Pushing above recommended row crop seeding rates may result in a slight yield bump, but probably not enough to justify the added cost.

Dropping slightly below recommended plant populations may not hurt production if stands are uniform, but below a certain level, yields decline significantly.

Louisiana State University Ag Center agronomist Dan Fromme looked at the effect plant population and stand uniformity have on cotton, corn, grain sorghum and soybeans during the recent Louisiana Agricultural Technology and Management Conference in Marksville, La.

Fromme works out of the LSU AgCenter in Alexandria, focused mostly on cotton, corn and grain sorghum.

Cotton

He says recent research has shown that cotton producers can reduce costs without sacrificing yield by cutting seeding rates. “Two to three plants per foot of row is the ideal final plant population,” he says. “To achieve this stand, seeding rates should be slightly higher based on the actual stated germination (according to the 2016 Cotton Varieties for Louisiana publication).”

Fromme goes on to say, “final stands of 1.75 plants per foot of row yields just as well and even one plant per foot that is spaced uniformly is adequate; however, it is a little harder to manage from a height and maturity standpoint.”

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When farmers plant also makes a difference. Fromme recommends April 10 to May 10 as an ideal planting range. “After May 20, yields go down by 18 percent. I feel comfortable with that 1.75 plants per foot for final stand if they are all healthy.”

Corn

Recommended seeding rate for corn is 32,000 to 34,000 in Arkansas and 32,000 to 36,000 for Louisiana. “We want a final stand of at least 26,000 to 27,000 plants per acre,” Fromme says. “Below that, corn yields go south fast.”

Stand uniformity also makes a difference. Research trials included hand-planted plots designed to simulate planting delays for portions of the stand — a 2-leaf delay and a 4-leaf delay. Yield loss may be substantial with a significant percentage of the plants delayed, especially as late as a 4-leaf delay.

With a 2-leaf delay on 33 percent of the crop, yield loss is 10 percent; at 16.5 percent delayed, loss is 5 percent; at 8.25 percent delayed, yield drops by 2.5 percent, and with just a 4.125 percent of delayed plants, loss is 1.25 percent.

The longer delay results in bigger crop losses. At 33 percent delayed population, yield loss is 23 percent. That drops to 11.5 percent with 16.5 percent of the crop delayed, to 5.75 percent at 8.25 percent of plants delayed, and 2.88 percent at 4.125 percent delayed plants.

Planting dates also affect corn yields. Trials at the LSU AgCenter in Alexandria indicate the effect of delayed plantings. Two planting date studies show a significant drop with late planted corn.

In a 2017 study, planting dates included Feb. 24, March 20, and April 6. Seeding rate of DKS 62-08 was at the recommended rate, 34,000 seed per acre.

Top yield, 181.9 bushels per acre, came from the March 20 planting date. The April 6 planting was almost 20 bushels per acres lower, 164.6. The February planting yield was only slightly below March 20 at 178.1 bushels.

“After March 20, yield drops by 6 percent or 1 bushel per day,” Fromme says.

A 2018 study looked at even later dates, result of weather delays. Plantings, again with 34,000 seed per acre of DKS62-08, went out on March 26, April 4, and April 27.

Results show a 0.9 percent yield loss after March 26, or slightly more than one-half bushel per day. March 26 yield hit 171 bushels. April 4 yield dropped to 166 bushels and the April 27 planting date made 154 bushels per acre.

Delay into May results in significant yield losses, according to AgCenter studies. Fromme says estimated yield for a March 19 planting date is 167 bushels per acre. That drops to 150 with an April 14 planting date and precipitously to 80 bushels with a May 16 planting date.

Plant population and planting date correlations show a combination of low seeding rate and late planting reduce production potential almost 50 percent at the extremes.

At 18,000 final plants per acre and a May 4 planting date, yield estimate is about 56 percent of normal.

At 31,000 final plants per acre and a March 23 planting date, expectation is for 100 percent of normal yield. Even with the May 4 planting date, expectations are for 82 percent of normal yield at recommended plant populations.

At 27,000 final plants per acre, the early planting date should make 98 percent of usual yield and 80 percent with the late planting.

Below that level, expectations drop quickly.

Grain Sorghum

A final plant population for grain sorghum for Texas and Arkansas is 50,000 per acre, according to the United Sorghum Checkoff Board.

Planting dates are earlier for south Louisiana, April 1 to May1. For north Louisiana, recommended planting dates are April 15-May 15.

Fromme says 50,000 plants for a final stand is not a problem.

“The best grain sorghum made at Dean Lee (research station) has been planted the first week of April. But sorghum producing states plant sorghum as soon as they plant their last bag of corn.”

He says Louisiana needs more information on planting dates for the state.

Soybeans

Soybean planting dates vary by group, Fromme says. Recommendations are: Group III: April 15 – May 10; Group IV: April 15 – May 10; Group V: March 25 – May 5.

“Seeding rates of 100,000 look just as good as 125,000, 150,000 and 175,000,” he says.

Replant Dilemma

For any crop, Fromme says producers often face difficult decisions on replanting skimpy stands. How much loss does it take to affect yield enough to justify extra costs to replant?

He offers some guidelines.

“Determine the stand density and condition of stand,” he says. “Then determine the yield potential of the current stand. Estimate the expected gross revenue from the sparse stand. Estimate the cost to replant.

“Next, estimate the yield potential and gross revenue from a replanted stand. Determine whether replanting will pay for itself.”

Getting a crop off to a god start is a critical factor in producing good yields, Fromme says. In addition to selecting the proper hybrids and varieties, staying as close as possible to recommended seeding rates and planting dates, as well as planting to assure a uniform stand improves the odds.