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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 Dep. of Range, Wildlife, and Fisheries, Texas Tech Univ., Lubbock, TX 79409-2122
* 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: DM, dry matter
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Mineral characteristics of individual Bothriochloa spp. have not been extensively researched and differences among Bothriochloa species are virtually unknown. Differences in mineral concentrations do occur both among forage species and among varieties within species sufficient to warrant selection for specific mineral traits (Hacker, 1982; Sleper et al., 1989). Soil moisture is known to influence mineral availability and uptake. In the semiarid environment of the Southern High Plains, the old world bluestems are grown under both dryland and irrigated conditions, providing a range of soil moisture conditions. Soil moisture can influence root proliferation, mineral transport mechanisms, microbial activities, and the oxidation state of minerals with oxidation–reduction potential.
Elkins et al. (1977) reported that concentrations of K, Ca, and Mg were increased in annual ryegrass (Lolium multiflorum Lam.), but not in arrowleaf clover (Trifolium vasiculosum Savi.) by increasing soil O2 and suggested that low soil O2 was a potential cause of hypomagnesmic tetany in grazing cattle. Rahaman et al. (1971) reported increases in most minerals in water-stressed forage. Kidambi et al. (1990) found that concentrations of Ca, Mg, and Zn in alfalfa (Medicago sativa L.) and sainfoin (Onobrychis viciifolia Scop.) were increased by an inadequate soil moisture supply. In alfalfa, P increased as available soil water declined. These authors suggested that soil moisture levels affected mineral concentration, but not relationships among minerals in these two forages.
Our hypothesis was that differences would occur among different species of Bothriochloa and that increasing surface soil moisture through irrigation would increase mineral uptake and decrease concentration within these forages. Thus, our objective was to investigate effects of Bothriochloa species, irrigation levels, and dryland conditions on concentration and uptake of selected minerals by three old world bluestems in a semiarid environment.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Four water treatments imposed in a split-plot arrangement within each forage species included dryland, and low, medium, and high irrigation levels. Thus, there were 36 subplots, and each was 10 by 15 m. Amount of water applied in the high irrigation level was targeted at 100% replacement of grass reference evapotranspiration (ETo; 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%). Irrigated treatments received water through a surface drip irrigation system with 16-mm tubing (Eurodrip, San Diego, CA) spaced at 1-m intervals. Emitters (457-mm spacing) provided a delivery rate per emitter of 1.52 L h–1 with about 207 kPa pressure. Irrigation water was applied during a period of 5 d every week throughout the entire growing season. Water treatments within each species replicate were located in sequence to minimize edge effects and climatic alterations among experimental units. Water treatments began in spring with emergence of photosynthetically active tissues and ended in autumn with occurrence of the first frost. A lone exception to this scenario occurred in 2001, when irrigation began on 21 June, immediately following installation of the irrigation system. Thus, only years 2002 and 2003 represented full season irrigation from April through October and were included in the current experiment.
Water applied during the 2002 growing season (irrigation and/or precipitation) was 219 mm (20% of ETo), 474 mm (44% of ETo), 727 mm (67% of ETo), and 1046 mm (97% of ETo) for dryland, low, medium, and high levels, respectively. In 2003, water applied was 184 mm (16% of ETo), 530 mm (47% of ETo), 882 mm (78% of ETo), and 1118 mm (99% of ETo) for respective treatments. All plots received an additional average of 182 mm precipitation during the dormant period (November–April) during both years.
Soil at the site was a nearly level (0–1% slope) Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll). These soils were high in K and Mg with concentrations of >600 mg kg–1 of soil. Soil minerals of medium and low concentrations included Ca and Zn, which averaged 3050 and 0.5 mg kg–1, respectively. The pH at the beginning of the experiment was 8.0. Plots were fertilized equally across all treatments to meet or exceed soil test recommendations such that N and other nutrients were not limiting (Philipp et al., 2005). In 2001, the initial year, N (60 kg ha–1) as urea [CO(NH2)2] was applied in early August. In 2002, N (60 kg ha–1) was applied at the beginning of the growing season as urea ammonium nitrate [NH4 NO3 CO(NH2)2 H2O] in solution in 38 mm of water to all plots by means of an existing underground drip irrigation system. After resuming irrigation treatments in spring 2002, the system was cleaned to remove excessive algae growth with a dicarbamide dihydrogen sulfate solution (CH4 N2O 1/2 H2SO4), effectively increasing total N applied to 140 kg N ha–1. Thus, in 2002, only 30 kg N ha–1 was applied as urea in August. In 2003, N (60 kg ha–1) was applied in spring and again in August. In August of all three years, N was applied by hand as urea. All plots were harvested as hay at the end of July of each year (mean harvest date of July 31). Biomass was also removed in late winter before spring growth.
