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Chapter 29
Herbicide absorption Herbicides are usually absorbed by the roots or leaves. The amount of herbicide absorption depends on the vegetative stage of the weed, since the leaf cuticle is thicker in old leaves in comparison with young ones. The number and size of stomata also affects herbicide entry into the leaf. Weeds differ in both the number of stomata/mm2 of leaf surface and the location of stomata (on the upper or lower leaf surface). The efficient use of herbicides depends on their ability to pass through the cuticle and plasmalemma. The entrance of herbicides is via stomata, if they are open during the time of herbicide application. Application of herbicides to green and tender shoots of weeds is frequently as effective as when applied to the leaves. The bark of woody shoots is usually the obstacle to entrance of herbicides. However, in some barks there are openings that allow the herbicide ingress. Lenticels may also constitute a route for herbicidal entrance. The most dangerous weeds causing losses to olive culture are Cynodon dactylon, Oxalis and Rubus fruticosus. Other weeds present in olive orchards include Sorghum halepense, Convolvulus arvensis, Cirsium arvense, and Amaranthus (see Table 29.1).
WEED MANAGEMENT Weed management varies from orchard to orchard and, of course, upon the weed species present in the orchard, the soil characteristics, the irrigation system and the age of the orchard.
Soil properties Soil texture and organic matter determine the weed species present in the orchard, the activity and adsorption of herbicides, the number of times soil should be cultivated and when planting should take place. In sandy soils less herbicide is used, and these soils require more cultivation to deter weeds. On the contrary, clay soils are cultivated less frequently and can adsorb a greater quantity of herbicide. Soil characteristics, where herbicides are applied, affect their runoff. Soil texture and organic matter content affect the speed of infiltration and runoff, which are greater in sandy soils (Galindo et al., 2006). Organic matter affects herbicide mobility and adsorption: soil compaction decreases the speed of herbicide infiltration and increases herbicide concentration in the water flow. Stability of soil aggregates affects the rate of infiltration, as does the slope of the orchard, since a greater angle of incline increases runoff.