Drought stress early in the season can complicate that, Coulter explained. "When it's dry, that makes it hard for the nodal roots to fully develop because they develop at around three quarters of an inch below the soil surface," he said.
By V5, corn plants with underdeveloped nodal roots can be spotted as the heavier plant begins to lean. Worse yet, these plants are less able to take up much needed water from deeper in the soil profile.
The good news is that if plants finally do receive adequate moisture, they can grow additional, better developed nodal roots in the freshly watered soil, Coulter said. "If we get moisture soon, those plants will be able to have more nodal root development and can recover to some degree," he said.
But they'll still be at higher risk for lodging later in the season, so even if rain comes, make a note of fields that showed this problem in June.
EAR SIZE ESTABLISHMENT -- V5 to V8
This is indeed a critical time for the corn plant, Coulter noted. The width of the corn ear, or the number of kernel rows, is being established between V5 and V8.
The good news is that much of that size determination is a function of genetics, Coulter said. "It's less influenced by the current growing environment, although that can have some impact," he explained.
KERNEL NUMBER ESTABLISHMENT -- V5 to V15
The potential number of corn kernels -- how many little seats the corn plant will set up in its ears' rows, ready to be filled -- is also starting to be established around V5. And unlike ear girth, this number can be readily affected by the environment.
But don't panic, Coulter said.
"If we have stress during that period, the number of kernels can be reduced a little bit, but it's typically not a huge problem," Coulter explained. "Because that's just the number of potential kernels." These tend to be more abundant than the actual final number of kernels, which are determined by how successful pollination is and how many are retained throughout the season, he said.
"That's why, ultimately, drought stress doesn't translate into large yield-reducing effects early in the season," he said.
SILKING AND POLLINATION
Around V13, the game changes, and heat and drought stress become a more serious yield threat, Coulter explained. "The critical period for corn is typically between 10 days before silking to 14 days after," he said.
High temperatures and moisture stress can delay those emerging silks, but at the same time, the stressed corn plant speeds up its pollen shed. "This little bit of mismatch can cause a quite a big yield reduction," Coulter said. See more on that dynamic here: https://www.agry.purdue.edu/….
So, expect greater yield loss from drought-stressed plants at V13 and beyond, with losses dependent on the intensity and duration of that dryness through July, Coulter concluded.
See his full comments here: https://blog-crop-news.extension.umn.edu/….
SOYBEANS: RESILIENT UNTIL SEED FILL
Soybeans are made of tough stuff, University of Minnesota Extension Soybean Agronomist Seth Naeve reminded readers in a recent university newsletter.
In the face of heat and dryness, they shy away from light, tilting their leaves strategically to avoid sunlight and slowing leaf growth to conserve water and energy, he noted. Instead, the plant puts that energy into its root development, sending them deeper in search of moisture.
"From a practical standpoint, significant drought stress during the vegetative period of soybean development primarily pauses plant growth," Naeve explained. "Leaf expansion is slowed or stopped and only under severe drought does photosynthesis slow or stop."
Surprisingly, despite this reduced growth rate, it's only at R1 that these drought coping mechanisms can begin to chip away at yield, Naeve said. Ultimately, the greatest yield reductions come when the plant must turn its attention to filling the seeds it has set, he added.
"(S)oybean is quite resilient to most stressors through early reproductive growth," he wrote. "... While stresses during seed filling can have a large effect on seed yield, little research, including that on drought, has demonstrated significant yield losses to stressors prior to seed filling."
(See a chart on soybean growth stages from Kansas State University: https://webapp.agron.ksu.edu/….)
See more from Naeve here: https://blog-crop-news.extension.umn.edu/….
See more on why corn leaves roll their leaves up under drought stress here: https://www.dtnpf.com/….
Emily Unglesbee can be reached at emily.unglesbee@dtn.com
Follow her on Twitter @Emily_Unglesbee
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The Link LonkJune 22, 2021 at 12:20AM
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Drought Stress: When Does it Start Stealing Yield from Corn and Soybeans? - DTN The Progressive Farmer
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