Seven cover-crop treatments were compared in two north Willamette Valley Pinot noir vineyards over two years to test if alleyway cover crops that are mowed in spring and summer compete with grapevines for water or nutrients. Five different cover-crop mixtures were compared to a clean-cultivated control and resident vegetation treatments in 2004 and 2005. Treatments were evaluated for biomass production, quantity of nitrogen (N) contributed to the vineyard floor, weed suppression, and effect on soil water content. Vine responses to the different floor-management strategies included measures of shoot growth, water and nutrient status, yield, and juice quality. Three treatments were evaluated for their effect on fine roots and colonization by arbuscular mycorrhizal fungi (AMF). Cover crops influenced soil moisture in a different manner at each site, although the lowest soil moisture was consistently found in the perennial grass and clover mixture. Cover-crop treatments had an impact on grapevine N status at one vineyard, altering leaf blade N concentrations at bloom and juice N concentrations at harvest, although different treatments did not alter N status consistently over time. Cover crops did not alter shoot growth, pruning mass, leaf water potential, fine root density, or colonization of roots by AMF and did not affect yield, cluster weights, juice soluble solids, pH, or titratable acidity. Results showed that alleyway cover crops managed by spring and summer mowing do not have consistent effects on grapevines in western Oregon vineyards and suggest that little competition occurs between cover crops and vines in the mixtures evaluated. Further examination of cover crops composed primarily of clovers or of perennial grasses is warranted.
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