Nitrogen-fertilised grass in a subtropical dairy system 3. Effect of stocking rate on the response to nitrogen fertiliser

Abstract

Two stocking rates, one as practised on farms (2 cows/ha) and the other 50% higher, were assessed for effect on pasture and milk yield response to applied nitrogen (N) fertiliser (0-600 kg N/ha. year) for Holstein-~Friesian cows grazing Rhodes grass (Chloris gayana) cv. Callide pastures. Pastures were grazed in combination with grazing oats for winter, with overall farm stocking rates of 1.17 and 1.37 cows/ha for ' the 2 treatments. Cows were maintained on these areas for 3 years. Cracked grain was given at 0.8 t/cow. year, and hay or silage supplements were given when green grass yield was <0.5 t dry matter (DM)/ha. The incremental response (P<0.05) in milk yield to each kg increase in level of applied N was 4.93 kg/ha at 1.17 cows/ha and 1.64 kg/ha at 1.37 cows/ha. The amount of conserved forage fed at the high stocking rate increased (530 and 970 kg/ha. year at 1.17 and 1.37 cows/ha), and financial margins over costs were reduced at the high stocking rate. The low milk response at the high stocking rate was associated with a low response in pasture growth. At <2 t pasture DM/ha on offer, incremental response to applied N declined, and there may have been an excessive loss of N through volatilisation in heavily grazed pastures. Milk yield per cow was closely related to total pasture yield on offer (P<0.01), and to leaf and stem yields (P<0.05). Relationships were stronger in summer and autumn than in spring. Over the full year, milk yield increased by 1.24 kg/kg leaf DM or 0.24 kg/kg total pasture DM on offer. At the higher stocking rate, surface soil (0-10 cm) concentrations of phosphorus and nitrate were higher than at the lower stocking rate. We conclude that in areas of moderate rainfall (<1000 mm/year) in the subtropics, high stocking rates resulting in low pasture yields and exposed ground surface will be associated with low efficiency of use of applied N.