The land suitability assessment part of the crop productivity model (Part I, Figure 2.1) allows the selection of single crops to be made, for each agro-ecological cell, according to their yield potentials in the cell. The overall objective of the crop productivity model is to quantify productivity potential of each agro-ecological cell from the whole growing period, or all growing periods per year, and not just single crops from a part of the growing period.
The next step in the model (Part II) is therefore the formulation of cropping pattern options. This is achieved by incorporating the features of multiple cropping using the inputs specific information on land suitabilities and crop options generated through Part I of the model. Once annual cropping pattern options have been formulated, it is then possible to specify cropping pattern constraints and fallow requirements of each cropping pattern.
Multiple cropping is the intensification of cropping in the space and time dimensions, i.e. growing two or more crops on the same land in a year (Table 5.1). The various patterns of multiple cropping reflect essentially two underlying principles - that of growing individual crops in sequence, i.e. sequential cropping; or of growing crops simultaneously in mixtures, i.e. intercropping. Double (and triple, etc.) relay and ratoon cropping work on the former principle, while mixed, row, strip, and alley cropping use the latter. There are several other forms of multiple cropping patterns, but these originate through synthesis of the sequential and simultaneous cropping practices. Some of the related terminology used in the multiple cropping systems is given in Table 5.2.
Crops are grown sequentially one after another so that time is used to obtain more production, or crops can be mixed and grown together simultaneously intercropped. With the latter, since the participating crops have different growth requirements, a mixture of crops of similar length to maturity can have higher productivity than a single crop. However, crops commonly used in mixtures usually differ in maturity, so their growth requirements are further separated in time, and competition between them is less.
The principle of yield increases resulting from a better use of time with crops in sequence is complementary to increases arising from a more efficient use of space with crops in mixture. Theoretically, therefore, maximum cropping should be obtained with sequences of ‘high-yielding’ crops in compatible mixtures. In practice, this pattern has evolved in relation to the traditional resources at low and intermediate inputs circumstances where several crops are planted and harvested in mixtures at different times.
TABLE 5.1
Definitions of the principle multiple cropping patterns (Andrews and Kassam 1976)
| Multiple cropping: | The intenS1fication of cropping in time and space dimensions. Growing two or more crops on the same field in a year. |
| 1. Sequential cropping: | Growing two or more crops in sequence on the same field per year. The succeeding crop is planted after the preceding crop has been harvested. Crop intenS1fication is only in the time dimension. There is no intercrop competition. Farmers manage only one crop at a time in the field. |
| 2. Intercropping: | Growing two or more crops simultaneously on the same field. Crop intenS1fication is in both time and space dimensions. There is intercrop competition during all or part of crop growth. Farmers manage more than one crop at a time in the field. |
TABLE 5.2
Related terminology used in multiple cropping systems (Andrews and Kassam 1976)
| 1. Sole Cropping: | One crop variety grown alone in pure stands at normal denS1ty. Synonymous with solid planting; oppoS1te of intercropping. |
| 2. Sequential Monoculture: | The repetitive growing of the same sole crop on the same land in a year. |
| 3. Sequential Multiculture: | The repetitive growing of different sole crops on the same land in a year. |
| 4. Cropping Pattern: | The yearly sequence and spatial arrangement of crops or of crops and fallow on a given area. |
| 5. Cropping System: | The cropping patterns used on a farm and their interaction with farm resources, other farm enterprises, and available technology which determine their make-up. |
| 6. Land Equivalent Ratio: (LER) | The ratio of the area needed under sole cropping to one of intercropping at the same management level to give an equal amount of yield. LER is the sum of the fractions of the yields of the intercrops relative to their sole crop yields. |
The practice of multiple cropping has been reviewed (Andrews and Kassam 1976; Kowal and Kassam 1978; Kassam 1980), and the important rainfed cropping patterns, generalized according to thermal zones and length of growing periods, are given in Table 5.3. Advantages from intercropping are numerous. These include better or fuller use of production resources of water, nutrients, heat, radiation, space and time; better distribution pattern of labour demands, better security of production, better control of pests, diseases and weeds in the absence or sub-optimal use of biocides, better control of soil erosion, and extra yield advantages (i.e. LER > 1.0). Any of these attributes, either singly or in combination, may make intercropping attractive to farmers; and even where there may be no extra yield advantages, intercropping may still be a normal practice because security of production is often a good enough reason for intercropping.
