Density and relative frequency effects on competitive interactions and resource use in pea-barley intercrops

Summary

Intercropping advantages may be influenced by both plant density and relative frequency of the intercrop components. In a field study barley (Hordeum vulgare L.) and pea (Pisum sativum L.) were sole cropped and intercropped at three densities, corresponding to i) half of the recommended, ii) the recommended and iii) twice the recommended plant density. Two intercrops combinations were included holding 1/3pea+2/3barley and 2/3pea+1/3barley relative to the sole crop densities employed.
For three growth intervals within the whole growing period, the relative performance and interactions between the two crops were assessed using the Relative Efficiency Index (REIc). The relative performance of the two species varied over the growing season and was influenced by plant density and relative frequency of pea and barley. Barley came off to a better start whereas pea was in general more growth efficient once the initial growth phases had been passed. This reversal in relative growth efficiency along with the observation that early barley dominance did not appear to suppress pea growth indicates that differences in phenology played a role in shaping the prevailing dynamics.
Whereas increasing plant density had a clear positive effect on the growth of pea, the growth of the initially stronger barley component of the intercrops responded quite differently. From being unaffected by changes in density at the middle of the growth season the effects of increasing plant density became severe at the end of the growing period, biomass yields falling with increased plant density. Calculations of the competitive ratio of barley relative to pea (CRBP) for the growth season as a whole, showed that barley was most competitive at low and recommended plant density (CRBP>1) whereas at high density the pea crop became the competitively superior component of the intercrops (CRBP<1). Changes in the relative frequency of barley in the intercrops affected its competitive impact on pea, competitive strength increasing with decreasing relative frequency of barley.
Using the Land Equivalent Ratio (LER) as a measure of resource complementarity we found that the LER ratios calculated on the basis of final biomass yields (from 0.9 to 1.2), expressed resource complementarity in almost all studied intercrops. However the degree of resource complementarity attained was not directly affected by changes in plant density or relative frequency.
When intercropping pea with barely at low and recommended density the pea increased its reliance on atmospheric nitrogen fixation compared to the pea sole crop, throughout the growth period, implying that the barley crop competed strongly for soil nitrogen. At high plant density barley crop growth was clearly suppressed by the end of the growing period and was thereby unable to push intercropped pea to increase nitrogen fixation. With respect to soil nitrogen uptake there were no effect of plant density but a strong effect of the relative frequency of pea in the intercrop, the greater the proportion the lower the uptake.
Changes in the competitive strength of the pea and barley crop over the growing season had a marked effect on the proportion of pea in the final grain yields of the intercrops. At low and recommended density the proportions of pea and barley in the final grain yield was not markedly different from the expected 1/3pea:2/3barley and 2/3pea:1/3barley proportions sown, however at high density the suppression of barley strongly increased the proportion of pea in the final grain yield.
With the exception of the barley sole crop weed, infestation levels decreased as density was raised, the suppressing effect of density was clearly stronger the greater the frequency of pea in the crop. Early germination, rapid initial growth and tillering ability are seen as likely explanations of the greater competitive strength of barley towards weeds.
From the present study we conclude that the added value of including sequential harvest when studying interspecies dynamics was well worthwhile, giving valuable information on interspecies dynamics that would have been hidden had we chosen to evaluate the outcome of the study on the basis of data from one final harvest. We found that the relative performance of the pea and barley crops varied over time and were clearly affected by changes in plant density and relative frequency. This points at the potential of employing density and relative crop frequency as “regulators” when specific intercrop objectives such as increased competitiveness towards weeds or specific grain yield composition are wanted.