Intercropping

Intercropping is defined as the growth of two or more crops in proximity, in the same field during a growing season, to promote interaction.

Each crop species has its own characteristic needs for light, water and nutrients according to its own ecological niche. In monocropping systems, all plants belong to the same species and compete for exactly the same resources. It follows that the simultaneous growing of different crops (with sufficiently different niches) on the same piece of land could bring more efficient exploitation of these resources (Hauggaard-Nielsen et al., 2001).

In intercropping, as in any bio-diverse ecosystem, the competition/complementarity between plants enhances the overall stability of the system, including a significant resilience against pests, diseases and weeds.

A common example of intercropping is legumes and cereals. Legumes are well known to help maintain soil fertility via symbiotic di-nitrogen (N2) soil fixation, which could be exploited by cereals for a complementary and more efficient use of N sources without compromising cereal N use, yield level and stability (Intercrop, 2008).

At the same time, some authors also warn that combinations of crops in intercrop systems must be carefully chosen and, that under certain conditions (if intercropping leads to

excessive competition for resources), intercrops might show reduced yields with respect to stand-alone crops (Santalla et al., 2001; Thorsted et al., 2006a, 2006b).

Environmental impact and economic performance

Intercropping pea and barley showed that productivity increased by 25 to 36 % compared to sole crops. This is explained by the intake of N by barley, generated by pea (while N generated by peas as a sole crop, is mainly available for weeds) (Hauggaard-Nielsen et al., 2001).

Intercropping legume and cereal crops may be favourable against pests. In Spain, where the weedy root parasite Orobanche crenata causes huge damage to legume crops and where standard agronomic practices (use of herbicides) do not sufficiently respond to the problem, intercropping of faba bean and pea reduces O. crenata infestation significantly. This was also confirmed under intercrop culture of faba bean and cereals. It was suggested that inhibition of the weed’s seed germination might be due to allelochemicals released by cereal roots (Fenández-Aparicio et al., 2007).

Intercropping may be also useful for reducing nutrient pollution from farming while maintaining yields. Whitmore and Schroder (2007) reported positive effects (reduced leaching) on soil nitrogen dynamics under competing intercrops. Under-sowing grass, between the rows of an established maize crop, appears to reduce nitrate concentrations in water draining from soils during winter by 15 mg/l compared with a conventional catch crop and by more than 20 mg/l compared with a fallow soil.

In terms of soil biodiversity, Schmidt et al. (2003) reported that temperate wheat–clover intercropping systems have been shown to support much larger earthworm (Lumbricidae) populations than conventional wheat monocropping systems in Ireland and Britain. In particular, earthworm populations seem to greatly benefit from the input of organic matter from the mixed winter wheat–white clover crop, given its quantity, nutritional quality and also continuity throughout the year. Earthworms are responsible for the creation and maintenance of the soil macro-porosity, as discussed for other practices.

An economic analysis based on prices and costs, including labour, revealed that the economic returns from field maize intercropped with some dry bean varieties were similar to the average income value of the bean sole cropping. Yet, the highest economic returns of the bean–sweet maize intercropping systems were superior to the average income value of single-cropped beans (Santalla et al., 2001).

Martin et al. (1987) studied the consequences of intercropping corn and soybean in Canada, confirming that intercropped corn is generally more cost-effective than mono-cropped corn.

Fertiliser cost of treatments was lower, generating an economic advantage of USD 130-260 /ha as shown in Table 3.13. Quality or percent of crude protein of the intercrop silage was also significantly higher than silage from mono-cropped corn.

Table 3.13: Effects of intercropping on yield, LER, protein content and cost effectiveness

Treatments Dry weight

yield (kg/ha)

LER Protein (%)

Cost effectiveness compared to control (C)

C Monocropped corn 13 946 - 9.38 same

120 kg N/ha (10 398) - (8.16) same

I-1 100 % corn-50 % bean

60 kg N/ha 15 186 1.21 9.10 USD 261

alternate rows - - -

-I-2 100 % corn – 100 % bean

60 kg N/ha 14 691 1.23 9.63 USD 217

alternate rows - - -

-I-3 67 % corn-67 % bean

60 kg N/ha 12 807 1.14 10.09 USD 150

within rows - - -

-I-4 67 % corn-67 % bean

60 kg N/ha 13 213 1.13 10.04 USD 132

alternate rows (10 353) (1.23) (8.75) (USD 135)

I-5 67 % corn-67 % bean

120 kg N/ha 12 997 1.13 10.76 USD 76

alternate rows (9 958) (1.20) (9.95) (USD 44)

LER=Land Equivalent Ratio (e.g. LER=1.21 indicates that the amount of land needed to produce a certain yield from pure stands is 21 % higher than the amount of land needed to grow the same yield under intercrops with the same species

Source: Martin et al., 1987.

Prins and de Wit (2006) argued that the commonly used Land Equivalent Ratio (LER) (see Table 3.13) is a poor measure for measuring cropping advantage of intercrops over sole cropping. Instead, net returns, weed suppression abilities and yield reliability of the different crops should be compared. Indeed, single-cropped grain legumes are highly susceptible to weeds and have low yield reliability. Other authors have supported this approach, stating that the feasibility of intercropping depends heavily on the profitability of the system in addition to increased yields (University of Manitoba, 2006).

Table 3.14 highlights the variability in net returns for a number of intercrops tested by the University of Manitoba in experiments in the US, ranging from USD 655 /ac to a loss of USD 82 /ac (University of Manitoba, 2006). Full-rate wheat was among the most consistently profitable treatments, and even half-rate wheat was more profitable than many of the intercrop combinations. Wheat-barley and wheat-spring rye were the more profitable cereal intercrop combinations. The wheat-mustard intercrop proved to be among the most profitable, while wheat-flax and wheat-field pea gave inconsistent but potentially promising results.

The cover crop treatments tended to have lower returns because the cover crops did not provide a marketable product, nor did they generally have significant positive effects on wheat yield. In fact, the cover crops resulted in negative returns in two cases (see Table 3.14).

However, not included in this analysis are the benefits that cover crops can provide to the subsequently grown crops. Legume cover crops, in particular, can provide significant nitrogen contributions to the soil, which are especially important in organic cropping systems.

Table 3.14: Net returns from intercropping systems

Source: University of Manitoba, 2006: Clearwater and Carman are the two test sites.

3.1.7 Grasslands