Plot Weed control method Weed count at start (per m2) Weed count after 6 weeks (per m2) A Hand weeding 35 8 B Mulching 32 12 C Herbicide application 38 3 D ...
Question 1 Report
| Plot | Weed control method | Weed count at start (per m2) | Weed count after 6 weeks (per m2) |
|---|---|---|---|
| A | Hand weeding | 35 | 8 |
| B | Mulching | 32 | 12 |
| C | Herbicide application | 38 | 3 |
| D | No weed control | 30 | 54 |
2 A farmer tested four different weed control methods on separate plots of maize. Each plot was the same size. Table 2.1 shows the number of weeds counted per square metre in each plot at the start and after 6 weeks.
Table 2.1
(a) State which weed control method gave the lowest weed count after 6 weeks. [1]
(b) Suggest one reason why Plot D, which had no weed control, showed a large increase in weed numbers. [1]
(c) Name the type of herbicide that kills weeds without harming the crop plants. [1]
(d) Explain the difference between a pre-emergence herbicide and a post-emergence herbicide. [2]
(e) State two reasons why a farmer with a small farm might choose hand weeding instead of using herbicides. [2]
(f) Give one reason why crop rotation can help to reduce weed problems over several seasons. [1]
(g) Suggest one reason why the farmer should wear protective clothing when spraying herbicides. [1]
(h) State one environmental problem that can be caused by the overuse of herbicides. [1]
Answer Details
(a) The weed control method that gave the lowest weed count after 6 weeks is herbicide application (Plot C), with only 3 weeds per m2 remaining. [1]
(b) Plot D (no weed control) showed a large increase because weeds were allowed to grow, flower and set seed without any intervention. [1] With no competition from control measures, existing weeds thrived and new seedlings germinated freely, causing the population to almost double from 30 to 54 per m2.
(c) The type of herbicide that kills weeds without harming the crop is a selective herbicide. [1] Selective herbicides exploit physiological differences between the crop and weed species (e.g. broadleaf weeds vs. grass crops) so that only the target weed is killed.
(d) Difference between pre-emergence and post-emergence herbicides:
- A pre-emergence herbicide is applied to the soil before the weed seeds germinate. It forms a chemical barrier in the topsoil that kills seedlings as they sprout. [1]
- A post-emergence herbicide is applied after the weeds have already emerged and are growing above the soil surface. It is absorbed through the leaves and kills established weeds. [1]
(e) Two reasons a small-scale farmer might prefer hand weeding to herbicides:
- Hand weeding does not require the purchase of expensive chemicals, reducing input costs on a small farm. [1]
- Hand weeding does not require specialist spraying equipment; only simple tools such as a hoe or bare hands are needed. [1]
Other acceptable answers: hand weeding provides employment for farm workers; it avoids the risk of chemical damage to the crop; a small area can be weeded by hand in a reasonable time.
(f) Crop rotation helps reduce weed problems because different crops compete with different weed species and changing the crop each season breaks the life cycle of weeds that are adapted to one particular crop. [1] Some rotation crops (e.g. leguminous cover crops) can smother weeds through dense canopy cover.
(g) The farmer should wear protective clothing because herbicide chemicals can be absorbed through the skin and cause harm to health. [1] Spray droplets can also irritate the eyes and lungs if inhaled.
(h) One environmental problem from herbicide overuse: herbicide residues can contaminate water sources such as rivers, streams and groundwater, harming aquatic organisms. [1] Other acceptable answers: herbicides can kill non-target plants, reducing local biodiversity; residues can persist in soil and affect future crops.
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