What are successive cereal crops?

Answer 1

IntroductionTwo major problems relating to growing cereals in Europe are a) overproduction and b) the use of N fertiliser and agrochemicals which can sometimes lead to pollution and are becoming unacceptable to the general public. Maximising yields is no longer the main aim of research programmes - what are needed now are systems of growing crops, especially cereals that are environmentally benign but remain profitable.

As part of a European Union funded project IGER and LARS together with partners in Ireland and Denmark have developed a system for growing cereal crops for either whole crop silage or grain production that requires greatly reduced level of N fertiliser and arochemeicals.

The systemThe system now developed is simple and straightforward. It depends on a permanent, perennial understorey of white clover ( Trifolium repens ) to provide N and a range of other benefits to the cereal crop. Initially a sward of pure white clover is established and there are a number of methods of doing this. In autumn the clover is defoliated either by grazing with sheep or by cutting and ensiling. Following this a cereals crop, usually winter wheat, is direct drilled into the clover understorey, usually in October. The cereal and clover develop together over winter and the bicrop is given 50 kg N fertiliser/ha in late spring (May). This compares with around 200 kg N/ha given to conventional crops. The N applied to the bicrop can be provided by slurry instead of using artificial fetiliser. We believe that the clover understorey fixes a substantial amount of N that the cereal can utilise.

The cereal plus clover understorey is allowed to develop and can be cut for silage or the cereal can be left to mature and a grain crop taken. Following harvest for silage or grain the clover understorey recovers quickly and is defoliated again in autumn. A further cereal crop is then drilled into the clover to repeat the cycle. Our research suggests that it is possible to grow four successive cereal crops but there would be a need to switch from wheat to another cereal, probably oats, after two years to reduce the risk of diseases.

Weeds, pests and diseasesIn conventionally grown crops agrochemicals are usually needed to control weeds, pests and diseases. However in our bicropping system the need for the use of these chemicals is greatly reduced.

Weeds: Few broad-leaved weeds are able to compete with the dense cereal clover crop canopy. However, grass weeds (especially Poa spp) have been very troublesome at some sites, leading to crop failure. The problem is however easily controlled by the application of paraquat in late autumn, just prior to drilling the cereal.

Pests: Aphids are the major pests of cereal crops but their populations do not develop in bicrops for a number of reasons. These include a) large numbers of predatory insects build-up in the perennial clover understorey b) 90% of aphids fall off their host plant every day and the clover makes it physically difficult for them to relocate their host c) differences in N nutrition of the cereal plants reduce aphid population growth.

Diseases: the major disease of wheat is Septoria, which is splash borne, but the clover understorey prevents rain from splashing the disease propagules up the cereal plant. Consequently the use of fungicides is reduced or obviated.

YieldsIn our recent work whole crop silage yields were some 98% of those from conventionally grown crops, but were achieved with greatly reduced level of N and agrochemical usage. Grain yields however have been disappointing and have averaged around only 60% of those of conventional crops. But the reductions appear to be due to late season plant competition form the clover. Further works is needed to investigate and develop appropriate methods of reducing this competition and we can envisage how this may be achieved. Other advantages of the systemThe perennial cover of vegetation virtually halts wind and water soil erosion, which would be very advantageous in many situations.

Earthworm populations are greatly increased with the consequent benefits to soil fertility and probably for many wild life species, although this has not been scientifically evaluated.