A STUDY OF THE APPARENT MODULUS OF ELASTICITY AND MAXIMUM CONTACT STRESS OF TIMOTHY STEM NODES

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1 A STUDY OF THE APPARENT MODULUS OF ELASTICITY AND MAXIMUM CONTACT STRESS OF TIMOTHY STEM NODES IN RELATION TO DISINFESTING HESSIAN FLY PUPARIA IN HAY REBALING PROCESSING G. Wang, L. G. Tabil, A. Opoku, D. Wang ABSTRACT. To disinfest Hessian fly in timothy hay bales by mechanical compression, the crushing strength of timothy stem nodes was investigated using an Instron testing machine. Unbaled first cut timothy hay of the 007 crop year was used in the test. Before testing, stem segments with nodes were equilibrated to moisture contents of 1.19% and 14.13% wet basis (w.b.). According to its position in the hay stalk, the node on each stem was designated as a top, middle, or bottom node. The force deformation curves for the nodes as affected by the node position on the stem, node size, and moisture content were obtained under a constant rate of deformation of 1 mm min -1. Equations based on Hertz's contact stress theory for cylindrical bodies radially compressed between two parallel flat plates were employed for calculation of the apparent modulus of elasticity and maximum contact stress of the timothy stem nodes. The apparent modulus of elasticity and maximum contact stress values were affected by node moisture content, size, and position on the stalk. The uniaxial compression test showed that the nodes had no elastic behavior. The apparent moduli of elasticity of timothy stem nodes for this study ranged from 4.86 to 39.8 MPa at 1.19% moisture content and from to MPa at 14.13% moisture content. Maximum contact stress of 8. MPa with a standard deviation of 3.5 MPa was obtained for timothy stem nodes at moisture content of 14.13% and size less than 1.60 mm. The maximum contact stress values obtained for the stem nodes in this study indicated that sufficient pressures were applied to Hessian fly puparia in the field tests (1.41 MPa) within 99% probability. These pressures were influenced by the moisture content, node size, and node position. The apparent modulus of elasticity and maximum contact stress values increased with increasing moisture content and decreased with increasing node size, and these values decreased from top position to bottom position on the stem. Keywords. Apparent modulus of elasticity, Compression, Maximum contact stress, Node, Timothy, Uniaxial compression. Timothy grass (Phleum pratense L.) hay is a common feed for dairy cows and other livestock. It is grown in Russia, the U.K., Canada, and other cooler climate areas. Timothy grass is grown in many regions of Canada, especially Alberta and Saskatchewan. High quality timothy hay is produced in western Canada and exported to Asia Pacific countries. The typical nutrient analysis of #1 timothy hay is 8.50% crude protein and 0.7% calcium with 1.4 Mcal kg -1 net energy available on an as fed basis, i.e., 11% wet basis (w.b.) moisture (CHA, 005). Submitted for review in January 008 as manuscript number BE 7374; approved for publication by the Biological Engineering Division of ASABE in July 009. The information contained in this article was partly presented at the ASABE/CSBE North Central Intersectional Conference as Paper No. RRV The authors are Guanghui Wang, Assistant Professor, Department of Agricultural Engineering, China Agricultural University, Beijing, China; Lope G. Tabil, ASABE Member Engineer, Professor and Head, and Anthony Opoku, Research Engineer, Department of Agricultural and Bioresource Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada; and Decheng Wang, Professor, Department of Agricultural Engineering, China Agricultural University, Beijing, China. Corresponding author: Lope G. Tabil, Department of Agricultural and Bioresource Engineering, University of Saskatchewan, Saskatoon, SK, Canada. S7N5A9; phone: ; fax: ; e mail: lope.tabil@usask.ca. Harvested timothy hay, however, may contain volunteer crops (hollow stemmed plants such as barley, oat, wheat, rye, and Agropyron species) that are hosts of Hessian fly (Mayetiola destructor) during harvesting and baling. Hessian fly infestation of cereal and forage crops can cause great damage to the crops, resulting in yield reductions. Dean and McColloch (1915) reported that 5% to 50% reduction in wheat crop stands may result from Hessian fly infestation. It is estimated that Hessian fly infestation can reduce soft red winter wheat dry matter yield by 14% to 46% (Buntin and Raymer, 1989). Smiley et