Farmers in north-central Iowa are experiencing slow corn and soybean emergence despite higher-than-average Growing Degree Units (GDUs) this season, according to a May 12 report. Phil Long, a regional agronomist with Liqui-Grow, said the region accumulated about 197 GDUs from April 10 to May 1, surpassing the 30-year average of 121 GDUs.
Long explained that although it typically takes about 130 GDUs for corn or soybeans to emerge, current field observations show many crops have not yet surfaced. “It takes about 130 or so GDUs to get corn or beans out of the ground,” Long said. “So why aren’t more crops emerged?”
He attributed the delay to differences between air and soil temperatures. While air temperature data suggest rapid development should be occurring, seeds respond primarily to soil heat rather than ambient conditions. “What’s most important to the corn and beans out there in the ground is soil GDUs,” Long said. “Even corn up to V6 is regulated primarily off the heat in the ground.”
Long also noted that cold nighttime temperatures have kept soil readings low despite warmer days: “We’re not getting that soil temperature up there very far,” he said. He added that factors such as heavy crop residue and persistent cloud cover further reduce how much warmth reaches seeds beneath the surface.
According to Long, fields with significant residue can see GDU accumulation drop by half compared with conventionally tilled areas. Overcast weather can reduce solar radiation by as much as eighty percent: “If there’s heavy cloud cover, that can reduce solar radiation by 80%,” he said.
As planting continues into mid-May, Long advised farmers to focus on seedbed micro-climates rather than just tracking air temperatures: “Although we’re ahead in terms of air temperature GDUs for this year compared to the ‘average’ year, we’re probably behind in terms of those seeds sitting in the ground,” he said.
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