Corn-soybean rotation resulted in higher corn and soybean yields than the respective monocultures. For both corn and soybean, the rotation effect - or perhaps more accurately, the continuous cropping yield decline - was observed to be greater in low-yielding environments than in high-yielding environments. Yields of corn in an annual soybean/corn rotation or first-yr after 5 yr of consecutive soybean were not different from each other. First-yr soybean after 5 yr of consecutive corn, yielded more than soybean in the corn/soybean rotation, and both of these sequences yielded more than the continuous soybeans. These results show that the commonly practiced corn-soybean rotation optimizes corn yields, but not soybean yields.

A first-century B.C. text states "some crops are to be planted not so much for the immediate yield as with a view to the following year." During the early 1980s it was documented that corn grown on land planted to corn the previous year yielded 10 to 15% less grain than corn rotated with certain other crops. Also, soybeans yielded 10 to 15% better when rotated with corn rather than grown continuously. Yield increases associated with crop rotation have been referred to as the "rotation effect."

Three of the longest on-going studies designed to evaluate corn-soybean cropping sequences in the northern Corn Belt were initiated in the early 1980s at two locations in Minnesota and one location in Wisconsin. The studies were established near Lamberton, MN in 1981 on a Webster clay loam soil, Waseca, MN in 1982 on a Nicollet clay loam soil, and near Arlington, WI in 1983 on a Plano silt loam soil. Recommended practices for optimum production were followed each year.

Results - Corn

Combining the data from all three locations for all years (29 environments), first-yr corn and corn in the corn-soybean rotation yielded significantly more than the other cropping sequences (Fig. 1). There was a 15% increase in first-yr corn yield compared to continuous corn. Corn yield from the soybean-corn rotation was 13% greater than from continuous corn. Second-, third-, fourth-, and fifth-yr corn yields were no different from continuous corn.

Figure 1.

In 15 of 29 environments there was a positive yield effect observed for both first-yr and annually rotated corn compared to continuous corn. In no environment was there a significant negative effect of rotations compared to continuous corn. There was a linear relationship between the corn yield advantage in the annual rotation and first-yr corn versus the yield of continuous corn (Fig. 2). As the yield of continuous corn increased (better environments), the yield advantage of rotation declined. In low-yielding environments, first-yr corn and corn grown in annual rotation often yielded greater than 25% over continuous corn. Whereas in high-yielding environments, the yield advantages for first-yr corn and corn grown in annual rotation were generally less than 15%.

Figure 2.

High-yielding corn environments were characterized by high, but not excessive rainfall, temperatures, and solar radiation during the growing season. Lowest-yielding environments occurred in the hottest, driest, and sunniest growing season (in 1988 at all three locations) and the coolest, wettest, and cloudiest growing season (in 1993 at all three locations).

Results - Soybean

When averaged across all years, first-yr soybean yields were the highest of any soybean cropping sequence at all three locations (Fig. 3). Combining the data from all three locations for all years (28 environments) resulted in an 18, 8, and 3% increase in first-, second-, third-yr soybean yields compared to continuous soybeans. Yields from the fourth- and fifth-yr soybeans were not different from continuous soybeans. Soybean yield in the corn-soybean rotation was 10% greater than in continuous soybeans.

Figure 3.

In 20 of 28 environments there was a positive yield effect for first-yr soybean compared to continuous soybeans, whereas in only nine of 28 environments there was a positive yield effect for annual rotation compared to continuous soybeans. In no environment was there a significant negative yield effect of rotation compared to continuous soybeans. With higher continuous soybean yields (better environments) the advantage of rotation declined (Fig. 4). In low-yielding environments, the yield advantages of first-yr soybean and soybean grown in annual rotation varied greatly and often exceeded 25%. Whereas in high-yielding environments, the yield advantages for first-yr and annually rotated soybean were generally less than 15%.

Figure 4.

Moderate rainfall, temperatures and solar radiation characterized high-yielding soybean environments. Low-yielding soybean environments included below normal early-season rainfall (1988 at Lamberton and Waseca, 1989 at Lamberton) or low early-season temperatures (1992 at Lamberton and Waseca, 1993 at Lamberton). Seasons when there was a large rotation effect included those when growing degree units from planting through July were low (1992 and 1993 at Lamberton and Arlington, and 1992 at Waseca).

Conclusions

First year and annual rotation corn yields increased by 15 and 13%, compared to continuous corn. Yield from second-, third-, fourth-, and fifth-yr corn was equivalent to yield of continuous corn. Following 5 yr of consecutive soybean, first-yr corn yield was greater than second-yr corn yield, but no further yield loss would occur if corn were grown on the same land for additional years. This statement could not be made for soybean. Soybean yields in the annual rotation and first-yr soybean increased by 10 and 18%, respectively, compared to continuous soybean yields. An increase of 8% in second-yr soybean compared to continuous soybean was observed. Third-yr soybean had a 3% yield increase over continuous soybean while the fourth- and fifth-yr soybean were equivalent in yield to continuous soybean. A 6% decrease in soybean yield was observed when soybean was grown in an annual corn-soybean rotation compared to first-yr soybean after multiple years of corn. These results show the prevailing practice of annually rotating corn and soybean does not give maximum soybean yields.