IRRIGATION AND CULM CONTRIBUTION TO YIELD AND YIELD COMPONENTS OF WINTER-WHEAT

Water is generally the limiting factor in U.S. Great Plains wheat (Tri ticum aestivum L.) production. With increasing demands for limited wet er, improving the efficacy of irrigation is critical. One technique is to irrigate during responsive stages of crop development, but few stu dies have examined this approach. This 2-yr study on a Nunn clay loam soil (fine, montmorillonitic, mesic Aridic Argiustoll) was designed to examine the effects of irrigation, based on stage of crop development , on winter wheat yield, yield components (on a plant basis), and spec ific culm responses. In the first year, the treatments were control (d ryland), and irrigation at late jointing. In the second year, the trea tments were dryland, irrigation at late jointing, irrigation at anthes is, and irrigation at both late jointing and anthesis. Irrigation at l ate jointing or anthesis significantly increased grain yield and the m ost important yield component (spikes per plant), as well as spikelets per plant, number of kernels per plant, and kernel weight per plant. The increased spikes per plant in the irrigation treatments, particula rly with late-jointing irrigation, was due to reduced tiller abortion. Increased yield was primarily due to the contribution of more seconda ry tillers (T10, T11, T24 T21, T34 and T31) that produced spikes. The contribution of main stems to the total yield decreased from 92% to at most 86% with irrigation, although the dry weight of main-stem spikes increased with irrigation. The contribution to total yield of the mai n yield-producing tillers, T1 and T2, decreased from 20 to 15% and 19 to 15%, respectively, with irrigation. As with main-stem spikes, irrig ation also increased T1 and T2 spike dry weight. Therefore, the produc tion of secondary spikes due to irrigation treatments was not at the e xpense of main stem or primary tilter spikes. If only one irrigation c an be applied, irrigation at late jointing is recommended for central Great Plains conditions, due to its greater effect on tiller survival. This implies that developmental and physiological processes at late j ointing are critical in determining final grain yield, and water stres s should be avoided at this growth stage.