Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 61

Table 2.6. Propagule weight of annual weeds and crops

Embryo plus

Propagule endosperm Propagule

Weed species a weight (mg) weight (mg) Crop species b weight (mg)

Abutilon theophrasti 9.5 5.1 Maize 250

Ambrosia artemisiifolia 4.4 2.4 Soybean 220

Brassica kaber 2.2 1.8 Wheat 39

Chenopodium album 0.74 0.47 Oat 35

Amaranthus retroflexus 0.44 0.29 Rye 27

Notes:
a Weeds are the five most common annual weeds in agronomic crops in New York state (Bridges,
1992).
b Crops are the five annual agronomic crops with the greatest hectarage in New York state (New
York Agricultural Statistics Service,1994).
Source : Adapted from Mohler (1996).

small-seeded weeds that emerge best from a depth of 0.5 to 1.0 cm,most agro-
nomic crops and many vegetable crops have much larger seeds (Table 2.6).
Consequently, they are usually planted at 3 to 5 cm.As discussed in Chapter 4,
this difference in emergence depth allows pre- and post-emergence cultiva-
tion in the crop row. In some systems, it also allows directed feeding of water
and nutrients to the crop (Chapter 5). In addition, the difference in seed size
between crops and weeds makes possible the use of crop residue and dust
mulches (Chapter 5) and greatly facilitates the use of crop competition for
weed management (Chapter 6). Conversely, the relatively small difference
between the seed size of crop species and large-seeded weeds like Xanthium
strumarium , Avena fatua , and Ipomoea hederaceae helps explain why these species
are so difficult to control.
Herbivores and damping off fungi have their greatest impact on weed
density during the establishment phase because very small plants have few
resources for defense and recovery. Cover by residue and the crop canopy is a
major factor regulating the effectiveness of naturally occurring generalist
seedling predators (see Chapter 5). Frequently, the inundative release of bio-
logical weed control agents is most effective when the weeds are small (e.g.,
Pitelli, Charudattan & Devalerio, 1998).
The susceptibility of a weed to physical disturbance decreases as it grows.
First, as the plant grows, stems and roots thicken and toughen with fiber.
Consequently, impact with a hoe or cultivator tine is less likely to cause fatal
breakage to a large old plant than to a small young one. Second, plants grow
by repeated addition of metamers, units consisting of a leaf, the subtended
bud(s), and an internode (White, 1979). Potentially, a weed can lose most of its