Chemical injury

Primary symptoms

Herbicide injury can cause malformation (e.g., leaf twisting) of young plants and yellowing, burning, and dying of the leaves.
Fertilizer burning can cause seeds to not germinate, or the seedling may emerge and then die. Burned plants may look wilted or stunted.

Confirm the problem by checking the tables below.


Causes of herbicide injuryAdditional evidence required
The wrong product was used. Ask the farmer what was used. Check to see if the product is approved for use on maize.
The application rate of the product was too high or product application overlapped. Ask the farmer how much chemical he used per tank of water. Look at his tank to see how big it is. Calculate the application rate he used. Check this against the recommended rate on the label or in Table 7 below.
Timing of application was incorrect. Ask the farmer when he applied the chemical. Relate this to an approximate growth stage and check against Table 7 below.
A nonselective herbicide was applied with no shield or with a shield which did not work. The damage could also be due to drift from adjacent fields. Ask to see the shield if one was used. Ask if the weeds were taller than the crop when the product was applied.
Causes of fertilizer burnAdditional evidence required
Product placed directly in hole with the seed. Ask the farmer how planting was done.
Fertilizer was placed too close to the plant when sidedressed, especially when plants were under moisture stress, Ask the farmer where product was placed, what soil moisture was like, and if there was wilting after the application.

Maize can be damaged by the improper use of agricultural chemicals such as herbicides, fertilizers, or insecticides. The damage is usually the result of applying the chemical carelessly, at too high a rate, at the wrong growth stage, or when the plants are under drought or temperature stress. Usually problems of chemical injury are the result of occasional accidents and are not major limitations to yield in an area, but it is important that an agronomist be able to recognize these problems.

Herbicide. Herbicide injury can cause malformation of young plants and yellowing, burning, and dying of the leaves. The malformation (such as twisted leaves which do not unroll properly, or distorted roots) results from applying herbicides from certain groups such as 2,4-D at the wrong growth stage or at too high a rate. This problem can occur with phenoxy herbicides (e.g. 2,4-D), dinitroanaline herbicides (e.g. pendimethalin), benzoic acid herbicides (e.g. dicamba), and amide herbicides (e.g. alachlor or metolachlor). Leaf chlorosis and death can result from overapplication of triazine herbicides (e.g. atrazine). Leaf burning results from getting a directed spray of a contact herbicide such as paraquat on the maize or from applying too high a concentration of another chemical. You can distinguish herbicide injury from foliar disease by looking for burning which occurs in patterns from the sprayer nozzle and which only affects leaves of a certain age which were exposed when the chemical was applied.

Fertilizers. If fertilizers (especially Nand K) are placed in contact with the seed or too close to young plants they can cause burning, especially if soil moisture is deficient. If the problem occurs at planting, the seeds may not germinate, or the seedling may emerge and then die. Burned plants may look wilted or stunted. When fertilizer is sidedressed, it should be placed no closer than 10 cm to the stem.

Foliar applied insecticides or fertilizers can occasionally cause burning of the maize leaves. This can be minimized by applying early in the morning or late in the evening, to avoid intense, direct sunlight on the leaves.

Is chemical injury a problem?


Estimate the proportion of plants affected by the problem. If more than 10% of the plants are damaged badly enough to reduce yields (that is, if more than 30% of the leaf area on those plants is damaged), a problem exists. It is very important to look at several fields belonging to different farmers to decide if there is a problem with the technology in use or if a specific farmer simply made a mistake during that particular season.

If the injury has reduced the plant stand, measure the remaining population density. If the reduction in stand causes the density to drop below 70% of the optimum for your variety, measured under non-limiting conditions, the injury is important. (See plant density).

Possible solutions: Chemical injury

  • Change to a safer product or use lower rates. Investigate "safeners" available for use with specific herbicides.
  • Change the timing or method of application of the product.
    • Use a better design of shield. If the weeds were taller that the crop at application, causing problems when the farmer tried to apply a directed spray, this indicates a land preparation problem (see land preparation).

Possible solutions: Fertilizer injury

  • Use a planting stick with two points to make a separate hole for fertilizer.
  • Sidedress fertilizer when soil moisture is adequate and place the product at least 10 cm from the stem.
  • Design experiments to examine the effect of timing on fertilizer application. In some environments the application of fertilizer at planting can be avoided.

Table 7. Common herbicides in maize, recommended range of concentrations*, and recommended time of application.

ChemicalRecommended rate (kg/ha)Recommended time of application
2,4-D amine 0.5-1.0 From emergence until maize has 6 leaves (30 cm tall)
2,4-D ester 0.3-0.6
Paraquat 0.2-0.6 When needed post-emergence but as a directed spray. Contact with maize will burn the leaves
Dicamba 0.25-0.36 As for 2,4-0.
Glysophate 0.34-1.12
for annual weeds
Never allow chemical to contact maize. Can be used very carefully as a directed spray to control problem weeds.
for perennial weeds
Atrazine 1.5-3 Pre-emergence. Can be applied post-emergence, but if applied with oil and crop is under stress, may damage maize.
Alachor 1.7-4.5 Pre-emergence. Can be applied early post-emergence to crop, before weeds emerge.
Metolachlor 0.6-1.2 Pre-emergence, or post-emergence before maize is 8 cm tall.
Pendimethalin 0.6-2.2 Pre-emergence.

* Note that the concentrations given here are as kg of active ingredient per hectare. You will have to calculate the amount of commercial product as: kg of commercial product = kg active ingredient / (fraction of active ingredient in the commercial product).