Farm Progress

What’s the correlation between plant stress and grain fill? Here is an agronomist’s breakdown of each grain-fill stage and how stress impacts yield potential every step of the way toward maturity.

Jill Loehr, Associate Editor, Prairie Farmer

May 31, 2017

4 Min Read
1 STEP CLOSER: Each passing grain-fill stage means less potential impact on final yield. Once corn reaches black layer, yield loss from grain fill is no longer a factor, says Brent Tharp, Wyffels Hybrids. Other issues, like stalk lodging or falling ears, impact yield.

Kernel abortion. Cannibalization. Sharp words describing two things that can go seriously wrong during grain fill. “If pollination is interfered, you can have 100% yield loss,” says Brent Tharp, agronomic manager with Wyffels Hybrids.

What can interfere with pollination? Drought, hail, disease and pest pressure may all lead to plant stress and potential yield loss, Tharp notes.

Drought is the major deal-breaker. Drought-stressed plants may have late silk emergence, Tharp explains. If corn pollen sheds before plants push out silks, corn misses the pollination window. Drought conditions may cause pollen to dry out and make it less viable, he adds.

Each passing grain-fill stage means less potential impact on final yield. Once corn reaches black layer, or physiological maturity, yield loss from grain fill is no longer a factor.

Tharp breaks down corn grain-fill stages, how stress impacts the plant at each phase and how much yield potential may be lost.

R1: Silk
• Approximate days to black layer: 55 to 65 days
• Description: 50% of ears have silks emerging from the tips; pollen shed from tassel will ideally sync or “nick” up with silking
• How stress impacts kernel development: poor or incomplete pollination
• Estimated yield loss (loss of leaves to whole plant loss): 97% to 100%

R2: Blister
• Approximate days to black layer: 45 to 50 days
• Description: pollinated corn kernels look like tiny blisters of clear liquid; if blisters aren’t visible, the corn didn’t pollinate
• How stress impacts kernel development: kernel abortion — caused by plants cutting off nutrient flow to kernels
• Estimated yield loss (loss of leaves to whole plant loss): 73% to 100%

R3: Milk
• Approximate days to black layer: 35 to 40 days
• Description: corn kernels expand and become more defined; milky white liquid pops from kernel when squeezed (Sweet corn enthusiasts may call this the “roasting ear” stage.)
• How stress impacts kernel development: kernel abortion and stalk cannibalization, which is when a corn plant takes sugar and starch from the stalk and delivers it to the kernels
“The corn plant will do whatever it takes to save the kernels,” Tharp says.
• Estimated yield loss (loss of leaves to whole plant loss): 59% to 75%

R4: Dough
• Approximate days to black layer: 30 to 35 days
• Description: kernels change sugars into starch form and milky liquid begins solidifying
• How stress impacts kernel development: reduced kernel size and weight
• Estimated yield loss (loss of leaves to whole plant loss): 41% to 50%

R5: Dent
• Approximate days to black layer: 20 to 25 days
• Description: conversion of sugars to starch begins to dry out kernel, drawing out moisture and forming a dent
“You can still have up to 40% yield loss at this stage if the plant dies,” Tharp explains. “Starch is still forming, and it’s still gaining yield.”
• How stress impacts kernel development: reduced kernel size and weight
• Estimated yield loss (loss of leaves to whole plant loss): 23% to 40%

R5.5: Half-milk line
• Approximate days to black layer: 14 to 18 days
• Description: dry starch accumulates from kernel cap down to tip, where kernel is attached to cob; the milk-line describes the progression of starch accumulation as the corn moves toward black layer
• How stress impacts kernel development: reduced kernel size and weight
• Estimated yield loss (loss of leaves to whole plant loss): 7% to 12%

R6: Black layer
• Description: hard starch layer reaches base of kernel — physiological maturity
“Think of it like a mom and her kids,” Tharp explains. “At this point, the kids are cut off, and the purse strings are cut.”
• How stress impacts yield: yield loss after black layer is a result of disease, falling ears or plant lodging, not from grain fill
• Estimated yield loss (Loss of leaves to whole plant loss): 0%

What can farmers do to protect corn during pollination? Fungicide treatments at tasseling will help mitigate risk from disease, Tharp notes.

Farmers should scout for insects that feed on silks, like Japanese beetles and corn rootworm beetles, and treat if thresholds are reached. (See story below: “Japanese beetle threshholds.”)

Protecting corn pollination from heat stress begins at planting, Tharp adds. Planting a mix of different genetics and maturities helps manage potential risk from drought stress during pollination. He recommends planting a hybrid maturity mix of 25% early, 50% mid and 25% late.

Preg-check your corn
Did corn pollinate, or did Mother Nature interfere? Tharp says there’s a way to be sure.

When corn reaches the late blister stage, carefully remove the husks from the ear, flip it upside down and shake it. If silks detach from the kernels, the corn pollinated successfully. Did the silks stick? Stress such as drought, hail or pests impacted pollination, he explains.

Japanese beetle thresholds

Avoid overestimating Japanese beetle populations by scouting several areas within the field and not field edges, says Brent Tharp, agronomic manager with Wyffels Hybrids. Consider an insecticide treatment during corn silking if scouting reveals that:

• There are three or more Japanese beetle adults per ear.
• Silks are clipped to less than one-half inch.
• Pollination is less than 50% complete.
• Japanese beetles are lurking and actively feeding.

About the Author(s)

Jill Loehr

Associate Editor, Prairie Farmer, Loehr

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