Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 155

measurements of how surface-applied 32 P was mixed by disking and rototil-
ling may indicate the way surface seeds are moved by these operations.
Typically, a single moldboard plowing of surface seeds results in a skewed,
bell-shaped distribution of seed density with depth (Figure 4.3).The direction
and degree of skewing has varied among studies (Mohler, 1993), perhaps due
to variation in depth of operation and degree of soil turning by the plows. A
variety of factors affect the degree of inversion. These include design of the
moldboard and coulters,speed of operation,amount of roots and crop residue
present, and soil structure, texture, and moisture.
In contrast with moldboard plowing, a single operation with other imple-
ments produces a monotonic decline in seed density (Figure 4.3). A second
operation (usually the following season) makes the distribution more
uniform with all implements (Figure 4.3), but seeds still tend to be concen-
trated near the surface with tine and disk type implements.
Using colored beads placed at different depths, Cousens & Moss (1990) and
Grundy, Mead & Burston (1999) developed data on movement to and from
multiple layers of the soil and then used these data to model seed distribu-
tions after multiple tillage events. The distributions predicted by Cousens &
Moss (1990) for a single cohort of seeds moved repeatedly by a single type of
implement were similar to seed distributions after multiple tillage events
observed in empirical studies (Figure 4.3) (Röttele & Koch, 1981; Van Esso,
Ghersa & Soriano, 1986). Cousens & Moss (1990) also considered the situation
where weeds shed seeds onto the surface each year. The stable distributions
predicted under the assumptions of the model after many cycles were similar
to those for a single cohort after one tillage operation (Figure 4.4). Published
seed distributions after many years of moldboard plowing with seed input
(Roberts & Stokes, 1965; Fay & Olson, 1978) are more uniform than predicted
by Cousens & Moss (1990). In real agricultural fields, the distribution of seeds
is rarely stable because years of high seed input usually occur sporadically
among years of successful control and little seed input.Following one or more
high-input years, the distribution would look something like those in Figure
4.3 for one tillage operation.After one or more years of good weed control,the
distribution would become more uniform.
Tillage and the emergence of weed seedlings
Germination of weed seeds is more likely near the soil surface because
seeds are more likely to experience light, fluctuating temperatures, and other
factors that commonly promote germination (see Chapter 2). Moreover, if a
seed does germinate, emergence from the soil is more likely if the seed is near