73 Persistence: the key to legume performance in summer-dry hill country M.B. DODD, G.W. SHEATH AgResearch Ltd., Ruakura Research Centre, Private Bag 3123, Hamilton mike.dodd@agresearch.co.nz Abstract In North Island hill country, a number of environmental and management factors interact to limit the performance of herbaceous legumes in pasture swards. Summer-autumn moisture stress appears to be the dominant influence, particularly because of its irregularity. Legume performance is a function of establishment success, persistence and productivity. Long-term persistence of genotypes in particular should be a primary focus in the development of new germplasm. Secondary to this is the importance of improving herbage productivity and quality. We present evidence for this view from field studies of white and subterranean clover, as well as preliminary examinations of other legume species. Key characteristics contributing to enhanced persistence are prostrate growth forms with high numbers of growing points below the grazing horizon; nodal rooting and root tolerance of low soil pH; flowering patterns that are timed to avoid the dry period, yet sufficiently flexible to cope with the unpredictability of that period; and seed mechanisms that protect against “false strike” following short- lived summer rains. Key words: clover, hill country, legumes, persistence, summer-dry Context - the hill country environment for legumes North Island summer–dry hill country pastoral environments (comprising over 4 million ha) present a challenging context for the use of temperate pasture legumes in grazed systems. Numerous factors interact to limit legume performance, such as seasonal moisture deficits, low soil fertility, soil acidity and variable grazing pressure. Complicating these factors is the high degree of spatial variability characteristic of hill country. This is driven by the range of slope and aspect combinations (as they influence temperature and moisture regimes), slope instability and the physical and chemical disturbances of grazing animals. Variability in the timing and intensity of rainfall events in the late summer-early autumn period, combined with low rainfall effectiveness in wetting steepland soils, can create an acute short-term drought situation (Bircham & Gillingham 1986). This affects growth and reproduction at a crucial stage in the phenology of pasture legumes. In addition, the between-year variability in weather patterns demonstrates that these hill country sites do not have a consistent climate. For example, summer/autumn moisture balances in the western Waikato region have ranged between +146 mm to -336 mm through the 1990s (Dodd et al. 2001). While nitrogen is often the most limiting nutrient for pasture growth in hill country (Chapman & Macfarlane 1985), phosphorus is the major nutrient constraint for legumes (Edmeades et al. 1984). White clover has been shown to be at a competitive disadvantage to browntop in accessing soil P and moisture (Jackman & Mouat 1974), a factor contributing to clover suppression in mixed swards (Harris 1974). Trace elements such as molybdenum can also be limiting for clover production (Jaiyou et al. 1993). The characteristically low pH (4.5–5.5) of many hill soils limits pasture production (Edmeades et al. 1984). Legumes are particularly sensitive to acid soils, mainly related to the sensitivity of the legume-rhizobia symbiosis. Another factor is the increase in the availability of metal cations (such as aluminium and manganese) with low pH (Edmeades et al. 1983). Red, white and subterranean clovers are moderately sensitive to Al in the soil solution relative to other temperate pasture species (Wheeler et al. 1992). Throughout the course of a year, grazing pressure is highly variable in hill country. Grazing affects the competitive balance of different species in the sward indirectly through modification of the shading environment of the sward (Hay & Hunt 1989). Grazing also has direct effects - under certain conditions of sward structure legumes are preferentially grazed; and the removal of reproductive structures is particularly detrimental to annuals (Sheath & Persistence: the key to legume performance in summer-dry hill country