Estimation of Spring Wheat Spike Water Concentration and Grain Maturity from Air Temperature1

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Abstract
The domestic agricultural industry and crop yield forecasters/modelers would benefit from an index that estimates spike and grain water concentration of spring wheat (Triticum aestivumL.) as a measure of maturity stage. A three‐part, 5‐yr field study was conducted on Williams loam (fine‐loamy Typic Argiborolls). First, the spike water concentration and content were measured in five trials with variables including cultivars, water, and fertility, and regressed on the growing degreedays (GDD) from anthesis to grain maturity. Second, the interrelationships of spike, grain, and chaff water concentration were determined in ‘Sinton’ wheat from 2000 g kg−1spike water concentration to maturity in four trials. Third, the effect of spike water concentration at windrowing on grain yield and quality was measured in Sinton wheat in three trials. Cumulative GDD accounted for 90% of the variability in spike water concentration and 49% of the variability in spike water content among all treatments. Spike water concentration was inversely related to grain dry matter assimilation, ranging from 3140 g kg−1near anthesis to 270 g kg−1at maximum grain dry matter. Spike water content averaged about 590 mg spike−1at anthesis, reached a maximum of about 750 mg spike−1at 200 to 600 GDD after anthesis, and then declined sharply. Linearily related grain water concentration was about 1.3 and chaff about 0.5 times the spike water concentration at 2000 g kg−1and less. Spring wheat windrowed at spike water concentrations of 600 g kg−1or less did not adversely affect grain yield or grain N concentration. This water concentration exists at about 670 GDD after anthesis. Windrowing as early as possible enhances the water conservation potential, with the primary benefit accruing to an autumn‐planted crop.

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