al. (004) quantified the apparent yield reduction in spring wheat infested with Hessian fly and developed a correlation between the percentage of infested plants and grain yields. They reported that 17% to 0% in grain yield reduction could be expected for each percentage point of plants infested with at least one puparium per plant. Japan and other countries in the Asia Pacific region are the main destination of exported Canadian timothy hay products. The strict phytosanitary regulations of Japan can cause timothy baled hay contaminated with volunteer crops to be rejected. Much of the cost is spent on the inspection fee. Recently, with the agreement between Japan and Canada allowing for inspection of timothy baled hay for host plants in Japan, the cost is shifting towards ship back insurance, which the Canadian exporter must purchase to cover the expenses of shipping rejected baled hay back to Canada. Thus, Transactions of the ASABE Vol. 5(5): American Society of Agricultural and Biological Engineers ISSN

2 to eliminate the business risk associated with the stringent plant quarantine regulations in Japan, the Canadian hay industry is trying to find alternatives for disinfesting timothy hay. Yokoyama et al. (1994, 1996) employed chemical fumigation to develop an effective method to control Hessian fly in timothy hay bales. Sokhansanj et al. (199) and Sokhansanj (1998) tested thermal treatment methods to disinfest Hessian fly pupae in alfalfa bales. However, the environmental and economic concerns associated with using chemical fumigation and thermal treatment have resulted in the need to find other ways to disinfest timothy hay bales. Yokoyama et al. (1993) reported that a pressure of 0.6 kpa (3.0 psi) was adequate to crush and cause mortality to Hessian fly pupae without affecting the quality of the feed and the health of the animals. They performed laboratory tests to determine the magnitude of pressure required to crush the pupae. One such disinfestation method is rebaling (high pressure hay compression) of field baled hay to crush the Hessian fly pupae in volunteer crops that may be mixed with the timothy hay (Opoku et al., 006; Tabil et al., 006). Hessian fly pupae look like flaxseed and are sometimes found behind leaf sheaths near the infested plant stem nodes. Therefore, adequate compressive force to crush the node will eventually crush and therefore disinfest the hay of Hessian fly. Shelef and Mohsenin (1967, 1969) studied the mechanical properties of Seneca wheat grain and yellow dent corn by applying uniaxial compression with an Instron testing machine. Niklas (1998) studied the mechanical failure of Arundinaria tecta (Walt.) Muhl. by applying axial compression and found a difference in Young's modulus values of the internodal wall tissues between the longitudinal and transverse directions. Koya and Faborode (005) developed a mathematical model for predicting the force required to break palm nuts based on Hertz's theory of contact stress. During the investigation, it was reported that there were three stages of biomaterial deformation: transient stage, apparent elastic stage, and failure stage. Niklas and Moon (1989) first used multiple resonance frequency patterns to study plant organs' modulus of elasticity. They chose the flower stalk of Allium sativum L. and reported that the average value of modulus of elasticity (E) was ± Pa from multiple resonance frequency measurements, compared to the E value of ± Pa from static loading tests. Sherif et al. (1976) developed an equation for the calculation of apparent modulus of elasticity when a cylindrical material was radially compressed between two parallel flat plates. The derivation of the equation was consistent with the assumptions used in Hertz's contact stress theory. The modulus of elasticity equation of Sherif et al. (1976) was subsequently applied to radial compression of corn cob (Anazodo, 1983) and potatoes (Solomon and Jindal, 003). Usually, a harvested timothy stem includes three or four stem nodes. Because of its hollow structure, the internode portion is easier to crush than the stem node. Meanwhile, it has been observed that Hessian fly puparium reside behind the leaf sheath near the stem nodes of cereal plants. Hessian fly puparia have survived mechanical compression tests designed to compress timothy and crush the puparia. One of the reasons for this insect survival is that the pressure used to compress the hay was insufficient to crush the timothy stem nodes and thereby crush the puparia. Determining