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Published in Agron. J. 106:1267–1274 (2014) doi:10.2134/agronj13.0488 Copyright © 2014 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. AbstrACt

Winter cereal production systems in the northern USA are inefficient with respect to the capture of photosynthetically active radiation (PAR) during the year. Our objectives were to determine radiation use efficiency (RUE) in winter wheat (Triticum aestivum L.) and triticale (3 Triticosecale Wittmack) at low (67–125 plants m 22 ), medium (116–170 plants m 22 ), and high (205–332 plants m 22 ) plant densities and RUE of interseeded red clover (Trifolium pratense L.) during two growth periods after cereal harvest. During the linear phase of cereal growth (GS 30–80), RUE averaged across plant density was 3.50 g MJ 21 for wheat and 3.21 for triticale in 2004 and 3.37 for wheat in 2006. In 2006, triticale RUE was similar at the low and medium plant density (3.28 g MJ 21 ) but lower at the high plant density (2.84 g MJ 21 ). Red clover RUE following wheat and triticale differed by growth period and exhibited varying levels of plant density dependence within growth period. Following wheat at the high plant density, RUE ranged from 1.40 to 1.97 g MJ 21 across years and growth periods. Following cereal harvest in mid-July until early October, red clover interseeded in wheat intercepted on average 65% (2004) and 35% (2006) of incident PAR. The wheat–red clover system was more robust than triticale–red clover for grain RUE and intercepting PAR after cereal harvest.

Persistent concern regarding surface energy balance closure encourages increased scrutiny of potential sources of error. Laboratory and field experiments addressed heat flow distortion and thermal contact resistance errors during measurement of soil heat flux (G) using the flux plate technique. Steady-state, one-dimensional heat flow experiments determined flux plate thermal conductivities (λ m ) and measured the effect of air gaps and thermal heat sink coatings on plate performance. Use of measured instead of manufacturer-specified λ m and plate dimensions in a heat flow distortion correction improved the consistency but not the average disagreement between imposed sand G and corrected plate heat flux density (G m ). Consistent underestimates of G in dry sand by 20 to 25% after heat flow distortion correction was attributed to thermal contact resistance effects. A convex air gap 0.1 to 1.32 mm thick across 5.9% of the plate face area reduced G m by up to 9.7%. A thin layer of a thermal heat sink compound with λ 0.18 W m -1 K -1 greater than the plate λ m (1.0 W m -1 K -1 ) did not increase G m in a day soil but Increased G m by ∼6% in quartz sand. A 6.5% increase in G m was also observed for plates treated with the same heat sink compound in a silt loam soil under field conditions. Thermal contact resistance errors are probably

Energy balance studies require knowledge of the heat flux at the soil surface. This flux is determined by summing the heat flux at a reference depth (z r ) some centimeters below the surface and the rate of change of heat storage in the soil above z r . The rate of change of heat storage, or heat storage for short (ΔS), is calculated from soil volumetric heat capacity (C) and temperature. The objectives of this study were to determine how choices regarding z r , C measurements, and AS calculations all affect the accuracy of ΔS data. Heat transfer theory and data from three field sites were used toward these ends. In some studies, shallow reference depths have been used and ΔS neglected. Our results indicate that when z r is sufficiently deep to permit accurate heat flux measurements, AS is too large to neglect. Three methods for determining C were evaluated: soil sampling, the Theta-Probe soil moisture sensor, and beat pulse sensors. When C was determined using all three methods simultaneously, the estimates agreed to within 6% on average; however, the temporal variability of C was best recorded with the automated heat pulse sensors. Three approaches for calculating AS were also tested. The common approach of letting C vary in time but neglecting its time derivative caused errors when soil water content was changing. These errors exceeded 200 W m 2 in some cases. The simple approach of assuming a constant C performed similarly. We introduce a third approach that accounts for the time derivative of C and yields the most accurate ΔS data.

