Research projects have identified resource-efficiency benefits in some crop-to-crop interactions that might aid farm-management decisions.

No-till-driven crop diversity is revealing that some crop sequences can produce benefits exceeding those attributed to rotation alone.

"When cropping is diversified, there is often a rotation effect, but crop synergism goes beyond that," says Randy Anderson, a USDA Agricultural Research Service agronomist based in Brookings, S.D.

"We've found that some crops can improve the use of resources, such as water and nitrogen, in continuous, long-term no-till situations."

Anderson first noted the crop synergism impact while conducting a rotation experiment in northeastern Colorado, where annual rainfall is about 16 inches.

Five years into the study, which included 15 different rotations, he noted an increase of water-use efficiency for wheat when corn and dry peas (terminated 7 weeks after planting) were included in the rotation.

When the rotation effect was factored out of his data, it was clear there was something extra going on in sequences utilizing corn and dry peas, he says.

Yield Advantages

In a 7-year study, corn following dry peas yielded 11% more than corn following soybeans across the entire period — but the yield gain was significantly higher during dry years. Anderson says the synergies he identified are most apparent during crop stress.

In another trial, grain yield was reduced 75% by foxtail millet interference when corn followed soybeans or spring wheat, but by only 50% when the corn crop was preceded by dry peas. In this 3-year study, corn yields varied from 95 to 150 bushels per acre in weed-free plots, but 11% yield benefit was consistent each year.

More good news for no-tillers, he says, is that the favorable impact of dry peas on corn persists for 2 years. Anderson conducted a study comparing corn-grain yield in various crop sequences, and in foxtail millet-infested conditions, as well as plots split into weed-free and weed-infested subplots.

In the second year after dry peas were grown, corn was more tolerant to weed interference and, in weed-free plots, yielded 8% more in a dry pea-soybean-corn sequence than a corn-soybean-corn rotation.

Even though corn following dry peas yielded more than corn following soybeans, there was no difference in plant height, development or nutrient concentration for corn-dry peas vs. corn-soybeans, he found.

Complex Causes

The synergism, Anderson theorizes, may be due to a change in corn physiology that improves growth efficiency and probably results from a multitude of factors.

Altered nutrient cycling, microbial changes and other interactions likely come into play, Anderson says. He cites a study that found rhizobacteria density 700 times higher on spring wheat roots when preceded by dry peas rather than wheat.

Rhizobacteria can suppress plant pathogens, produce growth-promoting substances and increase nutrient uptake. But he notes that a myriad of soil organisms are at work beneath the surface — a mere fraction of which have been identified, and there are countless interactive processes to be studied.

"Because of the complexity of soil biology, we may not be able to attribute crop synergism to a specific cause. But we've been able to identify trends that can help guide farmers' management decisions," Anderson says.

Synergies in crop sequences — such as those observed with dry peas and corn — appear to be somewhat rare, he says, and are more likely to occur in low-yield environments where stresses are apt to reveal improved resource-use efficiency. Low-rainfall areas like the western and northern Great Plains may gain the most advantages from synergistic-cropping sequences because of the higher likelihood of drought stress.

Improved water-use efficiency from crop synergism may be a big factor in helping crops gain an edge on weeds. In Anderson's research trials, a preceding dry-pea crop improved weed tolerance in both winter wheat and corn, while corn improved weed tolerance in subsequent soybeans.

A preceding dry-pea crop improved the water-use efficiency of winter wheat and corn and that, Anderson says, may be one reason for the enhanced weed tolerance.

Ongoing Research

Anderson's current research involves developing rotational schemes to take advantage of the crop synergism of corn and dry peas.

Pointing out that short intervals of growth with cover crops can produce beneficial microbial changes in soil, Anderson suggests that a "green fallow crop" of dry peas is worthy of consideration for growers in arid portions of the Great Plains.

The dry peas, he notes, could be planted in early April, allowed to grow for 6 to 8 weeks and then chemically killed. Winter wheat would follow the peas, with long-term, carbon-sequestration benefits and synergistic advantages. Such a practice, Anderson says, may even qualify for NRCS cost-sharing funds.