application of N that was applied following hay removal in July. However, similar to observations made in the first half of the growing season, CP values were below 60 g kg–1 in irrigated plots by October. It appeared that water treatments affected CP values much less than time of sampling reflecting advancing growth stage in all irrigation treatments.
Nitrogen fertilization as applied in this experiment should not have been limiting for growth of old world bluestems (Berg, 1990) and CP concentration in plant tissues was likely a reflection of N metabolism by C4 grasses. Crude protein in C4 grasses is typically marginal for animal dietary requirements (Minson, 1990). The C4 grasses generally respond to soil N with an increase in growth but concentrate N in plant tissues to a lesser extent than C3 grasses (Hallock et al., 1965). Species differences in CP were evident throughout the entire period of investigation. The finding that B. bladhii averaged higher in CP than the other old world bluestems tested was consistent with findings in grazing trials previously conducted at this location (D. Niemann, unpublished). It is not clear if these advantages would hold elsewhere. Sanderson et al. (1999) indicated that CP concentration of B. bladhii depended on soil type. Dewald et al. (1995) reported that CP in B. bladhii was similar to other old world bluestems at Woodward, OK. This would limit general conclusions regarding CP prediction for investigated species. Nevertheless, the consistently higher CP values under a range of irrigation levels as well as when grazed by lambs (D. Niemann, unpublished) at the same location suggests that higher CP in B. bladhii vs. the other old world bluestems was consistent under varying management strategies. Although B. bladhii was clearly higher in CP, nutritional requirements of most livestock would still indicate that supplementation with CP would be needed (Minson, 1990).
In comparison to the finding that CP declined by approximately 50% over 3 mo, TNC was more stable during the first 2 mo of the growing season but increased under all irrigation levels in July. Differences due to water treatments were more evident at hay cut in July and in regrowth during late summer. Coyne and Bradford (1987) suggested that on average, TNC concentrations during the growing season were higher in leaf sheaths and enclosed stems followed by stem bases and leaf blades. Higher leaf blade:stem-plus-sheath ratios were observed under limited irrigation, but according to Coyne and Bradford (1987) that should have led to a decrease in above-ground plant TNC concentration. Thus, it is likely that TNC in July was enhanced because of stress-induced cell sugar accumulation and relative change in accumulation of photosynthates vs. respiration compared with previous months and water treatments (Blaser et al., 1966).
Total nonstructural carbohydrates tended to increase toward the end of the growing season as well as increase in response to increasing water stress under medium and low irrigation. The lack of increase under dryland conditions in late summer likely reflected plant dormancy and senescence as water stress became more severe. Observed TNC in B. caucasica was somewhat higher than 57 g kg–1 reported by Allen et al. (2000) in Virginia under grazing. Forwood et al. (1988) reported even less (47 g kg–1) for B. caucasica in Missouri that was moreover less than in native big bluestem (Andropogon gerardii Vitman). Conversely, Niemann (D. Niemann, unpublished) found about 120 g kg–1 TNC in grazed B. caucasica in Texas. This content was reached in our research only at the end of the growing season under water-stressed conditions.
The increase in DMD under water-stressed conditions could be due in part to an accumulation of sugars as evidenced by the increase in TNC concentration. However, DMD was estimated on the basis of ADF; thus, additional factors influencing digestibility were involved. Animal trials are needed to more accurately assess apparent DMD.
Total yield of digestible DM may be more important than total biomass. However, the relatively small differences in DMD that ranged between 550 and 580 g kg–1 did not offset advantages in total seasonal DM production in response to irrigation treatment. Thus, DDM followed closely patterns of DM yield. Niemann et al. (2001) found similar gain per hectare and daily gains of lambs grazing B. caucasica and B. bladhii, which supports our findings of similar amounts of DDM per unit area for these two species.
In terms of the value of irrigation water invested, between 20 and 60 kg ha–1 $–1 can be expected with varying water supply. Averaged over the two full growing seasons in 2002 and 2003, a low-level irrigation seemed to be slightly more profitable than the other irrigation treatments. Moreover, B. caucasica appeared to have higher returns under water-stressed conditions (low irrigation) versus the other old world bluestems.
