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.

	1st plot	2nd plot	3rd plot	4th plot
1st year	Yam	Groundnut	Cassava	Maize
2nd year	Groundnut	Cassava	Maize	Yam
3rd year	Cassava	Maize	Yam	Groundnut
4th year	Maize	Yam	Groundnut	Cassava

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