MILK-PRODUCTION FROM COWS GRAZING ON TROPICAL GRASS PASTURES .1. EFFECTS OF STOCKING RATE AND LEVEL OF NITROGEN-FERTILIZER ON THE PASTURE AND DIET

A 3-year experiment was conducted at Kairi Research Station on the Atherton Tablelands, Queensland [Australia], to determine the effects of stocking rate and applied nitrogen fertilizer on the pasture yield and composition, diet selection by cows and soil fertility of Gatton panic (Panicum maximum cv. Gatton) pastures. Thirty-two Friesian cows were used in a 4 .times. 2 factorial design: four stocking rates (2.0, 2.5, 3.0, and 3.5 cows/ha), each at two rates of fertilizer application 200 and 400 kg N/ha year. The higher rate of fertilization increased the pasture green dry matter on offer at all samplings (P < 0.01); the increase ranged from 1106 kg/ha in summer to 548 kg/ha in spring. Green dry matter decreased (P < 0.1) with increasing stocking rate, with mean yields of 3736 and 2384 kg/ha at 2.0 and 3.5 cows/ha, respectively. Weed yields increased over the 3 years at the higher stocking rates for pastures receiving 200 kg N/ha .cntdot. year. The crude protein content of leaf and stem increased with increasing stocking rate and amount of applied nitrogen fertilizer. Values ranged from 12.1 to 26.5% of dry matter (DM) in leaf and from 3.7 to 13.8% DM in stem. In leaf, sodium concentration (range 0.05-0.02% DM) was increased, while phosphorus cocnentration (range 0.21-0.44% DM) was decreased by the higher rate of fertilizer application. Plant sodium and phosphorus levels were inadequate for high levels of milk production. Dietary leaf content and crude protein contents were consistently increased by both a reduced stocking rate, and the higher rate of fertilization. Cows were able to select for leaf and at the lowest stocking rate, leaf in the diet averaged 38%; while the leaf content of the pasture was 20%. Dietary leaf content ranged from 38 to 57% in summer and from 11 to 36% in winter. Dietary crude protein ranged from 13 to 15% in summer and from 7 to 11% in winter and was positively correlated with pasture crude protein content and dietary leaf percentage. Soil pH decreased (P < 0.05) from an overall mean of 6.3 in 1976 to 6.1 at 200 N and 5.8 at 400 N in 1979. Soil phosphorus status remained stable, while calcium and magnesium levels were lower (P < 0.01) after 3 years.