| LGP | Thermal zones | ||
| (days) | T1. T2, T3 | T4, T5 | T6, T7 |
| < 120 | SCas | SCas | SCas |
| (Is) 1 | (Is) | ||
| 120–210 | SCas | SCas | SCas |
| Is + Id | Is + Id | Is | |
| (Smo + Smu) | (Smo + Smu) | ||
| 210–270 | SCal | SCal | SCal |
| Is + Id | Is + Id | Is + Id | |
| Smo + Smu | (Smo + Smu) | ||
| 270–365 | SCal + SCp | SCal + SCp | SCal + SCp |
| Id + Is | Id + Is | Is + Id | |
| Smo + Smu | (Smo + Smu) | (Smo + Smu) | |
1 Brackets indicate minor status
SCas - Sole cropping of annual short-duration crops
SCal - Sole cropping of annual long-duration crops
SCp - Sole cropping of perennial crops
Is - Intercropping with crops of similar lengths of maturity
Id - Intercropping with crops of different lengths of maturity
Smo - Sequential monoculture
Smu - Sequential multiculture
5.1.1 Sequential cropping
Sequential cropping is possible in areas where conditions for crop growth exist beyond the duration of one crop, either due to longer growing period or due to more than one growing period.
In the frost-free areas in Kenya, the restriction to sequential cropping is one of availability of soil moisture. In the areas with a longer growing period, as in the moist sub-humid (growing period 210–270 days) and humid (>270 days) areas, crop growth is possible throughout most of the year. It is in such areas that a strong association with sequential cropping emerges, and sequential crops in both monoculture and multiculture are involved (Table 5.3). However, because of the cool temperatures in the thermal zones T6 and T7, sequential cropping is of minor importance because the annual crops that are adapted to the prevailing conditions are generally slow to reach maturity.
5.1.2 Intercropping
In areas with growing periods of less than 120 days, sole cropping of short duration annual crops is dominant in all thermal zones. Some simultaneous cropping is practiced with crops of similar length to maturity, but its status in thermal zones T1, T2, T3, T4 and T5 is a minor one. In the thermal zones T6 and T7 growing conditions only permit a moderate to marginal production from sole cropping of single crops.
In areas with growing periods between 120 and 210 days crop mixtures, including those involving crops of different length to maturity, are common in the thermal zones T1, T2, T3, T4 and T5. Because of the cool temperatures in T6 and T7, crop mixtures involving crops of similar length to maturity are common.
In areas with growing periods greater than 270 days, crop mixtures, especially those involving crops of different lengths of maturity, are common. In such areas, the slow growing and later maturing components generally tend to mature under better end-of-season moisture conditions. In these areas, multiple cropping both on the simultaneous and sequential principle is practised.
The cropping pattern options in Part II of the productivity model (Figure 2.1) are formulated by:
Firstly, fitting crop growth cycles into prevailing component lengths of growing periods for each agro-ecological cell. For all annual crops except wetland rice, cropping patterns are made up by fitting growth cycle lengths (full or partial) to total component length of growing period. Where crop growth cycle is curtailed due to inadequate length of growing period, it may still be considered acceptable provided the crop is able to offer a yield.
For wetland rice, growth cycle lengths are matched to the humid period of each component length. For areas with year-round humid lengths of growing periods, the humid period is also year-round. For normal lengths of growing periods, the humid period is approximately 45 days shorter than the total length. For intermediate lengths of growing periods, there is no humid period.
For cassava and the perennial crops of banana, sugarcane, oil palm, the length of crop growth cycle is taken to be equal to the length of the growing period, provided the minimum acceptable growth cycle length can be fitted.
The above matching process is applied, in each agro-ecological cell of the land resources inventory, to all component lengths of growing periods of each LGP-Pattern. This procedure allows the identification of cropping patterns for the complete range of annual cropping periods, including the worst and the best expected, in a given area; and within each year for the different expected lengths of growing periods, including the shortest and the longest.
Secondly, incorporating the ‘turn-around’ time between crops, within sequential cropping patterns, needed to harvest the first crop, prepare the land and sow the subsequent crop. The turn-around parameter is a model variable and can be modified as required. For annuals a turn-around period of 10 days has been assumed. For banana and sugarcane a turn-around time of 15 days has been assumed in the year-round growing period zones.
Thirdly, deciding for which levels of inputs and for which crops intercropping is acceptable. In the model this variable is formulated as follows. Intercropping is considered only at the low and intermediate level circumstance for all crops except wetland rice, sugarcane, banana and oil palm.
