Back to Basics: Pre-emergence Weed Control (1997, Vol. 20 No. 4) - SIA

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by Edmond Lewis
“Prevention is better than cure”. “One year of seeding is seven years of weeding.”
Familiar adages? And, what relevance to weed control in sugarcane?
There are perhaps many instances in which growers have just not been able to efficiently organize farm activity to make pre-emergence spraying possible so that by the time they get around to giving attention to a particular field, weeds are already full grown.
In recent years, in efforts to cut costs many growers took the decision to substitute an early post-emergence spraying for pre-emergence. The argument was, why waste money applying a chemical to bare soil. With a little luck there might not be rain for weeks. No rain means no weed growth and money would be saved.
In any event, if weeds appear then they can be easily attacked in the early growth stage before they cause crop loss.
But things rarely ever work out that way. The farm may be looking good until the rains set in. Then it might be four or even six weeks before sufficient drying out occurs for a resumption of field work. By which time, the weeds have taken over; it is months before the farmer catches up and the quality of control achieved is less than satisfactory.
Applying a pre-emergence herbicide protects the farmer from the catastrophe of being overrun by weeds. An earlier publication of Sugar Cane showed that uncontrolled weeds can result in yield being reduced by as much as a third.
There are considerable benefits from pre-emergent weed control:
  1. Studies show that the basis of success in any weed control programme is to apply the herbicide as pre-plant, or pre-emergent.
  2. With pre-emergence treatment germinating cane sprouts avoid competing with aggressive weeds which share fertilizer and soil moisture.
  3. The farmer avoids having to use higher rates of herbicides necessary to kill older weeds.
Early weed control is important for several reasons:
  • Even at higher rates, a herbicide does not always give good kill of older weeds making weed control cost higher, and the quality of control lower.
  • Weeds mature earlier than the cane and produce vast numbers of seeds capable of germinating immediately, or which can be stored in the soil for germination over future years.
  • Weeds, if allowed to become adult, choke the young, delicate cane sprouts leaving them slender, yellowish, weak, brittle, and easily breakable even in moderate winds.
In addition, in this age of environmental consciousness, it should be remembered that lower rates of herbicides, as applied at pre-emergence, are healthier for the environment!
The early herbicides, 2,4-D and diuron were applied as residual (pre-emergent) treatments for weed control in sugar cane. Over the past two decades, the ametryn/atrazine combination at pre-emergence has proven effective against grass, sedge, and broadleaf weeds under varying conditions. Effectiveness on mixed grass stands is, however, declining as it offers very little control of corngrass particularly during the spring. Its usefulness can be improved applied in a cocktail with pendimethalin (Herbadox) for better control of ricegrass, guineagrass and wild pangola in mixed grass stands on light, medium or heavy soils.
The use of pre-emergent diuron gives satisfactory control of nutsedge and cocograss which tend to evade most pre-emergent herbicides. Its advantage lies in its ability to be re-activated with each round of irrigation or rainfall.
The application of terbutryn (Igran), in most instances, is for late post emergent control of corngrass where there is potential for severe damage to cane especially on heavy soils. Its effectiveness at pre-emergence to control corngrass, guineagrass, sprangletop, and many other grasses, is not as well known.
Application of terbutryn at pre-emergence, or very early post emergence can offer earlier, effective, and economic control of a wide range of grass weeds and should therefore be used more often.
The newest addition to the pre-emergent armoury is isoxaflutole (Merlin), a water dispersible granular formulation which can be applied prior to irrigation. Like diuron, subsequent application of moisture to soil containing isoxaflutole activates it, and renews its pre-emergent action.
A cocktail with diuron increases the spectrum of weeds controlled, and extends the residual activity against corngrass, guineagrass, ricegrass, and sprangletop.
Weed Control Options
The range of herbicides currently on the market offers many options for early, reliable and cost effective control when used responsibly - with proper timing, correct rate of application, and good application technique. Table 1 summarizes the more effective cocktails for use within the various zones. The inclusion of 2,4-D, 2,4-D/dicamba, or 2,4-D/ioxynil in the mixture targets broadleaf weeds, and enhances the competence of most of these herbicides.
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