"The fast and the furious" may be a good way to describe the kind of soybeans that growers want in their fields when the plants hit the flowering stage, says Mark Westgate, an Iowa State University plant physiologist.
"The crop's potential for yield is set early on, by flowering time," points out Westgate, who is leading a three-year study on how soybean plants respond to water-related stress alone and in combination with diseases and herbicide injury.
Last year was the first year of the project. In field trials, Westgate studied the growth of soybean varieties that were either resistant or susceptible to soybean cyst nematode (SCN) and brown stem rot (BSR).
A main message from the first year's results was that reducing stressful roadblocks to plant growth early in the season can put soybeans on the fast track to better yields.
"During flowering, rapid crop growth resulted in higher yields," he says. "Higher yields were associated with greater biomass and faster canopy growth rates during pod set." In his studies, the canopies of SCN-resistant varieties were larger than susceptible varieties, and out-yielded them.
How plants approach flowering is crucial to the number of seeds they set, which, along with seed size, determines yield. "In our trials, the differences we saw in yield were primarily due to differences in the number of seeds produced," Westgate says.
Because any stress that stymies growth early on appears to hurt yield potential, farmers need to decide how to manage their fields as plants enter the flowering stage, Westgate adds. "Their choices of varieties, herbicides and how they address disease and pest problems early in the season contribute to the ultimate outcome."
The linchpin decision may be genetics. Westgate's studies confirm the importance of choosing a high-quality variety to match a grower's field conditions. "Variety selection sets up the potential for setting seeds and is a big factor in determining how much stresses will hinder plants as they begin to flower," he said.
For example, he found herbicide injury temporarily slowed the growth of most varieties, but particularly the SCN-susceptible varieties. The resistant varieties were better at getting back to speedy growth and ended up out-yielding the susceptible plants.
"The key is to minimize plant stresses that producers have control over," Westgate said. "So select the best variety for your particular field conditions. Avoid herbicides that might stunt plant growth. And don't be the first in the field or you may serve up a feast for the insects. These actions will help to maximize plant growth rate at flowering."
This year Westgate is again examining the interactions of water use, herbicides and SCN, and how they affect plant growth rate, canopy development and the potential for setting seeds. He will examine whether SCN or BSR pressure late in the season has an effect on seed size.
He's also exploring the effect of SCN on roots. "We believe the interaction between SCN and herbicide injury may reflect a change in infected roots' capacity to carry water. Susceptible varieties tend to wither and die earlier. We need to know if they have a premature shutdown of root function because of SCN or herbicides."
Westgate's overall mission is to understand the basics of soybean quantity and quality - yield and composition. He wants to learn how plants decide how many seeds to make and then what to put in them. The work may have implications for future soybean improvement.