Cropping programme and crop rotation

Planning » Crop Selection » Crop Rotation

Cropping intensity and production potential are in general higher for crops under irrigation than for crops under dryland farming. This calls for a balanced cropping programme, a sound rotation and strict plant protection measures.
When designing an irrigation scheme, the preparation of cropping programmes is the first step in calculating crop water requirements. Based on this, the capacity of the irrigation system and the area to be covered by the system can be determined, taking into consideration the water availability.

Crop selection

Besides water availability, other important factors to consider in crop selection are prevailing climatic conditions and soils, the farmer preference and marketing potentials. These affect the choice of crops and crop varieties and the planting time. Labour requirements and availability, market distances and needs, transport costs and reliability, and measures to combat pests and diseases must also all be considered as they determine the scale and frequency of production. These factors are often site-specific, which
must be taken into cognizance when a farmer produces different crop varieties.
Neither cropping programmes nor agronomic recommendations are fixed and this should therefore be
taken into consideration when designing the irrigation systems. For design purposes, a cropping pattern should be made in such a way that the farmers would be able to grow and irrigate all of their crops adequately. This involves careful investigation and planning, considering all factors mentioned earlier, and in-depth discussions with all farmers involved.
When a certain water supply is given, cropping patterns may need to be adjusted to avoid peak irrigation
requirements at periods of high evaporative demand and to avoid peak requirements of various crops occurring simultaneously. This includes consideration of shifting the sowing or planting dates, taking into account climatic constraints in a given area, manipulating the length of growing seasons by use of different crops and cultivars, and knowing sensitive crop stages in order to avoid yield reduction due to water stress during the critical period.

Cropping pattern

Once the crops have been selected, one can make up the seasonal cropping pattern indicating the place and the occupying area of each crop. A cropping programme diagram as shown in Table 3 is very useful. This diagram helps in establishing which crop will occupy what part of the available area during each season, also taking into consideration the crop rotation requirements (Section 3.2). While the time needed for land preparation and for harvest should not be included when calculating the crop water requirements, it is useful to indicate on the cropping programme diagram the time needed for so doing.
Planting or transplanting dates, the length of the period that the crop will be in the ground, the time and conditions needed for harvest as well as for land preparation for the next crop are all important. Repeating
this for the next season gives a clear picture of the yearly cropping pattern.

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Crop rotation

The place occupied by the different crops during consecutive years makes up the crop rotation (Table 4). In
order to reduce the risk of pests and diseases and to maintain soil fertility, crops are rotated within the field in such a way that crops belonging to the same family do not occupy the same area during consecutive years as they share same pests and diseases. Of special concern are crops susceptible to nematodes. In this case, a rotation cycle of minimum four years is required. Examples are tobacco, tomato, potato, eggplant, chilli (all of the Solanaceae family), okra and carrot.
A heavily nematode-infested soil will reduce the growth and yield of a whole range of crops grown on this soil. Cotton and onion are known to be good cleaning crops when grown prior to and following crops that are susceptible to nematodes. It is recommended that the stubble of susceptible crops be uprooted and burnt. Besides cotton and onion, some flowers, for example the marigold, have a nematode population reducing effect. Nowadays, chemicals are also available to control nematodes by soil treatment. However, these chemicals are quite expensive and very toxic. Their application requires expertise and know-how. The cheapest and most effective preventive control against nematodes is an adequate crop rotation.
The degree of nematode infestation will be a guide to determining which control method to opt for. Table 5 gives a rough indication of the rotation frequency of some crops.
Table 6 shows a number of vegetable crops that have been grouped according to their susceptibility to nematodes. It shows that crops from the onion family (Alliaceae) are, in general, tolerant of nematodes.
As nematode activity is reduced to a low level between May and September in the high altitude areas (> 1000 m above sea level), it is quite safe to plant susceptible crops from group A during this time as long as they do not precede summer-grown susceptible crops. In low altitude areas (< 1 000 m above sea level) nematodes are active all year round and susceptible crops can only be grown once every four years.

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