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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).

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 agronomic 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 cultivation 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 biological 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 shoot and still regrow into a full-sized plant, provided a single bud is left. Modular growth below ground similarly allows recovery from drastic damage to the root system. However, for most herbaceous dicot species, a seedling that is broken between the root and base of the cotyledons will not survive. At this stage the weed has only one shoot meristem, and its loss is fatal. Establishing monocot seedlings are somewhat less susceptible to damage than dicots because they lack the long hypocotyl between the root and shoot meristems, but they too may fail to recover following loss of a substantial portion of the cotyledon or primary root. Thus, very small weeds in the white thread and cotyledon stages are more easily controlled by mechanical means than are weeds that are more developed. Cultivation techniques specifically aimed at establishing weed seedlings are discussed in Chapter 4.

Surprisingly, the biology of mechanical weed management has been little studied. For example, conventional wisdom among farmers holds that rotary hoeing is most effective if the soil is not immediately wetted afterward by rain or irrigation, and some experimental evidence confirms this view (Lovely, Weber & Staniforth, 1958). However, the phenomenon has only been studied at the level of the field, and not at the level of the individual weed. To what extent is the elimination of weeds by a pre-emergence operation with a rotary hoe or tine weeder due to (i) direct damage, (ii) desiccation from loss of intimate contact with soil, or (iii) reburial of white thread seedlings that have already expended their seed reserves.?

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