Underground War On Weeds

Press, Volume CVIII, Issue 31802, 5 October 1968, Page 9

Underground War On Weeds

]\JOST commonly-used herbicides, such as MCPA and 2,4-D, are applied to a crop when both crop and weeds have emerged and have developed a number of leaves. At certain stages of crop and weed growth these chemicals are toxic to weeds but not to the crop, so that weeds can be selectively controlled. In recent years, a new concept of weed control has been developed in which the herbicide is applied to the soil and incorporated in the surface layer before sowing seed or planting transplants. The chemicals trifiuralin or benefin (commercially available as Treflan and Balan respectively) work in this manner, and were introduced at a re. cent meeting in Christchurch. In spite of repeated surface cultivation to kill weed seedlings and encourage further weed germination, it is virtually impossible to prevent the germination of weeds after sowing a crop. Although harrowing after drilling will control some weeds, a crop frequently suffers from weed competition in its early stages of growth. With peas, selective herbicides can be applied but results depend on the weed species present and climatic conditions at and after spraying. Furthermore, peas can be checked by chemicals applied under adverse weather conditions and wheel marks can cause damage, particularly to vining peas. Several of these problems can be overcome by the use of the recently developed trifluralin. It is applied either shortly before drilling or up to three or four weeks before sowing.

Rates of application vary depending on soil type from one pint of the formulated product on light, sandy soils, to 1) pints on medium soils and two pints on heavy, clay soils. It is not recommended for soils containing high organic matter (peat soils). Trifluralijf can be used near such

sensitive plants as vines and tomatoes and can be applied in 10 t'o 40 gallons of water per acre. As trifluralin is quickly degraded by sunlight and acts through the fine root hairs of seedlings shortly after they germinate, it must be thoroughly mixed in the surface sell to a depth of about two to three inches as soon as possible after application. Unless the soil is disturbed by further deeper cultivation the chemical will remain in the top soil to prevent the growth of a wide range of weeds for several weeks. It is not leached by rain nor is rain needed to activate it Incorporation to depths of greater than three Inches will dilute the chemical and can therefore reduce the efficiency of weed control. If incorporated too shallow, weeds may germinate below the treated layer of the soil. Weeds can germinate in the untreated soil inside large unbroken soil clods so fine soil tilth gives best results.

Where possible, the chemical should be applied and at least partly incorporated in the one operation. For example, harrows pulled behind the sprayer will provide initial protection from sunlight. Full incorporation to a depth of not less than 2in or more than 3in however, must be carried out either by rotovation or by double-discing along and across the field at four to six miles an hour with the discs set to cut 3in to 6in deep (discs only mix soil to about two-thirds of the full’ depth of the cut). Spike-toothed harrows (Dutch harrows) have also been used successfully. Trifluralin controls most grasses as well as fat hen, cornbind, wireweed, willow weed, red shank, chickweed and wild oats. It does not control established plants ahich can sometimes recover lowing cultivation) such as

Californian thistle, twitch and yarrow, nor such tolerant weeds as wild turnip, nightshade, tares, clovers, storksbill and annual nettle. Trifluralin has been tested In New Zealand and is registered by the Agricultural Chemicals Board for controlling weeds in canning peas and beans, field peas, swedes, turnips, chou moellier, and transplanted tomatoes. It has also shown promise in soya beans, cabbage, celery, carrots and other crops. Benefin (Balan), the second material introduced at the meeting, is chemically very similar to trifluralin and shows promise for controlling weeds in newly-sown lucerne and transplanted tobacco crops. It must be applied and incorporated in a similar manner. It is, clear from the information given at this meeting that, to gain the full benefits from the use of chemicals such as trifluralin, the farmer must know which weeds are likely to arise and must be prepared to apply and Incorporate the chemical accurately and effectively. The correct use of herbicides which can be applied before sowing promises to increase the benefits from weed control by eliminating weed competition during the establishment phase when crops are particularly sensitive to competition. Mr L. Prior, of Elanco Products Company, the manufacturers, outlined the main features of the two chemicals to the meeting; details of the use of them for weed control in various crops were given by Mr S. Collingwood, of Elanco, and Mr F. N. Sumich, of Neill Cropper and Company, Ltd, and Mr H. C. Hiatt, of Imperial Chemical Industries, reviewed present methods of weed control in peas and the possible advantages of weed control by soil incorporated herbicides in such crops. A film on the use of trifluralin in North America was shown.