To understand management practices like tillage, crop rotations and fertility, a range of conditions needs to be experienced over time. The weather in Nebraska is so diverse that a two- or three-year period may not give the true variability farms typically experience.

Some crop production questions can be answered with studies lasting one to three seasons. But when you want to know how soil fertility treatments impact production and the land, long-term studies are necessary. These are the type of studies best suited to University of Nebraska agricultural laboratories scattered around the state. One example is a long-term crop rotation, tillage and nitrogen study initiated in 1986 by now-retired Nebraska Extension soil scientist, Charles Shapiro, and his research team at UNL's Haskell Agriculture Lab near Concord.

"Our long-term study has no-till, plow and disk tillage treatments, with continuous corn compared to corn following soybeans," Shapiro says. The results of this study have been fodder for nearly every VIP summer tour at HAL since then, because of the long-term implications of the results.

"From a production standpoint, the biggest significant effect — and this is not surprising — is that the corn following soybeans is much better yielding than the corn following corn," Shapiro says. "And it yields more with less nitrogen."

Looking specifically at the 12-year period, between 2004 and 2016, for instance, average yield results for rain-fed corn following soybeans was 164 bushels per acre, compared to just 135 bushels on continuous corn.

With average rainfall at 30 inches per year, there is not much difference in yield between tillage treatments over a long period, he notes. No-till yields averaged 147 bushels per acre, compared to 144 bushels with a disk and 140 bushels with plow treatments.

Rainfall is a key factor. "The low rainfall years averaging 23 inches of rain is where the no-till really makes a difference," Shapiro says. Data shows corn yields for no-till in dry years at 80 bushels per acre, compared to just 60 bushels on plowed fields. The yield difference, as expected, is much less between tillage treatments in years of higher precipitation.

"The crop rotation effect is similar regardless," he says. Long-term data shows average yields in low precipitation years on corn following soybeans at 81 bushels per acre, compared to 66 bushels. Similarly, in higher-precipitation years, corn following soybeans yields an average of 184 bushels per acre, compared to 149 bushels on continuous corn.

"We did find that in years where there are two drought years in a row, the rotation difference switches and continuous corn is better the second year," Shapiro says. "I don't have the data to prove it, but it might be the corn stubble captures more snow than soybean stubble, which would provide slightly better soil moisture."

Shapiro adds that the study also initially included continuous corn with an intercrop of hairy vetch, which was administered by co-investigator Dan Walters, who passed away in 2011. The intercrop was discontinued in 1990 because it was too difficult to get the vetch established in the fall. "Looking back, we should probably have switched to rye," Shapiro says.

Adding five different nitrogen treatments within each of the tillage and rotation blocks into the study, there are more implications for producers. "Using the long-term average ratio of corn price per bushel to nitrogen price at 8:1, that means that a nitrogen application of 35 pounds per acre needs to produce a yield increase of 4.4 bushels per acre to pay for itself," Shapiro says.

Over the past 30 years, the study would indicate that the 105-pound-per-acre nitrogen rate would be the most profitable under most years, except the dry years, he says.

It is possible to draw all kinds of conclusions from the data. For instance, over the 30-year period, the mean average yield for the fields in a corn-soybean rotation with absolutely no nitrogen added was only 4 bushels less than the continuous corn fields with 140 pounds of nitrogen applied each year.

So, did nitrogen make a difference in yield at all over the past three decades?

"That was the long-term result, but farmers need to fertilize based on the rotation they are in and the continuous cornfields responded economically to added nitrogen, even when the yield level was lower than the corn following soybeans. If you look at the last 12 years, between 2004 and 2016, the differences do not hold up," Shapiro says.

"Greater yield potential needs more nitrogen. Better weed control in the no-till fields probably helped utilize the nitrogen better. Plus, the soils are fairly high in organic matter, between 4% and 5%, so they mineralize a lot of nitrogen and can support the zero rates," he explains.

Between tillage treatments though, there is very little yield difference, no matter what nitrogen rate is applied, he adds.