Explanation

(a)(i) Definition of crop rotation: Crop rotation is the planting of different types of crops on the same piece of land in a definite sequence. (ii) Design of a 4-year crop rotation system using yam, cassava, maize and Groundnut. 

NB: - For the purpose of the 4 crops given: 
- Yam and cassava must not follow each other 
- Groundnut and maize must not follow each other 

(b) Reasons why weeds are difficult to control 
- Weeds are aggressive and persistent in their growth/vigorous growth 
- Weeds have high reproductive capacity 
- Most weed seeds have long periods of viability 
-Weeds have efficient dispersal methods 
- Weeds have efficient means of propagation 
-Weed seeds have long period of dormancy 
-Some we seeds mature at the same time as the crops and are harvested and stored along with the crops Waiting for the next cropping season (
-Weeds have modified parts, leaves, stems, roots and seeds that help them survive very adverse environmental conditions/wide adaptation 
- Most weeds produce many seeds/are prolific 
-Short life cycle 

(c) Effects of diseases on crop production 
Reduction in yield/productivity, Increases the cost of production, Reduction in quality of crops, Causes death -of whole plants, Leads to retarded growth in crops, -Delayed maturity, Causes reduction in the income of the farmers, Damage to crop parts/reduction in photosynthesis 

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