Residual value of nitrogen fertiliser in a grazing system.

Abstract

Proc. Aust. Sot. Anim. Prod. Vol. 19 RESIDUAL VALUE OF NITROGEN FERTILISER IN A GRAZING SYSTEM W .K. EHRLICH and R. ir: COWAN Queensland Dept of Primary Industries, Mutdapilly Research Station, M/S 825, Ipswich, Qld 4305. Nitrogen fertiliser increases the quality and quantity of tropical grass and milk yield of cows (Cowan et al. 1987). This experiment was conducted to assess the impact on milk and pasture yields of reducing the applied nitrogen level for pastures with a history of high nitrogen fertiliser inputs. The experiment started in November 1989. Cows grazed dryland Rhodes grass (Chloris gayana cv. Callide) at 2 cows/ha in the summer and dryland oats (Avena sativa) at 2.5 cows/ha in the winter. Twenty multiparous Holstein-Friesians were allocated to 2 replicates of the following treatments; 150/150, 300/300, 300/150, 450/150 and 600/150. The first number of each pair refers to the rate of N (kg/ha) applied per year to Rhodes grass for 6 years before November 1989 and the second number to the rate of N applied per year subsequently. The nitrogen was applied in September, December and March. The oats were fertilised at 100 kg N/ha.year at planting. The data presented in Fig. 1 are for the summer growth period, January to April 1991 (the second year of the experiment). All cows received 5 kg cracked sorghum per day after morning milking. Fig. 1. Milk response in the second year of the experiment (m, previous fertiliser rate maintained; 0, previous rate reduced to 150 kg N/ha.year). Vertical bar is the s.e. of mean. Milk yield was highest for treatments which had previously received 450 or 600 kg N/ha.year. Production for the 300/150 treatment was below that of the 300/300 and 150/150 treatments. Pasture yield also declined for the 300/150 treatment when compared to 300/300, averaging 3950 and 5000 kg DM/ha respectively. Yields were intermediate for 450050 and 600/150 fertiliser treatments, averaging 4300 kg DM/ha. Leaf percentage in all pastures was low, in the range 1 l-18% DM. The results suggest there is a substantial residual effect of level of nitrogen fertiliser on pasture and milk yields. COWAN, R. T., LOWE, K. F., UPTON, P. C. and BOWDLER, T. M. (1987). Proc 4th AAAP An. Sci. Congress, Hamilton, N.Z. p. 146. 328