WEED CONTROL IN FIELD BRASSICAS AND ONIONS

New Zealand Journal of Agriculture, Volume 95, Issue 6, 16 December 1957, Page 607

WEED CONTROL IN FIELD BRASSICAS AND ONIONS

Recent Research Work

TRIALS in the 1956-57 season have confirmed the value of using ■ chemicals to weed field brassicas, particularly swedes and turnips, and have emphasised that onions are one of the most difficult crops to keep free of weeds.

IN FIELD BRASSICAS

THE trials show that in brassicas effective weed control depends on a number of conditions, the most important being that of establishing a good crop to obtain complete ground cover as soon as possible. The weedkillers used usually control the first crop of weeds to strike, but if the crop is slow to establish, successive crops of weeds may then compete with the crop. Of the chemicals used, brassicas show greatest tolerance to TCA and may be used at up to 201 b. per acre without detriment to the crop. To be effective this chemical must be applied to moist soil and further rain should follow the application. It should be applied normally as soon as the crops are drilled or broadcast. If no rain follows the application, the weeds germinate and compete with the crop. If sufficient rain follows, good control is obtained for several weeks, and if the crop is vigorous, it can establish weed free in this period. Used under optimum conditions TCA gives effective control of fat-hen, red-root, Prince of Wales feather, and willowweed, and to some extent will control spurrey. If spurrey is the dominant weed, IPC or CIPC under cool soil conditions may be used at rates of lib. carbamate equivalent to the acre. These chemicals should be applied as for TCA; that is, they should be applied as soon as a crop is drilled in and when rain is expected. If the seed is broadcast, the weedkillers should be applied immediately the seed is covered. It is necessary to apply the weedkillers with a minimum quantity of water. If the brassica seed is drilled into the soil, the weedkillers may be applied behind the drill as a directed spray only over the row or as a spray over the whole area behind the drill.

Another type of chemical, oktone, may be used for brassica weed control if the crops are drilled. The seed-bed should be prepared, weeds allowed to strike (lin. to 3in. high), and then the crop drilled. As soon as the crop is drilled, if the oktone is applied at 21b. to 41b. per acre in 10 to 15 gallons of kerosene, the weeds are killed without detriment to the crop. This chemical, unlike TCA, CIPC, and IPC, has no residual life in the soil, so that if the crop does not establish quickly, further germination of weeds may occur. Oktone should not be applied once the crop is through the ground; otherwise crop damage will occur. This treatment is independent of climatic conditions and will give a wide range of weed control. It is more suitable for rape and chou moellier, crops that form a complete ground cover.

SUMMARY

Crops of brassicas may be grown weed free. This depends on the effective use of chemicals and a . quick-establishing crop. If the crop is slow to establish, successive germination of weeds may occur after the effect of the chemical has worn off. The most successful chemical is TCA. If spurrey is the dominant weed, IPC and CIPC should be used. Oktone and diesel fuel oil may be used for crops that are drilled into the soil.

IN ONIONS

Further trials in the 1956-57 season have emphasised - that onions are one of the most difficult crops to keep weed free. Any crop that remains in the ground for a considerable period and does not form a complete ground cover is difficult to maintain free of weeds. The chemical required for weed control in onions is one that has long residual properties in the soil so that successive crops of weeds are destroyed. At present no chemical is sufficiently selective to give long-term weed control without damage to the onions. Monuron (formerly CMU) persists for long, periods in the soil, but rates that are selective for onions allow Scotch thistle, various flatweeds, docks, wireweed, willowweed, etc., to persist and grow at the expense of susceptible weeds that are controlled with monuron. Monuron also has the disadvantage that it can cause crop damage if heavy rain falls and leaching occurs. CIPC is more selective than monuron, but also fails to control all weed species. Scotch thistle, sow thistle, various flatweeds, clover, etc., survive. CIPC can also cause onion damage ’ or its herbicidal properties can be dissipated quickly under high soil temperatures. In a typical' trial at Opiki the main weeds were Scotch thistle, black nightshade, buttercup, cress, and willowweed. Monuron at 21b. per acre failed to control Scotch thistle and some willowweed which in the absence of other weeds grew larger than normal plants. CIPC failed to control Scotch thistle, to the detriment of the onion crop. A further factor in the use of monuron and CIPC is that both these chemicals have to be applied before the weeds have struck; otherwise the weeds soon become resistant to the low rates of application that have to be used.

Contact chemicals such :as . oktone or sulphuric acid control a wide range of weeds, but have no residual life in the soil. If oktone or sulphuric acid is used to kill weeds which have appeared before the crop has germinated or is applied to a stale seed-bed, the onion crop germinates and establishes weed free. In the stale seed-bed method the weeds are allowed to strike before the crop is drilled into the soil. If the onions have been established weed free with the use of either oktone or sulphuric acid, CIPC may then be used as a residual weedkiller with less likelihood of damaging the onions. High rates, 41b. to 81b., may be used. The CIPC should always be applied before the weeds have struck. If resistant weeds are present, such as various thistles, oktone or sulphuric acid may be used. J Oktone should be used as a directed spray at 21b. to 41b. per acre, in 10 to 15 gallons of diesel fuel oil. Used in 'this method oktone plus diesel fuel oil will give wide weed control. Sulphuric acid will not control plants with wax-like leaves such as portulaca.

RECOMMENDATIONS

Weedkillers will assist materially in the control of weeds in onions. The residual weedkillers probably are too toxic to be used as pre-emergence chemicals, but if the crop is established weed free with contact chemicals such as oktone or sulphuric acid, the residual chemicals can then be applied with greater safety and their effects may last several months. If resistant weeds are present, hand removal or control by contact chemicals such as directed sprays of oktone or diesel fuel oil or sulphuric acid will give adequate weed control. Unless cultivation benefits the crop, the use of correct weedkillers plus hand removal of any resistant weeds will give better weed control than cultivation alone plus hand removal of row weeds.

—B. A. J. SMITH