Summary
The objective of this field study was to determine early-season effects of N source, N, K, and P fertilization, and clipping (to simulate grazing) on potential tetany hazard of bromegrass (Bromus inermis L.) as indicated by the chemical composition of its forage. 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 per cent total N/per cent 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-week 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 have been 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.
This study indicated an increased potential tetany hazard resulting primarily from N fertilization with either NH4-N or NO3-N sources. However, the potential for increased forage and livestock-carrying capacity with N fertilization is very large. Therefore, management practices corroborated by livestock data are vitally needed to minimize tetany hazard while increasing bromegrass yields by N fertilization.
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Additional information
Contribution from Soil, Water, and Air Sciences, North Central and Northeastern Regions, ARS-USDA.
Follett, Power, and Grunes are soil scientists and Kleinis a biological laboratory technician. Follett is now National Program Staff Scientist, ARS, BARC-West, Beltsville, MD 20705. Power and Klein are at the USDA Northern Great Plains Research Center, Mandan, ND 58554, as formerly was Follett. Grunes is at the U.S. Plant, Soil and Nutrition Laboratory, Ithaca, NY 14853.
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Follett, R.F., Power, J.F., Grunes, D.L. et al. Effect of N, K, and P fertilization, N source, and clipping on potential tetany hazard of bromegrass. Plant Soil 48, 485–508 (1977). https://doi.org/10.1007/BF02187256
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DOI: https://doi.org/10.1007/BF02187256