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

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 and with two relative frequencies when intercropped. Earlier seedling emergence gave barley an initial growth advantage, assessed using the relative efficiency index (REIc), whereas pea was in general more growth efficient once the initial growth phase 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 increases in plant density had a positive effect on the growth of pea, the growth of intercropped barley was severely limited by increases in density at the end of the growing period and more so in the pea dominated intercrop. At the final harvest land equivalent ratios (LER) of 0.9-1.2 express resource complementarity in almost all studied intercrops, complementarity that was not directly affected by changes in plant density or relative frequency. Intercropped pea did not increase its reliance on atmospheric nitrogen fixation compared to the pea sole crop. 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 proportions sown; however, at high density the suppression of barley strongly increased the proportion of pea in the final grain yield. Weed infestation levels decreased as density was raised and the suppressing effect of density was clearly stronger the greater the frequency of pea in the crop. Earlier germination and tillering ability of barley are seen as likely explanations of lower weed load in the barley dominated crop treatments.