Heat flux plates are commonly used to measure soil heat flux, a component of the surface energy balance. The plate method is simple and precise, but several previous studies have demonstrated the potential for relatively large errors. Here we present the results of in situ tests of the plate method, and we describe some promising alternative methods. Summertime soil heat flux was measured with heat flux plates and with two alternative methods at each of three sites. In total, three alternative methods were used: a single probe gradient method, a three needle gradient method, and a self-calibrating plate method. The standard plate method underestimated the magnitude of the heat flux by 18 to 66% depending on the site and type of plate. Agreement between the alternative methods was good with discrepancies ranging from 2 to 6%. The plates underestimate flux apparently duetoacombinationoflowplatethermalconductivity,thermalcontact resistance, and latent heat transfer effects. The three needle gradient method for measuring heat flux performed well at all three sites, providing a good alternative to the standard plate method. The selfcalibrating plate method performed well at the one site where it was tested and may also be a good alternative. Increased adoption of these methods should lead to more accurate soil heat flux and surface energy balance data. S CIENTISTS in the disciplines of agronomy, forestry, ecology, and climatology often rely on surface energy

This study investigated the effects of simulated rainfall on N partitioning and concentrations of degradable (DIP) or undegradable (UIP) intake protein for wilting orchardgrass (Dactylis glomerata L.) and bermudagrass [Cynodon dactylon (L.) Pers.] hays. Orchardgrass forage was wilted to 674, 153, or 41 g kg 21 of moisture (WET-O, IDEAL-O, and DRY-O, respectively) in the field before applying the simulated rainfall (0, 13, 25, 38, 51, 64, or 76 mm). For WET-O, DIP (g kg 21 crude protein [CP]) increased cubically (P 5 0.020) with simulated rainfall, but the overall range of response was small (653–673 g kg 21 CP). Estimates of DIP (g kg 21 CP) for IDEAL-O and DRY-O decreased by 46 and 25 g kg 21 CP, respectively, between the 0and 76-mm rainfall increments; for IDEAL-O, these decreases occurred in a linear (P , 0.0001) pattern, whereas quadratic (P 5 0.009) and linear (P 5 0.029) effects were observed for DRY-O. Bermudagrass forage was field wilted to 761, 400, or 130 g kg 21 of moisture (WET-B, MID-B, and IDEAL-B, respectively) and evaluated similarly. For WET-B and MID-B, DIP (g kg 21 CP) was not affected (P . 0.05) by simulated rainfall. In contrast, quartic (P 5 0.019) and linear (P 5 0.002) effects were observed for IDEAL-B, but these responses were confined primarily to changes between the undamaged (0-mm) control and the initial 13-mm rainfall increment. On a practical basis, concentrations of DIP were, at most, altered only moderately in response to simulated rainfall and relatively little when forages were still too wet to bale.

Throughout much of the southeastern USA, hay harvest can be complicated by a high probability of rainfall events that may cause damage to the resultant hay crop. The objectives of this study were to investigate the effects of simulated rainfall and two postrainfall drying methods on losses of dry matter (DM) and changes in nutritive value for wilting orchardgrass (Dactylis glomerata L.) and bermudagrass [Cynodon dactylon (L.) Pers.] forages. Orchardgrass was wilted to moisture concentrations of 674 (WET), 153 (IDEAL), and 41 (DRY) g kg - 1 and subjected to 0,12, 25, 38, 51, 64, or 76 mm of simulated rainfall from a custom-built rainfall simulator. For IDEAL orchardgrass, DM loss reached a maximum of 88 g kg - 1 when 76 mm of simulated rainfall was applied. Dry matter loss, total N, and all fiber components except hemicellulose increased with rainfall amount, exhibiting single or multiple polynomial effects (P ≤ 0.048) in each case; however, responses were not consistent across these response variables. A second study was conducted with bermudagrass using similar techniques, except that the forage contained 761 (WET), 400 (MID), and 130 (IDEAL) g kg - 1 of moisture when simulated rainfall was applied. For IDEAL bermudagrass forage, DM losses increased in linear (P = 0.001) and quadratic (P = 0.003) relationships with simulated rainfall, but the maximum DM loss was only 21 g kg - 1 . For both forages, DM loss and deleterious changes in nutritive value generally increased with rainfall amount, but these responses appeared to be much greater for orchardgrass.

