Effects of soil and water management on the productivity of irrigated pastures in the Goulburn Valley

The water status and productivity of perennial pastures were monitored during irrigation cycles. Measurements of leaf water potential reflected the water status of the pastures and when this deteriorated with the developing shortage of water after irrigation, various responses of the pastures were recorded. These included the rate of leaf elongation, canopy conductance, and the rates of net photosynthesis and evapotranspiration. The Parameters of gas-exchange were Measured using open-system, field chambers. The studies were designed to characterise and Compare the responses of these Pastures as water deficit developed, and to quantify their,effects on productivity. Experimental plots included swards of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and paspalum (Paspalum dilatatum) both as monocultures and as mixtures of the three species. These swards were growing on two soil profiles, both of which were Lemnos loam. The first profile was in its normal state and had been under pasture for fifteen years. The second had been structurally and chemically modified in 1979 to minimise the limitations to plant growth. It had been re--sown to pastures in 1980. The productivity of monocultures and mixed swards of each species on each profile was investigated as water shortage developed. In all species the responses to water shortage were the same, involving a reduction in the rate of development of leaf area,, followed by a reduction in photosynthesis per unit leaf area as the lack of water became more severe. The sensitivity of each species was distinct, with white clover being the most sensitive, ryegrass intermediate and paspalum the least. On modified soil, all species were less affected than on the normal profile but the order of sensitivity was the same. Overall, these experiments showed that after a typical irrigation cycle, which is about 8 days, water shortage reduces the productivity of white clover by 50%, ryegrass by 20% and paspalum by 5-10%. These limitations to productivity were overcome to some extent by modifying the profile, so that after an equivalent period the productivity of white clover was only reduced by about 208 and paspalum was not affected at all. The ability of paspalum to maintain high levels of productivity during an irrigation cycle had the effect of promoting this species in a mixed sward, its dominance becoming greater as the shortage of water became more severe. In order to achieve balanced pastures of maximum productivity, farmers in the Goulburn Valley need to reduce the limits to growth imposed by the physical and hydraulic characteristics of the soils. This may be achieved by irrigating more frequently or by undertaking some form of profile modification