Forage & Grazing Lands

loaded and a portion of the nitrate is absorbed by the blood stream as nitrate and nitrite. Some of the nitrate Nitrate contained in forage may pose performance and health risks that is absorbed recycles back to the rumen through to ruminants. With timely and accurate assessment of forage nitrate levels, steps to reduce the risks of excessive nitrate intake by livestock saliva thereby adding again to the nitrate pool in the can be applied. Traditionally, plant nitrate is measured in the laborarumen. In contrast, absorbed nitrite inhibits the oxygen tory from finely ground oven-dried tissue, which is slower than nitrate transporting capacity of red blood cells by oxidizing the quick-test assays of plant sap. However, use of plant sap nitrate referrous iron of hemoglobin to ferric iron (methemogloquires calibration to nitrate in dried samples. Winter wheat forage bin) leading to chronic animal performance problems (Triticum aestivum L.) collected at jointing and heading from fields including suppressed appetite, rate of weight gain, and fertilized with 56 to 235 kg N ha 1 was used to compare a laboratory milk production (Hibbs et al., 1978; Osweiler et al., 1985, flow injection analysis (FIA) method (Cu-Cd reduction column) with p. 460–467; Undersander et al., 1999) and in severe nontraditional laboratory microplate (M-NaR) and field-test (F-NaR) cases, acute toxicity and possibly death. With timely and enzyme linked kits (nitrate reductase, E.C. 1.6.6.1), and two handaccurate assessment of nitrate concentration in forage held quick-test nitrate assays using a card mounted ion specific electrode (ISE-card) and test strip reflectance meter (TSR). Hot-water and water sources, potential risks of livestock exposure extracts of oven-dried samples and fresh samples macerated in propato excessive nitrate intake may be properly managed nol solution with a high-speed hand-held blender were prepared. or avoided. Compared with FIA, mean differences in tissue nitrate were nearly Typically, plant tissue nitrate levels increase with inalways greater (13–66%, P 0.05) with the other methods. For dried creasing amounts of N fertilizer applied to annual ceresamples, these differences were due partially to extract interferences als and cool-season grasses used as forages (Moeller that suppressed detection of nitrate with FIA and falsely elevated and Thurman, 1966; Wright and Davison, 1964). In the nitrate detection with the ISE-card. Interferences caused only a slight southern Great Plains, the primary cool-season forage underestimation of forage nitrate with TSR, and were nearly absent used for stocker cattle (Bos taurus L.) enterprises is with the M-NaR assay. The ISE-card was the most variable and deviwinter wheat. In many cases, producers grow wheat as ated the most from the FIA. Nitrate extraction over a nearly fourfold range was 18% less from fresh than oven-dried tissue. The quicka dual-purpose crop for both forage and grain (Pinchak test consumable cost per nitrate assay was similar for F-NaR and TSR et al., 1996; True et al., 2001). Compared with grainmethods, but the TSR was easier to use. Because a hand-held meter only and graze-only wheat systems, dual-purpose winter is not required with F-NaR, initial startup cost can be reduced. Both wheat diversifies farming choices and may reduce ecoTSR and F-NaR performed well for quick-tests of tissue nitrate. nomic risks (Redmond et al., 1995), but has its own recommended set of management practices to assure success. Dual-purpose and graze-only wheat should be N risk to ruminants can be affected by various planted earlier than grain-only wheat. To assure early animal factors such as rumen microbes, age and fall growth, fertilizer N needed to achieve a desired condition, environmental stresses, diet and water qualgrain yield plus additional N to account for N removal in ity, as well as several plant factors including nutriconsumed forage is usually applied at planting (Krenzer, ent management, species, growth stage, environmental 1994; Zhang et al., 1998). Oklahoma grown wheat fertilstresses, and nonstructural carbohydrate level (Alaized with 0 to 168 kg N ha 1 had leaf NO 3 –N values at boudi and Jones, 1985; Crawford et al., 1961; Wright and Feekes growth stage 5 (pseudostem strongly erect [Large, Davison, 1964). Forages containing less than 1000 mg 1954]) that ranged from 99 to 7960 mg kg 1 (Raun and NO 3 –N kg 1 (dry weight basis) usually pose no risk for Westerman, 1991). At this growth stage, two of the four cattle (Strickland et al., 1995; Undersander et al., 1999). Oklahoma site-year combinations fertilized with 90 kg Low levels of ingested nitrate are reduced by rumen N ha 1 had leaf NO 3 –N values exceeding a potentially bacteria to nitrite and then ammonia (Cowley and Colrisk onset level of 1000 mg kg 1. These nitrate levels lings, 1977), and any excess ammonia absorbed by the were present at a time shortly before cattle would norblood stream is excreted in the urine as urea. However, mally be pulled-off for dual-purpose wheat production. when high levels of nitrate are ingested, the capacity of Nitrate in plant tissue is readily water-soluble and is the normal nitrate conversion process becomes overmost often extracted from samples that have been ovendried and finely ground (Anderson and Case, 1999). In USDA-ARS, Grazinglands Research Lab., 7207 W. Cheyenne St., El Reno, OK 73036. Received 12 Feb. 2003. *Corresponding author Abbreviations: FIA, flow injection analysis; F-NaR abs, field nitrate ([email protected]). reductase kit used with spectrophotometer absorbance detection; F-NaR vis, field nitrate reductase kit used with visual reading; ISEPublished in Crop Sci. 44:218–226 (2004).  Crop Science Society of America card, ion specific electrode card; M-NaR, laboratory microplate nitrate reductase kit; NaR, nitrate reductase; TSR, test strip reflectometry. 677 S. Segoe Rd., Madison, WI 53711 USA