While increased irrigation levels affected leaf blade:stem-plus-sheath ratios negatively in almost every month of sampling, visually B. bladhii did not appear to follow this pattern. This species maintained apparent leafiness throughout the growing season with late-season stem elongation and development of an inflorescence in contrast with the other species when stem elongation began by early July. However, this growth habit which is characteristic of B. bladhii did not result in a measurable advantage in leaf blade:stem-plus-sheath ratio over the other species as measured in this experiment. While visually the plant canopy of B. bladhii appeared leafy in character, this species maintained relatively long stems throughout the growing season.
Coyne and Bradford (1985) also found that leaf blade:stem-plus-sheath:root ratios were relatively constant across B. caucasica, B. ischaemum, and B. bladhii. Although leaf blade:stem-plus-sheath ratios were similar, canopy characteristics of B. bladhii should be of advantage to grazing animals by presenting more leaf within the canopy strata that could be physically prehended by the grazing animal.
Mean stage weights and counts of B. bladhii clearly suggested that maturity increased with greater amounts of irrigation and provided further evidence of an increase in physiological maturity with increased irrigation as well as a change in leaf blade:stem-plus-sheath. The two distinctively different years characterized by relatively high and low seasonal precipitation, respectively, had little effect on tiller development. Mean stage weights and counts were similar between these two seasons across water treatments. Thus, while biomass accumulation in 2003 was reduced relative to 2002 because of more stressful conditions, tiller maturation seemed to be less affected. However, there was an overall decline in flowering over the entire period of investigation. Bothriochloa bladhii showed visually less inflorescence development especially in the last year of research. Reduced tillering could have been influenced by only two defoliation events in each growing season. Also, growth in experimental units appeared to become more variable toward the end of the investigation. We additionally observed that elongated tillers in some cases had up to nine nodes without developing an inflorescence, while others reached reproductive stages after developing the fifth node.
Globally, concerns for water quantity and quality are increasing. In the Texas High Plains, the Ogallala Aquifer is the major water-bearing unit (Weeks and Gutentag, 1984) with about 95% of water extracted used for irrigation (Gutentag et al., 1984; LERWPG, 2001). Rates of withdrawal far exceed potential recharge (HPUWCD, 2003) and current use is not sustainable. Shifting a portion of cropland into forage production can result in decreased total irrigation water required while improving profitability (Allen et al., 2005), decreasing soil erosion potential (Collins, 2003), and enhancing soil microbial activity and carbon sequestration (Acosta-Martínez et al., 2004). However, forage species and management practices are crucial to achieving this goal. Results of the current research show that water available for plant use has profound effects on growth and chemical composition of the three Bothriochloa spp. tested, likely due largely to effects on plant maturity, leaf blade:stem-plus-sheath ratios, and rate of senescence. Further research should address defoliation strategies aimed at more frequent harvests to adjust for different rates of plant maturation because of water treatments. While results of this research provide information on effects of dryland to full replacement of PET that are useful in understanding the relationships of water availability to nutritive value and morphology, only the low irrigation level (267 mm) approached an irrigation amount that might be used currently in the Texas High Plains. Allen et al. (2005) found that B. bladhii, as a part of an integrated cotton (Gossypium hirsutum L.)–forage–livestock system, was important in maintaining profitability and reducing total irrigated water use compared with a cotton monoculture. The B. bladhii was irrigated by subsurface drip irrigation with 270 mm water annually and provided 140 grazing d ha–1 for stocker steers (Bos primigenius f. taurus) and a seed harvest in October (21.1 kg PLS ha–1; mean of 4 yr). Thus, under pasture conditions, irrigating to replace about 30% of PET above precipitation appeared practical particularly when applied by subsurface drip irrigation. Surface applications of irrigation water can result in greater loss of water by evaporation than subsurface application (Bordovsky and Porter, 2003). In the research by Allen et al. (2005), total DM yield of B. bladhii from subirrigated pastures appeared similar to that achieved by surface-drip irrigation at about 60% of PET above precipitation in the current experiment. Further research is needed to optimize irrigation strategies with the benefits obtained in nutritive value through water stress demonstrated by the current research.
Finally, water stress may improve nutritive value through an increase in TNC and CP, a decrease in fiber components, and improvements in DMD in these forage species. Digestibility decreased with increasing irrigation, but DDM under full irrigation was not offset by higher DMD in plots that received limited irrigation. Where water for irrigation is not limited, increased rate of maturation may be offset by harvesting forage at an earlier growth stage. Further studies are needed with more frequent defoliation to define these relationships.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication November 19, 2004.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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