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 two stages, three at most: (1) pre-planting, (2) vegetative stage of corn, and (3) seed maturation stage. The third rarely occurs. It is the prevalence of weeds which really dictates if there is any need to spray. Further, failure in any scheduled spraying may necessitate repeat spraying.
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.
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.
Note: There are various possibilities that determine the exact timing of herbicide spraying. These are discussed in the page on corn planting.
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.
The necessity to control weeds when corn is established is the main reason why farmers have become dependent on chemical herbicides. Currently growing corn plants tell the farmer that weeding is necessary. And when weeds become visible in great number to the point that they may overtake the growth of corn plants, weeding has to be done with haste. In a place where labor cost for manual weeding is unusually high or availability of labor is uncertain whenever most needed, resort to herbicides becomes essential.
Herbicide spraying at any time starting from seedling emergence until the corn plants are firmly established is the most convincing reason why farmers prefer to plant corn varieties which are resistant or tolerant to herbicides. Otherwise if the variety planted is susceptible of damage, herbicides are excluded. The farmer in turn reverts to the previously stated situation of confronting the problem of labor.
As to the first herbicidal spray during land preparation before planting, farmers generally consider it as merely optional. For those who have little capital to spare or simply want to be economical, burning and repeat slashing are commonly practiced. The non-essentiality of applying herbicides before planting is even more highlighted by the innovative farmers' practice of planting corn on farm with existing weeds but still on low growth or recently slashed close to ground.
As to the above-stated third spraying, it is rarely practiced because the first two herbicide spraying tend to have drastically curbed the growth of weeds. Corn plants also increase shade below the canopies as they continue to produce leaves. In situations where weeds manage to grow and climb upward, they are sometimes removed manually. Or else most farmers reason out that it is not worth spending time and money anymore for corn plants that already approach maturity.
(Ben G. Bareja 2014, edited Apr. 21, 2019)