It’s been 30 years since agronomists at land-grant universities developed the Degree Day 60 or DD-60 program to provide cotton producers with benchmarks for measuring the growth and fruiting habits of their varieties.

Newer cotton varieties — with higher yields and improved fiber qualities — may have “outgrown” those measurements, according to Dr. Tyson Raper, Extension cotton specialist with the University of Tennessee Institute of Agriculture.

“The DD-60 models don’t quite fit what we’re seeing in the field,” said Raper, speaking at the Winter Production Meeting at UT’s Lone Oaks Farms near Middleton, Tenn. “We think those thresholds that were established in the mid-1980s may not apply. It seems like we’re maturing more quickly than we did in the past.”

Raper said he and other researchers have been looking at growth stage development for different maturities in studies at the University of Tennessee’s West Tennessee AgResearch and Education Center in Jackson for the past two years.

The studies included Deltapine 1612 B2XF and DP 1646 B2XF both years and DP 1555 B2XF in 2017 and DP 1851 B2XF in 2018. “We’re basically trying to look at very early maturing, early-to-mid- or mid-maturing and then a late-maturing variety,” he said.

Heat units

“We’re trying to figure out how many heat units it takes to get to each growth stage of interest — squaring, flowering, cutout, cracked boll and 60-percent open.”

He displayed a graph with DD-60s on one axis and the growth stages on the other from the 2017 study. Dotted lines on the graphs represented the upper and lower guidelines of development of varieties when the parameters were developed in the 1980s.

You’ll note we do a pretty good job of following in those guidelines with the early- and early-to-mid-maturing varieties,” he said. “The full-maturing variety falls a little bit above so it takes a few more heat units to mature that late-maturing variety than the other two.

“But they do a pretty good job of falling within the guidelines until we move away from cracked boll into 60 percent open. Then we see a hard deviation. What this data says is we don’t require as many heat units to open up that crop as what we thought and definitely not as many as we required in the mid-1980s.”

The researchers saw a similar trend when they continued the study in 2018, said Raper. “Basically, we were maturing those bolls out quicker than the (DD-60) model said that we should. One thing to notice is we see a deviation from emergence to square. It was a hot year in 2018, going from a relatively wet, cool April to warm very quickly.

No additional benefit

“We didn’t take advantage of those heat units. Between planting and squaring, sure we need some heat, but if it’s 95 degrees we’re not getting any benefit from those additional seven degrees above 86 degrees.”

Raper said he and other UT researchers are planning to re-evaluate the way they calculate DD-60s and adjust the DD-60 thresholds.

With help from Cotton Incorporated, university agronomists expanded the number of research sites to 12 in 2018 and could have as many as 15 locations across the Cotton Belt in 2019, he said.

Data from the studies locations are revealing some significant differences in how quickly varieties can move from one growth milestone to the next.

“Look at the separation we see even from just squaring,” he said. “Some locations are reaching squaring within 35 days after planting, Other locations will take almost 50. This is a function of environment.

“From a flowering standpoint, the first location reaches flowering in about 55 days,” Raper noted. “About 75 days is the latest a location will reach flowering. The environment dominates this maturity.”

Another graph shows the varieties are even closer together in reaching the milestones. “We’re looking at maybe two or three days to reach squaring. From a flowering stand point it may be four or five days,” he said.

“When we look at variety maturity, Deltapine 1612 vs. Deltapine 1646 or Deltapine 1646 vs. Deltapine 1851, these varieties are falling a lot closer than you think. I’m not telling you to plant your whole farm in DP 1851. What I’m saying is we have the potential with these later-maturing varieties to manipulate them to increase your yield potential.”