410 Nitrate Concentrations of Annual Forages Grown for Grazing in Nebraska.

Abstract

Samples (n=443) of annual forages, containing less than 25% DM, submitted for nitrate analysis to a commercial lab in Nebraska (Ward Laboratories, Kearney, NE) during 2016–2017, were classified into five groups based on the submitted description: 1) brassica (turnip, radish, collard; n=63), 2) mix (cover crop or multiple annual forage species; n=34), 3) small grain (oat, rye, triticale, wheat, barley; n=70), 4) millet (pearl, foxtail, german; n=40), or 5) sorghum/sudan (cane, sorghum, sudangrass; n=236). Brassicas contained the greatest (P < 0.01) concentration of nitrate (NO₃N; DM basis) at 4060 ± 334 mg/kg. The remaining groups did not differ (P ≥ 0.10), with NO₃N of 1806 ± 364, 1564 ± 172, 1391 ± 418 and 1008 ± 317 mg/kg for mix, sorghum/sudan, millet and small grain, respectively. The threshold for toxicity in ruminants is suggested to be greater than 2100 mg/kg NO₃N. A large proportion (48%) of the brassica samples were above the suggested toxicity threshold, while 20 to 28% of the other forages were above this threshold. In addition, six fields in which small grain (oats or rye) and brassica mixes were planted in the late summer were sampled in the late fall. Despite being planted in the same fields, the brassicas (turnips and radishes) contained significantly greater (P < 0.01; SEM±1170) NO₃N than the small grains (1611 mg/kg). Radish tops (9248 mg/kg) and roots (9073 mg/kg) did not differ (P = 0.92) in NO₃N and tended to (P ≤ 0.12) contain more than turnip tops (5932 mg/kg) and roots (6354 mg/kg) which did not differ (P = 0.78). In these six fields, all brassicas contained above 2100 mg/kg NO₃N. Both the commercial lab data, and the comparison of mixes in fields, suggest that brassicas accumulate more nitrate than annual grasses, and frequently exceed the level considered to be toxic using traditional guidelines.