Legumes benefit rotations
Including legumes in crop rotations pays off in multiple ways, according to a long-term field study at Iowa State University (ISU).
Since 2001, researchers have compared yields, inputs, soil health, profitability, and environmental benefits of three cropping systems. They compared a two-year corn/soybean rotation with a three-year rotation of corn/soybeans/oats plus red clover and with a four-year rotation of corn/soybeans/oats plus alfalfa/alfalfa. The three- and four-year systems periodically received cattle manure.
“The two-year corn/soybean system has been managed with conventional rates of mineral fertilizers and herbicides; whereas, the more diverse three-year and four-year systems have been managed with lower rates of agrichemicals,” says ISU agronomist Matt Liebman.
The diverse, low-input systems including legumes showed marked benefits. “Results of this long-term study indicate that diversification of conventional corn/soybean systems with small grains and forage legumes, coupled with integration of those systems with livestock, can allow for large reductions in the use of mineral fertilizers and herbicides,” says Liebman.
“The more diverse systems can also lead to less environmental damage, equivalent profitability, improved soil quality, and higher crop productivity,” he says.
For the purposes of the study, corn and soybeans in all rotations were grown using conventional tillage (chisel plowing after corn and surface cultivation after soybeans).
The oats included in both the three- and four-year rotations were used as a starter crop for both red clover and alfalfa. The oats and either red clover or alfalfa were seeded in late March or early April; no herbicides were used.
The oats were combined in July and the straw was baled.
“For the three-year rotation, we clipped the red clover six weeks after the oats were harvested to control weeds,” says Liebman. “We terminate the red clover by plowing in late November or early December.”
They leave the plowing rough over winter and use a field cultivator in spring to prepare a seedbed for corn.
After the harvesting of oats in the four-year rotation, the alfalfa is cut no later than September. This gives plants the time needed to regrow and store in the roots the energy needed to overwinter.
“In the second year, we’ll take three to five cuttings, depending on the weather,” says Liebman. The second-year alfalfa yields 4 to 5 tons per acre.
“If I were a farmer, I would maintain the alfalfa for one more year,” he says. “That would give me a better chance of recouping the cost of the alfalfa seed by spreading it over more years.”
Like the red clover, the alfalfa is terminated by plowing in late fall. Seedbed preparation for corn is the same as that used after red clover.
These are highlights of the benefits the study found in diverse rotations.
• Reduced use of nitrogen (N) and herbicide. “During the period of 2006 to 2016, mineral nitrogen fertilizer use was 86% and 91% lower in the three- and four-year systems, respectively, than in the two-year system,” says Liebman.
“Reductions in nitrogen fertilizer use were made possible through nitrogen fixation by legumes and by nitrogen recycling in manure applications,” he says.
Corn in the two-year rotation received 152 pounds per acre of N. Corn planted after red clover in the three-year rotation received 29 pounds of N per acre, and corn after alfalfa received 24 pounds per acre.
Herbicide use was also 96% and 97% lower in the three- and four-year systems, respectively, than in the two-year system.
• Higher yields. “Corn yield has averaged 4% higher, and soybean yield has averaged 16% higher in the more diverse systems compared with the two-year system,” says Liebman.
The oats in the three-year system yielded 93 bushels per acre and 98 bushels per acre in the four-year system.
• Weed populations unaffected by rotation. Across all cropping systems, weed biomass in corn and soybeans averaged 20 pounds per acre.
• Disease in soybeans reduced. “Incidence and severity of sudden death syndrome, a key disease affecting soybeans in the Corn Belt, have been markedly lower in the longer rotations than in the two-year rotation,” says Liebman.
• Soil health improved. “Three indicators of soil quality – particulate organic matter carbon, microbial biomass carbon, and potentially mineralizable nitrogen – were 22% to 51% higher in the three-year and four-year rotations than in the two-year rotation,” he says.
“We found a 30% increase in soil microbial biomass in both the three- and four-year rotations, with the alfalfa system having a slightly higher amount,” says Liebman.
The calculated rate of soil erosion in the three- and four-year systems was 25% less than in the two-year system.
• Reduced chemical contaminants in runoff. Concentrations of nitrate in drainage water collected from corn in the more diverse systems were 57% lower than nitrate in drainage water from corn in the two-year system.
“By reducing the amount of nitrogen, phosphorus, and herbicide running off and through the soil, we were able to reduce the potential for water contamination,” he says. “Fresh-water toxicity associated with herbicide use was 93% lower in the more diverse systems than in the conventional system.”
• Savings in fossil energy. The diverse cropping systems consumed 59% less fossil energy than the two-year system.
• Sustained profitability. “Increases in rotation length led to greater labor requirements and decreased gross revenue,” says Liebman. “However, production costs also dropped substantially as cropping system diversity increased. As a result, net returns to land and management did not differ among systems, though profitability tended to rise as rotation length increased.”
The profit, or net return to land and management, for the two-year system was $335 an acre per year. For the three-year rotation, it was $357 an acre per year, and for the four-year rotation, it was $361 per acre annually.
“With the diverse rotations, we have fewer crops of corn and soybeans but higher yields when we do grow those crops – with less fertilizer and herbicide,” says Liebman. “In our case, the profits balance out because of the lower input costs in the legume rotations.”
Options for genetics
The long-term Iowa State University (ISU) study evaluating the performance of diverse rotations also included a shorter-term component comparing the performance of transgenic crops with nontransgenic crops.
“Measurements of weed seed densities indicated that soil seed banks were neither increasing nor decreasing in any of the rotation systems and were unaffected by technology package,” says ISU agronomist Matt Liebman.
“Overall, our findings indicate that farmers have options when it comes to the seeds and herbicides they want to use,” he says. “Transgenic crops come with a higher cost, and data from our experiment suggest they don’t always add to profitability.”