Optimizing Water Use for Three Old World Bluestems in the Texas High Plains

This research was conducted for 3 years, 2001 through 2003, at the Texas Tech University Field Research Laboratory located in north east Lubbock County (101° 47’ west longitude; 33° 45’ north latitude; 993 m elevation). Soil at the research site was a Pullman clay (Fine, mixed, super active, thermic Paleustoll) with a bulk density of 1.2 Mg/m3 for the 0.0-0.3 m3 soil layer and 1.49 Mg/m3 for the 0.3-1.0 m layer (Baumhardt et al., 1995). The area has a semiarid climate with an annual mean precipitation of 470 mm and an average air temperature of 15.5 °C. The landscape was nearly level with 0 to 1 % slope (USDA, 1979).

Existing stands of WW-B. Dahl, Caucasian, and WW Spar old world bluestems were used. There were three replications of each forage treatment in a complete randomized block design. These forages were established in 1996 in paddocks (0.07 ha per replication). The three forages were replicated 3 times in a complete randomized block design. Water treatments included ‘dryland’, and ‘low’, ‘medium’, and ‘high’ irrigation levels. These treatment levels were placed in sequence to minimize micro-climatic alteration among experimental units. Amount of water applied in the high treatment was 100% replacement of potential evapotranspiration (PET) minus precipitation. Medium and low treatments were spaced evenly and calculated as 66 and 33% of the high treatment; the dryland treatment received no irrigation (0%). Each irrigation treatment plot was 10 by 15 m.

In June 2001, plots, except those assigned as dryland, were equipped with 10 lines of 16-mm irrigation tubing produced by Eurodrip (San Diego, CA) with a 457-mm emitter spacing and a delivery rate per emitter of 1.52 L h-1. The distance between irrigation tubes was 1 m. An appropriate pressure of approximately 207 kPa to run the irrigation system was maintained by pressure valves, which were connected to flowmeters that allowed exact monitoring of the amount of delivered water. 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 that began 21 June 2001, following installation of the irrigation system. Twice a year, 60 kg N ha-1 was applied to all plots. Other nutrients were applied according to soil test recommendations.

Measurement of WUE required calculation of PET, the amount of water input (mm) throughout the growing season, precipitation, soil moisture, and above-ground biomass expressed as DM yield (kg ha-1). Daily PET values were obtained from weather data using the Penman-Monteith equation (Allen et al., 1998) measured at the Texas Agriculture Experiment Station, which was located approximately 6 km in a south-western direction of the research site. Amount of precipitation was measured with 2 gauges installed approximately 150 m apart at the south-west and north-east corners of the research site and averaged. Precipitation was used to calculate replacements for the high treatment adjusted by amount of rainfall during each irrigation period of 1 week. Irrigation started at the same time of the day with application length chosen according to amounts of water needed to replace deficits for the different treatments. Each year, soil moisture was measured gravimetrically at the beginning of April and end of October to determine soil water depletion. One soil sample to a depth of 1 m in 20-cm increments was taken from each plot with soil augers specified for dry soil (JMC Backsaver Handle, Clements Associates, Inc., Newton, IA; Eijkelkamp Agrisearch, Giesbeek, The Netherlands). Gravimetric soil samples were placed in plastic bags, sealed, and stored in a cooling room with a temperature of 4 ºC until further analysis. Soil moisture was determined gravimetrically. Approximately 35 g of soil was oven dried at a temperature of 105 ºC for at least 24 h or until constant weight was reached and weighed (± 0.0001 g) again to obtain gravimetric water content. Volumetric water content values were obtained using soil bulk density values given by Baumhardt et al. (1995), which were measured at this site.

Above-ground biomass was determined by clipping three, 1.0 x 0.5-m quadrates randomly from within each treatment area in mid-July at hay cut stage and at the end of the growing season to estimate total seasonal DM yield and WUE. Forage from the entire experimental area was removed immediately after determining amount of the above-ground biomass in July by harvesting with a tractor-mounted shredder. All harvested forage was removed as hay. Standing biomass in autumn was determined as described above but was not removed March prior to spring growth. Stubble height after harvest in July and March was about 8 cm. Biomass plant samples were weighed (± 1.0 g) in the field immediately after clipping, oven dried (at 55º C) and weighed again after drying to constant weight to calculate percentage moisture and total above-ground biomass production. Biomass for the growing season was expressed as DM yield ha-1.

