This page can be found at: www.scri.ac.uk
Skip to content¦

Crop Mixtures

Towards ecologically balanced agriculture

A resource base for developing the use of crop mixtures

So what are mixtures?

In a natural ecosystem epidemics are the exception. In an intensive modern agricultural crop they are often the rule. This is largely because there exists a degree of natural balance between hosts and their
pathogens in natural ecosystems ensuring long-term survival of both. This balance is not a static one. Its dynamics are essentially the rate-limiting parameters of epidemics and can often be manipulated to tilt the balance more in favour of either host or pathogen. Spatial deployment of mixed host genotypes is one such manipulable parameter in epidemic control.

There are two key components which can be manipulated spatially:

  • the density of susceptible hosts
  • the barrier effect of resistant hosts.

A third component which may be important in some host–pathogen interactions is induced resistance. As levels of inducible resistance varies between genotypes, this may be optimised by careful choice of variety.

Winter barley mixtures graph

Crop monocultures are successful in obtaining maximum yield in high-input agriculture under near-optimal environmental conditions. They are suitable for risk-averse, highly profitable, non-sustainable agricultural systems where environmental considerations are not high priority.

Under more sustainable conditions, sub-optimal for maximum yield, crop mixtures are superior to monocultures, providing greater yield and quality stability, and better exploiting all the resources available through enhanced crop plasticity. They also offer the opportunity to reduce inputs, which can increase profitability and lower risk in a well designed programme. Alternatively their plasticity can be exploited to extend crop production into more marginal, higher risk environments. However, they are suited to all environments, including high input near-optimal ones, and also for growing specialist genotypes, for example, low glycosidic nitrile (GN) barley varieties along with other genotypes which compensate for their agronomic deficiencies.

Crop mixtures clearly have many advantages, but they are also perceived, by some end users, to have disadvantages, mainly lack of homogeneity. For example, barley maltsters assume, if they stick to a single variety and a restricted range of grain nitrogen values, that environmental variation can be disregarded.

Spring barley mixtures graph

However, the variation within a single ear, within a field and, certainly, between different fields indicates that end users are in effect dealing with an acceptable level of heterogeneity. In practice it has been shown that homogeneity is likely to be no greater in carefully chosen mixtures than in monocultures, and that some heterogeneity may be beneficial, resulting in, for example, enhanced spirit yield in barley (Newton et al., 1998). The other adverse perception relates to the difficulty of identifying and quantifying the components of a mixture. However, modern molecular techniques, especially genetic fingerprinting, permit genotypes to be unambiguously identified and quantified in a mixture to an accuracy of 1% (Habernicht and Blake, 1999).

In practice crop mixtures range from agro-forestry through to multilines, that is, from diversity at the plant order level down to the single plant resistance genes which determine resistance to a single genotype of a pathogen species. Within this range are some mixtures which are used very effectively even in high input western agriculture to control pathogens and thereby reduce pesticide inputs. Variety or cultivar mixtures enable not only diversification of resistance genes but also the combination of other mutually complementary characteristics not present together in single host genotypes. In cereals these are particularly effective. Mixtures of agronomically compatible fodder crop species are also grown such as different cereal species or legume and cereal combinations. Pathogen control is seldom complete, but it can be cost effective compared with fungicide, notwithstanding environmental considerations.

Habernicht D.K. and Blake T.K. (1999), J. Am. Soc. Brew. Chem., 57, 64-71. Newton A.C., Swanston J.S., Guy  D.C. and Ellis R.P. (1998), J. Inst. Brew., 104, 41-45.

Related SCRI Annual Report Articles