Effects of applying dairy wintering barn manure of differing C:N ratios directly to pasture on N mineralisation and forage growth

Two experiments were conducted to examine the effect of applying spent wintering barn bedding (termed ‘manure’ for the rest of this paper) to a pasture soil. An Incubation study compared rates of mineralisation of manures of differing storage durations (fresh, 2.5 months and 12 months storage) and C:N ratios (26:1, 22:1, 17:1 and 12:1), added to soil at a rate of 289 mg N kg-1 soil. The second component of this work was a field experiment conducted in South Otago in southern New Zealand. This experiment looked at pasture response to manure under three treatments; control; stored manure (c.70 kg N ha1; C:N of 17:1); and Urea (30 kg N ha-1). Incubation results suggested the addition of a nitrogen source, such as urea, to the manure product at a rate to bring the C:N ratio closer to 12:1 resulted in a priming of the manure and an increase in the N available to plants above that of the added urea and the manure. Field experiment results suggested that application to pasture of carbon-based manure with a high C:N ratio (above 18:1) resulted in a period of N immobilisation and pasture growth restriction. Knowledge of the C:N ratio of a manure product may help guide farmers in determining the best time to apply manure to soil to both maximise pasture growth and minimise potential nutrient losses to the environment.