the mechanical property of a single node will verify whether ade- Figure 1. Timothy stem node with the segments on both sides. quate pressure is being applied to the whole stem to compress and eliminate the Hessian fly puparia. The results will help researchers find the effective force or pressure necessary to disinfest the Hessian fly pupae through mechanical compression. The objective of this investigation was to determine the apparent modulus of elasticity and maximum contact stress of timothy stem nodes. MATERIAL AND METHODS HAY MATERIAL First cut timothy hay was used for this study. It was harvested on 4 July 007 by Elcan Forage, Inc. (Broderick, Saskatchewan, Canada), and the experiments were conducted from 5 to 30 August 007. The hay was collected from the field before baling. The first cut timothy hay was of premium quality. It had an initial moisture content of about 67% w.b. determined according to ASAE Standard S358. (ASAE Standards, 003). All moisture content values in this article are given in wet basis. PREPARATION OF SAMPLES Timothy grass with three nodes on the stalks was chosen for this study. Some of the stalks had four nodes, in which case the bottommost three nodes were used. These lower nodes were thicker and contained mostly dry matter. A total of 100 three node hay stalks were prepared. The nodes were cut from the stalks with 5 to 10 mm stem segments on both sides and kept in separate ziplock bags according to their original position on the stalk (top, middle, or bottom). The rest of the stalks were used for measurement of moisture content. The timothy stem nodes were then air dried to 1% and 14% moisture. According to the difference in color between the node and stalk (fig. 1), a Dremel cutter was used to separate the dried and moisture equilibrated nodes from the stalks. The nodes were cut to approximately cylindrical shapes. The node diameters and lengths were measured with a digital caliper. TESTING METHODS The tests were performed using an Instron testing machine (model 1011, Instron Corp., Canton, Mass.) with a 500 N load cell, computerized control, and a data acquisition system. A relatively slow loading rate (1 mm min -1 ) was used so that the forces applied could be considered as static. The average temperature and average relative humidity were 8 C and 33%, respectively, during the tests. The samples were radially compressed, and the compressed samples were assumed to be cylindrical in shape. Equations developed by Sherif et al. (1976) for calculating the apparent modulus of elasticity of 1808 TRANSACTIONS OF THE ASABE

3 a cylindrical material radially compressed between two parallel flat plates were used: 8(1 μ ) FZ E = (1) πdl where E = apparent modulus of elasticity (Pa) = Poisson's ratio (assumed to be 0.40 for timothy stem node) F = force (N) D = average diameter (m) L = length of sample (m) Z = R avg /b, where R avg is the average radius of sample (m), and b is the half width of the compressed sample (m). The value of Z was determined from equation and then substituted into equation 1 to estimate the apparent modulus of elasticity. The Z value was determined by using the Goal Seek feature in Microsoft Excel: x 1 1 = (ln Z) + () D Z where x is deformation (m). The maximum contact stress (S max ) was calculated using equation 3: F S = max (3) πlb Roundness is a measure of the sharpness of the corners of a solid. Mohsenin (1986) indicated that several methods had been used to estimate the roundness of solid materials. Thakor et al. (1995) calculated the roundness of canola seeds from the ratio of minor diameter to the major diameter: r Roundness ratio = (4) R where r = radius of the smallest (minor) dimension of the material R = radius of the longest (major) dimension of the material. NUMBER OF NODES FOR EACH TEST The number of specimens tested was decided according to ASTM Standard E1 00 (ASTM, 003). The sample size was calculated from: where n = size of the sample. 