Alan Schlegel
Agronomist-In-Charge, Southwest Research-Extension Center, Tribune

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Research on 4-year no-till crop rotations with wheat and grain sorghum was initiated at the Southwest Research-Extension Center near Tribune in 1996. Rotations in this research project include:

* Wheat-wheat-sorghum-fallow (WWSF)

* Wheat-sorghum-sorghum-fallow (WSSF)

* Continuous wheat (WW)

Available water is measured in the soil profile (0 to 8 ft) at planting and harvest of each crop. The center of each plot is machine harvested after physiological maturity, and yields are adjusted to 12.5% moisture.

Results and Discussion

Soil water

The amount of available water in the soil profile (0 to 6 ft) at wheat planting varied greatly from year to year. Soil water was similar following fallow after either one or two sorghum crops, and averaged about 9 inches across the 15-year study period (data not shown).

Water at planting of the second wheat crop in a WWSF rotation generally was less than that at planting of the first wheat crop. Soil water for the second wheat crop averaged more than 3 inches (or about 40%) less than that for the first wheat crop in the rotation. Continuous wheat averaged about 0.7 inches less water at planting than the second wheat crop in a WWSF rotation.

Similar to wheat, the amount of available water in the soil profile at sorghum planting varied greatly from year to year. Soil water was similar following fallow after either one or two wheat crops and averaged about 8.4 inches over 15 years. Water at planting of the second sorghum crop in a WSSF rotation was generally less than that at planting of the first sorghum crop. Averaged across the entire study period, the first sorghum crop had about 1.4 inches more available water at planting than the second crop.

Grain yields

Averaged across 15 years, recrop wheat (the second wheat crop in a WWSF rotation) yielded about 83% of the yield of first-year wheat in WWSF. Before 2003, recrop wheat yielded about 70% of the yield of first-year wheat. In 2003 and 2009, however, recrop wheat yields were much greater than the yield in all other rotations. For 2003 recrop wheat, this is possibly a result of failure of the first-year wheat in 2002, which resulted in a period from 2000 sorghum harvest to 2003 wheat planting without a harvested crop. However, this was not the case for the 2009 recrop wheat. Generally, little difference has occurred in wheat yields following one or two sorghum crops. In most years, continuous wheat yields have been similar to recrop wheat yields; however, in several years (2003, 2007, and 2009), recrop wheat yields were considerably greater than continuous wheat yields.

Averaged across 15 years, sorghum yields have been similar following one or two wheat crops. The second sorghum crop typically averages about 70% of the yield of the first sorghum crop.

Summary

In general, we have not seen the kind of yield declines we might have expected in the continuous wheat rotation. Second-year wheat and sorghum yields have also been a little better than expected. Among these four-year no-till rotations, the most favorable rotation at current wheat and grain sorghum prices (which are similar now in this immediate area) is probably the wheat-sorghum-sorghum-fallow. In that rotation, the wheat averaged 41 bu/acre, the first-year sorghum averaged 77 bu/acre, and the second-year sorghum averaged 50 bu/acre.