Ethephon [(2‐chloroethyl) phosphonic acid] has recently been introduced in North America as a regulator to control lodging in cereals. A 3‐yr study (1983‐1985) was conducted to determine how widely grown spring wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.) cultivars in the North Central United States responded to ethephon. Nine randomized complete‐block, split‐plot experiments with wheat, and seven with barley, were conducted at the Crookston (soil classification, Aeric Calciaquoll), Morris (Aeric Calciaquoll), St. Paul (Typic Hapludoll), and Waseca (Aquic Hapludoll) Experiment Stations in Minnesota. Ethephon was applied at a rate of 0.42 kg a.i. ha−1 in all years as well as 0.28 kg a.i. ha−1 in 1985. When lodging occurred, ethephon treatment at either rate lessened its severity. Ethephon shortened crop height, more so when applied at the higher rate. Effects of ethephon on grain yields varied from significant reductions (average 13% for wheat, 9% for barley) to significant increases (average 12% for wheat, 13% for barley). Increases were most common when control plots lodged, although higher yields in response to reduced lodging were not obligatory. When lodging did not occur, ethephon treatment tended to result in reduced yields. Among barley cultivars, ‘Robust’ was most likely to exhibit reduced yields. Genotypic variability for ethephon sensitivity among wheat cultivars was less evident. In most experiments, ethephon treatment lowered kernel numbers per spike or mass per kernel. We conclude that ethephon use is most reasonable when the production practices followed, or environmental conditions, assure the likelihood of significant lodging. Further research investigating cereal responses to lower ethephon rates, as well as interactions between ethephon and plant stress, is needed.

Campbell and Lipps, 1998;Yang et al., 1999). Resistance expression often differs among envi-The development of wheat (Triticum aestivum L.) cultivars resis-

Plants require a continuous supply of iron (Fe) to maintain proper growth. Low rates of Fe chelates applied to reduce Fe deficiency in soybean [Glycine max (L.) Merr.] probably do not satisfy this requirement. Our objective was to evaluate the effectiveness of high rates of Fe‐EDDHA in reducing Fe deficiency when applied to susceptible and resistant cultivars grown on soils where soybean historically has exhibited mild to severe Fe deficiency. Four cultivars (two resistant, two susceptible) and six rates of Fe‐EDDHA (0, 2.24, 4.48, 6.72, 8.96, and 11.20 kg ha−1) were evaluated at one location in 2002 and two locations in 2003. Severity of Fe deficiency varied markedly across environment and cultivars. Unifoliolate relative chlorophyll concentrations indicated that Fe deficiency can occur early in plant development and that planting seed Fe concentration (seed [Fe]) may be insufficient for early growth. Responses to higher rates of Fe‐EDDHA were environment and cultivar specific and occurred over an extended period, manifest at maturity. At lower rates (<6.72 kg Fe‐EDDHA ha−1), resistant cultivars exceeded susceptible cultivars in plant height, seed number, and grain yield, whereas at higher rates, susceptible cultivars often had values similar to resistant cultivars. Both resistant and susceptible cultivars exhibited linear responses to increasing rates when grown in harsh or intermediate environments, suggesting that even at very high rates of Fe‐EDDHA, Fe deficiency limited plant growth and grain yield. Seed [Fe] changed very little in response to added Fe. Plotting relative grain yield versus seed [Fe] for each environment illustrated the narrow range of seed [Fe] associated with wide ranges in relative yield and the large difference between resistant and susceptible cultivars regardless of relative yield.

Lodging can be a limiting factor of hard red spring wheat (HRSW) production. The main objective of this study was to determine the optimum timing and rate of trinexapac‐ethyl (TE) to improve straw strength, resistance to lodging, and related agronomic responses of HRSW. Field experiments arranged in randomized complete blocks were conducted from 2004 to 2006 in Crookston, MN. Five TE rates (0, 62.5, 93.75, 125.0, and 250.0 g a.i. ha−1) and one ethephon rate (280.2 g a.i. ha−1) were applied at Zadoks growth stage (GS) 30, 32, or 37. Measurements included crop injury, plant height, lodging, straw strength, acid detergent lignin (ADL) content, plant maturity, plant density, and yield. Increasing TE rates linearly decreased plant height and increased lodging resistance, straw strength, and ADL content. Lodging resistance was negatively correlated with plant height and positively associated with straw strength and ADL content. The TE rate of 125 g a.i. ha−1 decreased plant height by approximately 6%, and increased plant erectness by 9% and straw strength by 13%, without causing crop injury, delaying maturity, or affecting yield. Applications of TE at GS 37 resulted in less crop injury, shorter stand, and more erect plants than those at GS 30 or 32. These data suggest that the optimum application rate and timing of TE may be 125.0 g a.i. ha−1 at GS 37 for HRSW.

The importance of rapid, nondestructive, and accurate measurements of leaf area for agronomic and physiological studies is well known. Several mathematical formulas have been derived for estimating leaf areas for numerous crops, but there is little information available for soybeans [Glycine max (L.) Merr.]. The purpose of this study, therefore, was to develop prediction equations for estimating leaflet, trifoliolate, and total leaf areas of soybeans. Statistical analyses of soybean leaf areas were divided into three levels: leaflet, trifoliolate, and total leaf area. At each level, we compared the predictive abilities of three regression equations, each involving a different independent variable. On the basis of these results, we chose one independent variable at each level for subsequent regression analyses of various hypotheses. Prediction equations derived from independent variables involving measurements of length and width were superior at each level to those involving measurements of only length or width. Our data indicate, however, that considerable savings of time, with little loss of predictive ability, could be possible by measuring only length or width in each instance. With the use of independent variables involving measurements of length and width, regression analyses were performed to assess the effects of cultivars at each level and also the effects of leaflet orientation on the trifoliolate and the trifoliolate position on the plant at their respective levels. In general, these analyses indicated that a single regression equation could be used at each level. Leaflet, trifoliolate, and total leaf areas of the 12 cultivars we studied could be estimated by the following equations, respectively: A = 0.624 + 0.723LW (R2 = 0.985); A = 0.411 + 2.008LW (terminal leaflet) (R2 = 0.983); A = 6.532 + 2.045 (∑L1W1 terminal leaflets) (R2 = 0.965).