0 / ) n = ( av e (5) a = factor corresponding to a low probability that the difference between the sample estimate and the result of measuring all the units in the lot is greater than the maximum allowable error (E m ). It was assumed that the factor 1.96 would give a probability of 1 in 0 that the sampling error would exceed E m. The value of a was therefore selected as V 0 = advance estimate of the coefficient of variation, which was selected to be 0.35 (Patil et al., 1996). e = allowable sampling error, which was selected to be 0.15 (Patil et al., 1996). From equation 5, the required sample size (n = ) for each test was determined. A sample size of 5 was used in each test. O'Dogherty et al. (1995) studied the physical and mechanical properties of wheat straw stem and conducted about 10 to 5 replications for each combination of the experimental factors. The force deformation behavior of each node was measured using the Instron machine. The apparent modulus of elasticity and maximum contact stress were calculated using equations 1 and 3, respectively. The stem nodes used in this study exhibited size (diameter) variations. Therefore, after measuring and calculating the apparent elastic modulus and maximum contact stress, the results were separated into three groups based on the average node diameter: (1) less than 1.60 mm, () between 1.60 and 1.90 mm, and (3) larger than 1.90 mm. Each treatment had the same sample size of 5 nodes. Thus, in this study, the node samples were separated into two groups with different moisture contents, and each group was further subdivided into the three aforementioned sizes and three positions (top, middle, and bottom) on the stalk. Scheffe's multiple comparison test was used to compare the means using SPSS (SPSS, 005). RESULTS AND DISCUSSION GEOMETRICAL ATTRIBUTES OF NODES During the study, the sizes of timothy stem nodes were measured, and the roundness ratios were calculated (table 1). The minor and major diameters of the nodes increased with the node position from top to bottom. The roundness ratios were similar regardless of the node position. The transverse section of the node was approximately round. UNIAXIAL COMPRESSION TEST Typical compression curves of the nodes at two moisture contents between two parallel plates at a constant rate are shown in figure. The force deformation curves of the samples exhibited nonlinear characteristics. At the different moisture contents, the force deformation curves were differ- Minor Diameter Table 1. Sizes and roundness ratios of timothy stem nodes used in the experiments (n = 5). [a] Moisture Content = 1.19% Moisture Content = 14.13% Major Diameter Roundness Ratio Minor Diameter Major Diameter Roundness Ratio Position Avg. SD Avg. SD Avg. SD Avg. SD Avg. SD Avg. SD Top a a 0.05 Middle a a 0.07 Bottom a a 0.06 [a] Avg. = average; SD = standard deviation. Average values followed by the same letter in the same column are not significantly different at P = Vol. 5(5):

4 Force (N) % 1.19% Deformation Figure. Typical force deformation curves of timothy stem nodes at 1 mm min -1 loading rate. ent; the slopes were steeper for the samples at 14.13% moisture compared to the samples at 1.19% moisture. Qingxi et al. (007) tested the mechanical properties of rape stalk. The load displacement curves of the compressed sample showed non linearity behavior. The load deformation curves of palm nuts (Koya and Faborode, 005) were similar to those of the nodes in this study. Figure shows that the node at 14.13% moisture content reached its rupture point prior to that for 1.19%, and its short concave section indicated that it was tougher than the node at 1.19% moisture content. APPARENT MODULUS OF ELASTICITY The results of the apparent modulus of elasticity of the timothy stem nodes are presented in table. The two moisture contents used in these tests were values commonly encountered during double compression of timothy hay. The apparent modulus of elasticity of the nodes at 1.19% and 14.13% moisture shows some differences. Timothy stem nodes with higher moisture contents produced higher E values than those at lower moisture contents. It can be observed that the E values decreased with increasing node diameter. From equation 1, it can be found that sample (hay node) size does affect the E value. The larger the sample, the bigger the contact area becomes, resulting in decreased E values. For node diameters greater than 1.90 mm, the E value for the 1.19% moisture samples was the lowest. The E values of the nodes decreased from top to bottom. The higher E values at the top portion of the stalks were probably due to the smaller size compared to the middle and bottom nodes. Node position had an effect on the E values; for the same moisture content, the position of the node on the stalk significantly influenced E values. For the same node size and position, moisture content influenced E values as well. Comparing the same node sizes, E values decreased with decreasing moisture content, but this trend is not obvious. High moisture samples contain more water than low moisture samples, which produces