Considerations for planting wheat into grain sorghum, soybean, or sunflower residue

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Where row crops are just now being harvested and moisture is adequate, producers may be planning to plant wheat, even in late October. If wheat will be planted after sorghum, producers should make sure the sorghum crop has died before planting wheat, since living sorghum will continue to take up water and nutrients, and produce allelopathic toxins. If a freeze hasn’t killed the sorghum, it may need to be sprayed with glyphosate prior to planting wheat.

There are several other important factors to consider when planting wheat into sorghum, soybean, or sunflower residue.

• What seeding rate should be used? Wheat doublecropped after soybeans or sunflowers should be planted at the rate of at least 90 pounds per acre. For wheat doublecropped after grain sorghum, producers should use 120 pounds of seed per acre.
• What’s the best variety to use? Use the same variety you would use for full-season wheat.
• How much nitrogen (N) should be applied? Ideally, this should be based on the results of a soil profile N test. But there’s often not enough time to do this before planting the wheat. As a result, the N rate to use is usually based primarily on yield goals.

For wheat being planted into soybean residue, producers should use their normal rate of N. For wheat following grain sorghum or sunflowers, add an extra 30 pounds of N per acre to the normal N rate. If previous crop yields were severely reduced, it would be good if at all possible to take a profile N test to assess potential carryover nitrogen, which can be significant after a crop failure. 

Research by Ken Kelley (now retired) and Dan Sweeney at the Southeast Agricultural Research Center has shown how the higher nitrogen rate is helpful to wheat yields when planting no-till into sorghum residue. Their research was conducted for several years. A good example of the results can be found in the 2004 Agricultural Research report from the Southeast ARC: www.ksre.ksu.edu/library/crpsl2/SRP926.pdf

Effect of Previous Crop and Nitrogen Rate on No-till Wheat: Southeast Agricultural Research Center: 2003
	Wheat yield (bu/acre) when planted after:
N rate (lbs/acre)	Grain Sorghum	Corn	Soybeans
0	30.1	50.5	39.8
20	41.0	57.7	50.3
40	45.3	62.7	53.5
80	57.9	69.8	64.7
120	69.8	71.5	69.7

Source: 2004 Agricultural Research, Southeast Agricultural Research Center, SRP-926

The gain in grain yield observed with each incremental increase in nitrogen rate was generally greater for wheat planted no-till into grain sorghum residue than into corn or soybean residue. The higher nitrogen rate when planting into grain sorghum residue is needed to compensate for:

• The amount of nitrogen immobilization expected by the sorghum residue
• Possible allelopathic effect of grain sorghum residue on wheat
• The low fall tillering potential of late-planted wheat

If the wheat will be no-tilled, an additional 20 pounds of N is suggested when planting after any of these row crops. In any cropping system, it’s a good idea to use some starter fertilizer (such as 18-46-0, 11-52-0, or 10-34-0) if equipment is available. The remainder of the N needed can be applied during the late fall or winter months.

The extra N needed when planting in any of the situations mentioned above should be added to the topdressing done this winter or early spring, as long as the wheat crop seems to have at least average yield potential.

Jim Shroyer, Crop Production Specialist
jshroyer@ksu.edu

Dorivar Ruiz Diaz, Nutrient Management Specialist
ruizdiaz@ksu.edu

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