SAP TEST TO DETERMINE NITRATE-NITROGEN CONCENTRATIONS IN ABOVEGROUND BIOMASS OF WINTER COVER CROPS

In the San Luis Valley of south central Colorado, winter cover crops ( WCC) are used to reduce soil erosion and scavenge residual soil-N. Som e San Luis Valley farmers are beginning to use WCC as a source of over -winter or early-spring grazing. Common WCC used by farmers, wheat (Tr iticum aestivum L.) and rye (Secale cereale L.) are reported to accumu late high levels of nitrate nitrogen (NO3--N) in aboveground biomass t hat can be toxic to animals. Evaluation and calibration of a quick Car dy Meter(2) Sap Test (CMST) for determination of NO3--N status in the field will facilitate the management of these WCC. Field and growth ch amber studies were conducted to correlate the CMST with laboratory pro cedures and with plant and soil parameters. In field and growth chambe r studies, the CMST was correlated with standard dry tissue NO3--N lab oratory analysis (P<.001) and with soil inorganic N content (P<.05). T hese field and growth chamber studies show that the CMST can be a tool in helping farmers identify fields where WCC aboveground biomass is a ccumulating potentially toxic levels of NO3--N. Additionally, plant pa rameters such as nitrogen uptake, biomass, and grain yield of WCC grow n under growth chamber conditions were correlated with the CMST readin gs conducted at the growth stage, Feekes five (P<.05). The growth cham ber results suggest that if WCC are grown for grain production, the CM ST can help identify the needs for additional nitrogen (N) fertilizer application at Feekes five.