In Part IV of the crop productivity model (Figure 2.1), productivity potentials of land (agro-ecological cells) for each crop rotation option is quantified in three steps.
Step one quantifies the sequential crop yields of each of the crop rotation option. Step two encorporates the intercropping yield increments, and step three applies the production stability constraints (and any other constraints) as criteria for selecting optimum crop rotations and productivities.
Part IV also provides an estimate of potential crop residues, crop by-products and crop primary products that can be made available for livestock production.
Sequential cropping is possible in areas where there is either a long continuous growing period or where there are more than one growing period separated in time due to a marked bimodel (or trimodel) nature of rainfall distribution.
A sequential cropping pattern could be a monoculture or it could be a multiculture. In the former case (e.g. two crops of rice, or white potato), participating crops are of the same adaptability group (see Technical Annex 3). In the latter case, the second crop may be different but may belong to the same thermal adaptability group with a similar photosynthesis adaptability response to temperature (e.g. groundnut followed by cowpea, or pearl millet followed by lowland maize, or wheat followed by white potato) or a different thermal adaptability group (e.g. groundnut followed by lowland maize).
It is therefore an overriding condition that all crops participating in ecologically suitable and desired cropping patterns must first themselves be ecologically suitable. Accordingly, in Part I of the model, a crop type is only permitted to participate in the formulation of a reference crop rotation if its minimum yield (with climate, soil and landform constraints) for the chosen inputs level is more than 20% of its maximum attainable yield. Reference yields, including maximum yields, for situations with no thermal or soil constraints for all crops are given in Tables A4.3, A4.4 and A4.S in the Appendix for high, intermediate and low level of inputs respectively.
| Crop position | Crop yield as first crop (% of maximum) | |||
| 20–40 | 40–60 | 60–80 | 80–100 | |
| 2nd | 50 | 25 | 25 | 25 |
| 3rd | 75 | 50 | 25 | 25 |
| LGP (days) | Input/Relative crop yield | |||||||||
| Low | Intermediate | |||||||||
| <0.2 | 0.2–0.4 | 0.4–0.6 | 0.6–0.8 | 0.8–1.0 | <0.2 | 0.2–0.4 | 0.4–0.6 | 0.6–0.8 | 0.8–1.0 | |
| < 120 | 1.0 | 1.0 | 1.0 | 1.0 | 1.0 | 1.0 | 1.0 | 1.0 | 1.0 | 1.0 |
| 120–170 | 1.0 | 1.1 | 1.2 | 1.2 | 1.3 | 1.0 | 1.05 | 1.1 | 1.1 | 1.15 |
| > 270 | 1.0 | 1.2 | 1.3 | 1.3 | 1.4 | 1.0 | 1.1 | 1.15 | 1.15 | 1.2 |
The reference crop yields in Tables A4.3, A4.4 and A4.5 apply (after taking into account climate, soil and erosion constraints) when the crops are considered as single sole crops in the component growing period (i.e. no sequential cropping), or occupy the first position as sole crops in the annual sequential cropping patterns.
Two additional parameters are incorporated in the model (before single crop yields can be applied to quantify sequential crop yields) to take into account:
the increased agro-climatic constraints (e.g. increased pest and diseases, increased workability constraints) on crops when they are positioned second or third in the cropping sequence instead of being first; and
those situations when the yield formation period of the crop in the cropping sequence cannot be fully accommodated within the time available for cropping, with the consequence that there is a partial yield loss (as opposed to total crop loss).
To allow for the increased agro-climatic constraints for crops in cropping patterns with two or three crops, single sole crop yields of the second and the third crop are downgraded, as shown in Table 7.1 in comparison to the yield as the first crop. Of the 58 crop types considered in the model, cassava, sugarcane, banana and oil palm do not have the possibility of taking up a second crop position in an annual cropping pattern.
Where crops cannot complete their yield formation within the time available, yield reductions are made proportiontely to the decreases in the yield formation periods. The yield formation periods for cereal, legumes and roots and tuber crops are assumed as one-third, one-half and two-thirds respectively of their corresponding total length of normal growth cycle. This assumption is also made in defining the climatic adaptability of crops and in the calculation of net biomass and yields of crops (Technical Annex 3).
The extent of the extra contribution to production per unit area from intercropping has been described in Kassam (1980). In practice, farmers select compatible mixtures with LER values of greater than 1.0 except in situations where intercropping is still advantageous for reasons other than extra yields.
Reference LER values which have been applied in the model are given in Table 7.2. Based on evidence from surveys and experiment, it is assumed that intensifying crop production through intercropping would have its limits. At the high inputs level, the primary disadvantage of intercropping is the difficulty in mechanization, and in effectively conducting some of the cultural operations. This generally restricts the widespread use of intercropping in large farm systems, particularly when under such systems most of the advantages of intercropping no longer apply. It is, therefore, suggested that at high inputs level, there should be no extra yield advantages (LER=1.0) in production over and above that which is already reflected by sole crop yields.
At the low inputs level, the most complex patterns that are also potentially the more productive would eventually require so much labour and other resources that even the small farmer may only use them occasionally on a small part of his land. Further it is most likely that the extra yield advantages claimed under experimental conditions (e.g. LER=1.3–1.5 in LGPs 120–270 days, LER=1.85 in LGPs >270 days) with their accompanying ‘high’ inputs would decrease by about 50% under field conditions at the low inputs level. However, it is postulated that the extra yield advantages from intercropping would increase with the increase in length of growing period; and that the maximum advantages should be with mixtures where the individual component crops are very suitably adapted to the prevailing climate and soil environment.
