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Cover crops cut sidedress nitrogen without cutting cotton yield

Robbie Faust Georgia-cover-crop-cotton-Faust-1-a.jpg
Cotton is planted May 20222 for the second year of a two-year study taking place on Terrell County, Ga., farmer Robbie Faust’s farm, which includes a 70-acre pivot divided into 17-acre strips to plant multiple cover crop varieties and blends.
Georgia study tested cotton yield response to reducing sidedress nitrogen units in relation to the amount of nitrogen cover crops provided.

The farmer and the instigator were happy with the results. The cover crop alone returned enough nitrogen to more than pay for the cover crop seed. The grower also reduced sidedress nitrogen need, saving on overall fertility cost without sacrificing yield, plus the cover brought its additional benefits.

EDITOR’S NOTE: Last October, Southeast Farm Press published ‘Measuring the mass, nitrogen benefit of four early cover crops,’ which explained a study to measure the biomass of four cover crops planted Nov. 1 in south Georgia and the nitrogen from the cover available to the following crop. In this article, we circled back to update the findings from last year. It gets a bit number heavy but stick with it. Some interesting results.

2021 was the first year of the two-year study taking place on Terrell County, Ga., farmer Robbie Faust’s farm, which includes a 70-acre pivot divided into 17-acre strips to plant multiple cover crop varieties and blends. Seth McAllister, the University of Georgia Extension agent in Terrell, spearheaded the study.

For the first year, the cover was planted by Nov. 10, 2020.

The seed rate and cost were:

  • Wrens Abruzzi Rye was 67 pounds per acre and cost $25.46 per acre.
  • Dixie Crimson Clover was 18 pounds per acre and cost $18 per acre.
  • Rye+vetch was 35 pounds of rye and 8 pounds of Auburn Merit Vetch per acre and cost $28.50 per acre.
  • Four-way mix was Wrens Abruzzi Rye at 20 pounds, the clover at 5 pounds, vetch at 4 pounds and forage oats at 15 pounds per acre and cost $28.75 per acre.

The cover was rolled April 5, 2021. The cotton was planted the first week of May 2021. Samples of the cover were taken prior to planting.

The per-acre dry bulk and nitrogen credit of each:

  • The rye alone produced 3,925 pounds per acre and gave 10 pounds of N per acre.
  • The Crimson Clover alone produced 2,824 pounds per acre and gave 59 pounds of N per acre.
  • The rye plus vetch produced 5,308 pounds per acre and gave 75 pounds of N per acre.
  • The rye-clover-vetch-oats produced 6,173 pounds per acre and gave 66 pounds of N per acre.

As part of the study, they wanted to test yield responses to reducing the sidedress nitrogen units in relation to the amount of nitrogen the cover crops provided in each trial. They based the test off each trial receiving in total 110 units of nitrogen from a combination of the nitrogen credit from the cover and additionally applied commercial nitrogen.

They didn’t cut the sidedress nitrogen rate by the full amount of nitrogen reported in the samples. They gave Faust’s cotton a 10-unit buffer. For example, for the rye-clover-vetch-oats trial, instead of cutting 66 pounds of nitrogen at sidedress, they reduced it by 56 pounds. They did this for each cover mix, comparing the reduced-rate nitrogen yields to the full-rate check of 110 units of commercial, additional nitrogen application.

The cotton received variable-rate fertilizer applied at planting, which included between 30 units to 40 units of nitrogen.

The average per-acre cotton yield for each trial was:

  • Rye at 1,395 pounds. There was no reduced-nitrogen trial for the rye cover.
  • Crimson Clover at 1,325 pounds with the full rate of nitrogen and 1,255 pounds with the reduced sidedress application.
  • Rye plus vetch at 1,255 pounds with the full rate of nitrogen and 1,320 pounds with the reduced sidedress application.
  • Rye-clover-vetch-oats at 1,407 pounds with the full rate of nitrogen and 1,395 pounds with the reduced sidedress application.

Georgia-cover-crop-cotton-Faust-2-a.jpg

Cotton seedlings emerge from Robbie Faust's farm in  May 2022 in Terrell County, Ga., where the second year of the cover crop study is taking place. Credit: Robbie Faust

Faust has averaged better cotton in fields over the years, but last year, the cotton in the cover crop study was his best overall for 2021. Granted, the 2021 growing season wasn’t ideal for achieving the highest yield potential. Late-season rain brought hardlock and other harvest challenges. Most Georgia cotton growers took a yield-hit last year.

McAllister pointed out that the nitrogen credit from the cover crops became available throughout the growing season, peaking at optimum times during the cotton’s growth pattern.

Average adjusted revenue

UGA Extension economist Amanda Smith provided economic analysis of the study. In one measurement, she provided the average adjusted revenue per acre with a variable nitrogen cost comparing the full-rate nitrogen and the reduced-rate nitrogen treatments for each trial. The variable costs used for nitrogen were a low-end cost of 45 cents per pound, the actual last year’s average rate of 62 cents per pound and a high rate of $1.15 per pound, or closer to what nitrogen costs are this year.

For the rye-plus-vetch trial, the average adjusted revenue per acre for the full rate of nitrogen was $1,077 when using the low-end nitrogen cost, $1,058 when using last year’s average nitrogen cost and $1,000 when using the high-end nitrogen cost.

For the reduced-nitrogen treatments in the rye-plus-vetch trial, the average adjusted revenue per acre was $1,170 when considering the low-end nitrogen cost, $1,165 when using last year’s average nitrogen cost and $1,146 when using the high-end nitrogen cost.

“The nitrogen credits provided by the cover crop when averaged across the study more than covered the cost of the cover seeding alone, plus all the additional benefits such as improving soil health, weed management, erosion control, water infiltration and more,” McAllister said. “And we didn’t jeopardize yield or returns.”

Encouraged by the positive results from the first year of the cover crop study, Faust planted for this year additional acres of cover crops, or about 550 total cover acres, more than he's ever had on the farm. He planted some acres to a rye-vetch mix and some to rye-triticale mix.

“I think there is still some upside yield potential with using the cover and reducing nitrogen based on the credits on what we saw last year. I don’t believe we maxed out around that 1,300-pound or so point,” Faust said. “But we’ll see what happens this year.”

The study is being replicated this year with another cotton crop. The cotton in the study last year was planted behind corn.

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