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Conservation Tillage Fact
Sheet
Crop Rotation, Soil Water Content & Wheat Yields
Using These Relationships Can Help You Estimate Wheat
Yields Before You Plant
Cropping systems in the central Great Plains are diversifying and
intensifying from the traditional wheat-fallow system. But winter wheat
remains the major crop grown in this region. Adding additional crops in
rotation with winter wheat can influence the amount of soil water available
at wheat planting. This note describes those crop rotation effects on soil
water and the subsequent effect on winter wheat yields. |
Crop Rotation and
Tillage Influence Soil Water and Yield |
The amount of
soil water available at wheat planting time in late September is determined
by how much water was left at the time the previous crop was harvested, how
much rain fell between the previous crop harvest and the time of wheat
planting, and how efficiently the precipitation was stored in the soil. The
efficiency of precipitation storage during the non-crop period prior to
wheat planting is affected primarily by weed control method and the amount
of crop residues left on the soil surface. More crop residues on the surface
lead to greater precipitation storage efficiency as the soil surface is
shaded and remains cooler than bare soil, and as wind speeds near the soil
surface are slowed. Both effects reduce evaporation. Tilling the soil
destroys residues and stirs the soil surface which leads to increased soil
water loss.
Data averaged over a 9-year period at Akron, CO show that
weed control by tillage (CT) during the fallow period resulted in nearly 3
inches less available soil water at wheat planting than when no-till (NT)
methods were employed (Fig. 1, compare W-F (CT) with W-F (NT)). When corn
and proso millet were added to the NT cropping system, there were small
decreases in available soil water at planting. Available water following
millet and fallow is probably lower than when following corn and fallow
because of lower capacity of millet stubble to trap snow. When the fallow
period was replaced with proso millet (Fig. 1, compare compare W-C-F with
W-C-M), available water was reduced by nearly 4 inches.
The average wheat yield from W-F (NT) was 12 bu/a greater than from W-F (CT)
(Fig. 2). There was no reduction in yield when corn or proso millet were
added to the no-till system. When millet replaced fallow in the W-C-F
rotation (Fig. 2, compare W-C-F with W-C-M), wheat yield was reduced by
half. In these yield comparisons it is clear that yield differences are
related to available soil water differences.
Quantifying Soil Water Influence on Yield
Wheat yield is related to available water at wheat
planting according to two relationships (Fig. 3). The high response
relationship (Fig. 3a) occurs when environmental conditions in April, May,
and June are dry, average, or wet. Years that fall into this broad range of
moisture conditions are identified by the difference between pan evaporation
and rainfall during April, May, and June being less than 25 inches. This
situation occurs in 87% of the years at Akron. In these years wheat yield
increases 5.3 bu/a for every inch increase in available soil water at
planting. For example, 6.5 inches would be required for a yield of 40 bu/a,
while a yield of 60 bu/a would require 10.2 inches of available soil water
at planting.
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When the difference between
pan evaporation and rainfall during April, May, and June is greater than 25
inches, conditions are very dry. This situation occurs in about 13% of the
years at Akron. Under these conditions there is not enough rainfall during
the growing season to support a plant that can make efficient use of the
stored soil water, and the low response relationship between soil water and
yield applies (Fig. 3b). In these years wheat yield increases only 1.5 bu/a
for every inch increase in available soil water at planting, with 6.5 inches
of soil water producing only 23 bu/a, and 10.2 inches yielding only 29 bu/a.
Conclusion
Moving from conventional tillage to no-till fallow season weed control
results in an average of 2.8 inches more stored soil water which can produce
nearly 15 bu/a more yield in an average or wet precipitation year and 4 bu/a
more yield in a very dry year. Adding corn or proso millet to a no-till W-F
rotation will not significantly reduce wheat yield. However, substituting
millet for the fallow period (W-C-M vs W-C-F) will markedly reduce available
soil water at planting, resulting in a 20 bu/a yield decrease in most years
and a 6 bu/a yield decrease in a very dry year. Monitoring soil water at
wheat planting can provide producers with a projected wheat yield which may
assist with crop choice decisions in flexible cropping systems. |
Conservation
Tillage Fact Sheet #1-02. Published by USDA-ARS Central Great Plains
Research Station, 40335 County Road GG, Akron, CO 80720. By
D.C. Nielsen, Research Agronomist (e-mail
dnielsen@lamar.colostate.edu; website www.akron.ars.usda.gov) |
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