Improving Winter Wheat Yield in the Southeast by Modeling the Development and Mortality of Fall, Winter, and Spring Tillers Using Different Seed Populations and Nitrogen Management Strategies.

Vegetative growth in the form of tillers is crucial in the formation and development of wheat (Triticum aestivum L.).  Several studies have been conducted to characterize the development of tillers under certain environmental conditions as well as helping to identify main stem and tillers and how they should be measured (Klepper et al., 1983; Baker et al., 1986; Krenzer, et al., 1991).  By merging methods from both Haun (1973) and Jewiss (1972), Klepper et al. (1982) created a system for measuring main stem and tiller development under field and lab conditions.  They found that leaf number on the main stem and tillers increased at similar rates based on accumulated heat units.  However, these studies examined tiller development only.  None of these studies have examined the phenomena of tiller death and mortality nor the natural processes or environmental factors that lead to the decrease in tillers when the plant reaches first jointing stage. 

Research conducted from 2010 through 2012 evaluated tiller formation and mortality over two seeding rates with two types of nitrogen applications.  The objective was to determine what impact plant population and timing of nitrogen application may have on tiller development and decline.  Whole plant samples collected once a month from a 2 m area were used to measure leaf and tiller development and corresponding changes in whole plant biomass. In addition to the whole plant samples five plants were randomly chosen from each plot with main plants and seasonal tillers being marked with different colors with the goal of determining when leaves and tillers appeared throughout the plants life cycle.  Statistical analysis showed a significant (p < 0.0001) three way interaction among sampling date, tiller number, and seeding rate on leaf appearance and tiller number.  Nitrogen applied either in a single application at GS30 or split into an early application in January and another at GS30 did not have a significant effect on leaf or tiller development.  While the timing of tiller initiation or the rate of leaf development did not differ between seeding rates, the key differences were that at the low seeding rate more tillers were initiated in the spring and tillers that formed in winter or early spring developed more leaves with more tillers surviving to harvest.

The key factor in terms of leaf number was sampling date.  Regardless of seeding rate, green leaf number increased with the accumulation of growing degree days until the start of jointing.  Following GS30 leaf number on the main stem and, at the low density, tillers initiated in the fall continued to increase till the flag leaf emerged from the boot.  However, green leaf number in the tillers at the high density and tillers formed in the winter and early spring at the low density leveled off and then declined rapidly between GS30 and maturity.  At the high seeding rate this resulted in only the main stem and an occasional tiller reaching maturity; while at the low seeding rate only the main stem and the first two fall tillers remained at harvest.