Evaluating pea and barley cultivars for complementarity in intercropping at different levels of soil N availability

Two field experiments were carried out on a temperate sandy loam using six pea (Pisum sativum L.) and five spring barley (Hordeum vulgare L.) cultivars to determine cultivar complementarity in the intercrop for grain yield, dry matter production and nitrogen (N) acquisition. Crops were grown with or without the supply of 40 or 50 kg N ha⁻¹ in the two experiments. Cultivars were grown as sole crops (SC) and as mixed intercrops (IC) using a replacement design (50:50). The land equivalent ratio (LER), which is defined as the relative land area under SC that is required to produce the yields achieved in intercropping, were used to compare cultivar performance in intercropping relative to sole cropping.Barley was the stronger competitor in the intercrops and as a result barley grain yield and nitrogen uptake in IC were similar to SC. The per plant pea grain production and aboveground N accumulation in IC were reduced to less than half compared to SC pea plants due to competitive interactions.Application of N caused a dynamic change in the intercrop composition. Competition from barley increased with N application and the pea contribution to the combined intercrop grain yield decreased. The LER values showed that in the intercrop plant growth resources were used on average 20% more efficient without N application and 5–10% more efficient with N application.The choice of pea cultivar in the intercrop influenced the intercrop performance to a larger degree than the choice of barley cultivar. Furthermore, pea cultivar×cropping systems interactions was observed, indicating that cultivars performed differently in sole and intercrops. An indeterminate pea cultivar competed strongly with barley causing a greater proportion of peas in the intercrop yield, but caused a reduced N uptake and yield of barley. Determinate peas with normal leaves caused the highest degree of complementary use of N sources by allowing barley to exploit the soil N sources efficiently, while they contribute with fixed N₂. However, difference in performance among cultivars was observed. Using the indeterminate pea cultivar combined IC grain yield was in general lower than the greatest sole crop yield and vice versa for the determinate pea cultivars. Up to 22% (LER=1.22) greater combined IC grain yield was observed in several mixtures using determinate pea cultivars.From the present study, it is was concluded that there is a need for breeding suitable pea cultivars for intercropping purposes, since cultivars bred for sole cropping may not be the types, which are the most suitable for intercropping. For optimized N-use in pea–barley intercrops it is concluded that important traits for the intercropped pea are: (1) determinate growth, (2) a medium competitive root system for soil inorganic N and other nutrients during early growth, (3) high light absorption capacity by peas growing underneath the canopy of the higher barley component and (4) early establishment of symbiotic N₂ fixation to support a high growth rate during early growth stages.Fertilized pea–barley intercrops gave a 15% higher net income than fertilized barley sole cropping and is regarded as a better safeguard for the farmer’s earnings compared to pea sole cropping known for variable yields and poor competitive ability towards weeds.