Environmental factors frequently interfere with optimal planting dates. If moisture in the soil profile is lacking, growers choosing to sow too early run the risk of having wheat emerge and then perish due to drought stress. If moisture is not available in the top inch of the soil profile due to evaporative losses or excessive tillage, farmers are often tempted to sow wheat deep enough to reach moisture in the profile. In most circumstances, however, the better strategy is to "dust the wheat in" with the expectation that rainfall eventually will provide ample moisture for germination. Shallow sowing (one inch or shallower) is generally preferred because hot soil conditions reduce the coleoptile length of germinating wheat. Wheat seed planted deep to moisture may not produce a coleoptile long enough to break through the soil surface, resulting in poor emergence and stands (Figure 6.3 a & b).
Figure 6.3 (a & b) Coleoptile length varies by variety and soil temperature. Notice the accordionlike effect on wheat leaves that emerge below the soil surface.
Figure 6.4 Marginal increases (i.e., increase for adding one additional unit) in wheat forage yield (A) and marginal returns (B) for wheat seeding rates at Goodwell, OK in 2004.