high turgor pressure to resist deformation. Niklas (1989) reported that high turgor pressure increased the apparent elastic modulus of flower stalks. At the same positions, the high moisture samples had significantly high E values. However, this did not occur for nodes at the bottom position. The stem nodes in the bottom position have more compact structures, which mainly influenced the nodes' strength. Chattopadhyay and Pandey (1999) reported that, under the same loading rate and knife bevel angle, the modulus of compression of sorghum stalks increased at the seed stage compared to the forage stage. Shelef and Mohsenin (1969) studied the apparent modulus of elasticity of corn kernels at different moisture levels using uniaxial compression. They found that the apparent modulus increased with decreasing moisture content, and the apparent modulus of elasticity for corn kernels ranged from 3.38 kpa (at 1.88% w.b.) to kpa (at 0.73% w.b.). Anazodo (1983) radially compressed corn cobs and reported apparent elastic modulus values for the butt end, mid region, and tip end as 33.6, 40.8, and 44.0 MPa, respectively. MAXIMUM CONTACT STRESS The maximum contact stress of timothy stem nodes as affected by moisture content, node size, and node position is presented in table 3. When the node size increased from less than 1.60 mm to greater than 1.90 mm, the average S max decreased from 8. to 5.9 MPa at 14.13% moisture and from 5.85 to 4.33 MPa at 1.19% moisture. This trend was not significant between node sizes less than 1.60 mm and within 1.60 to 1.90 mm at 14.13%, and there were no significant differences between node sizes within 1.60 to 1.90 mm and larger than 1.90 mm at 1.19%. The stem nodes in the top positions had significantly higher maximum contact stress values than the nodes at the other two positions, but there was no significant variation for the stem nodes' S max values in the middle and bottom positions. The higher maximum contact stress for the top stem nodes was probably due to their smaller size (diameter) compared to the middle and bottom stem nodes. For the higher moisture content (14.13%), the top, middle, and bottom stem nodes had size diameters of 1.55, 1.74, and 1.89 mm, resulting in contacts areas of 0.33, 0.53, and 0.68 mm, respectively. The smaller diameter and the contact area of the top position stem nodes might have resulted in the higher maximum contact stress compared to the Table. Apparent modulus of elasticity of timothy stem node as affected by node position and node size at two moisture contents. [a] Moisture Content = 14.13% Moisture Content = 1.19% Sample Mean (MPa) SD (MPa) CV Mean (MPa) SD (MPa) CV <1.60 mm 5.73 a,b a,a mm a,b a,a >1.90 mm 37.8 a,b a,a Bottom x,x xy,x Middle xy,y x,x Top y,y y,x [a] SD = standard deviation; CV = coefficient variation, decimal. Means followed by the same lowercase letter in the same column (a,b and x,y) are not significantly different at P = Means values followed by the same capital letter in the same row (A,B and X,Y) are not significantly different at P = TRANSACTIONS OF THE ASABE

5 Table 3. Maximum contact stress of timothy stem node as affected by node position and node size at two moisture contents. [a] Moisture Content = 14.13% Moisture Content = 1.19% Sample Mean (MPa) SD (MPa) CV Mean (MPa) SD (MPa) CV <1.60 mm 8. b,b b,a mm 7.13 b,b ab,a >1.90 mm 5.9 a,b a,a Bottom 5.46 x,x xy,x Middle 6.40 x,y x,x Top 8.09 y,y y,x [a] SD = standard deviation; CV = coefficient variation, decimal. Means followed by the same lowercase letter in the same column (a,b and x,y) are not significantly different at P = Means followed by the same capital letter in the same row (A,B and X,Y) are not significantly different at P = other stem nodes. O'Dogherty et al. (1995) reported that the top internode portions of wheat stalks produced higher shear strengths compared to the lower portions. Similar results were reported by Annoussamy et al. (000). Moisture content also affected S max values (table 3). The high moisture nodes had higher S max values compared to the low moisture nodes. The higher turgor pressure of the highmoisture nodes resulted in higher S max values compared to the low moisture nodes. The same phenomenon was observed for the E values. The lowest S max value of 4.33 MPa was obtained at node diameters greater than 1.90 mm and moisture content of 1.19%. The node size affected the maximum contact stress, as indicated in table 3. Qingxi et al. (007) reported that the maximum values of compression resistance increased