Corn Farmers’ Practices in Herbicide Spraying
Under No-Till Farming
Ben G. Bareja, July 2014

2. Herbicide spraying - Three herbicides are commonly used to kill weeds in no-till corn farming: glyphosate, paraquat, and 2,4-D. In general, the farmers in this part of Sarangani use glyphosate throughout the growing season. However, under certain conditions, paraquat and 2,4-D are used as alternative or additive.

Glyphosate (N-(phosphonomethyl)glycine, ex. Roundup) is a nonselective, systemic (or translocated) post-emergence herbicide. Being nonselective, it is a broad-spectrum herbicide capable of killing grasses, sedges and broadleaved weeds except those genetically modified to resist or with natural tolerance to its toxicity. It is ineffective on roundup-ready or glyphosate-resistant varieties of corn.

As a systemic herbicide, it is first absorbed by the target plant and translocated throughout the plant body before it takes effect. Systemic herbicides are therefore capable of killing the entire plant. However, it takes several days for the symptoms of toxicity to show. 

Paraquat (1,1'-Dimethyl-4,4'-bipyridinum dichloride, ex. Gramoxone) is likewise a nonselective herbicide. In contrast to glyphosate, however, it is generally a contact herbicide which means that it takes effect immediately after spraying provided that there is light. Note: I remember from my Advanced Physiology of Herbicides that if you know the mode of action of paraquat, you can make it systemic.

2,4-D (2,4-dichlorophenoxyaceticacid) is a selective herbicide with systemic action. It is indicated for the control of broadleaf weeds. The numbers 2 and 4 in 2,4-D are useful guide that its killing effect, signified by the change in leaf color from green to yellowish or brownish, starts showing 2 to 4 days after spraying.

For economy, farmers desire that the number of herbicide sprayings be limited only to three stages: (1) pre-planting, (2) vegetative stage of corn, and (3) seed maturation stage. However, it is the prevalence of weeds which really dictates if there is any need to spray more. Further, failure in any scheduled spraying may necessitate repeat spraying.

Here’s how they control weeds by herbicide spray in no-till farming using glyphosate-resistant corn varieties under rainfed conditions:

1st  Herbicide Spray – Thick stands of weeds are first slashed, burned and allowed to regrow. The new regrowth should now be much reduced in thickness and allow thorough wetting. Roundup, or other glyphosate herbicide, is then sprayed on the young vegetation.

Dilution rate (dosage) is 100-150 ml of herbicide per 16 liters (1 knapsack sprayer) of water. The farmers do not use graduated cylinder or any volumetric measuring tool. Instead, they use the common sardine can (locally called tinapa, content 155 g) as dispenser. Spray delivery per hectare is around 10 knapsack sprayers give or take one or two loads. At near-full to full sardine can per knapsack load, about 1-1.5 liter of the herbicide is consumed in one hectare. Labor requirement per hectare is 1 man-day,  meaning one sprayman for one whole day.

This part of the farm was planted to glyphosate-resistant corn after the first herbicide spraying.

Farmers and spraymen do not practice sprayer calibration. For them it is sufficient to copy the dilution rate used in other farms if it produced successful results. Consequently, spray volume per hectare changes with such factors as efficiency of the sprayer, nozzle discharge, walking speed of the sprayman, and terrain of the land. Quite understandably as to hired spraymen who are paid on daily work, they prefer to use single nozzled sprayers. 

As alternative, farmers sometimes use paraquat instead of glyphosate in clearing the land in preparation for planting. There is also the practice of adding 2,4-D to glyphosate to enhance burndown effect. However, applying the mixture so close before or after planting will likely injure the emerging corn seedlings. 

Glyphosate-resistant corn close to harvest. Weed control was practiced by herbicide spray.

Note: There are various possibilities that determine the exact timing of herbicide spraying. These are discussed in the page on corn planting (click here to read).

2nd Herbicide Spray – If the land has been thourougly cleared during planting or herbicide was sprayed after planting, it will take time for new weeds to grow. The second spraying of herbicide is usually synchronized with fertilizer application about 30 days after planting  in order to eliminate weed competitors. 

Glyphosate is the preferred herbicide for spraying at this vegetative stage of growth of the corn crop. With the usual dosage, it is applied as blanket spray just before or right after sidedress application of fertilizer. But in areas with significant growth of broadleaf weeds (eudicot), farmers sometimes add 2,4-D to the spray solution based on their observation that  pure glyphosate is less effective on some broadleaves. 

3rd Herbicide Spray – At about 80 days or thereabout after planting, weeds may have grown. In particular, the three-lobed morning glory or Ipomaea triloba (locally called mote-mote and uyampong) may have started climbing by coiling around the corn stems. If not controlled, this vine may choke the corn plants and hamper harvesting. As chemical weed control, glyphosate or 2,4-D or a mixture of the two herbicides is sprayed. 

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