Effect of nitrogen potassium and phosphorus fertilization nitrogen source and clipping on potential tetany hazard of brome grass

Follett, R.F.; Power, J.F.; Grunes, D.L.; Klein, C.A.

Plant and Soil 48(2): 485-508


ISSN/ISBN: 0032-079X
DOI: 10.1007/bf02187256
Accession: 005256990

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Early-season effects of N source, N, K and P fertilization, and clipping (to simulate grazing) on potential tetany hazard of bromegrass (Bromus inermis) as indicated by the chemical composition of its forage was determined. Tetany is a metabolic disorder of ruminants resulting from forage with low Mg availability. Chemical components considered in the forage were inorganic cations, organic acids, aconitate and % total N/% total water soluble carbohydrate (N/TWSC). Differences between the sum (in meq/kg) of inorganic cations (Mg, Ca, K and Na) and inorganic anions (Cl, NO3, H2PO4 and SO4) in forage were defined as the concentration of organic acids (C-A). Soil was Parshall fsl, a Pachic Haploboroll. Yields and chemical composition of oven-dried forage from previously unclipped and reclipped plots were determined at 3 wk intervals beginning May 22 and June 12, respectively. A water budget was determined using soil-water and rainfall data. Forage yields were increased 2- to 3-fold by N fertilization with the NO3-N source generally outyielding the NH4-N source. A slight additional yield response to that obtained with N alone was obtained with K + P fertilization but not with K or P alone with or without N. Much less total forage was removed from reclipped plots than from unclipped plots. Forage Mg content was decreased only slightly by K or NH4-N fertilization. Soil analysis indicated that high NH4-N levels were present at the May 22 harvest. Magnesium and Ca concentrations were only slightly affected by N fertilization; however, K, K/(Ca + Mg), total N, C-A, and aconitate were increased. Reclipping increased Mg, N, K, N/TWSC, C-A, and aconitate. Estimates of blood-plasma Mg concentrations were obtained by using the data for plant N, K and Mg. These estimates did not indicate increased tetany hazard as a result of reclipping, but did indicate increased tetany hazard from N fertilization. Forage C-A and aconitate concentrations were decreased by fertilization with KCl which seemed to be caused by the increased Cl concentrations in the forage. Estimates of quantities of Mg, arriving at the root surfaces from the soil by mass flow, far exceeded amounts of Mg in the forage. Mass flow seemed to be the principal mechanism by which Mg and Ca arrived at root surfaces but this mechanism was much less important for K. An increased potential tetany hazard resulting primarily from N fertilization with either NH4-N or NO3 -N sources was indicated. The potential for increased forage and livestock-carrying capacity with N fertilization is very large. Management practices corroborated by livestock data are vitally needed to minimize tetany hazard while increasing bromegrass yields by N fertilization.

Effect of nitrogen potassium and phosphorus fertilization nitrogen source and clipping on potential tetany hazard of brome grass