Management and nitrogen fertiliser options for increased pasture productivity in dryland hill systems

Abstract

Legume productivity in dryland pastures is often less than 10% of the total annual growth and dominated by annual species. White clover content of these pastures is often less than 5% because the period of potentially most active growth coincides with that of low soil moisture levels. Therefore variations in summer rainfall have a dominant effect on the white clover content of the pasture in the following spring. Predictive relationships for white clover content are presented. Attempts to introduce alternative legumes into dryland hill pastures have generally been unsuccessful because of the harsh climate during late spring-summer and competition from resident species. It is unlikely that further research will change this situation. This paper compares improved legume productivity from dryland pastures with the use of N fertiliser as options to increase winter and spring pasture growth for increased economic gain. To optimise productivity and summer persistence of resident legumes, grazing management, especially in spring, must aim to avoid accumulation of surplus grass that will shade associated legumes. During winter and early spring, when soil moisture is adequate, pasture growth in dryland hill pastures is limited by inadequate soil nitrogen (N), due to low legume content and N fixation, even where soil phosphorus (P) is at moderate to high levels. Consequently, pasture responds well to applied N fertiliser. At Waipawa in Hawke's Bay, the magnitude of response to N fertiliser was inversely related to the legume content of the pasture and so was greatest (e.g. up to 47 kg DM/kg N) on steep north aspects with little clover, and much less (8-17 kg DM/kg N) on easy slopes. In winter, N fertiliser offers reliable and significant increases in pasture growth. To optimise economic returns from the additional pasture dry matter, winter stocking rates could be increased. However, a simulation study showed that the maximum economic benefit would be obtained by maintaining ewe numbers and increasing fecundity to produce more lambs. Key words: hill country, legumes, nitrogen fertiliser, pastoral dryland