To determine mineral concentrations, aboveground biomass samples were collected in mid-July, shortly before the scheduled biomass removal, and at the end of the growing season in October. Six forage samples were randomly collected from each plot at each sampling date. Samples were hand-clipped with shears at a height of approximately 8 cm, dried in a paper bag at 55°C to a constant weight, ground in a stainless steel Wiley mill (Comeau Technique Ltd., Vandreuil-Dorion, Quebec, Canada) to pass a 1-mm screen, and stored at room temperature for further analysis. Concentrations of Ca, K, Mg, P, S, Al, Fe, Cu, Zn, Na, and Mn were determined with an inductively coupled plasma atomic emission spectrophotometer following digestion with 3:1 HNO3:HClO4 (Muchovej et al., 1986). National Bureau of Standard samples (Apple Leaves No. 1515 and Tomato Leaves No. 1573A) were also analyzed to ensure accuracy. At the time of analyses, subsamples were dried at 105°C to determine the dry matter (DM) concentration of each sample, and all results were then corrected to a DM basis. Mineral uptake was calculated by multiplying the mineral concentration with the DM yield in kg ha–1.
Data were analyzed as a randomized complete block design with restricted randomization at the subplot level. The possible intercorrelation that may have resulted from a 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 or species combination) using the Mixed procedure in SAS (SAS Institute, 1998). Nonorthogonal contrasts were used to compare (i) dryland conditions vs. the mean of the irrigation treatments, (ii) linear effects of irrigation, and (iii) quadratic effects of irrigation. Mean comparisons among species were based on least square means. Differences among treatments were considered significant at P < 0.05 unless noted otherwise.
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RESULTS |
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Species Effects
Concentrations of Ca, K, Mg, Al, and Cu were greater in B. bladhii than in the other two species tested, while concentrations of P and Na in B. bladhii were greater than those observed for B. caucasica, but not for B. ischaemum (Table 1). Bothriochloa caucasica was greater in K concentration than B. ischaemum, and lower in Ca and S concentration than B. bladhii. Concentrations of S were smaller in B. caucasica than in either of the other species. Concentrations of Zn were greater for B. bladhii than in B. ischaemum. Iron concentrations did not differ among species.
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DISCUSSION |
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Mineral analyses of forage samples collected under field conditions are subject to bias from soil contamination (Cherney et al., 1983) and is often present to some degree. Soil contamination appeared to have little effect on results of the present experiment with the possible exception of samples from dryland plots. Soil contamination under dryland management may have occurred due to shorter plants and increased plant spacing as these stands thinned and exposed more soil, especially in B. bladhii. Samples collected on multiple dates from B. bladhii in a pasture adjacent to the experimental area were either washed or not washed and analysis showed no differences in Mg, P, K, Ca, Mn, S, Cu, or Zn (Cradduck, 2005). However, greater Al and Fe concentrations in unwashed than washed samples (Cradduck, 2005) indicated some soil contamination was present. Cradduck (2005) found Al concentrations of 440 and 380 mg kg–1 in unwashed and washed samples of B. bladhii, respectively, while respective values for Fe were 250 and 220 mg kg–1. The relatively low levels of Al and Fe present in the current experiment in irrigated forages suggest a similar low level of soil contamination and likely little effect on concentrations of other minerals examined. Furthermore, concentrations of Fe, with the exception of those in the dryland treatment, were within the 50 to 300 mg kg–1 suggested as typical of normal plants (Baker, 1983). The relatively higher Al and Fe concentrations observed in dryland forages could suggest higher soil contamination but the lack of differences in other minerals may not support this conclusion.
Mineral concentrations in forages are frequently influenced by physiological age and corresponding shifts in leaf-to-stem ratio, resulting in a decline of most minerals due to advanced growth stage (Minson, 1990). Kellogg et al. (1993) reported lower concentrations of minerals in mature than immature B. ischaemum. Concentrations of both macro- and microminerals declined during the growing season in B. bladhii, where samples were collected monthly from May to October from previously unclipped forage (Duch-Carvallo, 2005). A primary effect of increasing irrigation in this experiment was increased physiological aging and morphological stage of development (Philipp et al., 2005). However, with the exception of Fe, and the quadratic responses of K, P, and Cu, mineral concentrations in the current experiment did not decline with increased irrigation level, suggesting that soil moisture per se had a larger effect than did physiological age of the plant.