It is therefore suggested (as model variables in Table 7.2) that for LGP zones with less than 120 days, there would be no significant extra yield advantages (LER=1.0) from intercropping. The single sole crop yields are considered to adequately reflect the production potential.
For LGP zones with more than 120 days, the following has been applied in the model for all crops except wetland rice, sugarcane, banana and oil palm. For LGP zones 120 to 270 days under low inputs level, there is a 30% extra yield advantage (LER=1.3) from intercropping when attainable yields (from Part I of the model) of the individual participating crops are 80% or more of the maximum attainable yields. This yield advantage is reduced to nil (LER=1.0) for participating crops with attainable yields that are less than 20% of the maximum attainable.
For LGP zones with more than 270 days, there is 40% extra yield advantage (LER= 1.4) from intercropping when the attainable yields of the individual participating crops are 80% or more of the maximum attainable yields. This yield advantage is reduced to nil (LER=1.0) for participating crops with attainable yields that are less than 20% of the maximum attainable.
TABLE 7.3
Seed requirements (kg/ha dry weight of product)
| Crops | Level of inputs | ||
| Low | Intermediate | High | |
| Barley | 60 | 75 | 90 |
| Maize | 25 | 30 | 35 |
| Oat | 60 | 75 | 90 |
| Pearl Millet | 20 | 25 | 30 |
| Rice (dryland) | 25 | 30 | 35 |
| Rice (wetland) | 65 | 70 | 75 |
| Sorghum | 20 | 25 | 30 |
| Wheat | 70 | 85 | 100 |
| Cowpea | 30 | 35 | 40 |
| Green gram | 20 | 25 | 30 |
| Groundnut | 150 | 175 | 200 |
| Phaseolus bean | 30 | 35 | 40 |
| Pigeonpea | 25 | 30 | 35 |
| Soybean | 30 | 35 | 40 |
| Cassava | 0 | 0 | 0 |
| Sweet potato | 90 | 100 | 110 |
| White potato | 200 | 250 | 300 |
| Banana | 0 | 0 | 0 |
| Oil palm | 0 | 0 | 0 |
| Sugarcane | 250 | 300 | 350 |
For intermediate inputs level, yield advantages from intercropping are taken as half of those at the low inputs level.
For wetland rice, sugarcane, banana and oil palm, LER of 1.0 has been applied.
It is necessary to state the level of production stability, e.g. the tolerable difference between minimum (worst year) and maximum (best year) production; the tolerable soil erosion rate, desired from the cropping patterns and rotations that are selected to meet food and other demands specified by the objective function. These constraints are introduced as model variables in the selection of cropping patterns and crop rotations and the quantification of production therefrom.
In the model, the desired level of production stability between minimum and maximum production is set at 75%. This means that the production variations from year-to-year from the selected cropping patterns would not exceed 25%.
When the crop productivity model is applied to the land resources inventory, crop productivity potentials of each agro-ecological cell are quantified taking into account the requirements and constraints imposed at the various stages in the model. Results of the crop productivity assessment at district level are presented in Technical Annex 8.
TABLE 7.4
Calorie and protein nutritive values of 100 g of food product
| Crop | Food product | Calorie (Kcal) | Protein (g) | Conversion factor from dry weight to food product |
| Barley | Dry grain | 337 | 7.5 | 1.00 |
| Maize | Dry grain | 353 | 9.3 | 1.00 |
| Oat | Dry grain | 394 | 12.6 | 1.00 |
| Pearl millet | Dry grain | 338 | 8.0 | 1.00 |
| Rice (dryland) | Dry grain | 363 | 7.0 | 1.541 |
| Rice (wetland) | Dry grain | 363 | 7.0 | 1.541 |
| Sorghum | Dry grain | 354 | 10.7 | 1.00 |
| Wheat | Dry grain | 364 | 11.0 | 1.00 |
| Cowpea | Dry grain | 342 | 23.4 | 1.00 |
| Green gram | Dry grain | 340 | 22.0 | 1.00 |
| Groundnut | Dry unshelled nut | 384 | 16.2 | 1.542 |
| Phaseolus bean | Dry grain | 341 | 22.1 | 1.00 |
| Pigenpea | Dry grain | 343 | 20.9 | 1.00 |
| Soybean | Dry grain | 405 | 33.7 | 1.00 |
| Cassava | Fresh tuber | 110 | 0.9 | 2.863 |
| Sweet potato | Fresh tuber | 98 | 1.5 | 3.134 |
| White potato | Fresh tuber | 71 | 1.5 | 3.335 |
| Banana | Fresh fruit with skin | 60 | 1.0 | 3.706 |
| Oil palm | Oil | 884 | 0.0 | 1.00 |
| Sugarcane | Fresh cane | 28 | 0.3 | 10.087 |
1 Grain to paddy ratio = 0.65
2 Kernel to nut ratio = 0.65
3 Moisture content = 65%
4 Moisture content = 68%
5 Moisture content = 70%
6 Skin = 10% of total weight; moisture content = 70 %
7 Sugar in fresh cane = 10%.
To be able to use the crop productivity potentials for planning, it is necessary to take into account the waste factor and the amount of production which is required as seed and therefore not available to enter the animal and human food chain.
The waste factor, covering post harvest losses during food processing and in the food delivery system, has been taken as 10% and can be varied as required.
The seed factors for crops are given in Table 7.3. They are applied after applying the waste factor, to arrive at the net production available.
The demand for food is expressed in terms of calorie and protein. The calorie and protein conversion factors for food products are given in Table 7.4.
Once crop productivity potentials are quantified, it is possible to quantify crop residues, crop by-products (groundnut and soybean cake), and crop primary products (grain) which may be or need to be made available for livestock production (Technical Annex 5).
