Growing Ontario's Corn

Wheat Underseeded with Red Clover Maximizes Corn Yield
by Tony J. Vyn, Ken J. Janovicek and H. Wichers, University of Guelph

Recent results from Ontario's long-term rotation trials have once again confirmed that corn yields are higher following wheat underseeded with red clover than they are following corn, soybeans or even wheat alone in sequence. Ontario farmer decisions to reduce and, in some cases, to completely
eliminate wheat and/or underseeded red clover from their rotations has had negative consequences for short-term profitability and long-term productivity of their soils. This article summarizes the corn yield responses to rotation observed over the past eight years on medium- and fine-textured soils.

Corn Yields After Red Clover

Highest corn yields consistently occurred following wheat which had been underseeded with red clover (Tables 1 and 2). At the Chatham and Elora locations, corn yields following wheat were higher than those following soybeans only when wheat was underseeded with red clover. In fact,
corn yields achieved following underseeded clover were similar to those observed following up to two complete growing seasons of forage legumes (alfalfa or red clover) at these locations. This indicates the corn yield benefits associated with following forage legumes in a rotation were fully
realized even when red clover is incorporated in the same year it is underseeded.

Table 1 - Corn yield response to rotation on a Toledo loam near Chatham, Kent County, and on a Brookston clay loam near Maidstone, Essex County
Rotation*	Corn Yield ( bu/ac )
Rotation*	Loam (1990-95)	Clay loam (1990-93)
Continuous Corn	141	105
Soybean-Corn	156	118
SoybeanSoybean-Wheat (RC)** -Corn Wheat-Corn	151	126
Soybean-Wheat (RC)** -Corn	163	135
* All corn treatments were fertilized with 160 lbs/ac of N and fall moldboard plowed.Data courtesy of Doug Young, Ridgetown College, University of Guelph. ** Red clover plow-down was underseeded into wheat.

Underseeding wheat with red clover increased the following year corn yield by 9-12 bu/ac (seven per cent) over wheat alone at all three locationsin the moldboard system (Tables 1 and 2). Assuming that corn is sold at the 1997-98 Market Revenue price of $3.43/bu, this yield increase is
worth about $30 to $40/ac (or at least double the $15 seeding cost of red clover).

The yield benefits associated with red clover seem to persist for at least two years. For example, underseeding wheat with red clover at Elora increased second-year corn yields by nine bu/ac. The crop yield increases over three years associated with underseeding red clover one year out of four increased returns about $80/ac (assuming corn and soybeans are sold at market revenue prices).

Inclusion of wheat into a crop rotation, especially when underseeded, also increased corn yields in a fall chisel plow system (Table 2). The corn yield increases associated with underseeding red clover - relative to after wheat alone - were not as great as in the moldboard system.

Related Considerations

In addition to crop yield increases, underseeding red clover will often reduce the nitrogen fertilizer requirement for corn during the following year. Even short (e.g., up to 10-inchhigh) stands of red clover should be credited with supplying 40 lb/ac of N as long as the stand is relatively uniform.

An argument often used against underseeding red clover is the difficulty associated with consistently getting good stands. Without very good site specific maps and application equipment, it is virtually impossible to assign proper N fertilizer credits after spotty stands.

However, where red clover did establish, the crop yield increases will still occur. For example, even if red clover had only been established on half the field at the Elora site, the crop yield benefits in that area would still be worth $40/acre in a four-year rotation cycle.

Soil Structure

Planting more years of soybeans and fewer years of wheat in a rotation system will also result in a deterioration of soil structure and limit long-term soil productivity. In fact, the structure of soil in corn-soybean rotations can actually be poorer than soils in continuous corn
production.

For example, erosion following an intense June rainstorm following two years of soybeans at the Elora site was twice as high as following either continuous corn or wheat underseeded with red clover (just 1 year in 4).

Conclusions

The rotation benefit of including winter wheat is maximized when it is underseeded with red clover. Without underseeding, corn yields after wheat are 10 per cent higher than for continuous corn on loam and clay loam soils. With underseeded red clover, first-year corn yields were increased an
additional seven per cent (compared to after wheat alone) in all three experiments.

Since the soil structure and yield benefit of red clover persist well beyond the first year, and since profitability is assured even if only 50 per cent of the field has successful establishment and even if no fertilizer N credit is acknowledged, Ontario farmers should always underseed red clover into their winter wheat fields.

Table 2 - Rotation and fall tillage system effects on first- and second-year corn yields on a silt loam soil near Elora, Wellington County (1994-97)
Rotation*	Moldboard	Chisel	Moldboard	Chisel
First Year Yields
Second Year Yields
Continuous Corn bu/ac	155	142	151	141
Soybean-Soybean-Corn-Corn	159	149	145	136
Soybean-Wheat-Corn-Corn	157	158	141	142
Soybean-Wheat (RC)** -Corn-Corn	169	164	150	143
Rotation sequences were initiated in 1980; results in this table are those after four complete four-year cycles. All corn treatments were fertilized with 150 lbs/ac of N. * Red clover plow-down was