Phosphorus is concentrated in the plant in meristems and seeds, but accumulation of P in reproductive organs did not explain the P concentration increase under a high irrigation level. Increased flowering and seed heading with increasing irrigation was noted in all species, but B. bladhii developed only a few seed heads. However, this species showed a greater P concentration than B. caucasica.
Our results of increasing mineral concentrations with increased irrigation levels do not agree with Rahaman et al. (1971) or Kidambi et al. (1990). In the current research, decreasing soil moisture did not increase mineral concentrations until water stress of dryland conditions was reached. Interactions between nutrient uptake by forages and soil water deficits have been shown previously that included reduced uptake of N, P, and other minerals (Turner and Begg, 1978). These authors suggested that reduced growth as a result of moderate water deficit was in part due to a disturbance in mineral nutrition as well as direct effects of water deficits on plant growth and that unavailability of nutrients in the dry surface soil may limit growth and yield more than soil water deficit alone. Injection of nutrients, particularly N, at a depth of 45 cm more than doubled the yield of perennial ryegrass (Lolium perenne L.) compared with surface application of N when the soil surface was dry (Garwood and Williams, 1967). A similar response of alfalfa to deep placement of P under conditions of simulated surface drought has been reported by Simpson and Lipsett (1973).
In our experiment, irrigation treatments were surface applied; thus, with increasing irrigation there was a corresponding wetting of the surface soil where nutrient concentrations are generally greatest. Therefore, increases in both plant concentrations and uptake of most minerals with an increase in irrigation level may be due in part to an increase in soil mineral availability in the surface soil with increased surface soil moisture. The greater concentrations of most minerals in forage grown under dryland conditions was likely a concentration effect due to severely reduced growth at this water deficit level.
Concentrations of minerals within plants, true absorption of the mineral by animals, and selective grazing all impact mineral nutrition of grazing animals (Little, 1982). Mineral imbalances, deficiencies or excess, and low bioavailability of essential minerals result in negative economic impacts when animal performance and health are compromised. Mineral nutrition is complicated by the fact that plant requirements may not meet nutritional requirements of livestock (Van Soest, 1983).
On the basis of the mineral requirements for growing steers and heifers and gestating and lactating cows (National Research Council, 1996), dietary requirements for Ca, Mg, K, Fe, and Mn were met or exceeded by concentrations in all forage species. Bothriochloa bladhii appeared to provide greater concentrations of Mg, Ca, and K than the other two species.
Magnesium concentrations in these forages would have been marginal for lactating cows and supplementation should be provided to prevent grass tetany (Grunes et al., 1970). Minson (1990) reported that low serum Mg levels coincided with high CP concentration in perennial ryegrass (230 g kg–1) and orchardgrass (Dactylis glomerata L.) (260 g kg–1). These CP concentrations appear unlikely to occur in C4 grasses such as the investigated Bothriochloa species (Hallock et al., 1965). Crude protein concentration in the current experiment decreased with increased plant growth stage and was greater in B. bladhii than the other two species but was consistently <140 g kg–1 (Philipp et al., 2005). Nitrogen fertilization has been shown to reduce plant Mg absorption when applied in the NH4+ form, while NO3– can enhance plant absorption of Mg (Wilkinson, 1983). Much of the N applied in the current experiment was in the NH4+ form and may have reduced overall Mg uptake.
Potassium and Fe are rarely deficient in forages for grazing animals (Healy, 1973). Plant K requirements and the concentrations in plant tissues usually exceed animal requirements (National Research Council, 1996) and ingestion of soil, as well as plant concentrations of Fe generally meet this requirement (Healy, 1973). Concentrations of K found in our study exceeded requirements for beef cattle about two-fold (National Research Council, 1996). High K has been related to hypomagnesemic grass tetany (Sjollema, 1932; Fontenot et al., 1989), depressed plasma Mg levels, and reduced Mg absorption in the rumen (Suttle and Field, 1969; Tomas and Potter, 1976), further indicating a need to supplement Mg to lactating cows grazing these forages.
