Research in Kansas has shown that wheat planted directly into grain sorghum stubble yields less than wheat planted directly into soybean or corn stubble. When planting wheat into sorghum fields, producers are advised to use an additional 30 lbs/acre nitrogen (N) over the recommended rate based on soil test and expected yield. If the wheat is planted no-till, an extra 20 lbs/acre N is recommended on top of that.

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

This year, producers may have planted wheat into fields of grain sorghum that failed because of some combination of drought and heat. If most of the residue from the failed crop is remaining on the soil surface, the residue (especially the stalks) may well have higher levels of nitrogen than normal. The soil may also have more residual N in the soil profile than normal if the sorghum crop died prematurely. If the sorghum plants remained alive, however, they may have continued to take up N. Under these conditions, will it still be necessary to apply the additional 30 lbs/acre of N for the wheat crop planted into failed sorghum?

The short answer is “yes,” it would still be a good idea to use a higher N rate if the wheat is being planted into sorghum residue than if it were being planted into corn or soybean residue, or into fallow.

Even though failed sorghum residue likely contains more N than normal, resulting in a lower Carbon:Nitrogen ratio than normal, this residue can still cause N immobilization in the fall.

The amount of N left in the soil may be higher than normal if the sorghum crop failed, but there’s no way to know for sure without taking a profile N soil test. If there’s no time to do so before planting the wheat, then it’s best to assume there will be little or no extra N in the soil especially if the plants remained alive. If the plants died prematurely, there may be higher-than-normal levels of residual N in the soil and producers may not need quite as much extra N as normal, this is a situation where the use of soil nitrate test can be particularly important for an accurate N rate application.

In any case, the allelopathic effect of sorghum residue on wheat is unaffected by whether the crop failed or yielded normally. The wheat will need extra N to overcome this effect and produce normal yields. And the wheat will need some extra N to account for the late planting date, regardless of the condition of the preceding sorghum crop.

Therefore, it will still be a good idea to apply the recommended extra 30 lbs/acre of N to wheat being planted into failed sorghum. Is possible that residual nitrogen remains in the soil and a profile soil test will provide valuable information. That extra N 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.