The seed bank of planned volunteer crabgrass (Digitaria spp.) stands, whether in or on the soil, must be managed once it is established. Most stands are given minimum off-season conservation soil tillage (renovation) in the form of relatively shallow (2 to 4 inches) disking or sweep plowing sometime during fall to early spring, depending on the cropping approach (Dalrymple, 1996).

Stands may develop slowly if tillage is too deep because of moldboard plowing or offset diskings (8 to 12 inches; Dalrymple, 1983). However, once the seed bank is well distributed in the upper soil profile, the influence of tillage depth declines. Some crabgrass seeds remain viable in the soil for several years and successive tillage brings deeper ones up where they can germinate.

The objective of this study was to determine the depth at which the optimum percentage of seeds from volunteer crabgrass stands germinated and how tillage practices affected seed depth.

Two three-year-old producer fields of 'Red River' crabgrass managed for volunteer stands were used for the study and had undergone numerous diskings and draggings (homemade 10-inch I-beam drag). We assumed that the seed bank had been relatively well mixed within the upper 4 inches of the soil profile. The soil was Konsil loamy fine sand. Recent disking and dragging to facilitate volunteer management and subsequent forage production and gentle spring rains had left the soil firm and settled.

After the volunteer stand reached the one-leaf to early tillering stage, representative areas were sampled by taking several horizontal slices about 1 foot square by 4 inches deep. Several portions of the slice were gently washed over a 1/8-inch screen frame to remove plants with seed bracts still intact. The distance between the seed bract and the soil surface was measured to determine emergence depths. The soil surface was defined as the point where the white lower stem met the green upper portion. The data were tallied to determine the relative percentage of emergence from each ¼-inch depth increment.

'Red River' seedlings from the soil surface down to a maximum depth of 1.5 inches emerged (figures 1 and 2). Data from figure 1 illustrate the relative percentages of seedling emergence per ¼ inch of soil depth. More seedlings emerged as depth progressed from the soil surface to 0.5 inch. The highest percentage of seedlings (28) emerged from the 0.25- to 0.50-inch depth. Emergence declined steadily below that depth and no seedlings below 1.5 inches emerged. There was no way to determine the total percentage of seedling emergence in the sampled soil profile.

Cumulative percentage of the stand from various soil depths is presented in figure 2. Essentially half of the seedlings emerged from 0.5 inch or shallower; three-fourths, from 0.75 inch or shallower. Eleven percent emerged from the soil surface and 1 percent from 1.5 inches deep.

Although results of this study are confounded by the seed quantity at various depths, shallow average depth of seed (less than 0.75 inch) is important to reduce failure risk for good volunteer stands of small-seeded forages such as crabgrass, especially when soils are substandard or dry; in these cases the highest percentage of emerging seedlings will come from the soil surface or 0.25 to 0.50 inch deep. In practice, by employing relatively shallow tillage (2 to 4 inches) and more horizontal tillage (sweep plows), the manager of volunteering crabgrass stands can keep enough seed near the soil surface to produce excellent volunteer stands.

The same data apply to planting techniques. The seedbed should be fine, firm, and fresh without cracks, clods, and loose soil that would leave seed too deep after rains. A good technique is to prepare the primary seedbed, drag to smooth and firm, roll (cultipack), plant on the soil surface, and complete the planting by rolling. Often crabgrass is planted by being broadcast on the soil surface in completely grazed or otherwise harvested cool-season annual forages and crops and may be lightly trodden in under nonbogging conditions. More information on cool-season annual forage and crabgrass double cropping is available (Dalrymple et al., 1991).

Figure 1

Figure 2

Dalrymple, R. L., J. Baker, and S. Swigert. 1991. Crabgrass seminar and field day report. Publication no. CG-91, 97 pp. The Samuel Roberts Noble Foundation, Ardmore, Oklahoma.

Dalrymple, R. L. 1983. A summary of research and demonstration about using crabgrass as a forage. The Samuel Roberts Noble Foundation, Ardmore, Oklahoma.

Dalrymple, R. L. 1996. Managing for volunteer stands in crabgrass hay meadows. Stockman Grass Farmer Magazine 53 (3): 31-33.