Danish Experiences with Cereals in Monoculture1
Authors: Jensen A. 
Published: April 2008

Long‐term experiments show that monoculturing cereals will lead to yield depletion. The two main factors responsible for this loss are take‐all (Gaeumannomyces graminis (Sacc.) v. Arx & Olivier) and eyespot (Cercosporella herpotrichoides Fron). Of less importance is the cereal cyst nematode since Danish breeders produced new barley varieties that are highly resistant against this nematode. These parasites, however, do not account for all the losses occurring and other yield decreasing factors must be present, the nature of which remains to be revealed.

Experiments with barley grown in monoculture show that the yield, although declining steadily the first 2–4 years, remains stable after that, whereas take‐all declines. Adding straw to the soil or burning it has little effect on yield or diseases. The same is true for post‐harvest soil treatments. It is expected that post‐harvest crops such as Italian ryegrass and white mustard do have a beneficial effect and their value, especially as crops that can be sown after harvest, is being investigated. Breeding for resistance against take‐all is cumbersome and of doubtful economical value. Chemical control of take‐all is not feasible, but eyespot can be controlled efficiently with Benomyl.

Long‐term experiments show that continuous cereal growing may result in yield decrease. In barley, which is dominating in Denmark, the decrease on fertile soils is usually of minor importance. Experiments have further shown that the biggest yield decrease occurs in the 3rd to 5th year of barley monoculture.

The influence of different treatments of straw on yield and diseases has been insignificant according to experiments carried out during 7 years on 4 experimental stations.

Post‐harvest soil treatment has very little effect on diseases and yield except on land heavily infested with weeds. Post‐harvest crops such as Italian ryegrass and white mustard are of great interest and their value is further investigated, especially as crops sown after harvest.

The cereal root nematode is still a threat to the growing of oat and spring wheat. High yielding barley varieties which are resistant to the 2 pathotypes occurring in Denmark are a useful tool in reducing the risk inherent in continuous barley growing. Decline in the population of nematodes has been observed when using monocultures of different host‐plants.

Resistance against take‐all (G. graminis) is thoroughly investigated in Denmark. The conclusion is that breeding for resistance is very difficult and of doubtful economical value. Chemical control of take‐all in the field seems to be impossible in contrast to the good results obtained when testing several chemicals in the laboratory. The control of eyespot (C. herpotrichoides) with Benomyl has been highly successful in winter wheat and winter rye in some experiments, but an economical use demands a rather safe method of forecasting attacks in the different fields.

The problem of take‐all decline and yield stabilization by continuous cereal growing has attracted the interest of many research workers. In Denmark we have tried to investigate this phenomenon in different soil types placed under the same climatic conditions. The importance of take‐all as the most serious pathogen in barley monoculture is doubtful according to our experience gained from thousands of observations. We are therefore investigating intensively which factors could possibly be held responsible for the clearly noticeable reduction in yield.