Previous methodologies to measure losses of dry matter (DM) in wilting hays subjected to natural or simulated rainfall have relied generally on gravimetric techniques, resulting in variable and questionable estimates of DM loss. The objective of this study was to evaluate the use of fiber components and acid detergent insoluble ash (ADIA) as internal plant markers for accurately predicting losses of DM in bermudagrass [Cynodon dactylon (L.) Pers.] and orchardgrass (Dactylis glomerata L.) forages that were damaged by simulated rainfall. For both forages, concentrations of neutral detergent fiber (NDF), acid detergent fiber (ADF), hemicellulose (HEMI), cellulose (CELL), lignin, and ADIA generally increased with the amount of simulated rainfall in primarily linear patterns. Recoveries of all internal markers were high (≥952 g kg -1 ) and were not affected by simulated rainfall for either forage (P ≥ 0.06). Predicted losses of DM increased in primarily linear patterns with simulated rainfall for both forages when concentrations of NDF, ADF, HEMI, CELL, and ADIA were used as internal markers. Linear regressions of predicted losses of DM on values determined gravimetrically were good (r 2 ≥ 0.73; P ≤ 0.03) when concentrations of any fiber constituent or ADIA were used to calculate losses of DM; however. NDF was an especially effective internal marker (Y = 1.12X - 5; r 2 = 0.97; P < 0.01).

Water use efficiency (WUE) represents a given level of biomass or grain yield per unit of water used by the crop. With increasing concern about the availability of water resources in both irrigated and rainfed agriculture, there is renewed interest in trying to develop an understanding of how WUE can be improved and how farming systems can be modified to be more efficient in water use. This review and synthesis of the literature is directed toward understanding the role of soil management practices for WUE. Soil management practices affect the processes of evapotranspiration by modifying the available energy, the available water in the soil profile, or the exchange rate between the soil and the atmosphere. Plant management practices, e.g., the addition of N and P, have an indirect effect on water use through the physiological efficiency of the plant. A survey of the literature reveals a large variation in measured WUE across a range of climates, crops, and soil management practices. It is possible to increase WUE by 25 to 40% through soil management practices that involve tillage. Overall, precipitation use efficiency can be enhanced through adoption of more intensive cropping systems in semiarid environments and increased plant populations in more temperate and humid environments. Modifying nutrient management practices can increase WUE by 15 to 25%. Water use efficiency can be increased through proper management, and field-scale experiences show that these changes positively affect crop yield.

Maintenance of crop residues on the soil surface is promoted as a management practice for reducing soil erosion by wind and water. Crop residue on the soil surface, however, also influences turbulent exchange processes that affect surface water and energy balances. At present, there are very limited data on the aerodynamic properties of residue-covered surfaces. The objective of this research was to derive estimates of the aerodynamic properties (momentum roughness length z 0 , zero-plane displacement d, and drag coefficient C d ) of fields with standing corn (Zea mays L.) stubble. Wind speed and air temperature measurements were made at six heights above two no-tillage fields near Ames, IA, in the spring and fall of 1994 and in the spring of 1995. Both sites had standing corn stubble approximately 0.3 m tall at a density of greater than 60 000 stalks ha -1 and nearly 95% residue cover. An iterative, least-squares linear regression technique was applied to wind profiles obtained under neutral atmospheric conditions to determine z 0 , d, and the friction velocity (u*). Mean values of z 0 and d for each of the three measurement periods varied from 11.8 to 24.3 and 119 to 169 mm, respectively. The ratios z 0 /h s and d/h s , where h s is the mean stubble height, averaged 0.058 and 0.53, respectively, for all profiles. The magnitude of z 0 and d and their weak dependence on wind speed are consistent with turbulent flow over a surface with sparse, rigid roughness elements. The z 0 and C d were higher for the fall measurement period, suggesting that the loose, fresh residue presented a comparatively rough surface that was then smoothed by weathering and compaction beneath snow.

To fully characterize the water and energy balances of a field crop it is necessary to separate soil evaporation from total canopy evapotranspiration. An alternative to measuring soil evaporation directly is to measure the vapor exchange at the soil surface in order to assess mass transfer there. The objective of this research was to develop a method for measuring vapor exchange coefficients beneath plant canopies. The system consists of a Mariotte siphon water supply apparatus and a felt-covered vapor source plate, both of which were tested under laboratory and field conditions. A detailed description of the devices, their construction and operation, and a sample data set are presented. The system is suitable for long-term field installation and is capable of accurately measuring vapor transfer rates over short time intervals.