T he EPIC plant growth model was developed to estimate soil productivity as affected by erosion throughout the U.S. Since soil productivity is expressed in terms of crop yield, the model must be capable of simulating crop yields realistically for soils with a wide range of erosion damage. Also, simulation of many crops is required because of the wide variety grown in the U.S. EPIC simulates all crops with one crop growth model using unique parameter values for each crop. The processes simualted include leaf interception of solar radiation; conversion to biomass; division of biomass into roots, above ground mass, and economic yield; root growth; water use; and nutrient uptake. The model has been tested throughout the U.S. and in several foreign countries.

This study was designed to determine the interval of sensitivity of maize (Zea mays L.) yield components to moisture stress, and to evaluate that interval using estimates of plant available water (PAW). Individual maize plants were grown in containers in a glasshouse. For each treatment, water was withheld until the accumulated water use in well-watered control containers was 20 L, approximately twice the PAW in each container. Containers were well watered at all other times. Containers were weighed to determine water use rates and to estimate PAW. Moisture stress was assumed initiated when water use rates declined below the average for well-watered containers. The interval when kernel number was sensitive to moisture stress began 2 to 7 dafter silking and ended 16 to 22 dafter silking. Stress initiated prior to silking but relieved within 2 d after silking did not reduce kernel number, kernel weight, or plant yield. The fewest number of kernels, 45% of the control, occurred for stress during the 7-d period after silking. Kernel weight was reduced by stress during the grain filling period, and the lowest weight, 51% of the control, occurred for stress 12 to 16 d after silking. Water use rates in treatment plant containers were compared to estimates of the soil moisture stress index (SMI) determined as the percentage of PAW in the containers. Water use rates declined when SMI declined below thresholds of between 0.20 and 0.30. These thresholds were similar to those reported for other crops. Thus, this analysis demonstrated that parameters based on PAW can be useful for evaluating the timing of moisture stress on maize yield components.

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Crop models can be evaluated based on accuracy in simulating several years' yields for one location or on accuracy in simulating long‐term mean yields for several locations. Our objective was to see how the ALMANAC (Agricultural Land Management Alternatives with Numerical Assessment Criteria) model and a new version of CERES‐Maize (Crop‐Environment Resource Synthesis) simulate grain yield of rainfed maize (Zea mays L.). We tested the models at one county in each of nine states: Minnesota, New York, Iowa, Illinois, Nebraska, Missouri, Kansas, Louisiana, and Texas (MN, NY, IA, IL, NE, MO, KS, LA, and TX). Simulated grain yields were compared with grain yields reported by the National Agricultural Statistical Service (NASS) for 1983 to 1992. In each county we chose a soil commonly used in maize production, and we used measured weather data. Mean simulated grain yield for each county was always within 5% of the mean measured grain yield for the location. Within locations, measured grain yield was regressed on simulated grain yields and tested to see if the slope was significantly different from 1.0 and if the y‐intercept was significantly different from 0.0, both at the 95% confidence level. Only at MN, NY, and NE for ALMANAC and at MN, NY, and TX for CERES was slope significantly different from 1.0 or intercept significantly different from 0.0. The CVs of simulated grain yields were similar to the those of measured yields at most sites. Also, both models were appropriate for predicting an individual year's yield for most counties. Values for plant parameters, such as heat units for development and the harvest index, and values for soil parameters describing soil water‐holding capacity offer users reasonable inputs for simulating maize grain yield over a wide range of locations.

There have been many investigations into the time interval when maize (Zea maysL.) yield is most susceptible to stress. Knowledge of this interval is important for understanding yield potential, modeling, irrigation scheduling, and the study of the partitioning of assimilate into grain. The objective of this experiment was to determine the growth stage when shading stress affected number of kernels per ear. Three commercial maize hybrids were grown on a Houston black clay (fine montmorillonitic, thermic Udic Pellustert) under irrigation near Temple, TX in 1982 and 1983. Plants were stressed using horizontal shade panels that reduced the light by 79% for approximately 13‐day intervals. In order to define when number of kernels was affected, the panels were moved three times each week to provide an overlapping series of treatments. The number of kernels of each treated plant was measured and treatment means were compared with unshaded controls. The growth stage when shading decreased the final kernel number was closely associated with early kernel development, occurring near the end of the lag period of grain filling. Duration of the sensitive period varied from 6 to 23 days. It was speculated that a stress applied in the first 1 to 2 weeks after pollination caused a temporary shortage of assimilate for the ear, severely limiting endosperm cell number of some tip kernels. These kernels would not fill later, even if stress was relieved.

No abstract

Dedicated energy crops and crop residues will meet herbaceous feedstock demands for the new bioeconomy in the Central and Eastern USA. Perennial warm-season grasses and corn stover are well-suited to the eastern half of the USA and provide opportunities for expanding agricultural operations in the region. A suite of warm-season grasses and associated management practices have been developed by researchers from the Agricultural Research Service of the US Department of Agriculture (USDA) and collaborators associated with USDA Regional Biomass Research Centers. Second generation biofuel feedstocks provide an opportunity to increase the production of transportation fuels from recently fixed plant carbon rather than from fossil fuels. Although there is no "one-size-fitsall" bioenergy feedstock, crop residues like corn (Zea mays L.) stover are the most readily available bioenergy feedstocks. However, on marginally productive cropland, perennial grasses provide a feedstock supply while enhancing ecosystem services. Twenty-five years of research has demonstrated that perennial grasses like switchgrass (Panicum virgatum L.) are profitable and environmentally sustainable on marginally productive cropland in the western Corn Belt and Southeastern USA.

Crop models need accurate simulation of leaf canopy development. The thermal interval for leaf tip appearance (phyllochron) is critical for predicting the duration of vegetative development. The phyllochron in maize is shorter in temperate than in tropical and subtropical environments. As existing data has been evaluated in a narrow range of environments, the underlying mechanisms that affect phyllochron have not been adequately examined. The objectives of this study were to quantify the response of phyllochron to environmental variables and determine its stability across maize cultivars, to aid modelers in developing tools which accurately predict phenology. Maize was grown in field experiments at Wageningen, The Netherlands, Temple, Texas, USA, and three sites in Mexico, and in controlled environments at Wageningen. The experiment at Temple included grain sorghum and shading treatments to alter irradiance of the crop. Detailed data on leaf production and environmental conditions were collected. These data were combined with published data from field studies. Maize phyllochron acclimated to temperature and increased as mean daily temperature before tassel initiation increased from 12.5 to 25.