with increasing rape stalk diameter. Kamst et al. (00) found that the value of the compressive strength of rice grains increased with moisture content in the 4% to 6% range, but the compressive strength decreased with moisture content at moistures higher than 6%. Using a high pressure rebaler system, Tabil et al. (006) conducted experiments to crush and totally disinfest Hessian fly pupa in timothy hay bales at compression pressures of 10.3 and 1.4 MPa. Pressure sensor films were inserted at different locations in timothy hay bales to analyze the pressure distribution during compression. The results showed that most locations in the bale experienced pressures much higher than 00 kpa, with only 0.0% of the bale location having lower than 00 kpa. Yokoyama et al. (1993) reported that a pressure of 0.6 kpa was sufficient enough to crush and disinfest hay of Hessian fly puparia. The maximum contact stress values of timothy hay nodes in this study ranged from 5.99 to 4.33 MPa at 1.19% moisture content and from 8. to 5.9 MPa at 14.13% moisture content. In table 3, it can be observed that the highest mean value of maximum contact stress is 8. MPa with a standard deviation 3.5 MPa. Using confidence interval calculation, with a value of Student's t for two sided limits at 99% probability level and 4 degrees of freedom of.80 (t 0.01,4 =.80), it can be concluded that the 99% confidence interval is between 6.37 and MPa. Thus, there is 99% certainty that the pressures (10.3 and 1.4 MPa) used in the field compression studies of Tabil et al. (006) were much higher than the maximum contact stress of the node. The pressure transferred to the inner portion of the bale has to be estimated or measured to determine whether this transferred pressure is enough to crush the nodes of timothy hay stems. During rebaling, the timothy stalks were compressed totally, including the stem nodes and internodes. This compression procedure was similar to the experiment conducted here. During compression, the stalks were strained and compressed against each other as a result of the pressure applied by the piston. This pressure applied by the rebaler system could almost totally crush the timothy stem nodes. CONCLUSION In this work, the crushing strength (through maximum contact stress) of timothy stem nodes was determined at different node sizes, positions on the stalk, and moisture contents. The following conclusions are made: The apparent modulus of elasticity and maximum contact stress values were affected by node moisture content, size, and position in the stalk. The apparent moduli of elasticity of timothy stem nodes in this study ranged from 4.86 to 39.8 MPa at 1.19% moisture content and from to MPa at 14.13% moisture content. The maximum contact stress values obtained for the stem nodes in this study indicated that sufficient pressures were applied to the Hessian fly puparia in the field tests within 99% probability. These pressures were influenced by the moisture content, node size, and node position. As the moisture content increased from 1.19% to 14.13%, the apparent modulus of elasticity and maximum contact stress values increased. With increasing node size, the apparent modulus of elasticity and maximum contact stress values decreased. Node position affected the apparent modulus of elasticity and maximum contact stress values. Values decreased from top to bottom. The results from this study could help researchers to find the effective force or pressure to disinfest timothy hay of Hessian fly pupae through mechanical compression. ACKNOWLEDGEMENTS The authors wish to acknowledge Canadian Hay Association and China Scholarship Council (CSC) for the research funding. Thanks also to Bill Crerar (Department of Agricultural and Bioresource Engineering, University of Saskatchewan), Manfei Zhao (Beijing Institute of Computer Technology and its Application, Beijing, China) for assistance in data analysis, and Mark Shaw (Department of Agricultural and Bioresource Engineering, University of Saskatchewan) for technical support. Acknowledgement also goes to Elcan Forage, Inc. (Broderick, Saskatchewan, Canada) for providing the raw material for the experiment. Vol. 5(5):

