Agronomy of rainfed cropping systems includes all practices controlled by the farmer that contribute to productivity of crops. Many management decisions are influenced by climate, inherent soil properties and socio-economic constraints.
Each decision on crop and cultivar, land preparation, fertilisers and other agronomic practices will have impact on other factors as well. Intercropping is the major system in rainfed agriculture, although ratooning is practiced under unfavourable rainfall during the season.
Crops and Cultivars:
Crop combinations depend on climate, local preferences and other site specific factors.
Farmers combine crops with differences in:
1. Crop duration.
2. Plant morphology.
3. Root system.
4. Stress tolerance.
5. Density response.
6. Resistance to pests and pathogens.
7. Yield stability.
The logic behind such combinations is that the spatial and temporal differences of two or more combined crops utilise available resources more efficiently than either crops alone. The primary objective of subsistence farmers in crop combinations is to minimise the risk of total crop failure or to maximise the yield of primary crop and harvest the second crop(s) as bonus. The crop combinations are usually aimed at achieving the objective.
Time of Planting:
Planting crops at the earliest opportunity is important to allow growth during the most favourable period under rainfed conditions and to gain time for sequential cropping. Sowing component crops at different times ensure full utilisation of growth factors and minimise competition for growth factors.
However, in India, all crops are shown simultaneously at the start of rains due to risk of no opportunity for planting second crop later because of uncertainty of rainfall and difficulty of using equipment for planting second crop.
Plant Population and Row Arrangement:
Most cereals show plasticity in their yield response to plant population. This enables choice of a density that is less competitive to the associated crop in intercropping while maintaining high proportion of the potential yield.
The requirement of the component crops population is determined by the associated species and the temporal difference between the two crops. In systems where temporal difference is wide (sorghum + pigeon-pea, groundnut + pigeon-pea), both the component crops can be planted at 100 per cent of the sole optimum population.
Conversely, the system with closely maturing crops (cereal + legume, groundnut + sunflower) may not require additive population, though the total population of both the crops may be higher than for either of the sole crops.
In general, replacement series (where a proportion of one crop is substituted for a portion of the other) is followed for simultaneous planting of component crops and less frequently an additive series (where sole crop densities of two or more crops are planted).
Optimum row arrangements in a system with temporal species differences can be worked out rather easily. The arrangement that utilises a high proportion of the early crop to maximise its yield and allows the late maturing component to fully cover the ground should normally give the highest productivity.
However, in systems based on spatial differences where the competitive balance of the components is critical for yield advantage, a number of factors such as plant population, genotypes, moisture, fertilisers, canopy size of the two components, etc., needs to be considered. Generally, the dominant crop (cereal) has to be spaced sufficiently wide enough to minimise yield reduction in relatively less dominant component crop such as legumes.
The fertiliser needs of a system may be increased, unaltered or reduced, compared to those of sole crops, depending on the component crops involved. When both components respond to a particular nutrient, the intercrop system requirement may be higher than the sole crop needs as in the case of two cereal component crops, and higher phosphorus as in the case of two legume component crops.
Cereals intercropped with legumes at full population responds to nitrogen in similar way to sole cropped cereals because of competition from legumes. Nitrogen needs of cereals intercropped with legumes have been reported to be less than for sole cropping due to nitrogen transfer of some of the fixed nitrogen by legumes to the associated cereal during the season.
In general, there is no necessity for any additional nitrogen either to cereal + legume or legume + legume systems. Additional application of nitrogen and phosphorus is necessary for all intercropping systems.
Examples of fertility response equations developed specifically for intercropping situations are scarce. Nutrient supplementation index (NSI) has been proposed to adjust fertiliser requirements. NSI express the per cent of usual uptake for a given nutrient by sole crop A which should be added to the intercrops to meet the combined requirements of crops A and B.
where, NSI (A) = NSI of the crop A for a given nutrient,
NA = nutrient uptake by sole crop A for the same land area (kg ha-1),
Na = nutrient uptake of mixture A for the same land area as sole crop A, and
Nb = nutrient uptake of B in mixture for the same land area as sole crop B.
The NSI attempts to adjust total fertility inputs into cropping system, based on the relative uptake of each component crop as sole crop.
The nitrogen uptake by sole and intercropped finger-millet crop is 185 and 163 kg ha-1 respectively. Uptake of nitrogen by sole pigeon-pea and as intercrop with finger-millet is 101 and 40 kg ha-1 respectively. Calculate the NSI of finger-millet?
To satisfy both pigeon-pea and finger-millet requirements for nitrogen in the mixture, there is need to add an additional 9.7 per cent of that is required by sole finger-millet. Similarly, NSI for pigeon-pea is
Methods of fertiliser application are important where the components have different requirements, as with nitrogen in cereal and legume systems. Nitrogen should be applied to the cereal as far away from legume as possible so that nitrogen fixation by legume is not affected. If both the components require the same nutrient, as with nitrogen when both the component crops are cereals and phosphorus in cereal and legume systems, the nutrient can be applied in one application to both.
Intercrop weed management combines two qualitatively different aspects of plant/plant interactions. To increase crop yields, complementarity in patterns of resource use by component crops must be emphasised. The goal is to minimise the degree of overlap in resource use by inter-sown crop species such that more resources are exploited and more yield can be harvested per unit ground area.
In contrast, to achieve weed control, the similarity of requirements of crop and weed species, the consequent competition for limited resources and the suppression of growth and yield of the associated species are emphasised.
Weed scientists and farmers work to create an environment that is detrimental to weed and favourable to crops. Intercropping has potential as a means of weed control as it offers the possibility of a mixture of crops capturing a greater share of available resources than in sole cropping, pre-empting their use by weeds.
Additive mixture can control the weeds more efficiently than replacement mixtures. Smother intercrops and live mulch intercrops are high density additive mixtures that appear to offer great promise as means of weed control.
Although, crop rotation has lost prominence in modern agriculture, there are still situations where the residual effects of some crops on succeeding crops are of practical importance. Legumes in rotation provide nitrogen to succeeding crops. Legumes favourably respond to residual phosphorus.
Several reports pointed out poor performance of crops following sorghum. Wilt caused by Fusarium udum is aggravated if sole cropping of pigeon-pea is continued. It can be reduced by intercropping with sorghum. Intercropping groundnut with sorghum or pearl-millet reduce the incidence of groundnut rust and bud necrosis.
Regrowth from sorghum and pearl-millet can be managed to yield another grain or fodder crop. Ratooning sorghum for fodder and grain is more popular than other cereals. Rainy season sorghum on light soils can be harvested for fodder at 40 to 50 days during the year of prolonged drought and ratoon crop for grain.
Ratoon systems with pearl-millet are not as widely used as with sorghum because the millet is an early maturing crop which is removed from the field early to allow planting of alternate crops and due to less vigorous re-growth from its stubble.
Chief advantages of ratooning are to avoid planting of another crop and save time and costs for seed and land preparation. It also reduced labour inputs during the post-harvest period when demand for labour is relatively high. In rainfed areas where potential for planting sequential crops is limited, ratoon crops may provide a partial second crop.