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Potential Tetany Hazard of N-Fertilized Bromegrass as Indicated by Chemical Composition¹

The objective of this field experiment was to determine the effect of N fertilization on yield and chemical composition of smooth bromegrass (Bromus inermis L.) and the potential for grass tetany hazard in the northern Great Plains as indicated by chemical composition of bromegrass forage. Chemical components of forage considered in relation to the hazard of tetany (a metabolic disorder of ruminants resulting from forage with low Mg availability) were inorganic cations, organic anions, aconitate, and % total N/% total water-soluble carbohydrate ratio (N/TWSC). Soil was Parshall fine sandy loam, pachic haploborall. Yields and chemical composition of oven dried forage from plots not previously harvested were determined at approximately 3-week intervals beginning May 9. Differences between the sum (in meq/kg) of inorganic cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) and inorganic anions (C1^-, No₃-, H₂PO₄^-, SO₄^2-) in forage was defined as the concentration of organic anions (C-A).

Mature forage yield obtained from the unfertilized check plot treatment on July 29 was only 29 and 22% of yields obtained from plot treatments fertilized with 90 and 270 kg N/ha, respectively. The K/(Ca+Mg) ratios and K concentrations increased during May and early June, resulting in a K/(Ca+Mg) ratio near or above 2.2 during June and early June in oven dried forage from fertilized treatments. Potassium, expressed as a fraction K/C of the total cations (C), accounted for 35 to 74% of the cationic charge. Fertilization with N increased total N and K concentration and K/C in the forage. As K/C increased, Mg/C and Ca/C decreased and K/(Ca+Mg) increased. Aconitate and C-A concentration correlated highly with K concentration and were increased by N fertilization. Aconitate levels exceeded 1% on May 28; the 270 kg N-treatment remained above 1% through July. Nitrogen fertilizer increased N/TWSC in spring-harvested forage, compared to unfertilized forage, and greatly accentuated the peak N/TWSC values occurring in late spring samples.

This study indicated that although potential for increased forage and livestock-carrying capacity with N fertilization is tremendous, N-fertilization may result in a potential tetany hazard to ruminants. Therefore, management practices are needed which minimize tetany hazard while bromegrass yields are increased by N fertilization.

Key Words: Hypomagnesemia • Grass tetany • Organic acids • Range • Forage • Organic Anions • Ionic balance • Carbohydrates

² Follett and Power are soil scientists at the USDA Northern Great Plains Research Center, Mandan, ND 58554. Grunes is a soil scientist, and Hewes is a former biological science technician at the U.S. Plant, Soil, and Nutrition Lab., Ithaca, NY 14850. Mayland is a soil scientist at the USDA Snake River Conserv. Res. Center, Kimberly, ID 83341.

Received for publication January 10, 1975.