Crop Protection: Weed Control

12 Grade 10 Agriculture questions on Crop Protection: Weed Control, with worked marking schemes. 12 questions are available on this topic in full.

1. Question 1

   3 marks

   Explain three reasons why weeding in a maize crop should be carried out early, while the crop is still young.

   (3 marks)

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2. Question 2

   1 mark

   Maize fields in parts of Migori are infested with striga. Which practice would reduce the striga population in the field over several seasons?

   A. Planting the maize earlier at the onset of the rains each season

   B. Intercropping the maize with Desmodium, which makes striga seeds germinate with no host present

   C. Uprooting the striga plants only after they have flowered and set seed

   D. Ploughing the field deeply immediately before planting each season

   (1 mark)

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3. Question 3

   4 marks

   Outline four precautions a farmer should observe when spraying a herbicide in the field.

   (4 marks)

   Marking scheme

   Award 1 mark each, maximum 4: wear protective clothing such as overalls, gumboots, gloves, goggles and a nose mask; read the label and mix at the recommended rate and concentration; spray on a calm day so that the chemical does not drift onto neighbouring crops; do not spray when rain is expected shortly, as the chemical will be washed off and wasted; do not eat, drink or smoke while spraying, and wash thoroughly afterwards; use a clean sprayer, or one kept for herbicides only, so that traces do not damage other crops; observe the stated interval before harvesting the crop or grazing livestock on the land; bury or dispose of empty containers safely, away from water sources.

4. Question 4

   1 mark

   A farmer repeatedly ploughs a field infested with couch grass, yet the infestation becomes worse each season. What explains this?

   A. Couch grass seeds are carried from field to field on the tractor tyres

   B. The plough cuts the underground rhizomes into pieces, and each piece grows into a new plant

   C. Ploughing buries the crop seed too deeply for it to compete with the weed

   D. Ploughing releases nutrients from the soil that favour couch grass more than the crop

   (1 mark)

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5. Question 5

   1 mark

   Glyphosate sprayed onto the leaves of couch grass kills the underground rhizomes as well. Why does this happen?

   A. It is a contact herbicide and burns every plant tissue that it touches

   B. It is a systemic herbicide, absorbed by the leaves and translocated to the rhizomes

   C. It sterilises the soil so that nothing can grow in it for a season

   D. Rain washes the chemical from the leaves down into the soil where the rhizomes lie

   (1 mark)

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6. Question 6

   1 mark

   A maize crop at knee-high stage in Narok is heavily infested with black jack. Which control measure would remove the weed without destroying the maize?

   A. Flooding the field for several days

   B. Burning the vegetation in the field

   C. Spraying a selective herbicide that kills broad-leaved weeds but does not harm cereals

   D. Spraying glyphosate over the whole field

   (1 mark)

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7. Question 7

   5 marks

   Describe five cultural practices, other than mulching, that a smallholder farmer can use to control weeds.

   (5 marks)

   Marking scheme

   Award 1 mark each, maximum 5: timely planting at the onset of the rains, so that the crop germinates and covers the ground before the weeds establish; use of the correct, closer spacing so that the crop canopy closes early and smothers the weeds; crop rotation, which breaks the cycle of weeds that are adapted to one particular crop; planting clean, certified seed that is not contaminated with weed seeds; growing a cover crop or a smother crop such as sweet potatoes in the rotation; thorough seedbed preparation with enough time between ploughing and planting for the germinating weeds to be destroyed; flooding the field, as is done in paddy rice, to drown the weeds; cleaning implements, ploughs and machinery before moving them from an infested field to a clean one; using only well-decomposed manure, since fresh manure carries viable weed seeds.

8. Question 8

   6 marks

   Explain six features of weeds that make them difficult to eradicate from a farm.

   (6 marks)

   Marking scheme

   Award 1 mark each, maximum 6: they produce very large numbers of seeds from a single plant; their seeds can remain dormant but viable in the soil for many years and germinate long after the parent plant is gone; many reproduce vegetatively from rhizomes, stolons, bulbils or tubers, so pieces left in the soil grow into new plants; their seeds are efficiently dispersed by wind, water, animals, farm implements and contaminated crop seed; they grow rapidly and mature earlier than the crop, setting seed before weeding is done; they tolerate harsh conditions such as drought, poor soils and heavy grazing better than crop plants; some closely resemble the crop at the seedling stage, so they are left standing during weeding; some bear thorns, stinging hairs or an unpleasant smell that discourages hand weeding.

9. Question 9

   2 marks

   Distinguish between a pre-emergence herbicide and a post-emergence herbicide.

   (2 marks)

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10. Question 10

    1 mark

    Which one of the following weeds obtains its food from another living plant?

    A. Wandering jew

    B. Oxalis

    C. Dodder

    D. Black jack

    (1 mark)

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11. Question 11

    1 mark

    A farmer covers the soil of a kale plot with a layer of dry grass. How does this layer suppress weeds?

    A. It attracts insects that feed on weed seeds lying in the soil

    B. It shuts out the light that weed seedlings need, so they fail to establish

    C. It makes the soil too acidic for weed seeds to germinate

    D. It raises the soil temperature to a level that kills weed seeds

    (1 mark)

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12. Question 12

    4 marks

    State four ways in which weeds reduce the yield obtained from a maize crop.

    (4 marks)

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