Weed management in pineapple is especially important during early growth, because weeds compete for water, nutrients and light, are hosts for pineapple pests and viruses and interfere with production operations. Weed management includes soil tillage, mulches, and the use of pre-emergence (applied prior to weed-seed germination) and post-emergence herbicides (Fig. 9.14; Kasasian, 1971; Glennie, 1991). The efficiency of the pineapple weed-management system is affected by plant density, the degree of mulch cover, soil type and natural rainfall and/or the method of irrigation. Because the pineapple plant is relatively slow in establishing a complete ground cover, eliminating weed cover may result in high levels of soil erosion (El-Swaify et al., 1993).
Prior to the introduction of pre-emergence herbicides in the 1950s, weed management was primarily physical removal by tillage. The introduction of pre-emergence herbicides revolutionized weed control in pineapple, particularly the grasses. In general, the perennial grasses are much more difficult to control than the broad-leaved weeds. Important pineapple herbicides have been diuron, bromacil, amytryn, atrazine and paraquat (Glennie, 1991). No one herbicide will control all weeds in all situations.
Each production area has its own particular spectrum of weeds, sometimes determined by historical weed-control practices (St. John and Hosaka, 1932; Barbier and Trapin, 1956; Py, 1959; Silvy, 1962), e.g. wild sugar cane (Saccharum spontaneum L.) in the Philippines (Sison and Mendoza, 1993). Species that are particularly difficult to manage are Panicum maximum var. maximum, Sorghum halepense and the paspalums, Paspalum dilatatum and Paspalum urvillei. The sedge Cyperus rotundus (nut grass) is also a serious pest. Significant broad-leaved weeds are the morning glories, Ipomoea cairica, Ipomola plebeia, Ipomola indica, Ipomola purpurea and Ipomola triloba. Perennial weeds (e.g. S. spontaneum, S. halepense, Imperata cylindrica) are destroyed by deep ploughing.
Pre-emergence herbicides are used most commonly in pineapple production. Effectiveness is dependent on proper seed-bed preparation, including no live weeds, adequate soil moisture, complete soil coverage and no subsequent soil disturbance following application (Dalldorf, 1985). Split applications of pre-emergence herbicides may be made to pre-mulched or mulched and postplanted fields to ensure adequate pre-emergence protection through the preplanting/planting period. Weed 'escapes' during the first crop are controlled with bromacil at 2-4 kg ha-1 or dalapon at 6-10 kg ha-1. C. rotundus and Panicum repens appearing in the next cropping cycle are controlled with bromacil at 2-4 and 10 kg ha-1, respectively. Other weeds (I. triloba, Mimosa invisa, Crotalaria mucronata, Digitaria sanguinalis, Eleusine indica, Paspalum conjugatum) are controlled by pre-emergence sprays of bromacil at 1-2 kg ha-1, diuron at 0.75-1.5 kg ha-1, ametryne or atrazine (Mendoza, 1979). A summary weed-control programme is identified for Brazil (Reinhardt and Cunha, 1999).
Once plants begin to grow, herbicide applications should be directed away from the plants and on to the soil and developing weeds. This is particularly true with herbicides such as bromacil (Dalldorf, 1985).
Preventing grasses from producing seed both in the field and on field borders is critical to economically effective weed management. The adage of '1 year's seeding means 7 years' weeding' holds true (Broadley et al., 1993).
In contrast to grasses, broad-leaf weeds are relatively easy to control. However, some broad-leaved weeds, such as Emilia sagittata, may cause secondary damage in low population densities as alternative hosts for the yellow-spot virus.
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