Legumes take up nitrogen from the atmosphere efficiently and release it so it can be used by the next spring's grain, while grasses prevent N leaching by absorbing soil nitrate (NO3-N). Adjustment of grain yield and N effects of an undersown crop can be achieved through density of sowing and use of nitrogen fertilizer.
Invariable cereal cropping, nutrient leaching and the price of nitrogen fertiliser have posed increasing challenges to grain growing during recent decades. In his doctoral thesis, Hannu Känkänen, Senior Scientist at MTT Agrifood Research Finland, studied how these disadvantages could be addressed by implementing a model of an adaptive undersowing system.
"The research was carried out between 1991 and 1999 at four sites in south and central Finland, on various clover and grass species and their mixture. Simultaneous broadcasting of seeds was also studied," Hannu Känkänen says.
The field experiments focused on measuring the effects of undersown crops on grain yield of the main crop, on biomass and nitrogen yield of undersown crops as well as on soil nitrate levels. Suitability as an undersown crop as well as the effects on the growth of a main crop and the cycle of nitrogen are highly dependent on the species.
"Legumes took up nitrogen from the atmosphere efficiently and released it for the next spring cereal, while grasses prevented N leaching by absorbing soil nitrate," Känkänen explains.
Clovers circulate nitrogen efficiently
Autumn-oriented growth rhythms of the studied legumes were optimal for undersowing, whereas those of grasses were less well suited to the system. Perennial grass species had fewer negative effects on the main crop than annuals but their capacity to take up nitrogen in autumn was rather low. The most efficient species for absorbing soil nitrate in autumn was Italian ryegrass (Lolium multiflorum Lam.). Timothy was at its most efficient in taking up nitrogen the following spring.
"Red clover (Trifolium pratense L.) and white clover (Trifolium repens L.) are well suited to annually repeated undersowing, supplying fixed N for cereals and increasing their grain yield without markedly increasing the risk of N leaching," says Känkänen.
Consequently, fertilisation costs can be reduced by using clovers. A mixture of red clover and timothy is a good choice when increased plant diversity, nitrogen benefits and low risk of N leaching are targeted simultaneously.
Ambient conditions a challenge
The use of an adaptive undersowing system, as a whole, is determined according to its targets, other management practices and the prevailing conditions. A decisive question is whether the aim is to take up nitrogen from the atmosphere or absorb it from the soil, or whether the primary consideration is increased plant diversity and improved soil quality over the long term.
"Adjustment of grain yield and N effects of an undersown crop can be achieved through seeding rate and use of nitrogen fertiliser. In practice, implementation of the system requires continuous assessment of growing conditions and actual growth and, over time, consideration of improvement of soil quality."
The doctoral thesis by M. Sc. (Agr. & For.) Hannu Känkänen in the field of biology of plant production, more specifically crop science, entitled "Undersowing in a northern climate: effects on spring cereal yield and risk of nitrate leaching" will be publicly examined at the University of Helsinki, Faculty of Agriculture and Forestry, on 16 April 2010.
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