Water use efficiency was calculated by using measured total seasonal DM yield during the growing season divided by the amount of water used to obtain this yield. The water amount was calculated by measuring irrigation water applied throughout the growing seasons plus precipitation and soil water depletion. Soil water depletion was estimated by subtracting water remaining in the soil profile to a depth of 1.0 m at the end of the growing season from soil water content at the beginning of the season at the same depth. Precipitation that occurred within 2 wks before final above-ground biomass harvest was excluded from the soil water balance; likewise, any precipitation that occurred between biomass harvest and time of soil sampling was also subtracted. Thus, WUE was calculated with WUE = D/W, where D = total seasonal DM yield in kg ha-1, and W = (precipitation + irrigation + soil water depletion) in mm measured for each irrigation level and dryland during the growing season. Additionally, the slope of WUE was calculated resulting from an increase in total seasonal DM yield per increment of irrigation level. Also, forage mass based on dry matter was determined in May, June, August, and September by clipping two, 0.5 by 1.0-m quadrates randomly from within each treatment. Drying procedures were similar to those used for determination of WUE and total seasonal DM yield.

Plant material for forage nutritive value was sampled monthly within each species and water treatment plot replication from May through October. Thus, a total of 36 plots were sampled 6 times over the growing season. From each plot, 6 random forage samples at each sampling dates were taken at a clipping height of approximately 8 cm and composited in a paper bag for drying. Samples for determining nutritive value were placed immediately in a dryer and dried at 55º C until constant weight was achieved. Samples were then removed and ground in a Wiley mill (brand name and location) through a 1-mm screen and ground forage samples were placed into plastic bags (Whirl-Pak, Nasco Fort Atkinson, WI), holding approximately 50 g of ground material and stored at room temperature for further analysis.

Analysis for forage nutritive value included neutral detergent fiber (NDF), acid detergent fiber (ADF), crude protein (CP), and total non-structural carbohydrates (TNC). These parameters were predicted by creating a regression equation based on the near-infrared absorption patterns (near-infrared absorption technique, NIR) of all forage samples from each date of collection and a calibration set of the entire sample population analyzed by means of conventional wet chemistry procedures. Fiber components such as NDF, ADF, hemicellulose, (NDF-ADF), cellulose, and lignin were determined based on standard procedures described by Van Soest (1963), and Goering and Van Soest (1970). Percentage CP was estimated according to A.O.A.C. (1995). Dry matter digestibility (DDM) was determined as an estimate of ADF [DDM% = 88.9-(0.779 ADF%)] according to Linn and Martin (1989). Percentage TNC of samples was analyzed based on the procedure described by Mounsif (1986) with the following modifiactions. The spectral absorbance of glucose standards of 0.1, 0.2, 0.3, and 0.4 mg ml-1 was measured with a spectrophotometer (Bausch & Lomb, U.S.A.) and a regression model was built based on the resulting spectra of these standards. Ground forage samples were weighed as close as possible to 0.5 g and boiled in a reflux apparatus with 60 ml of 0.2 N HCL for two hours. Samples were then cooled to room temperature and filtered into a 100 ml volumetric flaks. Distilled water was added to a volume of 100 ml, from which 1 ml was taken and diluted with 4 ml of distilled H20 before mixing on a vortex mixer for 30 seconds. Form this solution, 1 ml was taken again and mixed with 10 ml of anthrone solution (330 ml conc. H2SO4, 5.0 g thiourea, 0.25 anthrone) and placed on a heater block set at 97º C. Samples were heated for 15 minutes, then removed and cooled to room temperature. Absorbance of these samples was read together with the prepared glucose standards at a spectrometer setting of 612 nm. The content of TNC of each sample was estimated with the regression equation according absorbance of the glucose standards.

Effect of species and water treatments on response variables were determined by analyzing variances within and among treatments (Steel and Torrie, 1960). The Proc GLM procedure of SAS (SAS Institute, 1998) was used to calculate the slope of WUE and to determine linear, quadratic, or cubic effects of water treatments on RWC, canopy light interception and reflection. The Proc GLM procedure was also used to analyze species effects on forage mass. The Proc Mixed procedure was used to determine species effects on total seasonal DM yield. This procedure was also used to determine to effects of water treatments on WUE within each species.