Concentrations of P, Cu, S, and Zn did not meet nutritional requirements for any of these classes of beef cattle but again, B. bladhii appeared to more nearly meet requirements for P, Cu, and Zn than the other species. Nitrogen, P, Cu, Co, and Na in forages, particularly tropical grasses, are frequently limiting for animal production (Hacker, 1982). Phosphorous deficiency frequently occurs in grazing ruminants, especially cattle, and particularly in tropical countries (McDowell, 1985) due in part to distribution of P within the plant with greater concentrations in meristems and seeds than in vegetative plant parts (Minson, 1990). Sodium was likely deficient for animal nutritional needs with concentrations in our experiment of <100 mg kg–1. Duch-Carvallo (2005) and Cradduck (2005) found Na concentrations in B. bladhii averaged about 100 mg kg–1 when grown in an adjacent area on similar soil types. Dietary Na recommendations of beef cattle range from 600 to 1000 mg kg–1 (National Research Council, 1996). Concentrations of Na differed little between leaf and stem or between grazed vs. nongrazed B. bladhii (Duch-Carvallo, 2005).
In our research, S was low for animal nutritional needs in all three species. Bothriochloa caucasica had greater concentrations than either B. bladhii or B. ischaemum. Low uptake of S by warm season grasses has been suggested previously to limit animal performance (Reid, 1988). Research in Virginia has indicated that S fertilization of corn (Zea mays L.) and sorghum (Sorghum bicolor L.) increased DM digestibility and utilization of crude protein (Buttrey et al., 1986; Ahmad et al., 1995) by lambs fed these forages as silage, even when there was no corresponding increase in forage production.
Crozier et al. (1997) found that B. caucasica was an acceptable hay for yearling Arabian geldings but would likely require supplementation with crude protein, P, S, and Cu to meet their nutritional needs. Contrary to the current study, Zn concentrations were found to be greater in B. caucasica than in alfalfa or in tall fescue (Festuca arrundinacea Schreb.) grown in Virginia and was the only one of these three forages that met the Zn requirement for the horses. Reid (1988) also observed greater Zn concentrations in B. caucasica than in cool-season grasses in West Virginia. In our study, comparisons were made only among species of Bothriochloa, and B. bladhii appeared to maintain greater concentrations of Zn than the other two species but concentrations did not approach the 56 mg–1 kg–1 of Zn reported by Crozier et al. (1997).
In the current research, forages were fertilized so that N should not have been limiting for plant growth but crude protein was consistently low for animal nutritional needs and ranged from 
120 g kg–1 in May to 60 g kg–1 in July (Philipp et al., 2005). It is possible that low concentrations of both P and S limited crude protein metabolism in the plant and affected protein quality negatively.
Previous studies have suggested the Ca–P ratio can range between 7:1 and 1:1 as long as P is adequate in the diet (National Research Council, 1996). In the three forage species investigated, Ca–P ratios were consistently near 7:1 and P did not meet nutritional needs, suggesting that cattle grazing these forages would benefit from P supplementation.
All three forage species under all water treatment regimes were below the 1000 mg–1 kg–1 maximum tolerable level for Al (National Research Council, 1980).
Phosphorus and N uptake generally followed aboveground biomass production. In 2002 and 2003, biomass DM production peaked in July under a high irrigation treatment with 14.4 and 10.6 Mg ha–1 DM, respectively (Philipp, 2004). Soil accumulation and plant uptake of P and N are of particular interest from an environmental perspective. Philipp (2004) found that 77% of the total seasonal biomass for these grasses was produced by the July sampling date. Thus, more P and N could be taken up in aboveground biomass in the first half of the growing season than in late season growth regardless of irrigation level. Maximum DM yields were observed in B. caucasica with 18 Mg ha–1 under high irrigation averaged across 3 yr (Philipp, 2004), but even under full irrigation, total P uptake was relatively low.
On the basis of the results of this research and nutritional recommendations (National Research Council, 1996), it appears that beef cattle grazing these three old world bluestems would benefit from supplementation with P, S, Cu, and Zn, but the addition of S is likely to be more effective through fertilization than through a mineral supplement to the animal (Buttrey et al., 1986). Salt (NaCl) is a commonly recommended supplement to grazing animals and would likely be appropriate for livestock grazing these Bothriochloa species as well. If the grazing animals are cows in early lactation, Mg supplementation would also be warranted. Of the three old world bluestems tested in this study, B. bladhii would have provided to a grazing animal greater concentrations of most minerals measured than either B. caucasica or B. ischaemum, but further research to determine bioavailability is needed. While water treatments did influence concentrations of most minerals, neither dryland conditions nor irrigation treatments caused concentrations of any mineral to meet or fail to meet the nutritional needs of beef cattle.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication November 23, 2005.
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