Influence of fertiliser nitrogen on nitrogen fractions and nonstructural carbohydrates in grazed grass-clover herbage

Abstract

Contents of N and nonstructural carbohydrate fractions were measured over a year in herbage from a grazed ryegrass-white clover pasture, which had been fertilised with limeammonium nitrate at an annual rate of 0 (N₀), 112 (N₁₁₂), and 448 (N₄₄₈) kg N/ha. Total N content of the mixed herbage was often greater than 4% dry weight, and was highest in early spring and lowest in late summer. Levels in many cuts were increased significantly by the N₄₄₈ treatment; the N₁₁₂ treatment had little effect. The non-protein N and nitrate N contents were also highest in the N₄₄₈ herbage, particularly in early spring; the level of nitrate N was also high in autumn. Most of the nitrate N occurred in the grass component. Total water-soluble carbohydrates, and those soluble in hot water but insoluble in cold water, usually comprised less than 12% and 1% dry weight respectively of the mixed herbage. Total water-soluble carbohydrates were negatively and significantly correlated with both total N and “protein” N contents. The total N/total water-soluble carbohydrate ratio was, with one exception, greater than 0,3. No clear seasonal trends in the carbohydrate fractions were evident. Effects due to N treatments were generally small although sometimes significant for rhe hot-water-soluble fraction. The highest level of fertiliser N brought about high total N/total water-soluble carbohydrate ratios, and high herbage nitrate N contents, namely 0.66% dry weight in early spring and an average of 0.45% in August–January. The implications of these results for animal health are discussed.

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T. W. Collie

Deceased