Nitrogen-fertilised grass in a subtropical dairy system. 1. Effect of level of nitrogen fertiliser on pasture yield and soil chemical characteristics

Abstract

The effects of five levels of nitrogen (N) fertiliser on pasture yield and composition and soil chemical characteristics of Rhodes grass (Cholris gayana) cv. Pasture under grazing and cutting in a subtropical environment of 800 mm annual rainfall were studied over 6 years. Pasture received annual basal dressings of superphosphate (250 kg/ha) and potassium chloride (125 kg/ha for plots, 125 kg/ha . alternate year for grazed paddocks). Urea was applied in 3 equal dressings in September, December, and February, at rates equal to 0, 150, 300, 450, and 600 kg N/ha. year. Stocking rate was 2 Holstein-Friesian milking cows/ha throughout the year, and separate areas of grazing oats (0.4 ha/cow), cracked grain (0.8 t/cow.year), and hay or silage were used to supplement pasture. Under both cutting and grazing, pasture dry matter (DM) yield increased linearly with applied N to about 300 kg N/ha. year, with little further increase at higher levels. Under grazing there was evidence of a decrease in yield at 600 kg N/ha.year, due to total death of the pasture following frosting in winter and the need for regeneration from seedlings in spring; this regeneration was slowed by the large amount of surface litter. Grass N contents decreased and phosphorus and potassium contents decreased with increasing levels of applied N. All 3 nutrients increased from year 1 to 6. Leaf content of grazed pasture was highest during spring (>30% DM) and declined through to autumn (<20% DM), with no consistent effect of N level. Leaf content was consistently higher under cutting (>50%) but was not altered by level of applied N. Soil nitrate-N levels increased (P<0.05) with level of applied N, from 4 to 42 mg/L at 0 and 600 kg N/ha. year, respectively. After 6 years of fertilisation at 300 and 450 N/ha.year, nitrate-N levels were similar to those for 600 kg N/ha.year. Soil pH decreased (P<0.05) with applied N, by 0.15 and 0.28 units annually for 150 and 600 kg N/ha.year, respectively. We conclude that in this environment large responses in pasture growth occur under both cutting and grazing to levels of applied N to about 300 kg N/ha.year, with little response beyond this level.