PSII-8 Impact of Grazing High Nitrate Forages on Serum Nitrate Concentration and Production Performance in Beef Heifers

Abstract

Abstract The objective of this study was to compare nitrate concentrations in soil, forage, and the serum of animals grazing high (HNO3) and low (LNO3) nitrate forages. The fall before the study began one-half of a 6-ha field was amended with manure to increase nitrate levels in the soil. Forage barley (Hordeum vulgare L. var. Vernal) was planted in spring at 45 kg/ha. The 6-ha field was divided into twenty-seven 0.2-hectare paddocks (n = 13 HNO3; n =14 LNO3). Twenty-four crossbred yearling heifers were stratified by body weight (389.8 ± 2.8 kg) and randomly assigned to HNO3 (n = 12) or LNO3 (n = 12) with three replicates per treatment. Heifers entered paddocks when forage was 30 to 46 cm tall and removed when residual height was 10 cm. Heifers grazed for a total of 58 days. Each grazing period lasted 7-8 d. Soil samples were collected at the beginning of the study and sent to a commercial laboratory for soil analyses. Forage samples were collected prior to animal introduction to each grazing paddock, and before re-grazing. Heifers were weighed on d1 and every 14 d. Serum samples for nitrate analysis were collected on d 28, 42 and 57 of the trial. Forage samples were analyzed by near infrared spectrometry. Serum samples were analyzed for nitrate concentrations by ion chromatography. Data were analyzed using PROC GLM of SAS. Analysis of body weights and average daily gain included repeated measures. Soil, nitrate-nitrogen (ppm) and nitrate-nitrogen content (kg/ha) were greater (P < 0.001) in HNO3 compared to LNO3 paddocks. Soil P (P < 0.03) and K (P = 0.0002) levels were also greater in HNO3 than LNO3 paddocks. Average forage nitrate ion concentrations were greater (P < 0.0001) for HNO3 (1.58%) compared to LNO3 paddocks (0.42%). Forage nitrate ion concentrations increased (P < 0.002) with increasing grazing days. Peak nitrate ion concentration in HNO3 paddocks was 2.45%. Forage ADF, TDN, NDF, NEm, NEg were similar (P ≥ 0.07) between treatments. Forage crude protein was greater in HNO3 compared to LNO3 paddocks (P < 0.0001). Serum nitrate levels were undetectable (< 2 ppm) regardless of treatment. Heifer bodyweight and average daily gain were similar (P > 0.7) across treatments. Heifers gained an average of 1.1 kg/d resulting in a linear increase (P < 0.04) in body weight. In summary, grazing vegetative forage containing toxic concentrations of nitrate did not result in an increase in the serum nitrate concentration. Animal performance also did not differ across dietary treatments. This is suggestive of adaptive changes that prevented toxicity and compromised production performance for heifers grazing high nitrate forages. Supported by the Mary A. Turner Research Endowment