6 REFERENCES Anazodo, U. G. N Mechanical properties of the corn cob under quasi static radial compression. Trans. ASAE 6(4): Annoussamy, M., G. Richard, S. Recous, and J. Guerif Change in mechanical properties of wheat straw due to decomposition and moisture. Applied Eng. in Agric. 16(6): ASAE Standards S358.: Moisture measurement - forages. St. Joseph, Mich.: ASAE. ASTM E1 00: Standard practice for calculating sample size to estimate, with a specified tolerable error, the average for a characteristic of a lot or process. In 003 Annual Book of ASTM Standards, 14.0: West Conshohocken, Pa.: ASTM. Buntin, G. D., and P. L. Raymer Hessian fly (Diptera: Cecidomyiidae) damage and forage production of winter wheat. J. Econ. Entomology 8(1): CHA Timothy hay. Lethbridge, Alberta, Canada: Canadian Hay Association. Available at: Accessed 0 September 007. Chattopadhyay, P. S., and K. P. Pandey Mechanical properties of sorghum stalk in relation to quasi static deformation. J. Agric. Eng. Res. 73(): Dean, G. A., and J. W. McColloch The Hessian fly situation in Kansas. Circular No. 37. Manhattan, Kans.: Kansas State Agricultural College. Kamst, G. F., C. Bonazzi, J. Vasseur, and J. J. Bimbenet. 00. Effect of deformation rate and moisture content on the mechanical properties of rice grains. Trans. ASAE 45(1): Koya, O. A., and M. O. Faborode Mathematical modeling of palm nut cracking based on Hertz's theory. Biosystems Eng. 91(4): Mohsenin, N. N Physical Properties of Plant and Animal Materials. nd ed. New York, N.Y.: Gordon and Breach Science. Niklas, K. J Mechanical behavior of plant tissues as inferred from the theory of pressurized cellular solid. American J. Botany 76(6): Niklas, K. J Modes of mechanical failure of hollow, septate stems. Annals of Botany 81(1): Niklas, K. J., and F. C. Moon Flexural stiffness and modulus of elasticity of flower stalks from Allium sativum as measured by multiple resonance frequency spectra. American J. Botany 75(10): O'Dogherty, M. J., J. A. Huber, J. Dyson, and C. J. Marshall A study of the physical and mechanical properties of wheat straw. J. Agric. Eng. Res. 6(): Opoku, A., L. G. Tabil, and S. Talebi Crushing force, deformation modulus, and crushing strength of timothy hay nodal stem segments. ASABE Paper No St. Joseph, Mich.: ASABE. Patil, R. T., S. Sokhansanj, M. H. Khostaghaza, and L. G. Tabil, Jr Compression characteristics of alfalfa cubes. Canadian Agric. Eng. 38(3): Qingxi, L., L. Yitao, S. Caixia, and T. Boping Research on the physical mechanics properties of the rape stalk. ASABE Paper No St. Joseph, Mich.: ASABE. Shelef, L., and N. N. Mohsenin Evaluation of the modulus of elasticity of wheat grain. Cereal Chem. 44(4): Shelef, L., and N. N. Mohsenin Effect of moisture content on mechanical properties of shelled corn. Cereal Chem. 46(3): Sherif, S. M., L. J. Segerlind, and J. S. Frame An equation for the modulus of elasticity of a radially compressed cylinder. Trans. ASAE 19(4): Smiley, R. W., J. A. Gourlie, R. G. Whittaker, S. A. Easley, and K. K. Kidwell Economic impact of Hessian fly (Diptera: Cecidomyiidae) on spring wheat in Oregon and additive yield losses with fusarium crown rot and lesion nematode. J. Econ. Entomology 97(): Sokhansanj, S Thermal kill of Hessian fly (Mayetiola destructor (Say)) pupae baled in timothy (Phleum pretense L.) in a commercial forage dryer. In Drying '98: Proc. 11th Intl. Drying Symposium (IDS' 98), C: Thessaloniki, Greece: Ziti Editions. Sokhansanj, S., V. S. Venkatesan, H. C. Wood, J. F. Doane, and D. T. Spurr Thermal kill of wheat midge and Hessian fly. Postharvest Biol. and Tech. (1): Solomon, W. K., and V. K. Jindal Comparison of mechanical tests for evaluating textural changes in potatoes during thermal softening. J. Texture Studies 33(6): SPSS for Windows. Release Chicago, Ill.: SPSS, Inc. Tabil, L. G., A. Ghazanfari, M. Shaw, A. Opoku, W. J. Crerar, and J. Whistlecraft Disinfestation of timothy hay bales from Hessian fly (Mayetiola destructor (Say)) puparia using high compression rebaling machines. Canadian Biosystems Eng. 48: Thakor, N. J., S. Sokhansanj, I. McGregor, and S. McCurdy Dehulling of canola by hydrothermal treatments. J. American Oil Chemists' Society 7(5): Yokoyama, V. Y., J. H. Hatchett, and G. T. Miller Hessian fly (Diptera: Cecidomyiidae) control by compression of hay for export to Japan. J. Econ. Entomology 86(3): Yokoyama, V. Y., J. H. Hatchett, G. T. Miller, and P. L. Hartsell Hydrogen phosphide residues and efficacy to control Hessian fly (Diptera: Cecidomyiidae) in compressed hay export to Japan. J. Econ. Entomology 87(5): Yokoyama, V. Y., G. T. Miller, P. L. Hartsell, and J. H. Hatchett Complete mortality of Hessian fly (Diptera: Cecidomyiidae) puparia in two large scale tests to confirm the efficacy of a multiple quarantine treatment for compressed hay exported to Japan. J. Econ. Entomology 89(3): TRANSACTIONS OF THE ASABE