Rye (Secale cereale L.) is an important cover crop that provides many benefits to cropping systems including weed and pest suppression resulting from allelopathic substances. Hydroxamic acids have been identified as allelopathic compounds in rye. This research was conducted to improve the methodology for quantifying hydroxamic acids and to determine the relationship between hydroxamic acid content and phytotoxicity of extracts of rye root and shoot tissue harvested at selected growth stages. Detection limits for an LC/MS-MS method for analysis of hydroxamic acids from crude aqueous extracts were better than have been reported previously. (2R)-2-beta-D-Glucopyranosyloxy-4-hydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA-G), 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA), benzoxazolin-2(3H)-one (BOA), and the methoxy-substituted form of these compounds, (2R)-2-beta-D-glucopyranosyloxy-4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA glucose), 2,4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA), and 6-methoxy-benzoxazolin-2(3H)-one (MBOA), were all detected in rye tissue. DIBOA and BOA were prevalent in shoot tissue, whereas the methoxy-substituted compounds, DIMBOA glucose and MBOA, were prevalent in root tissue. Total hydroxamic acid concentration in rye tissue generally declined with age. Aqueous crude extracts of rye shoot tissue were more toxic than extracts of root tissue to lettuce (Lactuca sativa L.) and tomato (Lycopersicon esculentum Mill.) root length. Extracts of rye seedlings (Feekes growth stage 2) were most phytotoxic, but there was no pattern to the phytotoxicity of extracts of rye sampled at growth stages 4 to 10.5.4, and no correlation of hydroxamic acid content and phytotoxicity (I50 values). Analysis of dose-response model slope coefficients indicated a lack of parallelism among models for rye extracts from different growth stages, suggesting that phytotoxicity may be attributed to compounds with different modes of action at different stages. Hydroxamic acids may account for the phytoxicity of extracts derived from rye at early growth stages, but other compounds are probably responsible in later growth stages.

-In the humid, temperate mid-Atlantic area of the USA, crop production that leaves the soil uncovered can lead to undesirable soil and nutrient losses to the surrounding Chesapeake Bay watershed. To cope with this issue, winter annual cover crops could provide soil cover both during winter months and, as surface residue in no-tillage cropping systems, during summer months. Legume cover crops such as hairy vetch can produce abundant biomass and N by the time summer crops are planted in spring. Although N mineralized from a legume cover crop can contribute to meeting the N requirement of crops such as corn, it also may not be used efficiently by crops and could be lost into the local environment. This research was conducted to determine whether hairy vetch or a hairy vetch-rye mixture that was allowed to produce high levels of biomass with a high N content (200 to 250 kg/ha) could meet the N requirements of no-tillage sweet corn and to determine the efficiency of N use relative to that of fertilizer N. Our results show that marketable yield of sweet corn was approximately doubled by hairy vetch in 2 of 3 years compared to an unfertilized, no-cover crop control. However, in 2 of 3 years, hairy vetch and the vetch-rye mix reduced yield by 19 and 34%, respectively, compared to a no-cover crop control with fertilizer N. Reduced plant population that reduced the number of ears per ha accounted for the yield reduction by these cover crops compared to the fertilized no-cover crop control. Fertilizer N was 1.5 to 2 times more efficient than hairy vetch at producing sweet corn ear mass per unit of N input but combinations of fertilizer N with cover crops were less efficient than either alone. Results suggest that growing sweet corn without tillage in high biomass levels of cover crops can interfere with crop establishment, reduce the efficiency of crop production, and allow for potentially high N losses into the environment. cover crops / nitrogen use efficiency / sweet corn / Zea mays L. / hairy vetch / Vicia villosa Roth / rye / Secale cereale L.

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