For decades, mono-cropping without rotation has been known to often result in reduced yield. Indeed, long-term studies at the University of Kentucky show that first-year, corn-after-corn commonly experiences a "yield penalty" that growers should factor into their economic analysis.
"One of the interesting things about this yield penalty is that it appears to be greater as corn yield increases," says Paul Vincelli, University of Kentucky plant pathologist. "This suggests that, in the future, the 'rotation effect' will be not diminished — but rather enhanced — in the presence of better varieties, better management and excellent corn-growing conditions."
Vincelli, agronomist John Grove and entomologist Ric Bessin say that late this season, they observed little foliar disease in any of the plots in these long-term studies. While diseases like gray leaf spot and northern leaf blight certainly can be very damaging in continuous corn, they say these diseases were much too limited in these trials — well below 1% severity during late dent — to affect yield.
"We also observed that there were more green leaves on corn in rotation than on corn following corn, as well as higher yields," Vincelli adds.
The facts from these trials, the researchers say, are:
1. There was no significant pressure from foliar diseases associated with corn residue (gray leaf spot, northern leaf blight).
2. Plants in continuous corn plots exhibited earlier senescence, working their way up the plant (which are symptoms commonly associated with root infections or root-feeding insects on many crops).
3. Foliar nutrient analyses suggest that nutrient deficiencies were not the cause of the differences in leaf greenness observed in the university's experiments comparing corn-after-corn vs. rotated corn.
"These facts lead us to wonder about the role of root-attacking microorganisms in the yield penalty observed in many university trials, whether fungi, nematodes or others," says Vincelli, adding root-attacking insects, such as western corn rootworm, are also likely to be involved with long-term continuous corn, though not likely in this trial.
Research on whether soil-borne diseases account for part — or all — of this yield penalty in corn have been surprisingly limited, the researchers say.
Ohio State researchers found evidence that a soil-inhabiting Pythium species might become damaging to corn roots on poorly drained soils under continuous corn, but these weren't clearly proven to be causing reduced yields.
Researchers at the University of Georgia showed that soil-borne pathogens were involved in yield declines observed in a cropping system where corn was grown as a continuous double-crop over several seasons, but that was a production system very different from the Corn Belt.
"Unfortunately, these and other studies we have seen have not clearly identified root diseases as a significant factor in the yield penalty in continuous corn grown in the Corn Belt," Vincelli says.
Western corn rootworm has been a pest since the early 1980s in Kentucky with active infestations restricted to continuous-corn fields. Typically, a few years are required to build an economic infestation, but the more years a field remains in continuous corn the greater the likelihood of rootworm problems, Bessin says.
Initially, when corn rootworm moved into Kentucky, the entomologist says injury to corn was very easy to see as lodging and 'goose-necking' of corn were often in evidence. However, today's hybrids tolerate rootworm feeding better with reduced levels of lodging.
To diagnose a rootworm infestation, Bessin says root samples need to be dug, roots washed and examined for the characteristic root pruning, scarring, and proliferation of secondary root regrowth.
"Having stated this, corn rootworms are not the sole cause of yield penalties with continuous corn; they may not even be the most common cause," Bessin says. "While corn rootworms have the potential to reduce yields in continuous corn, it's unlikely in this case given the insecticide treatments applied in this study and that this was the first year of corn following corn."
While the researchers say they can't really present any solutions at this time to growers on the cause of the "yield penalty," they likely will look more and more at the potential role of root diseases in the "yield penalty" of continuous corn?