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. Some 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 (Triticum aestivum L.) and rye (Secale cereale L.) are reported to accumulate high levels of nitrate nitrogen (NO3--N) in aboveground biomass that can be toxic to animals. Evaluation and calibration of a quick Cardy Meter(2) Sap Test (CMST) for determination of NO3--N status in the field will facilitate the management of these WCC. Field and growth chamber studies were conducted to correlate the CMST with laboratory procedures and with plant and soil parameters. In field and growth chamber studies, the CMST was correlated with standard dry tissue NO3--N laboratory analysis (P<.001) and with soil inorganic N content (P<.05). These field and growth chamber studies show that the CMST can be a tool in helping farmers identify fields where WCC aboveground biomass is accumulating potentially toxic levels of NO3--N. Additionally, plant parameters such as nitrogen uptake, biomass, and grain yield of WCC grown under growth chamber conditions were correlated with the CMST readings conducted at the growth stage, Feekes five (P<.05). The growth chamber results suggest that if WCC are grown for grain production, the CMST can help identify the needs for additional nitrogen (N) fertilizer application at Feekes five.